At The Repair Bench
Chris Prioli, AD2CS
A Monthly Column Describing A Recent Repair Bench Event
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Troubleshooting Resources
The link below is the full version of AD2CS Basic Troubleshooting of Electronic Devices and Equipment four part article starting in the September 2022 CrossTalk
W2MMD Clubhouse Test & Repair Bench
Equipment and Supplies List
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Kenwood TM-241A
Kenwood TM-241A 2M Transceiver - December 2022
Every now and then, a repair comes along that is both easy and difficult at the same time. This month’s repair case history is one of those occasions. This is the story of a simple - and not so simple - repair of a Kenwood TM-241A fifty-watt 2-meter mobile transceiver.
The radio came to me with the complaint of being inoperative on transmit, which was easily verified with a simple output test into my Bird 43 directional wattmeter and a dummy load. The Bird 43 showed zero output power from the radio. During this test, however, I also noted that the unit failed to maintain its last-used settings, which told me that there was a second problem with the radio, and therefore the need for some deeper troubleshooting.
Armed with a TM-241A service manual and schematic, I set out to isolate the output power problem first. It took only a few minutes with an oscilloscope to determine that the signal was present at the input of the final amplifier, but that there was no output from that amplifier stage. A quick check of the power supply voltages to the final amp IC (Kenwood calls this the power module) showed that the operating voltages were correct, meaning that the IC was most likely a failed device. A quick online search showed that the power module, a Toshiba S-AV17, was available from many sources, including East Coast Transistor, an authorized Kenwood parts distributor. In the interest of sticking with original factory replacement parts, I ordered the IC from ECT and waited for it to come in.
Before ordering the IC, I went ahead and checked out the most likely cause of the radio failing to store any settings - a failed memory “keep-alive” battery. Kenwood did not make it very easy to replace this battery, which is a CR2032 coin cell with an insulating outer ring and welded tabs for connection to the printed circuit board. When installed, this coin cell is sandwiched between two insulators, one of which is double-sided adhesive foam which is used to affix the coin cell to the PCB.
Every now and then, a repair comes along that is both easy and difficult at the same time. This month’s repair case history is one of those occasions. This is the story of a simple - and not so simple - repair of a Kenwood TM-241A fifty-watt 2-meter mobile transceiver.
The radio came to me with the complaint of being inoperative on transmit, which was easily verified with a simple output test into my Bird 43 directional wattmeter and a dummy load. The Bird 43 showed zero output power from the radio. During this test, however, I also noted that the unit failed to maintain its last-used settings, which told me that there was a second problem with the radio, and therefore the need for some deeper troubleshooting.
Armed with a TM-241A service manual and schematic, I set out to isolate the output power problem first. It took only a few minutes with an oscilloscope to determine that the signal was present at the input of the final amplifier, but that there was no output from that amplifier stage. A quick check of the power supply voltages to the final amp IC (Kenwood calls this the power module) showed that the operating voltages were correct, meaning that the IC was most likely a failed device. A quick online search showed that the power module, a Toshiba S-AV17, was available from many sources, including East Coast Transistor, an authorized Kenwood parts distributor. In the interest of sticking with original factory replacement parts, I ordered the IC from ECT and waited for it to come in.
Before ordering the IC, I went ahead and checked out the most likely cause of the radio failing to store any settings - a failed memory “keep-alive” battery. Kenwood did not make it very easy to replace this battery, which is a CR2032 coin cell with an insulating outer ring and welded tabs for connection to the printed circuit board. When installed, this coin cell is sandwiched between two insulators, one of which is double-sided adhesive foam which is used to affix the coin cell to the PCB.
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Accessing the battery is the not-so-easy part and involves removal of the front bezel of the radio, followed by the removal of a metal structural cover, and finally the front panel display unit printed circuit board. At this point, the battery voltage can be measured easily. When measured in-circuit, the coin cell showed a voltage of only 0.36 volts. With the battery basically “dead”, it was necessary to continue the disassembly. To do so, the main front panel PCB is removed from the chassis to allow access to the coin cell, which is mounted between this PCB and the main chassis. Care must be taken when de-soldering the coin cell so as not to overheat and damage the PCB.
