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Sansui 990DB bias / trimpot issue

R

R&B Stereo

New Member
Greetings AK friends,

Some background for context: I am fully restoring a 990DB. During the process, and testing along the way, replacing the bias/offset trimpots on the driver board caused the DBT to 'pulsate'. I came across a Youtube of this guy who faced the same issue and pointed towards the trimpots. I re-installed the original ones and it operated again as it should. I eventually complete the restoration, and all is good. Sounds great. But time to (re) check DC offset and Bias and set to factory specs. DC offset only needed slightly adjusting on one channel. Then on to the bias. Adjusted one channel slightly to the required 30mV, then the other and last one... was around 60mV and would barely adjust when trying to dial the pot. The voltages eventually went a little erratic (nothing truly abnormal there when dialing) but eventually I get sparks out of it!! Ouch.

So, I decide to re-install the new trimpots, obviously no longer trusting the reliability of the old ones. And I re-experience the pulsating BDT!! Darn! Note that I have the 4K7 (DC offset) and 100R (Bias) ones. A later version of the ones referred to in the (available) service manual that refers to 4K7 and 1K ones. My new ones are 502 (5K) and 101 (100R). I turned the bias pots fully counterclockwise (ie full 100ohms resistance) and that stopped the DBT pulsating. Barely dialing clockwise and I get the pulsating back. Very disappointing to say the least. I can't get the unit get out of protection under DBT (I never did, I believe, but then the youtuber I watched had his click....), so I don't dare plug it in the wall to do measurements and adjust as it stands. I am using a 100W bulb in the DBT, by the way. Perhaps I need a higher W bulb for this unit to confirm it gets out of protection?

Thoughts and wisdom welcome.
 
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I can only find one version of the Service Manual - which indicates the bias trimmer is 1KΩ, the extreme (perceived) compression of the adjustment range you mention, provided by the 100Ω trimmer seems to confirm this. What were the values of the old bias trimmers? - copy their values not what you see on a schematic.

The pulsating DBT bulb is a symptom of too much current being drawn by the unit, whether this be bias current or something else. At this point, I would strongly advise NOT increasing the DBT bulb wattage, and as you are aware definitely not plugging the unit into the wall until you have full bias control over both channels.
 
R&B Stereo said:
Greetings AK friends,

Some background for context: I am fully restoring a 990DB. During the process, and testing along the way, replacing the bias/offset trimpots on the driver board caused the DBT to 'pulsate'. I came across a Youtube of this guy who faced the same issue and pointed towards the trimpots. I re-installed the original ones and it operated again as it should. I eventually complete the restoration, and all is good. Sounds great. But time to (re) check DC offset and Bias and set to factory specs. DC offset only needed slightly adjusting on one channel. Then on to the bias. Adjusted one channel slightly to the required 30mV, then the other and last one... was around 60mV and would barely adjust when trying to dial the pot. The voltages eventually went a little erratic (nothing truly abnormal there when dialing) but eventually I get sparks out of it!! Ouch.

So, I decide to re-install the new trimpots, obviously no longer trusting the reliability of the old ones. And I re-experience the pulsating BDT!! Darn! Note that I have the 4K7 (DC offset) and 100R (Bias) ones. A later version of the ones referred to in the (available) service manual that refers to 4K7 and 1K ones. My new ones are 502 (5K) and 101 (100R). I turned the bias pots fully counterclockwise (ie full 100ohms resistance) and that stopped the DBT pulsating. Barely dialing clockwise and I get the pulsating back. Very disappointing to say the least. I can't get the unit get out of protection under DBT (I never did, I believe, but then the youtuber I watched had his click....), so I don't dare plug it in the wall to do measurements and adjust as it stands. I am using a 100W bulb in the DBT, by the way. Perhaps I need a higher W bulb for this unit to confirm it gets out of protection?

Thoughts and wisdom welcome.
Click to expand...
Measure the value of the old trimmers to be sure you have the correct values of the new ones. Measure from outside pin to outside pin.

When I change trimmers I measure the values of the old ones and then set the new ones to about the same value.

If it comes out of protection with the old trimmers it should come out of protection with the new ones.

Check the offset voltages, if it is in protection you have to do it before the relay.

100 Watt bulb is just fine, it should power up and come out of protection if all is well.
 
There are two different versions of the F-2624 board--one uses 100 ohm trimmers (newer) and one uses 1k ohm trimmers (older). You have to use the same ones as your board has. It doesn't matter what the schematic says.
 
mrk229 said:
Measure the value of the old trimmers to be sure you have the correct values of the new ones. Measure from outside pin to outside pin.

When I change trimmers I measure the values of the old ones and then set the new ones to about the same value.

If it comes out of protection with the old trimmers it should come out of protection with the new ones.

Check the offset voltages, if it is in protection you have to do it before the relay.

100 Watt bulb is just fine, it should power up and come out of protection if all is well.
Click to expand...
Much appreciated. My main issue is not able / trusting the pot that cause sparks. I can measure the other ones removed (I should have thought of measuring them from the outset though, unsure if they may have moved from manipulation). One other thing I can't recall is if it went out of protection under DBT in original configuration. But all I know it was working just fine, plugged in the wall. Now, are you suggesting that if the offset is adjusted correctly on the new pots (let's say, same resistance level from the old pots when working), it should (assumingly) get out of protection under DBT? even if, say, bias is set to full resistance where I don't get the pulsating DBT?

Separate thought... I might need to check the power outputs in case any suffered from the trimpot that went bonkers.
 
