Stereo repair is kind of having the opposite effect
June 7, 2018 6:01 PM Subscribe
Fixing an old stereo has turned into a nightmare. Is there any place in the Bay Area where I could use or rent an oscilloscope?
It’s a Kenwood KR-7200 from around 1973. A friend gave it to me with a scratchy right channel, and I thought it would be fun to fix it. I was right, at first. I replaced a bunch of capacitors, and cleaned up the sound from the tape out, but the amp still had an intermittent channel.
I replaced the capacitors in the power supply, and now there’s a loud hum and the volume knob (already cleaned with Deoxit D5) makes a whooshing sound. And the RF unit just died suddenly.
I think I know where most of the problems are*, but I need to test it to be sure. Is there any place around here where I could use an oscilloscope to trace the signal? I’m really committed to fixing this, but I’ve never used a scope, and I don’t want to take the plunge on buying one just yet.
I’m in the East Bay, but I’m willing to drive a little bit if it could mean fixing this thing.
* I think at least part of the problem is that the amp board sits in a slot, and I think some of the pins aren’t making good contact. If anyone has any suggestions for how to improve those connections, I am all ears.
It’s a Kenwood KR-7200 from around 1973. A friend gave it to me with a scratchy right channel, and I thought it would be fun to fix it. I was right, at first. I replaced a bunch of capacitors, and cleaned up the sound from the tape out, but the amp still had an intermittent channel.
I replaced the capacitors in the power supply, and now there’s a loud hum and the volume knob (already cleaned with Deoxit D5) makes a whooshing sound. And the RF unit just died suddenly.
I think I know where most of the problems are*, but I need to test it to be sure. Is there any place around here where I could use an oscilloscope to trace the signal? I’m really committed to fixing this, but I’ve never used a scope, and I don’t want to take the plunge on buying one just yet.
I’m in the East Bay, but I’m willing to drive a little bit if it could mean fixing this thing.
* I think at least part of the problem is that the amp board sits in a slot, and I think some of the pins aren’t making good contact. If anyone has any suggestions for how to improve those connections, I am all ears.
The Oakland Public Library has a tool lending library. Their online list of tools is not exhaustive, but includes a multimeter.
posted by unknowncommand at 7:17 PM on June 7, 2018
posted by unknowncommand at 7:17 PM on June 7, 2018
Go to Audiokarma.org and post in the Solid State section or the Kenwood forum and someone will walk you through it. I doubt an oscilloscope would help.
If it wasn't humming before you recapped the power supply, and it is now, there's probably a cap in backwards or a bad solder joint or something in there. Pin that down and go from there. The schematic and service manual should be on hifiengine.com if you need them.
If you still want a scope, there should be plenty of Audiokarma folks in the Bay Area who can help.
posted by Slinga at 8:01 PM on June 7, 2018 [1 favorite]
If it wasn't humming before you recapped the power supply, and it is now, there's probably a cap in backwards or a bad solder joint or something in there. Pin that down and go from there. The schematic and service manual should be on hifiengine.com if you need them.
If you still want a scope, there should be plenty of Audiokarma folks in the Bay Area who can help.
posted by Slinga at 8:01 PM on June 7, 2018 [1 favorite]
Forgive me if my advice is all obvious - I don't have any idea what your skill level with radios is. If you were working on the power supply and now there's a loud hum where there wasn't before, you have introduced a new power supply problem.
I'm assuming you have a schematic and a multimeter - if so, you can check for power supply voltages at opportune spots, and go ahead and check with both the DC and AC settings on your meter. The DC should match what the schematic says, more or less, and the AC should be a small number proportionally, representing (something like) the ripple. If you got a bad new cap, or don't have one soldered in right, you'll see a huge AC component on the supply rail that's causing the hum now.
I think most of your problems are probably connectors and pots/switches. Deoxit might not fix your volume control - you may actually have to replace it. Or if you can manage to get it apart, you could try scrubbing the tracks with deoxit and a q-tip. They can be stubborn. Was scratchiness the only original problem?
