AU-9900 Diagnosis Help

[lgdavis]

Wow, you guys are good. @lgdavis, thank you for your suggestions! @petehall347, your quick look was right. ZD03 was installed backwards. I’d replaced it while looking over this board but I can’t remember why. I removed ZD03, tested it, reinstalled it correctly. Now amp passes dim bulb, but after a few seconds, where I normally would expect the relay to click, I hear a buzzing/fast hum coming from the amp instead, and relay doesn’t move. No speakers connected so no idea what’s making the noise. I turned it off right away, so didn’t get to check voltages (wasn’t sure it was safe). Do you guys think I could have damaged other components with the ZD03 mistake?

So now you have the B1 and B2 voltages back, correct ?
I don't think having the diode in backwards damaged anything, but you might have messed something else up when you pulled this board. Check all the connectors and make sure they're all in the right places.
Remember, the protection circuit is on it also (I'm assuming this buzzing is the protection relay (?). Put your finger on it to check.
PS- next time you check a diode, just lift one leg to check. That way you can't get it in backwards .

 

[Hunleyj]

So now you have the B1 and B2 voltages back, correct ?
I don't think having the diode in backwards damaged anything, but you might have messed something else up when you pulled this board. Check all the connectors and make sure they're all in the right places.
Remember, the protection circuit is on it also (I'm assuming this buzzing is the protection relay (?). Put your finger on it to check.
PS- next time you check a diode, just lift one leg to check. That way you can't get it in backwards .

Thank you so much! Yes, voltages are back! Posted them below. It WAS the relay making the buzzing noise, because I’d left the amp connected to the DBT! Jesus.

Now I get sound from the amp, it’s just quiet (at close to max volume it’s at conversation level). If I turn the input signal (iPad) up, the amp distorts even though the volume isn’t very high. Still, it’s something. Getting closer now! I’ve thoroughly cleaned all switches and pots, so don’t think that’s the cause.

F-2568:
1: 62.95
2: 0.0
3: 0.0
4: -63.98
5: -34.88
6/7: 0.0
8: 33.66
9: 0.0
10: 0.07
11: 0.275
12: 36.9
13: 36.9
14: 18.8
15: 28.9
16: 0.51

 

[lgdavis]

Thank you so much! Yes, voltages are back! Posted them below. It WAS the relay making the buzzing noise, because I’d left the amp connected to the DBT! Jesus.

Now I get sound from the amp, it’s just quiet (at close to max volume it’s at conversation level). If I turn the input signal (iPad) up, the amp distorts even though the volume isn’t very high. Still, it’s something. Getting closer now! I’ve thoroughly cleaned all switches and pots, so don’t think that’s the cause.

F-2568:
1: 62.95
2: 0.0
3: 0.0
4: -63.98
5: -34.88
6/7: 0.0
8: 33.66
9: 0.0
10: 0.07
11: 0.275
12: 36.9
13: 36.9
14: 18.8
15: 28.9
16: 0.51

I never thought about the DBT, lol.
Anyway, glad it's working. A great feeling, isn't it?
Check the attenuator that's behind the volume control knob. That might be why it's low.

 

[Hunleyj]

I never thought about the DBT, lol.
Anyway, glad it's working. A great feeling, isn't it?
Check the attenuator that's behind the volume control knob. That might be why it's low.

Checked that, but it’s on zero. I’m going to keep digging. Appreciate all of your help, the amp has come a long way because if it!

 

[Hunleyj]

Well, I figured out the main amp is working nicely. “Main In” inputs deliver clear, powerful sound when switch set to “separated”. I’ve checked all components in the F-2568 pre-amp power supply, all good. Any suggestions for the next board to look at? In the block diagram, F-2579 is the last board before the driver unit, not sure if I should work backwards or start from F-2070 at the beginning of the diagram.

 

[lgdavis]

Well, I figured out the main amp is working nicely. “Main In” inputs deliver clear, powerful sound when switch set to “separated”. I’ve checked all components in the F-2568 pre-amp power supply, all good. Any suggestions for the next board to look at? In the block diagram, F-2579 is the last board before the driver unit, not sure if I should work backwards or start from F-2070 at the beginning of the diagram.

