Sansui 3000a protect light

Ok, so replacing those old trimmers is probably a very good idea! Would old coupling caps be cause for concern as well?
 
Yes, and do check the coupling caps. They're at the age where they need replacement. Biased up properly once serviced, you should be good for a long time. Do begin with voltages at fuse 3 and 4.
 
Yes, and do check the coupling caps. They're at the age where they need replacement. Biased up properly once serviced, you should be good for a long time. Do begin with voltages at fuse 3 and 4.
can you point out the coupling caps ? maybe my schematic is wrong .
 
Yes, and do check the coupling caps. They're at the age where they need replacement. Biased up properly once serviced, you should be good for a long time. Do begin with voltages at fuse 3 and 4.
Ok, they are both on the same channel tho? I can check the voltages on all 4.
 
Correct me if I'm wrong, but the 3000A is capacitor-coupled. You can't measure DC at the speaker terminals. You need to measure off of the collector of the output transistors. There are 2 for each channel, and one should be set 1/2 voltage of the the other one. For example, start with the left side. One should be somewhere around 70v or so, then set the other left one at 35v. Then do the right side transistors.

Also, the DC must be set before setting bias because adjusting DC balance WILL affect bias. If you turn up the DC balance voltage, the bias will lower, and vice-versa.
 
Correct me if I'm wrong, but the 3000A is capacitor-coupled. You can't measure DC at the speaker terminals. You need to measure off of the collector of the output transistors. There are 2 for each channel, and one should be set 1/2 voltage of the the other one. For example, start with the left side. One should be somewhere around 70v or so, then set the other left one at 35v. Then do the right side transistors.

Also, the DC must be set before setting bias because adjusting DC balance WILL affect bias. If you turn up the DC balance voltage, the bias will lower, and vice-versa.
The unit has no DC offset adjustment,
I believe by adjusting both rails of each channel equally, the DC should be minimized.....I think?? You need 2 meters to adjust bias, as both adjustments affect each other.
Original design wouldn't be considered cap coupled ,would it? The service bulletin safety mod added in the caps afterwards to prevent speakers from catching fire.
 
I believe by adjusting both rails of each channel equally, the DC should be minimized.....I think?? You need 2 meters to adjust bias, as both adjustments affect each other.
Yes, Kev touched on this in post #16 - seems like 2 meters would be a very good idea.
Original design wouldn't be considered cap coupled ,would it? The service bulletin safety mod added in the caps afterwards to prevent speakers from catching fire.
That is my understanding. (personally I wouldn't even consider putting one of these into regular use without performing the Service Bulletin modifications).
 
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On my 4000, adjusting the bias didn't affect the DC balance all that much.

IIRC, the DC affects the bias directly. (Although conversely. )

I only had one MM, so it was a pain going from the fuse holder to the transistor collectors, but it can be done. Work carefully.
 
We realise this, my point was how on earth is the DC getting through the output caps.....
If you don't have a load on the speaker terminals the caps can have a DC voltage there because there is nothing to discharge the cap except the input resistance of your meter which could be over 10M Ohms. You have to check the DC offset at the input side of the output coupling caps. There are 2 adjustments, which are really both bias adjustments, and they affect each other. You get the bias close on both transistors, then tweak one of the adjustments for the DC offset to be 0V.
 
On my 4000, adjusting the bias didn't affect the DC balance all that much.

IIRC, the DC affects the bias directly. (Although conversely. )

I only had one MM, so it was a pain going from the fuse holder to the transistor collectors, but it can be done. Work carefully.
The 4000 is totally different. Bias has no effect on offset on the 4000.
 
Good luck finding a replacement trimpot for the ones used on the 3000A. They are high wattage because of the higher current running through them. Just clean them.
The best solution for the 3000A is to replace the power amp completely with a new board or module and get rid of the interstage transformers. That's what I did on mine. The tuner on this model is superb so it's worth upgrading the amp to make it reliable.
 
If you don't have a load on the speaker terminals the caps can have a DC voltage there because there is nothing to discharge the cap except the input resistance of your meter which could be over 10M Ohms. You have to check the DC offset at the input side of the output coupling caps. There are 2 adjustments, which are really both bias adjustments, and they affect each other. You get the bias close on both transistors, then tweak one of the adjustments for the DC offset to be 0V.
Ahhh. Ok ! Now I have something . Makes good sense.... I initially had a load on speakers, but I was also checking the dc at speaker outputs....
I had adjusted bias to 25mv at emitter resistors? Fuse method is a pain! Does 25mv sound reasonable??
 
If you don't have a load on the speaker terminals the caps can have a DC voltage there because there is nothing to discharge the cap except the input resistance of your meter which could be over 10M Ohms. You have to check the DC offset at the input side of the output coupling caps. There are 2 adjustments, which are really both bias adjustments, and they affect each other. You get the bias close on both transistors, then tweak one of the adjustments for the DC offset to be 0V.
I thought he had a load on it which is why I was wondering whether the mod was actually implemented, how else would there be that much DC on the output?
 
I thought he had a load on it which is why I was wondering whether the mod was actually implemented, how else would there be that much DC on the output?
I did have dummy loads attached for the first measurements, but from what Dr.Audio states, I need to be measuring st the inputs of the coupling caps?
 
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