Just wanted to know if B- was present. If it wasn't that would sure expalin it. Other things can cause it , too. In the 400/700, the VAS used to develop cold solder joints on the collector heat sink. That would stick it to the rail and kill the speaker. You just need to check everything in the samll signal path and feedback and look for something out of place.
Try cutting the trace between pin 6 of the opamp and the base of Q1. Connect a 10k resistor between the base of Q1 and the -15V opamp supply. If it sticks to the -65v rail, the rest of the amplifier is working and the opamp is bad. Use an LF351 or LF411. If it doesn't do this, something about Q1 or Q3 isn't right. Check both for forward bias by measuring the drop across their respective emitter resistors.
I have no idea how to check for forward bias. I've checked for cold solder joints, there's nothing.
I didn't think this would be difficult, there aren't that many parts on the boards. But it very much is difficult, for me at least. I can replace parts and do the most minor of troubleshooting, but mostly I can just replace parts.
Charles.
I didn't think this would be difficult, there aren't that many parts on the boards. But it very much is difficult, for me at least. I can replace parts and do the most minor of troubleshooting, but mostly I can just replace parts.
Charles.
Right now you have it sticking to the +65 rail, right? The procedure I outlined should send it to the other rail if Q1 and Q3 are working properly. Both will go from fully OFF (the situation you have now) to fully ON. There would be voltage across the resistor connected to the emitter, and around 0.65V from base to emitter. Anything significantly different indicates a failure in that stage. Could be the transistor, solder joint or resistor that looks perfectly good but isn't.
Hi,
The reason I asked about the negative -65 is because to be able to balance the output by adjusting the R24 to zero volt you need both voltages. If the negative -65 it is missing then the output will swing to +65 volt. You need the +/- 65 to be able to balance the bases of Q4 and Q9. Check the voltages of Q2 base and the collector. Also the base voltage of Q3. The collector of Q3 need to be negative.
The reason I asked about the negative -65 is because to be able to balance the output by adjusting the R24 to zero volt you need both voltages. If the negative -65 it is missing then the output will swing to +65 volt. You need the +/- 65 to be able to balance the bases of Q4 and Q9. Check the voltages of Q2 base and the collector. Also the base voltage of Q3. The collector of Q3 need to be negative.
Well, red is not negative rail. What happens is immediately on power up the positive rail fuse blows.
All the output transistors on this channel tested good. But for ***** and giggles I removed them anyhow, and then powered it up. R12 immediately fried.
I tried giving it some time, but it's still giving me hell. I'm getting ready to use it as target practice.
Charles.
All the output transistors on this channel tested good. But for ***** and giggles I removed them anyhow, and then powered it up. R12 immediately fried.
I tried giving it some time, but it's still giving me hell. I'm getting ready to use it as target practice.
Charles.
If you are blowing fuses and the outputs test good, make sure there isn't something stupid going on - like an open bias stack. On one of my 400's, the leads broke off the TO-92 mouned to the heat sink. It was just too flimsy to handle the repeated disassembly/assembly process. I replaced it with an isolated case TO-126 (and re-biased) and dared it to ever do it again.
Check Q4. It's probably dead.
It's a good idea to install a light bulb on the primary side of the transformer. This will prevent blowing all your outputs.
Outputs aren't blowing. Outputs aren't even installed. I *JUST* replaced Q4 with a brand stinking new part. BRAND NEW. Every transistor was replaced. Why would a brand new part be bad and cause a resistor to fry?
Especially since it tested good before I put it in, and a quick check with the multimeter doesn't show any shorted leads?
Charles.
Especially since it tested good before I put it in, and a quick check with the multimeter doesn't show any shorted leads?
Charles.
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