I'm confused: If you're getting "clear and powerful sound", that's the objective, right ?
So what are you troubleshooting. ?

 

[Hunleyj]

Sorry, I was trying to describe above what happens when I use “main in” and pre-amp is separated out via the switch. When preamp is connected, I just get really low volume. Looking at the block diagram, I assume everything to the right of (and including) driver is working. Not sure if I should go backwards from there, or start at the beginning.

 

[KeithD]

Are you using a source with variable output, and is that turned all the way up when connected to the pre-amp (Aux, or tape in, etc.)? If you put a source on the Main In jacks and turned that all the way down to avoid blowing speakers, etc, then when connecting that same source to the input jacks, you'll need to turn the source back up to normal levels. You might have done this, but not turning up the source would fit what you are hearing.

 

[lgdavis]

Are you using a source with variable output, and is that turned all the way up when connected to the pre-amp (Aux, or tape in, etc.)? If you put a source on the Main In jacks and turned that all the way down to avoid blowing speakers, etc, then when connecting that same source to the input jacks, you'll need to turn the source back up to normal levels. You might have done this, but not turning up the source would fit what you are hearing.

@KeithD has a good point, check that first...

Another check: Is this low level the same if Tone/Filters are on or switched out ?
Reason I ask: In the "Defeat" position, the only active stage (other than the Power Amp) is F-2574.
Switch S01, on that board, sends it's output (when in Defeat) directly to the Connected/Separated switch, and into the Power Amp.
So if it's the same with or without Tone & Filters, you have isolated your problem to F-2574.
Or, possibly a bad switch in the path prior to it. I know you cleaned them, but it's still possibile that one didn't "clean up" from spraying it.
You might re-spray the switches, and exercize them vigorously (> 30 times )

 

[KeithD]

@KeithD has a good point, check that first...

Another check: Is this low level the same if Tone/Filters are on or switched out ?
Reason I ask: In the "Defeat" position, the only active stage (other than the Power Amp) is F-2574.
Switch S01, on that board, sends it's output (when in Defeat) directly to the Connected/Separated switch, and into the Power Amp.
So if it's the same with or without Tone & Filters, you have isolated your problem to F-2574.
Or, possibly a bad switch in the path prior to it. I know you cleaned them, but it's still possibile that one didn't "clean up" from spraying it.
You might re-spray the switches, and exercize them vigorously (> 30 times )

Agree, if it is not your source, then as Igdavis says, put the tone/filter toggle into defeat, then move every switch one by one, listening carefully for any signs of "life" in the signal. On mine I had a problem with one channel being low/out. It ended up being the -20 db muting switch was dirty.

 

[Hunleyj]

Agree, if it is not your source, then as Igdavis says, put the tone/filter toggle into defeat, then move every switch one by one, listening carefully for any signs of "life" in the signal. On mine I had a problem with one channel being low/out. It ended up being the -20 db muting switch was dirty.

Thank you @KeithD amd @lgdavis. I did what you suggested. Here’s what I found:

Volume remains low no matter where the Tone & Filter switch is set. However, I noticed that when volume is at zero and:
- T&F on “filter only”, I can hear very faint noise from both speakers, sounds like a helicopter.
- T&F on “defeat”, the helicopter noise gets a little louder in left speaker.
- T&F on “tone & filter”, helicopter noise gets even louder, comes from both speakers again, although now louder on right. Here’s a clip of the R channel noise at this level: https://drive.google.com/file/d/13wGiafNc78jAYrPdIRxa05GHAAupIYSK/view?usp=drivesdk

Helicopter noise appears only when protection clicks off, and it’s volume doesn’t change when volume control is moved.

I did adjust each switch while T&F was set to defeat. No real signs of life, although there was a tiny bit of static while moving mute.

The other thing I noticed was that, when I touch the metal post of volume pot (front cover is still off while I’m working on this), I hear a buzzing noise. Doesn’t happen with other metal posts, only volume. It sounds loud in the clip only because phone is right at the speaker. https://drive.google.com/file/d/1WyCL3vd44SGnWpgZgwp0z8wa7j2h_9P5/view?usp=drivesdk

I think I should now focus on F-2574, right?

I will also re-spray the switches (this will be third time) and try again tomorrow.