An important point is to be certain to use JIS screwdrivers for this job, especially for the tiny screws that secure the metal front structure to the main chassis. It is extremely easy to strip the heads of these screws if the wrong screwdriver is used. Polarity alignment of the coin cell is aided by indications on the PCB referring to the + and - terminal connect points.
I ordered the coin cell from East Coast Transistor in the same order as the power module, and then I sat back and waited for the parts to come in. Delivery took eight business days. The parts arrived well-packaged and in good condition, though they had to travel via ground transportation due to the fact that the coin cell is a lithium battery and thus falls under certain transportation restrictions.
Installation of the coin cell and reassembly of the front panel was basically a reversal of the disassembly process. I noted that the control pushbutton extenders have a tendency to fall out when reassembling the unit. Apart from that hiccup, it is a straightforward process.
Installation of the power module, on the other hand, required removal of the main PCB from the radio chassis. I had elected not to remove this board until the new parts arrived, primarily so that the removal procedure would be fresh in my mind when it came to reassembly time.
I had taken several photographs prior to disassembly, which is a standard practice for me. This provides a reference for reassembly. In this case, it helped me to resolve a puzzle in that there is one plug with seven wires for which I could not seem to find the connect point. For several minutes, I struggled with trying to remember disconnecting the plug, but I simply did not remember unplugging it. Finally, by referring back to my photos, I realized that this is a plug that goes nowhere. It was simply hanging free underneath the loudspeaker behind the front panel. The other end of this harness is a seven-wire plug that connects to the front panel PCB.
Removing the PCB is not complex at all, and replacement of the power module on the PCB requires the normal care about excessive heat. Be aware that there are two mounting tabs on the S-AV17, which are secured to the rear of the chassis via machine screws, thus providing heat sinking for the IC. When reinstalling the PCB with the new power module, be sure to coat the rear surface of the IC with silicone thermal transfer grease for best heat transfer to the chassis. Again, be sure to use a JIS screwdriver for this task as well.
After reassembly was completed, it was time to test the operation of the repaired unit. The first thing that I checked was the ability of the radio to “remember” the last used settings. This was no problem; all worked as expected. Next up was the power output test. When tested with my faithful Bird 43 into a dummy load, the radio showed an output of >47 watts into a dummy load.
While the customer had made no mention of the settings memory issue, I would not have considered the radio to have been repaired completely if the battery had not been replaced. Had the customer squawked about the additional and not requested repair expense, I would probably have attempted to work out a parts/labor split with the customer for that specific part of the repair bill. As it turned out, the customer was satisfied with the overall repair and its cost as presented.
The moral of the story here is that there is often more to repair than just what the customer reports. A thorough repair tech will go the extra mile, though it is usually best to discuss any additional repairs with the radio’s owner before proceeding with those additional repair items.
See you next month…
An important point is to be certain to use JIS screwdrivers for this job, especially for the tiny screws that secure the metal front structure to the main chassis. It is extremely easy to strip the heads of these screws if the wrong screwdriver is used. Polarity alignment of the coin cell is aided by indications on the PCB referring to the + and - terminal connect points.
I ordered the coin cell from East Coast Transistor in the same order as the power module, and then I sat back and waited for the parts to come in. Delivery took eight business days. The parts arrived well-packaged and in good condition, though they had to travel via ground transportation due to the fact that the coin cell is a lithium battery and thus falls under certain transportation restrictions.
Installation of the coin cell and reassembly of the front panel was basically a reversal of the disassembly process. I noted that the control pushbutton extenders have a tendency to fall out when reassembling the unit. Apart from that hiccup, it is a straightforward process.
Installation of the power module, on the other hand, required removal of the main PCB from the radio chassis. I had elected not to remove this board until the new parts arrived, primarily so that the removal procedure would be fresh in my mind when it came to reassembly time.