Whatever you do, don't turn any of the adjustments on the RF/IF/discriminator transformers. Don't disturb any inductors that have their coils bent/stretched/wonky looking. Be aware that tuning caps have sliding/rotating parts just like volume controls, but they're more likely to be bushings or ball bearings. You may need to clean them just the same, though. Totally dead radio reception can certainly be caused by something this simple, because it can completely kill the RF or the local oscillator. Also, do not replace any non-electrolytic caps unless you want to get a sweep generator to go with your new scope! (Never turn down the opportunity to buy tools...)
posted by fritley at 8:07 PM on June 7, 2018 [2 favorites]
I'm assuming you have a schematic and a multimeter - if so, you can check for power supply voltages at opportune spots, and go ahead and check with both the DC and AC settings on your meter. The DC should match what the schematic says, more or less, and the AC should be a small number proportionally, representing (something like) the ripple. If you got a bad new cap, or don't have one soldered in right, you'll see a huge AC component on the supply rail that's causing the hum now.
I think most of your problems are probably connectors and pots/switches. Deoxit might not fix your volume control - you may actually have to replace it. Or if you can manage to get it apart, you could try scrubbing the tracks with deoxit and a q-tip. They can be stubborn. Was scratchiness the only original problem?
Whatever you do, don't turn any of the adjustments on the RF/IF/discriminator transformers. Don't disturb any inductors that have their coils bent/stretched/wonky looking. Be aware that tuning caps have sliding/rotating parts just like volume controls, but they're more likely to be bushings or ball bearings. You may need to clean them just the same, though. Totally dead radio reception can certainly be caused by something this simple, because it can completely kill the RF or the local oscillator. Also, do not replace any non-electrolytic caps unless you want to get a sweep generator to go with your new scope! (Never turn down the opportunity to buy tools...)
posted by fritley at 8:07 PM on June 7, 2018 [2 favorites]
Thought of one other thing - if you find a local radio guy who just works on antique AM radios, he'll still have a signal generator that will make modulated (AM) RF at your 10 MHz-ish IF frequency, because they generally were made for aligning shortwaves too.
Wait, I'm assuming only FM is dead but you didn't say that. If both AM and FM reception is gone you've got a fairly basic problem somewhere, like you already said about those connectors.
posted by fritley at 8:18 PM on June 7, 2018 [1 favorite]
Wait, I'm assuming only FM is dead but you didn't say that. If both AM and FM reception is gone you've got a fairly basic problem somewhere, like you already said about those connectors.
posted by fritley at 8:18 PM on June 7, 2018 [1 favorite]
The other advice in this thread is good- if you get it back to the point where you're dealing with your original channel out issue, if this model has a protection relay (you'll know it has one if there's a delay in the sound coming on when you first power the receiver up, then hear a click- that's the relay) dirty contacts on that are one of the top causes of channels intermittently cutting out on receivers and amps. You will need to remove the relay cover and polish the contacts (if you can disassemble it) or just replace it. Contact cleaning sprays don't do much for protection relays.
posted by Larry David Syndrome at 8:22 PM on June 7, 2018 [1 favorite]
posted by Larry David Syndrome at 8:22 PM on June 7, 2018 [1 favorite]
I think at least part of the problem is that the amp board sits in a slot, and I think some of the pins aren’t making good contact. If anyone has any suggestions for how to improve those connections, I am all ears.
If you're talking about an edge connector on a PC board, the standard trick I learned for fixing Apple ][+ expansion cards will probably work: polish the edge connector fingers with a pencil eraser.
Don't use an ink eraser - way too abrasive. Standard white office eraser is the business.
Once you've shined the fingers up with the pencil eraser (and you should stop halfway through and look at your working zone under a decent light; you'll be amazed how much shinier they look where the eraser has been at them) make sure you blow all the tiny rubber crumbs off the board with compressed air, preferably working well away from the motherboard so that no crumbs end up falling into the slot connector.
