anatech told you to leave the good channel alone. Why didn't you follow his advice? I'm shaking my head...
Best regards!
Best regards!
man I give up, gone over every connection, replaced all the transistors, measured all the parts again in the feedback circuit, put a new nichicon 470uf in there, still have 220mv which rises to 239 when a finger is placed on top of Q5 & 6.
checked earthing connections, seems all good. I am lost at this point.
checked earthing connections, seems all good. I am lost at this point.
I recapped the whole amp, all caps, replaced all important resistors, some were a little out, it was not original, only the trannies in left channel were changed which brought the dc down from 50 odd to 7 which it still is.
all I did after running it for the past few days was swap out the 470uf caps for some new black gate NP I found in my stash, didn't touch anything else, figured they would be the best quality I could put in the feedback circuit. and now here I am.
your comment doesn't really help me much though does it....
Hi Sonic Art,
Good plan to leave it. Now, time to think. You shouldn't scrap it, that doesn't teach you anything and frustration is a negative motivator. Did you measure base to base? The reading will be in mV (it better be!).
Something changed, had to. So step one is to remove those capacitors completely. Bipolars and any bypass caps. Your gain will drop to about unity now (that's okay). Measure DC offsets. Also have a look at solder connections. Clean the flux up (solder bridges have a habit of hiding in there). Lacquer thinner works very well on a toothbrush, or cotton swab, what we call a "Toothpick". Make sure the stick part is wood or paper/cardboard. The plastic ones dissolve and make a mess.
One thing I was taught that I found very helpful was, if something changes - I did something. There are times a component fails, but I was taught to assume I did something wrong first. Not unless the source of the trouble is dead obvious.
-Chris
Good plan to leave it. Now, time to think. You shouldn't scrap it, that doesn't teach you anything and frustration is a negative motivator. Did you measure base to base? The reading will be in mV (it better be!).
Something changed, had to. So step one is to remove those capacitors completely. Bipolars and any bypass caps. Your gain will drop to about unity now (that's okay). Measure DC offsets. Also have a look at solder connections. Clean the flux up (solder bridges have a habit of hiding in there). Lacquer thinner works very well on a toothbrush, or cotton swab, what we call a "Toothpick". Make sure the stick part is wood or paper/cardboard. The plastic ones dissolve and make a mess.
One thing I was taught that I found very helpful was, if something changes - I did something. There are times a component fails, but I was taught to assume I did something wrong first. Not unless the source of the trouble is dead obvious.
-Chris
when you say bypass caps are you talking about caps added across the bipolar or caps that are in the original circuit? I have not added anything across the bipolar, only some 0.1uf 100v across the two 100uf 80v caps.
I measured every component, pulled the mica from the feedback circuit and checked, lifted diodes that couldn't be checked on the board.
not sure what you mean by base to base, can you explain this simply. do you mean from the base pin of one ic to the other in the pair? or base to ground comparing the two?
board is clean, I clean boards after any work with pcb cleaner, static brushes and compressed air.
I have stared at the board for hours, checked every solder joint, of course I could have missed something, I don't discount anything with electronics!
left channel is still perfect at 6-7mv.
I will pull the 470uf bipolar in the morning and then check the dc with it out, and base dc if I understand it correctly once you reply.
I measured every component, pulled the mica from the feedback circuit and checked, lifted diodes that couldn't be checked on the board.
not sure what you mean by base to base, can you explain this simply. do you mean from the base pin of one ic to the other in the pair? or base to ground comparing the two?
board is clean, I clean boards after any work with pcb cleaner, static brushes and compressed air.
I have stared at the board for hours, checked every solder joint, of course I could have missed something, I don't discount anything with electronics!
left channel is still perfect at 6-7mv.
I will pull the 470uf bipolar in the morning and then check the dc with it out, and base dc if I understand it correctly once you reply.
Hi Sonic Art,
Great! Don't give up, there will be a valuable lesson somewhere in here.
Measure between the bases of your input pairs. So between the bases of Q3 and Q4 (which is also Q5 and Q6). This tells you if they are balanced. If the bases are not balanced, then the diff pair is trying to do it's job but can't. That leads you to Q7, Q8 or Q10, Q11 most likely. oR some component in that circuit.
Great! Don't give up, there will be a valuable lesson somewhere in here.
Measure between the bases of your input pairs. So between the bases of Q3 and Q4 (which is also Q5 and Q6). This tells you if they are balanced. If the bases are not balanced, then the diff pair is trying to do it's job but can't. That leads you to Q7, Q8 or Q10, Q11 most likely. oR some component in that circuit.
I appreciate the time your taking here, I'm still not 100% on this, measure from one base pin to the other, or base to ground and compare the two in each pair, and this is in running state.
what am I wanting to see here in voltage? I just want to be sure what I understand is correct, doubt is playing its part at this stage!
what am I wanting to see here in voltage? I just want to be sure what I understand is correct, doubt is playing its part at this stage!
Okay, that is what I suspected - and very good. Your diff pairs are balanced. However I don't see how you are getting a higher DC voltage at the bases than the output. Whatever is on the output goes through R25 with no path to common anymore. Therefore you should see whatever is on the output on the base of Q4 and Q6 (they are tied together).
I wonder, check the currents through the two tail current generators by measuring across R9 and R8. They should be approximately the same.
I wonder, check the currents through the two tail current generators by measuring across R9 and R8. They should be approximately the same.
R9 has 1.85 mV across it? That might be about 3.3 uA current. If you had 1.85 volts across it you may have 3.3 mA tail current, I would expect around 1.1 to 1.25 VDC across that resistor. 1.96 mA tail current with that. Or do you mean you measured 1.036 VDC across R9 (1.85 mA current flow)?
Okay, so we know the current source for Q5 and Q6 is not operating. R8 should be 560 ohms, measure out of circuit.
You cannot have 56 VDC between the bases of Q4 and Q6 as they are directly connected together - unless the trace burned open and that would be very obvious.
Okay, so we know the current source for Q5 and Q6 is not operating. R8 should be 560 ohms, measure out of circuit.
You cannot have 56 VDC between the bases of Q4 and Q6 as they are directly connected together - unless the trace burned open and that would be very obvious.