I had taken several photographs prior to disassembly, which is a standard practice for me. This provides a reference for reassembly. In this case, it helped me to resolve a puzzle in that there is one plug with seven wires for which I could not seem to find the connect point. For several minutes, I struggled with trying to remember disconnecting the plug, but I simply did not remember unplugging it. Finally, by referring back to my photos, I realized that this is a plug that goes nowhere. It was simply hanging free underneath the loudspeaker behind the front panel. The other end of this harness is a seven-wire plug that connects to the front panel PCB.
Removing the PCB is not complex at all, and replacement of the power module on the PCB requires the normal care about excessive heat. Be aware that there are two mounting tabs on the S-AV17, which are secured to the rear of the chassis via machine screws, thus providing heat sinking for the IC. When reinstalling the PCB with the new power module, be sure to coat the rear surface of the IC with silicone thermal transfer grease for best heat transfer to the chassis. Again, be sure to use a JIS screwdriver for this task as well.
After reassembly was completed, it was time to test the operation of the repaired unit. The first thing that I checked was the ability of the radio to “remember” the last used settings. This was no problem; all worked as expected. Next up was the power output test. When tested with my faithful Bird 43 into a dummy load, the radio showed an output of >47 watts into a dummy load.
While the customer had made no mention of the settings memory issue, I would not have considered the radio to have been repaired completely if the battery had not been replaced. Had the customer squawked about the additional and not requested repair expense, I would probably have attempted to work out a parts/labor split with the customer for that specific part of the repair bill. As it turned out, the customer was satisfied with the overall repair and its cost as presented.
The moral of the story here is that there is often more to repair than just what the customer reports. A thorough repair tech will go the extra mile, though it is usually best to discuss any additional repairs with the radio’s owner before proceeding with those additional repair items.
See you next month…
ICOM IC-706 HF/6M/2M Transceiver - November 2022
Sometimes you wish that the customer would just be honest with you and tell you what really happened, and this month’s At the Repair Bench is an example of one of those times. It all began with a phone call from a gentleman out in western Pennsylvania. I should have realized that something was wonky when he couldn’t tell me who it was that referred him to me - or maybe he wouldn’t say. Anyway, he asked if he could ship his faithful Icom® IC-706 to me for repair, saying only that the front panel was “dead”. Naturally, I agreed to look at it, so he shipped it in.
He did an over-the-top job of packing the radio and mic, going so far as to buy some Lowe’s sheet foam and cut custom blocks to surround the radio, and then gluing the blocks together to make two half shells that fit the radio quite well, and also fit the carton perfectly. Kudos on that part! He lost some points, however, when I got into the repair… but I am getting ahead of myself.
After unpacking the radio, I put it on the bench and connected it to my power supply and dummy load, and powered it on… or at least I tried to power it on. Nothing happened. The unit was stone cold dead and unresponsive. I took the cover screws out and lifted the top cover, and I saw immediately what the problem was.
The Icom® IC-706 has a removable front panel, which connects behind the panel to a set of eight spring contacts, which in turn connect to the main PCB via a “flex circuit” or Kapton cable. Connection to the main PCB is made through the use of a top-entry edge connector that is surface-mount soldered to the main PCB. This connector was off the PCB and floating free inside the radio, tethered to the end of the Kapton cable.
Here is the part where the owner lost points… someone had been inside the radio and most likely pulled that connector off the board. How do I know this? Simple… the speaker connection (the speaker is mounted to the top cover) was unplugged.
Sometimes you wish that the customer would just be honest with you and tell you what really happened, and this month’s At the Repair Bench is an example of one of those times. It all began with a phone call from a gentleman out in western Pennsylvania. I should have realized that something was wonky when he couldn’t tell me who it was that referred him to me - or maybe he wouldn’t say. Anyway, he asked if he could ship his faithful Icom® IC-706 to me for repair, saying only that the front panel was “dead”. Naturally, I agreed to look at it, so he shipped it in.
He did an over-the-top job of packing the radio and mic, going so far as to buy some Lowe’s sheet foam and cut custom blocks to surround the radio, and then gluing the blocks together to make two half shells that fit the radio quite well, and also fit the carton perfectly. Kudos on that part! He lost some points, however, when I got into the repair… but I am getting ahead of myself.
