SA-8100 protection circuit issue

need to get the negative voltage back to q6 base .. it has to be off . wish my memory worked properly .. been through this recently and totally forgot the culprit ... it may well be q6 but i cant see why . just needed some sanity ...
 
From previous measurements posted, it appears that the base voltage on Q5 is much too low which is causing Q5 to turn on (it should normally be off). The amp will remain in protection until the Q5 base voltage is raised above the emitter voltage. Perhaps you could try lifting one side of D5, D6 and D7 one at a time and check the base of Q5 to see if the voltage goes up. This may provide an indication of what is pulling down the Q5 base voltage.

Les.
 
Thought I'd check the readings again in the signal path for what it's worth (expected, actual):

Q7 C, +8V, +8.58V
Q7 E, +7.9V, +8.38V
Q7 B, +8.6V, +9.02V


Q5 E, +17V, +6.83V
Q5 B, +20.6V, +6.15V
Q5 C, -0.7V, +5.86

Q1 C, +20.6V, +5.87V
Q1 B, ??, +5mV
 
Agreed. Thought I'd check the voltages on the sides of D5,6,7 that are not connected to Q5 base.

D5 (connected to Q1 C):+5.8V (should be +20.6V)
D6 (connected to Q4 C):+16.6V (should be +20V)
D7 (connected to Q3 C):+5.5V (should be +20V)
 
OK, pulled one end of D5, base of Q5 did not change. Pulled one end of D7, and base of Q5 went to +18.6V! Collector of Q5 is now -1.2V, and emitter is +16.4V. Put Q6 back in, and base of Q6 is at -1.2V, collector is +8.95V, and emitter is +1.7mV.

I assume this means a bad D7 and should be replaced? Could this have been the source of my original problem of the amp dropping into protection for 2-3 seconds after about 10 minutes of on time?
 
Actually, I've got a spare 1N4004. Can I put that in to replace the 1S2473 that was in D7?
 
Before replacing D7, recheck Q3 as well as C3 and C4 for proper placement. With one leg lifted, you should be able to perform a diode test with your DMM but it's most likely fine. A 1N4148 is a typical replacement for that diode but a 1N4004 would also work if necessary.
 
I did not replace Q3, only Q1, 2, 6, 7. I have the original 2SC857 transistors back in Q1 &2, so I'll swap those for the replacement KSC1845FTA. I'll scrounge around and see if I have an 1N4148.
 
We don't have any 1N4148s here. I do have 1N4004 and 1N4001. We also have others, is there a close equivalent other than 1N4148?
 
OK, replaced D7 with a 1N4001, and same issue as with the original diode. Does not come out of protection voltages at Q5 were as before. This kind of makes sense, since when I tested D7 it was functioning properly.

Pulled the 1N4001 back out, and it comes out of protection. I'm thinking this is not an issue with D7, but with D7 out the protection circuit is working.

I put the 1845s back in for Q1 and 2. Voltage at base of Q5 is -1.0V, so closer to the spec of -0.7V, but that is with D7 disconnected, so what to do next?
 
Changed Q3 and Q4. Put in KSC1845FTA for both. Turned on without D7, and unit came out of protection. Put D7 back in, and no go. Unit does not come out of protection, and same low voltages at Q5 as before. At this point the only transistor I have not changed is Q5.

Checked voltages:
E C B
Q3 -0.6 +3.6 0
Q4 -0.6 +16.6 0
Q5 +4.8 +0.65 +3.5

Clearly Q3 C is low. Checked R13 without removing one end, and got around 11 kohm and slowly rising. Should be 15k, but I'd probably have to pull it to be sure I'm getting an accurate reading.

Q3 C is connected to R13 and D7, so doesn't it have to be one of those two?
 
Pulled D6 and D7. Came out of protection when turned on.

Checked voltages:
E C B
Q3 -0.6 +5.4 0
Q4 -0.4 +1.2 0
Q5 +16.8 -1.12 +25.1
 
Back
Top Bottom