R&B Stereo said:
Much appreciated. My main issue is not able / trusting the pot that cause sparks. I can measure the other ones removed (I should have thought of measuring them from the outset though, unsure if they may have moved from manipulation). One other thing I can't recall is if it went out of protection under DBT in original configuration. But all I know it was working just fine, plugged in the wall. Now, are you suggesting that if the offset is adjusted correctly on the new pots (let's say, same resistance level from the old pots when working), it should (assumingly) get out of protection under DBT? even if, say, bias is set to full resistance where I don't get the pulsating DBT?

Separate thought... I might need to check the power outputs in case any suffered from the trimpot that went bonkers.
Click to expand...
If the unit was functional before the trimmer swap then IMO it is a good baseline starting point to set the new trimmers to the same vale as the old ones.

The offset voltage(s) may be what is keeping it in protection. Most new trim pots I have encountered are set at 50%.

In the situation you are in you have to check and adjust the offset before the relay.

Easy place to do that is at the fuses on the protection board.

9090pr.jpg
 
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mrk229 said:
If the unit was functional before the trimmer swap then IMO it is a good baseline starting point to set the new trimmers to the same vale as the old ones.

The offset voltage(s) may be what is keeping it in protection. Most new trim pots I have encountered are set at 50%.

In the situation you are in you have to check and adjust them before the relay.

Easy place to do that is at the fuses on the protection board.

View attachment 3607874
Click to expand...

Thanks mrk229. I was able to test one of the old offset pots at 1K5 Ohms (the other strangely reading above the rated 4K7 ohms), and one of the old bias pots - not the one that sparked on me - at around 50R ohms. So, I set the new pots around those values. The good news is that I didn't get DBT pulsating, but still can't get it out of protection under DBT. Would you be able to elaborate a bit more how I would go about testing the way you suggest at the fuses before the relay? Thanks!
 
R&B Stereo said:
Thanks mrk229. I was able to test one of the old offset pots at 1K5 Ohms (the other strangely reading above the rated 4K7 ohms), and one of the old bias pots - not the one that sparked on me - at around 50R ohms. So, I set the new pots around those values. The good news is that I didn't get DBT pulsating, but still can't get it out of protection under DBT. Would you be able to elaborate a bit more how I would go about testing the way you suggest at the fuses before the relay? Thanks!
Click to expand...
Find those fuses I pointed out in an earlier post on the protection board. I believe on the 9090DB you can't see the fuses from the bottom?

Look for the green highlighted areas. 3 & 4 are the amplifier outputs coming into the board. They will have wires coming from the F-2656 PS/Driver/Amp board.

#3 is left and #4 is right. + meter lead to one of them and the minus to the chassis. Adjust the appropriate offset trimmer to get as near 0 vdc as possible, they can be twitchy and they may wobble around a bit. Ideally you can get them to + or - 10mV or less.

If they both will adjust that low then in theory it should come out of protection if the protection circuit is OK.

9090pr.jpg
 
Ok here we go... In a technical discussion is important to have direct experience or deep knowledge of topic being discussed.

F-2624 BIAS:
The version of the schematic uses a Vbe Circuit with the usual 1K trimpot for bias.
Newer versions uses those silly STV-3H diodes and use 100Ω trimpot.

OP:
Post a decent quality photo of your board to make visual inspection and a least to tell you what version do you have and fuse resistors are still present. Points if you use an external service with modern resolution to see better

Read post 1 an 2 of the thread, Not suggesting shotgun approach but good information there.

Restoring the f2624 board in Sansui QRX9001/8001/999's and 9090db/8080db models

This is a long posting about repairing and rebuilding the f2624 driver board. We use a different approach than usual. We don't jump in and try to find the problem. We change 24 parts, that really need to be changed, before doing anything else. It is so long it is in two parts. Rebuilding the...
audiokarma.org
 
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Good afternoon y'all. Thanks again for all the answers and assistance so far. I got the 990DB back on the bench this weekend but haven't been able to get it out of protection yet. I followed mrk229's advice to measure the DC offset at the said fuse points. I was able to get one channel to zero (had to play a little with the DC and Bias trimpots) but unable on the channel that experienced the original trimpot 'fireworks'. The best I could get down to was 400mV. So, I am going to assume there is another issue somewhere, and perhaps someone here can give me a hint. As a reminder, this unit was working fine after its full restoration but would not get out of protection after the incident of trying to adjust dc offset (still will original trimpots) and experienced sparks. I re-inspected the protection board, where there is known 180ohm resistors to fail, but those had been replaced as part of the restoration already (and still tested fine). Thanks in advance.
 
R&B Stereo said:
Good afternoon y'all. Thanks again for all the answers and assistance so far. I got the 990DB back on the bench this weekend but haven't been able to get it out of protection yet. I followed mrk229's advice to measure the DC offset at the said fuse points. I was able to get one channel to zero (had to play a little with the DC and Bias trimpots) but unable on the channel that experienced the original trimpot 'fireworks'. The best I could get down to was 400mV. So, I am going to assume there is another issue somewhere, and perhaps someone here can give me a hint. As a reminder, this unit was working fine after its full restoration but would not get out of protection after the incident of trying to adjust dc offset (still will original trimpots) and experienced sparks. I re-inspected the protection board, where there is known 180ohm resistors to fail, but those had been replaced as part of the restoration already (and still tested fine). Thanks in advance.
Click to expand...
Bias is bias, it should not be manipulated after being set. It certainly should not be used to affect the offset voltage.

In fact if all is well in the circuits bias should not have much affect at all on the offset voltage unless either it was set very low or perhaps there is a weak (low gain) output transistor.

If the offset trimmers won't dial it down to zero there is something wrong somewhere.
 
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