After unpacking the radio, I put it on the bench and connected it to my power supply and dummy load, and powered it on… or at least I tried to power it on. Nothing happened. The unit was stone cold dead and unresponsive. I took the cover screws out and lifted the top cover, and I saw immediately what the problem was.
The Icom® IC-706 has a removable front panel, which connects behind the panel to a set of eight spring contacts, which in turn connect to the main PCB via a “flex circuit” or Kapton cable. Connection to the main PCB is made through the use of a top-entry edge connector that is surface-mount soldered to the main PCB. This connector was off the PCB and floating free inside the radio, tethered to the end of the Kapton cable.
Here is the part where the owner lost points… someone had been inside the radio and most likely pulled that connector off the board. How do I know this? Simple… the speaker connection (the speaker is mounted to the top cover) was unplugged.
The repair was simple enough. Fortunately, no damage was done to the PCB - all of the pads were intact and in fact had plenty of solder on them. All I had to do was to reflow the solder on the connector pins once I put the connector in position. Of course, I had to remove the FL-100 CW Narrow Filter and the FL-223 SSB Narrow Filter to allow clear work access to the connector location on the PCB. The Kapton cable itself was unhurt, so after I resoldered the connector in place, I was able to simply re-insert the Kapton cable into the connector slot.
Was it embarrassment? Was it ignorance? Who knows? All that I know is that the radio performed properly once the repair was made, and I repeated the inbound packing job for the outbound trip back to the owner. A simple repair with a nebulous cause… but one thing is certain. That connector most likely did not fall off by itself.
I would much rather have the customer tell me the truth as to what is going on when a unit comes in for repair, as it takes a lot of the guesswork out of the equation. See you next month!
Was it embarrassment? Was it ignorance? Who knows? All that I know is that the radio performed properly once the repair was made, and I repeated the inbound packing job for the outbound trip back to the owner. A simple repair with a nebulous cause… but one thing is certain. That connector most likely did not fall off by itself.
I would much rather have the customer tell me the truth as to what is going on when a unit comes in for repair, as it takes a lot of the guesswork out of the equation. See you next month!
Heathkit® HP-23B Power Supply - October 2022
Every now and then, I will come across a repair that should have been avoidable with proper equipment maintenance. Unfortunately, some maintenance is beyond the skill set of the equipment owner. This month’s repair is just such a repair.
The Heathkit® HP-23B is a multi-output power supply used in conjunction with several of that company’s ham radio equipment offerings. The PSU provides outputs of 700VDC at 250mA, 350VDC at 150mA, 250VDC at 100mA, -100VDC at 20mA, and 12.6VAC at 5.5A. The incoming power is a standard 120VAC at about 350 watts. The unit is heavy, weighing in at about sixteen pounds.
Every now and then, I will come across a repair that should have been avoidable with proper equipment maintenance. Unfortunately, some maintenance is beyond the skill set of the equipment owner. This month’s repair is just such a repair.
The Heathkit® HP-23B is a multi-output power supply used in conjunction with several of that company’s ham radio equipment offerings. The PSU provides outputs of 700VDC at 250mA, 350VDC at 150mA, 250VDC at 100mA, -100VDC at 20mA, and 12.6VAC at 5.5A. The incoming power is a standard 120VAC at about 350 watts. The unit is heavy, weighing in at about sixteen pounds.
www.radiomuseum.org
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www.radiomuseum.org
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This particular HP-23B came to me with the complaint that it would repeatedly trip the chassis-mounted 2.92-ampere circuit breaker. The owner would press the reset button, and almost immediately it would trip out again. This behavior led me to believe that there was either a dead short circuit somewhere, or a condition that would mimic a short circuit very closely. Time for some detective work.
The circuit breaker is installed in the power transformer primary winding circuit, so I started out by applying some judicious use of the ohmmeter to the primary circuit, only to find that all was as it should be. No problem there, so it was off to the secondary side.
