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SAE 2200 amplifier 1vdc offset - how would you proceed

T

Trebolo

Member
I have a SAE 2200 that works, but it has 1.0 vdc to 1.4 vdc offset on channel 2. Bias adjustment does not do anything. The amp works and sounds ok, I do not want to run long enough to see how it sounds , but it does work ok. The VU meter shows the offset also about 30% of the vu meter lights up on that channel. As soon as the protect relay kicks in the offset shows up. It does not trip protection. I started wiggling stuff around to see if I could find a bad solder joint or loose connection and it seems that there may be a issue with a diode on power supply. If I push just right on diode or the wire from transformer to the diode the offset goes away. May or may not be the diode , could be in the area, I was only able to get it to go away a couple times so I cant figure out what was making it go away. Could a leaky diode on power supply cause this or one that's going bad? Ill replace them as soon as I get new ones , but I wanted to see if I could get some ideas on what may be causing this from someone who has more experience than me. I would normally not blame this on power supply, but it went away a couple times when i was pushing on stuff.
 
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The main amplifier bridge rectifier is a big block not much there to wiggle, monitor your main rail voltages while you are wiggling stuff. All of the other rectifier diodes are for housekeeping duties such as the protection circuit and the LED display and have nothing to do with the functioning of the basic amplifier. There is no DC offset adjustment on this model only idle current for the output stage. How are you attempting to adjust the idle current? Is the main rectifier block circuit board mounted?
 
I measure 1.0 - 1.1 vdc on the 62 ohm emitter resistor. Also on 5.1k resistor in the feedback path. This is the channel that has never been repaired. It does not trip the relay even when it goes up th 1.4 vdc. Seems to drift erratically between 1.0vdc - 1.2vdc occasionally up to 1.4. Since it moves so fast i cant really see if anything happens when adjusting bias pot. I just pulled the rectifier diodes to replace and they measure between .450 and .380 vdc. I am replacing now. Not sure why the bias goes to 0 when i push stuff around on the psu board. It only happened a couple times, maybe a bad diode or bad winding in transformer. Can a bad rectifier mess with the bias circuit?
 
Trebolo said:
I measure 1.0 - 1.1 vdc on the 62 ohm emitter resistor. Also on 5.1k resistor in the feedback path. This is the channel that has never been repaired. It does not trip the relay even when it goes up th 1.4 vdc. Seems to drift erratically between 1.0vdc - 1.2vdc occasionally up to 1.4. Since it moves so fast i cant really see if anything happens when adjusting bias pot. I just pulled the rectifier diodes to replace and they measure between .450 and .380 vdc. I am replacing now. Not sure why the bias goes to 0 when i push stuff around on the psu board. It only happened a couple times, maybe a bad diode or bad winding in transformer. Can a bad rectifier mess with the bias circuit?
Click to expand...
 
I cant erase that previous message sorry.......So i was messing with the output idle current , no adjust for bias..... I should have figured that out before i started turning stuff.
 
There is no bridge rectifier . Just diodes 1n5403's and I am only messing around in that area due to the 1 VDC going away when I was moving stuff near the diodes or 40VDC green wire from transformer
 
So I need to figure out how to adjust the IDLE current on this amplifier also , as well as removing the 1vdc on channel B
 
i see some terminations of wording needs addressing here .
simply ..
idle current = bias
bias current= idle current .
dc offset = voltage above 0v at speaker output .
they are slightly related but not enough to get them mixed up .
some of us can get confused if correct termination of words is used .
 
I wonder if I was causing a new problem when wiggling stuff around that made the 1vdc go away. So maybe I should ignore that and just try to find the more obvious causes
 
ok back to the problem in hand .
i cant comment further right now but i have a theory about power board .i just need to see how its wired . and if it might alter voltages on one channel .
 
petehall347 said:
i see some terminations of wording needs addressing here .
simply ..
idle current = bias
bias current= idle current .
dc offset = voltage above 0v at speaker output .
they are slightly related but not enough to get them mixed up .
some of us can get confused if correct termination of words is used .
Click to expand...
Thank You, I am self taught. I have spent years learning , but not so much communication with others. I will make the adjustments to my vocabulary!
 
I changed rectifier diodes out and no change. When I measured them after removal they were off due to heat. So same now. No Change. I am going to stop messing with it and see what you guys think , thank you for your time. It really is invaluable on the internet!
 
interesting design .not one i am familiar with . i would start by checking resistor values around the differentials
also the 5v zener right between them
 
there is the 1v then . now to figure out why . new matched pairs of those 4 transistors may well cure it if all resistors in spec .
see what voltage at r2 before proceeding further .
 
Power supply problems affect both channels, so that is nearly always a dead end.
petehall347 said:
there is the 1v then . now to figure out why . new matched pairs of those 4 transistors may well cure it if all resistors in spec .
see what voltage at r2 before proceeding further .
Click to expand...
This nearly always cures off set problems
However, you shouls straighten out the 51V zener strangeness. You should get approximately + and - 25.5 volts to ground on either end of the zener. (They won't be exactly equal, but should be close.) If you are getting supply voltages someone may have put a regular diode in backwards. (just a guess....)
Also, quick check of meter - many DMMs read high if battery is low - quick check is fresh alkaline cell should be 1.6 volts.

My 2 cents.
 
Okay, all the resistors test good. Should I swap the zeener with the other side and see if that changes? Do you think it could be a bad zeener? If not i am going to replace the input pairs with ksa992 and 2sc2240's? Good idea? Then I can bond together.
 
sregor said:
Power supply problems affect both channels, so that is nearly always a dead end.

This nearly always cures off set problems
However, you shouls straighten out the 51V zener strangeness. You should get approximately + and - 25.5 volts to ground on either end of the zener. (They won't be exactly equal, but should be close.) If you are getting supply voltages someone may have put a regular diode in backwards. (just a guess....)
Also, quick check of meter - many DMMs read high if battery is low - quick check is fresh alkaline cell should be 1.6 volts.

My 2 cents.
Click to expand...
When I measured the zeener I measured across the diode.
 
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