SA-6500II power supply problem s

The Fuxtor

Addicted Member
Ok, I got this unit and it has power supply problem/s in the regulated power supply side. Transformer , rectifiers and filters all test good.
Checking for 34v at emitter of Q23 shows only 22.1dcv. I am at a loss where or what the problem is here?
Took some voltages , maybe the more experienced can deduce something I am not seeing....
Q21 - E 15.9v ( supposed to be 33v)
C 0.99v .(? v)
B. 15.1v .(33.2v)
Q22 - E 15.8v .(33v)
C . 0.95v. (1.3v)
B . 15.25v .(?v)
Q23 . E . 22.1v .(34v)
C. 39.7v .(42v)
B. 23.5v . (?v)
Q26 . E . 16v. ( 33.2v)
B. 16v .( 29.6v)
C . 23.1v. ( 34v)
Q27 . E . 0v . ( ?v )
C. 23.5v . (35v)
B . -8.8v . ( -5v )

Charles
 
Nobody? Need to post a few more voltages?? I'm very stuck as I haven't ever encountered problems in these areas before. :oops:
 
First thing I would check is the cathode of zener D14 which should be near +35V. If it isn't it will likely need to be replaced

If the zener is good, then I would focus on Q27. According to the SM, Q27 provides power off muting. With -8.8V on it's base, it should be reversed biased and not affect the base voltage of Q23 under normal operation. However if it is leaking, it could possibly pull down the base voltage of Q23.

I think I would remove Q27 and power up on the DBT with speakers disconnected. Then measure the base of Q23 which should be closer to +32 or 33V. If it is good with Q27 removed, then you could try replacing Q27.

Les.
 
First thing I would check is the cathode of zener D14 which should be near +35V. If it isn't it will likely need to be replaced

If the zener is good, then I would focus on Q27. According to the SM, Q27 provides power off muting. With -8.8V on it's base, it should be reversed biased and not affect the base voltage of Q23 under normal operation. However if it is leaking, it could possibly pull down the base voltage of Q23.

I think I would remove Q27 and power up on the DBT with speakers disconnected. Then measure the base of Q23 which should be closer to +32 or 33V . If it is good, then you could try replacing Q27.

Les.
I have checked the zener diode you mentioned, but I'm only getting 22v ? I have pulled it out of circuit and it tests good in diode function? Think I would need to test it under some voltage to know if it's good? How does one test zeners?
Edit: btw thanks for helping me out!
 
In this case, the in-circuit test is the best way of checking D14. The diode should reach it's nominal breakdown voltage if there is sufficient current. You could check R97 (750ohms) with one leg lifted to make sure it is within spec since R97 supplies current for D14.
 
In this case, the in-circuit test is the best way of checking D14. The diode should reach it's nominal breakdown voltage if there is sufficient current. You could check R97 (750ohms) with one leg lifted to make sure it is within spec since R97 supplies current for D14.
I've checked R97 it's good.... It appears I would need 2 zeners to make up D14.
I should be getting 35v at one end of R97? Think I was only seeing 22v there too?
 
R97 is connected to the cathode of D14 so the voltage will be the same.

Yes, you can use two zeners in series to make up a 35V zener. Do you happen to have a 13V zener on hand?
 
R97 is connected to the cathode of D14 so the voltage will be the same.

Yes, you can use two zeners in series to make up a 35V zener. Do you happen to have a 13V zener on hand?
I don't think so...guess I should order some up..!
 
Last edited:
As a temporary measure, you can try adding a resistor in series with the zener to increase the voltage to the base of Q23. To play it safe, start with a smaller resistor (100 ohms or less) to see what the affect is on the voltage. You don't want the voltage going too high. A 1K resistor should put you just above 30V at the base of Q23.
 
As a temporary measure, you can try adding a resistor in series with the zener to increase the voltage to the base of Q23. To play it safe, start with a smaller resistor (100 ohms or less) to see what the affect is on the voltage. You don't want the voltage going too high. A 1K resistor should put you just above 30V at the base of Q23.
Would the resistor be in series before or after the zener or does that matter??
 
It doesn't matter as long as they are in series. Also, if you happen to have another zener that is close, (12-14V) that would be good as well.

If using a resistor, don't go too large or it will put 42V on the emitter of Q23.
 
It doesn't matter as long as they are in series. Also, if you happen to have another zener that is close, (12-14V) that would be good as well.
Lots of resistors, I'll see what the results are with that first, if results are positive maybe I can pull a zener from another project unit...
 
It doesn't matter as long as they are in series. Also, if you happen to have another zener that is close, (12-14V) that would be good as well.

If using a resistor, don't go too large or it will put 42V on the emitter of Q23.
Ok, I used a 500 ohm resistor which gives me 34.4v at the zener,
Q23
B .15.3v
C. 22v
E . 14.8
Q27
B. -8.8v
C. 22v
E. 0v
I checked R116 and I get 34v on one side but only 22v on the other? I pulled all these resistors previously and they tested good. Any suggestions now?
 
Are you sure about the collector of Q23? Your previous measurement was 39.7V .

We may have to pull Q27. I would do that on the DBT first without speakers connected.
 
Are you sure about the collector of Q23? Your previous measurement was 39.7V .

We may have to pull Q27. I would do that on the DBT first without speakers connected.
Ya, that's odd? I will pull q27 as you suggest! I will double check that voltage first...
 
The cathode of D5 is connected to the collector of Q23. They should be the same voltage.
 
Back
Top Bottom