I decided to eliminate the simplest circuit first, which is the 12.6VAC output. This circuit is simply taken from an isolated secondary winding and carried to the output plug as the two secondary leads from the winding - no additional components. There was no short circuit there, and the winding resistance was reasonable. I moved on to the DC outputs.
The three DC output circuits are easily isolated by opening a single wire lead in each circuit, which would then allow for resistance readings to ground at various points in those circuits. It did not take very long to identify an “almost” shorted 125µF filter capacitor in the medium/low voltage output circuit. This capacitor had extremely high leakage and would obviously require replacement.
I next removed and bench-tested all of the electrolytic capacitors in the unit, and I found relatively high leakage in three of the four 125µF filter capacitors and in one of the 40µF electrolytics used in the -100VDC bias circuit.
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There is a company called Hayseed Hamfest (www.hayseedhamfest.com) who provides re-cap kits for this power supply. The beauty of their kit is that the unit retains its original appearance, though it functions like brand new. The kit is offered in multiple formats - either with or without the smaller capacitors, and in standard or in increased working voltage versions. I ordered up the standard voltage kit in the complete (all capacitors) version and sat back and waited for it to come in. |
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The installation of the capacitors was a straightforward repair, as all of the parts are intended to fit exactly in place of the originals so as to preserve the factory appearance of the unit. Post-repair testing showed all to be functional and the voltages to be at the manual specified levels. I wrapped and boxed the PSU and shipped it back to the owner (after he paid my bill, of course!)
This put another one in the “WIN” column, but it did reveal an important point. When it comes to older electronic equipment, periodic maintenance should probably include at least the testing of, if not the actual replacement of, any and all capacitors that are likely to age poorly. This obviously includes filter capacitors, as they do a huge part of the job when making clean DC from the transformed AC supply.
See you next month!
Simpson 260 Series 5 Analog VOMM - September 2022
A few weeks ago, I received a carton in the mail, with a plea for help tucked into the carton alongside a Simpson 260 Series 5 analog VOMM. (Yes - VOMM is correct in this case, as the meter is a volts, ohms, and milliamperes meter.) The owner wrote that it had quit working in all modes and he had no idea why. Of course, I was up for the challenge, and it turned out to be quite interesting.
I first verified that there was no response in the meter on any function or scale. I then took off the rear cover - four screws and it was off. The first thing that I noticed was that there was no battery installed for the ohmmeter function. The 260 uses a total of five cells – four “AA” cells and one “D” cell to power the ohmmeter. Naturally, I installed a set of cells and tested the ohmmeter function again, to find that it was still inoperative.
A few weeks ago, I received a carton in the mail, with a plea for help tucked into the carton alongside a Simpson 260 Series 5 analog VOMM. (Yes - VOMM is correct in this case, as the meter is a volts, ohms, and milliamperes meter.) The owner wrote that it had quit working in all modes and he had no idea why. Of course, I was up for the challenge, and it turned out to be quite interesting.
I first verified that there was no response in the meter on any function or scale. I then took off the rear cover - four screws and it was off. The first thing that I noticed was that there was no battery installed for the ohmmeter function. The 260 uses a total of five cells – four “AA” cells and one “D” cell to power the ohmmeter. Naturally, I installed a set of cells and tested the ohmmeter function again, to find that it was still inoperative.
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I began a more thorough examination of the meter componentry, looking for burned components or broken wire solder points. What I found instead was quite surprising, and goes to show that you cannot always believe what people say. A close inspection revealed that the PCB on which most of the meter’s components are installed was cracked along its left (from the rear) side, about an inch in from the edge of the board and at an angle from the outer corner towards the center.
I removed the PCB to examine the opposite (foil) side, and found that five traces were broken along the crack. This might turn out to be a simple repair. I began by applying some cyanoacrylate glue to the crack to strengthen the board. When the glue had cured, I simply solder-bridged the cracks in the foil traces, a task made easier by the fact that the traces were solder-covered. I re-installed the “AA” and “D” cells, and tested the meter. Full operation was restored! I did some cleaning of the switches and pots inside the meter with some DeoxIT® Gold, cleaned up the exterior, and secured the back to the meter assembly.
