• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sansui 990db/9090db DC on output

Brandon1250

Brandon1250

New Member
Hello all. I’m kinda in a situation that has me completely confused with this 990db I have.

Getting right to it; This unit was given to me with absolutely no back story. Got it to the point where I felt confident plugging it in and putting it on the dim bulb tester.

No smoke but of course, just a wonderful red blinking light. Checked outputs, all good. Rebuilt slotted amp card, still in protect. Decided to check the blue and gray wire underneath the receiver, 0.02-/+ on the blue wire, 45v on the grey wire.

At that point I decided to play some music through the blue wire with a little test speaker connected to the blue wire. I have sound! At that point I thought maybe it’s one of my output transistors. Swapped them all out from one channel to the other. Not the problem. Okay.. must be on the amp card…Nope. Thanks to somewhere on AudioKarma, a post mentioned that you can flip the audio card 180 and it will still work, just inverses channels… and sure enough, still didn’t change anything.

That means the problem is on the actual power supply board, not the amp card. I even tried powering it up without the output transistors with the amp card in, and I’m still getting 45v on the grey wire, no matter what. I did attempt to remove the grey wire between the protection board & the power supply board and of course it did go out of protection, but of course still has 45v on that channel.

Any help would be appreciated greatly. Other than shooting the parts cannon at the power supply board, I don’t really know where to go from here.

Edit…Also bias and offset are good on both channels of the amp card. I was able to dial in both channels with the card flipped.
 
Last edited:
Register to hide this ad
Brandon1250 said:
Hello all. I’m kinda in a situation that has me completely confused with this 990db I have.

Getting right to it; This unit was given to me with absolutely no back story. Got it to the point where I felt confident plugging it in and putting it on the dim bulb tester.

No smoke but of course, just a wonderful red blinking light. Checked outputs, all good. Rebuilt slotted amp card, still in protect. Decided to check the blue and gray wire underneath the receiver, 0.02-/+ on the blue wire, 45v on the grey wire.

At that point I decided to play some music through the blue wire with a little test speaker connected to the blue wire. I have sound! At that point I thought maybe it’s one of my output transistors. Swapped them all out from one channel to the other. Not the problem. Okay.. must be on the amp card…Nope. Thanks to somewhere on AudioKarma, a post mentioned that you can flip the audio card 180 and it will still work, just inverses channels… and sure enough, still didn’t change anything.

That means the problem is on the actual power supply board, not the amp card. I even tried powering it up without the output transistors with the amp card in, and I’m still getting 45v on the grey wire, no matter what. I did attempt to remove the grey wire between the protection board & the power supply board and of course it did go out of protection, but of course still has 45v on that channel.

Any help would be appreciated greatly. Other than shooting the parts cannon at the power supply board, I don’t really know where to go from here.

Edit…Also bias and offset are good on both channels of the amp card. I was able to dial in both channels with the card flipped.
Click to expand...
There, 45v on the grey wire is the signal to the problem. That means 45v on one channel to the speaker coming from a full-on output transistor. Check for solder bridges on the main power board and for previous work done.

There is no way that with 45v on the gray wire should it come out of protection unless there is something awry with the protection circuit.
 
The OP powered up the receiver without the output transistors and with the amp board installed and still gets 45v on the grey wire. So the issue is either on the amp board or before it.
 
I think skywatcher and eiraved are on it.

I had a transistor shorted internally on my driver board, among other issues that either caused the transistor to short or the other way around. It had me chasing my tail with the protection circuit for quite a while before I found it.

Problem is, the F-2624 board has several transistors on it. That’s a lot of desoldering and re-soldering to check them as they can’t be checked in circuit.

I can’t remember from you post, but did the fault change channels when you reversed the board? If it did, that will at least tell you which side of the board its probably on.

Once I figured out which transistor it was, I just went ahead and replaced pretty much all the components on both channels of the board. Cheap insurance for the future IMHO.

But don’t just “shotgun” the board though. Figure out what is causing the fault first or you may end up rebuilding the board and still have the fault…
 
How does the power supply board look? Any previous repairs, bridged solder or excessive flux on the top or bottom of it?
 
eiraved said:
How does the power supply board look? Any previous repairs, bridged solder or excessive flux on the top or bottom of it?
Click to expand...
Another good consideration. I actually had a transistor shorted internally on the power board too. Thats another board I went ahead and rebuilt when I found the transistor fried. Power board is easier to rebuild too, not nearly as many components as the driver board.

This thing lead a hard life before I got it….
 
tourmax said:
I think skywatcher and eiraved are on it.

I had a transistor shorted internally on my driver board, among other issues that either caused the transistor to short or the other way around. It had me chasing my tail with the protection circuit for quite a while before I found it.

Problem is, the F-2624 board has several transistors on it. That’s a lot of desoldering and re-soldering to check them as they can’t be checked in circuit.

I can’t remember from you post, but did the fault change channels when you reversed the board? If it did, that will at least tell you which aide of the board its probably on.

Once I figured out which transistor it was, I just went ahead and replaced pretty much all the components on both channels of the board. Cheap insurance for the future IMHO….
Click to expand...
I always fully rebuild the F-2624 including the signal diodes and Zener, and test all the resistors that are not fusibles (replacing all the fusibles regardless). The traces are very delicate.
 