BTW: I’ve read the separate/connected switch S-01 on F-2579 is known for failing. I tested it for continuity, it seemed to work. Continuity basically followed between center posts and whatever side the switch was moved to. Do you think that’s enough to rule out that switch, or should I test it some other way? That’s the only switch I can’t test while amp is on (per KeithD’s suggestion).

 

[KeithD]

Set the pre-out/main-in switch to separated, then put L/R stereo cable connecting the pre-out/main-in RCA jacks. So, you are "jumpering" over the switch. That should tell you if the switch is the cause of volume loss.

 

[Hunleyj]

Set the pre-out/main-in switch to separated, then put L/R stereo cable connecting the pre-out/main-in RCA jacks. So, you are "jumpering" over the switch. That should tell you if the switch is the cause of volume loss.

Thanks for the idea! Tried it, no change in volume, so likely something on F-2574. Luckily it’s a small-ish board!

 

[Hunleyj]

OMG, I just bumped the tuner in/out switch sideways (not up/down) and heard actual volume for a moment! If I just apply the tiniest bit of pressure to right and slightly down, I get volume in channel R, sometimes in both! The helicopter noise is still there but I am happy to chase that down later. You guys are the best! I’m going to work on this tomorrow. Thanks again! I’ll report back.

 

[lgdavis]

Bingo!
Good luck with the switch.
Many guys restoring this amp here are totally disassembling and cleaning their switches.
That may be the way to go..

 

[Hunleyj]

Bingo!
Good luck with the switch.
Many guys restoring this amp here are totally disassembling and cleaning their switches.
That may be the way to go..

Update: cleaned switch again and now have sound. I really appreciate you guys sharing your pointers with me! Any ideas on how to tackle that helicopter noise from the link above?

 

[lgdavis]

Update: cleaned switch again and now have sound. I really appreciate you guys sharing your pointers with me! Any ideas on how to tackle that helicopter noise from the link above?

Likely a grounding issue.
Based on what I've read on this forum, the grounds for the preamp are daisy-chained, and go to a screw on the front panel. If this connection is flaky, strange issues arise.
This screw is near the Tuner switch, possibly it's mounting screw (not sure), so the hum may be related to your recent discovery - maybe..
Do a search for the user "dr*audio", and read his AU-9900 restoration threads. He talks about this, and has pics of it.
Also, some of the boards get a ground thru their knob\switch shafts. Are all yours tight?

 

[Hunleyj]

Likely a grounding issue.
Based on what I've read on this forum, the grounds for the preamp are daisy-chained, and go to a screw on the front panel. If this connection is flaky, strange issues arise.
This screw is near the Tuner switch, possibly it's mounting screw (not sure), so the hum may be related to your recent discovery - maybe..
Do a search for the user "dr*audio", and read his AU-9900 restoration threads. He talks about this, and has pics of it.
Also, some of the boards get a ground thru their knob\switch shafts. Are all yours tight?

Hi @lgdavis. Yes, all are tight. I’ve read elsewhere on AK that seriously aged capacitors can cause that noise, I might as well go through and replace these 40+ year-old caps while I’m in here. The ones I’ve tested have all been fine, but there are plenty more. Fingers crossed!

 

[Hunleyj]

I just noticed something odd. When I set the DC offset, I turn the amp on its side, so I can easily make the adjustments. I just found that when I turn the amp back to flat, the offset changed. On both sides. L goes from about zero to 225mV, R goes from about zero to ~30mV. Moved it a few times leaving DMM connected to make sure I wasn’t touching/moving volume control, etc. (nope). Is this a thing, or am I finally losing my mind? I make sure to move it gently. In order to make sure I set it correctly, I have to leave it flat, hanging a few inches off the table, and crawl underneath with a flashlight. That can’t be right.

 

[lgdavis]

Yes, you should be thinking about replacing capacitors.
When I did mine, replaced all electros and the big filter caps - and also the protection relay.
The four trimmer pots on the Driver board (F-2583)were also changed out,
because I didn't trust those old pots to be stable.
That may be the issue you're getting with the offset (225mv is definitely too high),
the capacitors could be a factor but wouldn't cause changes with physical movement.
Most likely a flaky connection on the board, back to the magnifying glass, maybe?
Read the AU-9900 restore threads on here, tons of useful info.