The meter’s folding handle was quite misshapen (read : bent). I decided to remove it and repair it. The handle is secured by a shoulder bolt on either side of the case, so I removed those bolts. The handle itself is a sandwich of metal encased by a plastic covering, making it a simple task to straighten the handle completely, and then to re-bend it to its proper contour. I then re-installed the handle and the job was done.
I do not know how the PCB got broken, but it is quite obvious to me that the unit had been dropped at some point in its history, based upon the way the handle was bent. I also have a hard time believing that the owner was unaware of the broken PCB, especially as he had removed the battery for shipping the meter. He also did not take any care in packing the meter for shipment – he simply stuck it in a carton with its test leads, but with no wrapping or packing at all, leaving the meter free to bounce around in the carton. Needless to say, it did NOT go back to him the same way.
I removed the PCB to examine the opposite (foil) side, and found that five traces were broken along the crack. This might turn out to be a simple repair. I began by applying some cyanoacrylate glue to the crack to strengthen the board. When the glue had cured, I simply solder-bridged the cracks in the foil traces, a task made easier by the fact that the traces were solder-covered. I re-installed the “AA” and “D” cells, and tested the meter. Full operation was restored! I did some cleaning of the switches and pots inside the meter with some DeoxIT® Gold, cleaned up the exterior, and secured the back to the meter assembly.
The meter’s folding handle was quite misshapen (read : bent). I decided to remove it and repair it. The handle is secured by a shoulder bolt on either side of the case, so I removed those bolts. The handle itself is a sandwich of metal encased by a plastic covering, making it a simple task to straighten the handle completely, and then to re-bend it to its proper contour. I then re-installed the handle and the job was done.
I do not know how the PCB got broken, but it is quite obvious to me that the unit had been dropped at some point in its history, based upon the way the handle was bent. I also have a hard time believing that the owner was unaware of the broken PCB, especially as he had removed the battery for shipping the meter. He also did not take any care in packing the meter for shipment – he simply stuck it in a carton with its test leads, but with no wrapping or packing at all, leaving the meter free to bounce around in the carton. Needless to say, it did NOT go back to him the same way.
NanoVNA - August 2022
Fairly recently, another member – Rich Subers W2RHS – and I entered into a complex trade agreement wherein he would give me an unbuilt MFJ antenna tuner kit for me to build, and in exchange, I would replace his failed NanoVNA with a new and slightly larger one. Rich threw in the failed NanoVNA, so I figured I would give it a try to see if I could repair it.
This particular NanoVNA is encased in a multi-piece 3-D printed plastic enclosure, which does a good job of protecting the unit. It would also serve to keep things in place post-repair. Rich had told me that the unit worked, even though there was no display on the screen, as he could connect it to his phone or PC and use the functions of the NanoVNA with no trouble via the software. I suspected that the problem was in the display backlight wiring.
We were fairly sure that the problem was a bad solder joint underneath the display, which was attached to the main PCB with some strong adhesive. The first question was whether or not I could successfully separate the display and the main board. I tried using some monofilament line to cut the adhesive, but the line kept breaking, Ultimately, I tried using some AWG24 solid copper wire, which did the trick nicely.
Once I had the unit open, I powered it up and starting putting localized pressure on the end of the Kapton cable that connects the display to the main board. There are about twenty individual connections carried in that cable, so I had to determine which ones were faulty. It soon became evident that there were multiple joints open, as pressing on various connections would illuminate different LED’s in the four-LED backlight system.
These wire connections are on a pitch or spacing of about 0.050”, so I chose NOT to attack this job with a soldering iron. Instead, I fired up my hot air reflow gun and heated the area with the hot air while placing light pressure across the entire Kapton cable end. This approach was quite successful, restoring the unit backlight to full operation. The best part was that no undue damage was done to the Kapton cable, and I did not have to worry about solder bridges.