You're unlikely to find small surface-mount components on a 1970s stereo, but for future reference if you find yourself using this technique on a more modern edge connector like a PCIe card or RAM DIMM, you need to be very careful to confine your rubbing to the connector fingers themselves. If you're a bit careless, it's pretty easy to "erase" little capacitors and resistors by accident.
the volume knob (already cleaned with Deoxit D5) makes a whooshing sound.
Potentiometers are pretty cheap, and 45 years is time for more wear and oxidation than sprays will easily deal with to set in. I'd replace it.
posted by flabdablet at 6:33 AM on June 8, 2018
If you're talking about an edge connector on a PC board, the standard trick I learned for fixing Apple ][+ expansion cards will probably work: polish the edge connector fingers with a pencil eraser.
Don't use an ink eraser - way too abrasive. Standard white office eraser is the business.
Once you've shined the fingers up with the pencil eraser (and you should stop halfway through and look at your working zone under a decent light; you'll be amazed how much shinier they look where the eraser has been at them) make sure you blow all the tiny rubber crumbs off the board with compressed air, preferably working well away from the motherboard so that no crumbs end up falling into the slot connector.
You're unlikely to find small surface-mount components on a 1970s stereo, but for future reference if you find yourself using this technique on a more modern edge connector like a PCIe card or RAM DIMM, you need to be very careful to confine your rubbing to the connector fingers themselves. If you're a bit careless, it's pretty easy to "erase" little capacitors and resistors by accident.
the volume knob (already cleaned with Deoxit D5) makes a whooshing sound.
Potentiometers are pretty cheap, and 45 years is time for more wear and oxidation than sprays will easily deal with to set in. I'd replace it.
posted by flabdablet at 6:33 AM on June 8, 2018
Contact cleaning sprays don't do much for protection relays
or for any contact with an opening and closing rather than a sliding motion, really.
posted by flabdablet at 6:34 AM on June 8, 2018
or for any contact with an opening and closing rather than a sliding motion, really.
posted by flabdablet at 6:34 AM on June 8, 2018
Response by poster: OK, based on what people are saying, I won't worry about a scope for now.
To answer some of the questions -- yeah, it definitely seems like I must have introduced a problem in the power supply. It's not the caps I put in (I replaced them again and I still have the same problem). Traces look fine, but I'll double check. I checked rail voltages, and most of them are pretty close to spec (at least within 1 volt). The one exception is the rail that goes to the amp circuits, which should be +45 V, is +41 V.
The thing is, I can plug in headphones into the tape rec jack, and it sounds totally fine (great, in fact, with no hum or noise on the line whatsoever). Could the hum/buzz be coming from just the rails leading to the amp circuits? It almost sounds like a grounding problem, but I can't figure out where.
There's also some hiss on the amplified signal. I'm hoping it's not the big 6800uF filter caps, but it very well could be. It would be a huge hassle to replace them (everything is connected by wire wrap), but first I need to at least get back to where I started.
I'll try to take apart the volume pot if I can. I'm worried it'll be annoying to try to replace it, because it's a combo of the volume and balance knobs. So it's a 3 gang potentiometer, but one of them has a tab (not sure what you'd call it) to click at the center line, and I'm not sure how easy that particular combo is to replace.
I'm hoping it's not a problem with the relay. I've had trouble opening it before, and I'm not sure if I'll be able to find a replacement with the right pin configuration. There's definitely a loose connection somewhere, and it makes sense that it could be in the relay. I'm still leaning towards it being the slot that the amp board sits in, because if I move that slightly, it can sometimes knock the signal in. I'll try that eraser trick on the connector pins and see if it makes a difference. My biggest worry is that the slot itself is loose and isn't making good physical contact with the board, but I don't know how likely that is.
Thanks for all the advice, I really appreciate it. I went into this with fairly limited experience, and it's been a crash course for sure.
posted by shapes that haunt the dusk at 12:45 PM on June 8, 2018
To answer some of the questions -- yeah, it definitely seems like I must have introduced a problem in the power supply. It's not the caps I put in (I replaced them again and I still have the same problem). Traces look fine, but I'll double check. I checked rail voltages, and most of them are pretty close to spec (at least within 1 volt). The one exception is the rail that goes to the amp circuits, which should be +45 V, is +41 V.
