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In Reply to: RE: Troubleshooting suggestions for a subwoofer plate amp? posted by fredtr on July 22, 2020 at 13:03:05
The 28mV was without a woofer connected.
With a woofer connected and switching on power the woofer cone does not move.
Sorry, no scope.
Follow Ups:
It doesn't sound like DC to speakers is the problem. Regarding bad output stage transistor, it is possible, but you will usually see DC or very weak, much distorted lower bass. Can you post some pictures? One thing that I will look for are power supply test points. I tried to find info on it, one thing mentioned, it varies B+.
I did find this about the amplifier: "monitors the woofer's voice-coil condition in real-time"
Were there any additional connections to the original woofer?
No, just a two conductor red/black connection.
I'll work up some pictures.
Here they are. Tough to see, everything is packed in tight.
The power supply is sandwiched in there. I've worked on similar, this was my approach. I disassembled it, looking first for power supply test points. If they are there, or silk screened output connections, I would measure those voltages and see if it makes sense. Since the power supply is servo'd, I would check to see if that was working, by playing music and watching the level with a scope. Sorry, I know you don't have one, here on CL scopes are inexpensive and it will be a big help on this problem. And useful for many other things.
The other thing I would do, while it was disassembled is a close visual inspection, looking for cracked or mis-colored resistors. Resistors will change color when they overheat. Check all diodes and transistors in-circuit with an ohm meter. Measuring in circuit will show some parallel paths, so in some cases you will probably unsolder some legs to determine if it is a parallel path or a bad component.
Here's some bad news, the last time I did work on a similar amplifier, I ended up tracing out some of the circuit. So you might reach out to whoever supports these and ask for a schematic.
Because of the symptom, if you have another 8" or greater speaker, preferably with a different impedance laying around, you may consider trying it. I know you said that you are sure your speaker is ok, this is a means of getting more information about the problem. What I read is that the signal to the speaker is being monitored. Depending on the thresholds for this circuit, different than expected impedance may give you some odd effects.
Another thing you can do, less info than a scope, download some wav files at different low frequencies and run the amp with it from your phone or whatever to determine if the problem is across the bandwidth or at specific frequencies. It will also give you a better idea of the nature of the distortion. Not as good as a scope, but you never know what you will hear unless you try it. Speaking of which, there are probably scope apps that use the mic as the input. Use the wav to drive and look at that, close to as good as a scope.
On the two wiring harnesses from board to board.
From power supply board:
Black: 12.8 VDC
Brown: -13.0 VDC
Red: 116.0 VDC
Orange: Gnd
Yellow: Gnd
Green: -115.6 VDC
Blue: -115.6 VDC
Purple: -115.6 VDC
Primary/Control board:
Green: Gnd
Yellow: -13.0 VDC
Orange: 12.8 VDC and 27.5 VAC
Red: Variable 0.0 - 1.4 VAC - Signal
Brown: 12.0 VDC and 25.4 VAC
Black: -11.4 VDC
Voltages sound ok, but surprised they are only using +/- 13 Volts. You have one big advantage, you have a working unit. I would use it for comparative anatomy, make measurements on both and look for differences.
Actually no...the replacement amp was completely different and I sold it several years ago.
Oh well. Good luck on the repair. Inspect carefully, my experience is that about half of problems have visual indications. I know I made a lot of different suggestions about how to diagnose the problem, lots of ways to skin a cat.
Very good suggestions. Thanks for all the help. I'll let you know if I make any progress.
Ewalaa, I think Rivervalley has the right idea. My plate amp,the BASH, has a volume pot and cross over pot that might cost 10 cents They are miniature pots and I sprayed them with contact cleaner and rotated them back and forth for a minute. I would like a better pot in the future but this worked. My friends M K woofer's plate amp had the same cheap volume pot problem and the cleaning did the trick on his sub too. I have looked all over for a plate amp with a decent volume or level control but I don't think they exsist. I would love to be proved wrong as I won't be happy until I can get a better volume/ level control in mine...Mark Korda.
I gave them a good dose of contact cleaner. No luck.
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