I flipped the display back in place, where the original adhesive bonded back together and held the display tightly. I then reassembled the plastic enclosure and the job was complete. Just for fun, I tried a calibration and then swept some coax, some lamp cord, and a couple of antennas. All worked as it should.
Because Rich had a new unit and did not want this one back, I casually offered it to the first taker on a Tuesday Noonday Net, and it was gone inside of five minutes. Now, another member, Anthony Cerami N2OAC, is the proud owner of a resurrected NanoVNA.
Fairly recently, another member – Rich Subers W2RHS – and I entered into a complex trade agreement wherein he would give me an unbuilt MFJ antenna tuner kit for me to build, and in exchange, I would replace his failed NanoVNA with a new and slightly larger one. Rich threw in the failed NanoVNA, so I figured I would give it a try to see if I could repair it.
This particular NanoVNA is encased in a multi-piece 3-D printed plastic enclosure, which does a good job of protecting the unit. It would also serve to keep things in place post-repair. Rich had told me that the unit worked, even though there was no display on the screen, as he could connect it to his phone or PC and use the functions of the NanoVNA with no trouble via the software. I suspected that the problem was in the display backlight wiring.
We were fairly sure that the problem was a bad solder joint underneath the display, which was attached to the main PCB with some strong adhesive. The first question was whether or not I could successfully separate the display and the main board. I tried using some monofilament line to cut the adhesive, but the line kept breaking, Ultimately, I tried using some AWG24 solid copper wire, which did the trick nicely.
Once I had the unit open, I powered it up and starting putting localized pressure on the end of the Kapton cable that connects the display to the main board. There are about twenty individual connections carried in that cable, so I had to determine which ones were faulty. It soon became evident that there were multiple joints open, as pressing on various connections would illuminate different LED’s in the four-LED backlight system.
These wire connections are on a pitch or spacing of about 0.050”, so I chose NOT to attack this job with a soldering iron. Instead, I fired up my hot air reflow gun and heated the area with the hot air while placing light pressure across the entire Kapton cable end. This approach was quite successful, restoring the unit backlight to full operation. The best part was that no undue damage was done to the Kapton cable, and I did not have to worry about solder bridges.
I flipped the display back in place, where the original adhesive bonded back together and held the display tightly. I then reassembled the plastic enclosure and the job was complete. Just for fun, I tried a calibration and then swept some coax, some lamp cord, and a couple of antennas. All worked as it should.
Because Rich had a new unit and did not want this one back, I casually offered it to the first taker on a Tuesday Noonday Net, and it was gone inside of five minutes. Now, another member, Anthony Cerami N2OAC, is the proud owner of a resurrected NanoVNA.
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Heathkit HD-1234 - July 2022
The typical ham wouldn’t usually think of an antenna coax switch as being a repairable item, but sometimes it is - especially when the name “Heathkit® ” is involved. I recently encountered a six-position (four live connections) switch, a Heathkit® HD-1234, that had two dead positions. Its owner had added another antenna, and so needed to use an additional position on the switch. Of course, I decided to give it a shot. This unit has a six-position switch mechanism which connects each of the four live connectors to a common connector in turn as the switch is rotated. The device is of a hexagonal shape, |
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with an SO-239 connector on each of five of the sides, and a ground post on the sixth side. The way the unit is designed, when a given position is selected, all of the other positions’ connectors are grounded. The connectors are labeled “1” through “4”, “C”, and “G”. The internal switching mechanism has no physical stop, which means that it can be rotated a full 360° if so desired.
The complaint was that positions “3” and “4” were defunct, with no connection to the common connector or to ground when selected. At first, it seemed like a straight-forward open circuit problem, such as a broken solder joint at each of the non-working connectors. Close examination, however, showed that all of the solder joints were intact. On the off chance that there were hidden defects there, I went ahead and reflowed the solder on all of the connectors, but to no avail. The problem still existed.
I decided that the problem had to be inside the switch itself, so I disassembled the unit, de-soldering all of the connectors from the switch. The switch is a double-sided rotary switch on a ceramic substrate, with a contact disc on either side of the ceramic base, and a fixed brush at each of the connector positions. In its assembled condition, only the ground side of the switch substrate or base is visible; the detent plate at the top conceals the upper active connection disc and brushes.