The thing is, I can plug in headphones into the tape rec jack, and it sounds totally fine (great, in fact, with no hum or noise on the line whatsoever). Could the hum/buzz be coming from just the rails leading to the amp circuits? It almost sounds like a grounding problem, but I can't figure out where.
There's also some hiss on the amplified signal. I'm hoping it's not the big 6800uF filter caps, but it very well could be. It would be a huge hassle to replace them (everything is connected by wire wrap), but first I need to at least get back to where I started.
I'll try to take apart the volume pot if I can. I'm worried it'll be annoying to try to replace it, because it's a combo of the volume and balance knobs. So it's a 3 gang potentiometer, but one of them has a tab (not sure what you'd call it) to click at the center line, and I'm not sure how easy that particular combo is to replace.
I'm hoping it's not a problem with the relay. I've had trouble opening it before, and I'm not sure if I'll be able to find a replacement with the right pin configuration. There's definitely a loose connection somewhere, and it makes sense that it could be in the relay. I'm still leaning towards it being the slot that the amp board sits in, because if I move that slightly, it can sometimes knock the signal in. I'll try that eraser trick on the connector pins and see if it makes a difference. My biggest worry is that the slot itself is loose and isn't making good physical contact with the board, but I don't know how likely that is.
Thanks for all the advice, I really appreciate it. I went into this with fairly limited experience, and it's been a crash course for sure.
posted by shapes that haunt the dusk at 12:45 PM on June 8, 2018
If all else fails consider Sacramento Radio Expo. The guy is outstanding and priced reasonably for repairs... I had him go through a Marantz 1060 and was quite pleased. He also tested 12 EL 34 tubes pulled from a Mesa Baron, tested under load, to make sure they were good to go... Again, very reasonable.
Good luck.
posted by WinstonJulia at 4:17 AM on June 10, 2018
Good luck.
posted by WinstonJulia at 4:17 AM on June 10, 2018
Response by poster: OK, quick follow up. I found where a wire had broken, resulting in a ground short. Fixing that immediately solved most problems. There's still some hiss, and I'm guessing that must be the big filter caps, because I've replaced pretty much everything else.
I did open up the relay and clean the contacts (using a slice of pencil eraser, which worked wonderfully!), and that seemed to improve performance. I also cleaned off the pins and socket for the board using the eraser trick.
I also, and this is embarrassing, cleaned off the RCA jacks at the back that I'd forgotten to clean. That, uh, also improved things.
Both channels work, but the R channel is loud and distorted, and the voltage is higher than it should be. I'm hoping it's just because the bias trim pot for that channel is dead, but it could also be a transistor problem. There's a slight hum in the L channel, so there might still be a short somewhere.
Anyway, that's a very long way of saying thanks, and that I appreciate being steered away from a costly or time-consuming oscilloscope when it was not necessary.
posted by shapes that haunt the dusk at 9:34 PM on June 11, 2018
I did open up the relay and clean the contacts (using a slice of pencil eraser, which worked wonderfully!), and that seemed to improve performance. I also cleaned off the pins and socket for the board using the eraser trick.
I also, and this is embarrassing, cleaned off the RCA jacks at the back that I'd forgotten to clean. That, uh, also improved things.
Both channels work, but the R channel is loud and distorted, and the voltage is higher than it should be. I'm hoping it's just because the bias trim pot for that channel is dead, but it could also be a transistor problem. There's a slight hum in the L channel, so there might still be a short somewhere.
Anyway, that's a very long way of saying thanks, and that I appreciate being steered away from a costly or time-consuming oscilloscope when it was not necessary.
posted by shapes that haunt the dusk at 9:34 PM on June 11, 2018
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As far as the contacts go, as long as they are making full and complete contact and they arn't bent, swabbing the pins with some automotive brake cleaner (the non chlorinated kind) or acetone and then a a swab of Caig De-Oxit works well if you can get at the pins.
posted by Dr. Twist at 7:06 PM on June 7, 2018