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Troubleshooting suggestions for a subwoofer plate amp?

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Posted on July 21, 2020 at 19:30:37
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 This is nothing more than a COVID project.

I have a subwoofer plate amp removed from from a long gone Onix UFW-10. It powers up, passes signal, and all controls function normally, but the output has a distortion which rides along with the music/bass. The bass output is strong but mirrored by a scratching noise. It sounds almost like a blown driver would sound, but the woofer being used is good.

I don't have a schematic, and I don't see any bulging caps or missed solder joints. A google search brings up a hum and power supply cap issue, but this one is dead quiet without a signal.

My guess is a bad output transistor, but it would take quite a bit of disassembly to get to them in order to test or replace. Do you think that I'm on the right track?

Thanks,
Ed

 

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RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 21, 2020 at 20:40:23
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 I'm not that familiar with class D amps, but it doesn't sound like it's an output transistor. More like a regulating type or even a driver transistor.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 21, 2020 at 21:16:24
rivervalley817
 'but the woofer being used is good'

sounds like a voice coil rubbing. why are you sure it's not the driver?

is it a 10" unit too? where did that come from?

in any event, try the plate amp with one of your other speakers

if it doesn't sound 'scratchy' hook it back up to your project driver after turning the driver 180 and run it for a bit ... sometimes they'll re-seat in the coil gap

be well,

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 05:11:27
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 I agree, that's exactly what it sounds like, but the woofer connected to another amp determines that it is not the driver. I have several woofers on hand, and all produce the noise when connected to this amp.

Thanks,
Ed

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 05:23:16
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 Good thought. Thanks.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 11:21:42
rivervalley817
 ok, assuming you thoroughly cleaned the input jacks, the next area to check are all the 'pots' on the plate amp. as they can typically contribute that type of noise signature ... if they have any gaps in their housings squirt a few drops of deoxit in there and 'work' them several times, if not try a penetrating oil on the shafts applied in a vertical orientation and work them ...

if that doesn't correct it just from a cost / labor perspective it's new plate amp time ... maybe head to Parts Express?

regards & good luck!

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 11:54:45
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 First thing is to see if it is feeding DC to the speaker. If you don't have a voltmeter, just watch the woofer, starting with amp powered off. If when you turn it on, with null input, does the speaker push in or out and stay that way? If it does then you do have DC, which is an output stage problem.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 11:56:31
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 Thanks, I'll try that.

This amp is the leftover from a replacement and was in my junk bin. I thought I would take a shot at getting it to work while having more time at home.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 12:10:02
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 Thanks, I read 29mV DC at output with no input.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 12:45:25
rivervalley817
 hmm, well that's enough to cause harmonic distortion under a load and could be enough to put the voice coil out of gap

what became of the original woofer? I think they used pretty nice drivers
with a cast frame and metalized cone?

if that stopped working there's probably enough DC output to fry voice coils happening

then the amp would need [at least] filter caps replaced ... not expensive but a pain in the rear

let's see if Fred or airtime weighs in

regards,

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 13:03:05
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 That's not too bad if the speaker is disconnected when you are measuring it. Was 28mv measured with woofer disconnected?

And with woofer plugged in do you see the cone move in or out and stay there when you power up?

Do you happen to have a scope?

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 13:13:43
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 The 28mV was without a woofer connected.

With a woofer connected and switching on power the woofer cone does not move.

Sorry, no scope.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 13:33:01
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 Shortly after purchase in 2005, the subwoofer (original amp and driver) began making the noise. It was a known issue to AV123 and they sent me a replacement plate amp. I swapped in the new amp and all was well.

The original amp went in the junk bin and the subwoofer (replacement amp and driver) worked fine for years. I've since sold it and believe it lives on today.

With time on my hands, I've dug out the original amp and taking a shot at fixing it. In my mind it seems possible since it functions and makes music, noisy music. If I can find a fix for the noise, I'll have a nice subwoofer amp to play with.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 13:35:15
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 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+.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 13:47:54
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 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?

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 13:56:49
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 No, just a two conductor red/black connection.

I'll work up some pictures.

 

RE: Troubleshooting suggestions for a subwoofer plate amp?, posted on July 22, 2020 at 14:00:08
rivervalley817
 'It was a known issue to AV123'

yeah, I've bumped into this issue with a few sealed sub-woofers that enclose the plate amp in a plastic cowl to ensure a better air seal

that's a recipe ripe for heat related failure points ...

it's probably a capacitor issue and a tough one to solder your way out of

particularly since you mention a known issue and replacement amp

their cabinets and drivers are what made the unit good not the plate amp

with regards,

 

RE: Amp photos, posted on July 22, 2020 at 14:36:37
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
























Here they are. Tough to see, everything is packed in tight.

 

RE: Voltage readings, posted on July 22, 2020 at 18:06:24
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 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

 

RE: Voltage readings, posted on July 22, 2020 at 21:19:56
mark.korda@myfairpoint.net
Audiophile

Posts: 1253
Location: Maine
Joined: August 16, 2011
 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.

 

RE: Amp photos, posted on July 23, 2020 at 08:01:14
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 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.

 

RE: Voltage readings, posted on July 23, 2020 at 13:37:43
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 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.

 

RE: Voltage readings, posted on July 23, 2020 at 14:38:19
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 Actually no...the replacement amp was completely different and I sold it several years ago.

 

RE: Voltage readings, posted on July 24, 2020 at 08:56:55
fredtr
Audiophile

Posts: 1987
Location: Phoenix
Joined: January 4, 2005
 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.

 

RE: Voltage readings, posted on July 24, 2020 at 18:12:02
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 Very good suggestions. Thanks for all the help. I'll let you know if I make any progress.

 

RE: Voltage readings, posted on July 24, 2020 at 18:13:12
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 I gave them a good dose of contact cleaner. No luck.

 

An update, posted on July 25, 2020 at 20:05:30
Ewalaa
Audiophile

Posts: 504
Location: Central VA
Joined: December 10, 2000
 I added a "preamp out" to the control board and the preamp section passes a clean signal. I fed the preamp isolated signal to an external power amp driving a passive sub and the crossover, phase, gain, and PEQ function properly.

So worst case, I have a nice sounding active crossover with a single band PEQ. That's enough to save it from the dumpster.





 

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