Skywatcher said:
There, 45v on the grey wire is the signal to the problem. That means 45v on one channel to the speaker coming from a full-on output transistor. Check for solder bridges on the main power board and for previous work done.

There is no way that with 45v on the gray wire should it come out of protection unless there is something awry with the protection circuit.
Click to expand...
When I disconnected the gray cable between the protection circuit and the power supply board, the 45v was still there, so I’d assume that rules out the protection circuit.
eiraved said:
The OP powered up the receiver without the output transistors and with the amp board installed and still gets 45v on the grey wire. So the issue is either on the amp board or before it.
Click to expand...
That’s what I assumed as well. However once I flipped the amp card around, the problem remained on the same gray wire (channel), which would lead me to believe there’s something going wrong with the power supply board.
tourmax said:
I think skywatcher and eiraved are on it.

I had a transistor shorted internally on my driver board, among other issues that either caused the transistor to short or the other way around. It had me chasing my tail with the protection circuit for quite a while before I found it.

Problem is, the F-2624 board has several transistors on it. That’s a lot of desoldering and re-soldering to check them as they can’t be checked in circuit.

I can’t remember from you post, but did the fault change channels when you reversed the board? If it did, that will at least tell you which side of the board its probably on.

Once I figured out which transistor it was, I just went ahead and replaced pretty much all the components on both channels of the board. Cheap insurance for the future IMHO.

But don’t just “shotgun” the board though. Figure out what is causing the fault first or you may end up rebuilding the board and still have the fault…
Click to expand...
The problem stayed on the same Grey wire channel no matter which side I flipped the card.
eiraved said:
How does the power supply board look? Any previous repairs, bridged solder or excessive flux on the top or bottom of it?
Click to expand...
Looked pretty stock to me. It’s a little dirty, but nothing that would leave me to believe it should be passing 45v of rail voltage.

Thanks everyone for the helpful advice. I really appreciate it.
 
Brandon1250 said:
When I disconnected the gray cable between the protection circuit and the power supply board, the 45v was still there, so I’d assume that rules out the protection circuit.

That’s what I assumed as well. However once I flipped the amp card around, the problem remained on the same gray wire (channel), which would lead me to believe there’s something going wrong with the power supply board.

The problem stayed on the same Grey wire channel no matter which side I flipped the card.

Looked pretty stock to me. It’s a little dirty, but nothing that would leave me to believe it should be passing 45v of rail voltage.

Thanks everyone for the helpful advice. I really appreciate it.
Click to expand...
The problem with the protection still functioning is what I was talking about. It should NOT click the relay with 45v on either wire no matter what.
 
Skywatcher said:
The problem with the protection still functioning is what I was talking about. It should NOT click the relay with 45v on either wire no matter what.
Click to expand...
Sorry for the confusion, it does not click when the grey wire is connected. I am getting readings straight from the output of the board, not the speaker terminals. Blue, 0.2v & grey 45v, at the speaker terminals, 0v both channels.
 
Can confirm D07/08 have the cathode facing down towards the connector. I have an original board in hand, and the silkscreen is wrong.
 
Replaced all glass zener diodes on the amp card, made sure to keep orientation correct at the bottom diodes (opposite from silk screen), still no dice. The amp card at this point is almost 100% rebuilt, including resistors, capacitors, diodes, and transistors. The only thing left is something in the power supply board.

Looking at the schematic, there’s not a whole lot going on between the amp and the power supply as far as channels go.

Not sure where to go from here.
 
Small update; removing F01 and F02 does get rid of the high voltage from the grey wire, and will allow protection to turn off, however no sound.

I’m starting to wonder if the diodes that are under that odd metal assembly might be a possible problem, as those fuses are directly connected to those.
 
Brandon1250 said:
Small update; removing F01 and F02 does get rid of the high voltage from the grey wire, and will allow protection to turn off, however no sound.

I’m starting to wonder if the diodes that are under that odd metal assembly might be a possible problem, as those fuses are directly connected to those.
Click to expand...
Did you check the resistors mentioned in post #2?
 
Brandon1250 said:
Small update; removing F01 and F02 does get rid of the high voltage from the grey wire, and will allow protection to turn off, however no sound.

I’m starting to wonder if the diodes that are under that odd metal assembly might be a possible problem, as those fuses are directly connected to those.
Click to expand...
What happens when you remove F04 / F05? Does it come out of protection? If you only remove one fuse (either F01 or F02) does it come out of protection?

The diodes you speak of are very robust. I'd be very surprised if they went bad.
 
CrSens said:
Did you check the resistors mentioned in post #2?
Click to expand...
I have not yet. That will be my next thing to check.
eiraved said:
What happens when you remove F04 / F05? Does it come out of protection? If you only remove one fuse (either F01 or F02) does it come out of protection?

The diodes you speak of are very robust. I'd be very surprised if they went bad.
Click to expand...

Removing F04 and or F05 does not change the 45v DC offset on the grey wire, nor does it go out of protection with either unplugged.

Both F1 and F2 must be removed to show no dc offset on the grey channel (and go out of protect) . As soon as I put the fuse back in on either slot; bam..45v.


What I did notice when attempting to bias the amps, the fuse holder for the good channel was drawing current (bias current, around 38mV) while the bad channel drew 0 across the fuse socket, no matter where the bias adjustment was.

Of course swapping the card 180, I could then bias the other channel on the amp card just fine.
 
You must log in or register to reply here.
Back
Top Bottom