Sansui 9090DB Driver Board Repair

I have spent more time listening to this unit since I replaced the fuse resistors. The unit seems to be working alright, but I have noticed that the right channel power meter measures lower than the left channel. I had thought this was due to them being out of calibration, but adjusting the balance to make the meters closer also seems to improve the sound quality. I suspect that the right channel is actually operating lower than the left.

As a first step I went through the entire unit and cleaned all of the switches and pots with deoxit and faderlube. I have also cleaned up all of the boards throughout the unit. I found lots of flux left on the solder side of all the boards. I also discovered several solder balls that were left over in this flux.

Unfortunetly the same issue remains. Should I do anything else before moving on to replacing the other components discussed on the driver board?
 
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I have checked over the resistors several times for correct placement. I have also checked the solder side for solder bridges. I don’t see anything wrong with my work.

I tried using another receiver as a pre amp for the 9090db, but the issue remained.

I have not touched the relay yet. I suspect this unit spent most of its life sitting idle. Is it possible that the relay is dirty? The pots that I cleaned were much easier to turn after cleaning and lubricating. Maybe the relay needs some similar attention?

If I am reading the schematic correctly, it looks like the signal that feeds the power meter adjustment pots is also after the relay? So I think a bad relay could cause low output and also a low reading on the power meter like I am experiencing.
 
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I was able to pop the cover off of the relay and inspect the contacts. I cleaned the contacts up with some deoxit. I don’t measure any resistance across the contacts.

I also removed all 8 output transistors and used the 6-way test to check them. They all tested fine using that method. I replaced the mica insulators and used new thermal compound on them.

I measure the offset and bias and everything measures within specifications.

Output in the right channel is still lower than in the left. Any other suggestions on what to check?
 
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How far off centre do you need to move the balance control to improve things? You could try reversing the pre-out/main in connections using RCA cables to determine whether the issue is in the driver board or the pre-amp..
 
I remember posting quite a while back describing an adjustment in the volume pot which can be used to resolve minor imbalance issues but I don't have time to look for it right now but it'll be there somewhere..
 
How far off centre do you need to move the balance control to improve things?

Somewhere between 1.5 - 2 tics seems to bring both the sound and the power meters together nicely. Adjusting the volume up and down does not seem to change anything.

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I’ve measured the voltages on the driver board. The rails seem a little high 60v and -63v, but they are equal in both channels.

I’m reluctant to change any more parts, but I’m wondering if I should change out the coupling caps C1 and C2 as a next step? I thought about swapping them between the two channels, but I’d rather not heat up the solder pads on the driver board anymore than I have too.
 
I went ahead and replaced C1 and C2, but the issue remains. Is it possible that a transistor was damaged when the fuse resistors failed? Should I pull and test the transistors on the driver board next?

It seems like my bias or offset should be off if I had a bad transistor though?
 
I just want to make sure I have done everything that I can before I shotgun the driver board. I would hate to introduce another issue by trying to fix this one and overlook something.

What I don’t know is, did the fuse resistors failing cause another component to fail? The 9090db works perfectly as a pre amp when I use my Pioneer as the amp. Clearly the issue is with the driver board. But is it something else that was damaged when the fuse resistors failed, or something I did wrong installing the resistor replacements? I have been over the resistor replacement work a dozen times and I don’t find any issues.

Maybe I should have taken those hybrid jokes earlier in this thread more seriously?
 
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Don't start shotgunning anything. The meter circuit goes through the Dolby board and it's push switches. There is lots of opportunity for one meter to read differently than the other. The push switches are very hard to clean and may be your problem. Here's a link to a thread for checking the operation of the Dolby function including it's use of the power meters.

As far as the sound level difference, have you checked it with head phones? Your fixation on the meters may be affecting your hearing.

- Pete
 
Hello Pete. Thanks so much for the feedback.

Both power meters come up to center evenly while using the built in test tone. The right power meter is low in all of the other Dolby tests spelled out in that link. The sound is fine during all of the tests.

Headphones! Why didn’t I think to try that? I hate to admit it but I think you may be right, my eyes could be deceiving me here. If I adjust the balance to where I think the channels sound balanced while playing music and the meters are level, then it is clearly not balanced anymore when I change to the test tone.

If the issue is due to a dirty push switch then shouldn’t the meters be balanced when using the pioneer as a Pre-Amp?
 
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Whether you are putting a signal through the preamp or not, the power meter circuit goes through the Dolby push switches. It also goes through the meter calibration trimmers on the protection board. Dirt in any of those points can affect the meter reading.

- Pete
 
Whether you are putting a signal through the preamp or not, the power meter circuit goes through the Dolby push switches. It also goes through the meter calibration trimmers on the protection board. Dirt in any of those points can affect the meter reading.

- Pete

Thank you very much for this input! I had thought that using the pioneer as a Pre-Amp was ruling out dirty switches, so this is very good information for me. I think my next move should be another round of switch cleaning with deoxit.

Does the fact that the meters are balanced when using the test tone help to rule anything out?

In my first attempt at cleaning I was jamming the deoxit straw in to the front of the switch near the spring to get the deoxit inside of the switch. Should I be spraying it into the push switches in another spot? Is it possible/necessary to disassemble the push switches?
 
I made some resistance measurements today. I measured across the meter adjustment pots on the F-2657 Relay board.

Pin 1 - Pin 5 = 376 ohms
Pin 2 - Pin 6 = 392 ohms

I suspect that the difference in resistance is probably due to the position of the adjustment pots? I have not touched them at all.

I also measured from Pin 5 and 6 of the Relay board all the way back to the meters. I think this would include the Dolby switches. I see zero resistance when the meter switch is active. And both measurements go to open when I change the button to Rec Cal Tone position. Is this the only switch involved? None of the other switches seem to have any effect on the resistance. Does this rule out a dirty switch?

I’m not sure if these resistance measurements help or not. I hoped that I would find high resistance across one side of the switch, but maybe my measurements are overlooking something?
 
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Interesting, it didn't occur to me to just do a simple circuit check. If you have true zero resistance from pins 5 and pins 6 on the protection board and each of the power meters, the switch and the rest of the circuit is OK. The trimmers could certainly be off but to adjust them you need a dummy load to simulate a 50 watt load.

- Pete
 
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