This meant that I would need to disassemble the switch to get at the upper contact disc. No problem — it is held together by a pair of machine screws with spacers, washers, and nuts. Once the switch was apart, I began to investigate just why there was no continuity at those two switch positions. What I found was a bit of a surprise. The whole problem was a heavy build-up of oxidation on the brush contacts at those two positions. The fix was fairly simple - some cotton swabs, a toothbrush, and some DeoxIT® Gold. Twenty minutes of cleaning on both sides of the ceramic disc, and the switch contacts were operational again. Another ten minutes, and the switch was reassembled and ready to go back into the unit, with a fresh application of DeoxIT® X10S on the shaft and bushing to aid in long-life operation.
I cleaned up all of the parts, including wire-brushing the threads on the SO-239 connectors. Then I re-assembled the entire unit, and soldered the connectors to the switch terminals. Testing with an ohmmeter showed good clean continuity and zero resistance through all positions of the switch. Passing 30 VDC through the switch from common to each output in turn showed zero voltage drop internally in the switch.
So what happened here? Why did only two of the switch positions have this heavy oxidation? As it turns out, the owner has had this switch in service for over thirty-five years, but he has never used anything but switch positions “1” and “2”, and never turned the switch all of the way around the dial - he simply switched it back and forth between the two positions he used. This led to a lack of “scrubbing” at the unused positions, and age and time took over from there.
Moral of the story? Sometimes, it pays to take the long way around. Exercising the switch through its entire range of travel from time to time will help to keep the contacts clean and oxidation free, allowing the switch to perform up to it design specifications. See you next month!
The complaint was that positions “3” and “4” were defunct, with no connection to the common connector or to ground when selected. At first, it seemed like a straight-forward open circuit problem, such as a broken solder joint at each of the non-working connectors. Close examination, however, showed that all of the solder joints were intact. On the off chance that there were hidden defects there, I went ahead and reflowed the solder on all of the connectors, but to no avail. The problem still existed.
I decided that the problem had to be inside the switch itself, so I disassembled the unit, de-soldering all of the connectors from the switch. The switch is a double-sided rotary switch on a ceramic substrate, with a contact disc on either side of the ceramic base, and a fixed brush at each of the connector positions. In its assembled condition, only the ground side of the switch substrate or base is visible; the detent plate at the top conceals the upper active connection disc and brushes.
This meant that I would need to disassemble the switch to get at the upper contact disc. No problem — it is held together by a pair of machine screws with spacers, washers, and nuts. Once the switch was apart, I began to investigate just why there was no continuity at those two switch positions. What I found was a bit of a surprise. The whole problem was a heavy build-up of oxidation on the brush contacts at those two positions. The fix was fairly simple - some cotton swabs, a toothbrush, and some DeoxIT® Gold. Twenty minutes of cleaning on both sides of the ceramic disc, and the switch contacts were operational again. Another ten minutes, and the switch was reassembled and ready to go back into the unit, with a fresh application of DeoxIT® X10S on the shaft and bushing to aid in long-life operation.
I cleaned up all of the parts, including wire-brushing the threads on the SO-239 connectors. Then I re-assembled the entire unit, and soldered the connectors to the switch terminals. Testing with an ohmmeter showed good clean continuity and zero resistance through all positions of the switch. Passing 30 VDC through the switch from common to each output in turn showed zero voltage drop internally in the switch.
So what happened here? Why did only two of the switch positions have this heavy oxidation? As it turns out, the owner has had this switch in service for over thirty-five years, but he has never used anything but switch positions “1” and “2”, and never turned the switch all of the way around the dial - he simply switched it back and forth between the two positions he used. This led to a lack of “scrubbing” at the unused positions, and age and time took over from there.
Moral of the story? Sometimes, it pays to take the long way around. Exercising the switch through its entire range of travel from time to time will help to keep the contacts clean and oxidation free, allowing the switch to perform up to it design specifications. See you next month!