Pearl Problem after R14 in one channel

[Mikelo]

Everything was working perfectly until today
when the left channel went down. I have
located approximately where the problem is.
The voltages are all ok until until right after the
1K resistor (R14), but that resistor measures
the proper resistance. Is there a way of testing
the FET and Q4 transistor?

Thanks for any help I can get.
Michael

[Panelhead]

Originally posted by Mikelo
Everything was working perfectly until today
when the left channel went down. I have
located approximately where the problem is.
The voltages are all ok until until right after the
1K resistor (R14), but that resistor measures
the proper resistance. Is there a way of testing
the FET and Q4 transistor?

Thanks for any help I can get.
Michael

Michael,
Dig around and measure the voltages around that resistor. It something opened up the voltage at C15 will be a little higher than the other channel.
The voltages from one channel to the other may not match exactly. The 2SK389 are not matched and the current draw will set the voltages.
What are you measuring at the top and bottom of the ZTX450? Also what is the coltage on top of the R14?

George

[Mikelo]

Right channel (which is fine):
R14 top: 28.5 volts
R14 bottom: 17.7 volts
R14 resistance: .99K

Q4(ZTX450) measuring point: 13.3 volts
Q4 middle: 13.9 volts
Q4 top: 17.7

R24 (100K): 99.4K

Left channel:
R14 top: 28.1 volts (this is what it measured
when the left channel was still fine.)
R14 bottom: .81 volts
R14 resistance: .99K

Q4 measuring point: .78 volts
Q4 middle: 1.4 volts
Q4 top: .81 volts

R24: 1.8K

Note that R14 measures the proper resistance in both channels,
but R24 (100K) is now way off in the left channel. If that's
the culprit, why did it suddenly go south?

Again, appreciate very much all advice and help!!!!

Michael

[Panelhead]

Originally posted by Mikelo
Right channel (which is fine):
R14 top: 28.5 volts
R14 bottom: 17.7 volts
R14 resistance: .99K

Q4(ZTX450) measuring point: 13.3 volts
Q4 middle: 13.9 volts
Q4 top: 17.7

R24 (100K): 99.4K

Left channel:
R14 top: 28.1 volts (this is what it measured
when the left channel was still fine.)
R14 bottom: .81 volts
R14 resistance: .99K

Q4 measuring point: .78 volts
Q4 middle: 1.4 volts
Q4 top: .81 volts

R24: 1.8K

Note that R14 measures the proper resistance in both channels,
but R24 (100K) is now way off in the left channel. If that's
the culprit, why did it suddenly go south?

Again, appreciate very much all advice and help!!!!

Michael

Try replacing the 2SK389. Looks like a short. This increased current draw is whats causing so much voltage drop across R14. I suspect R24 is fine, it is paralleled across a low reistance now.
R14 will not hold up long in this condition. It is dropping a little over a half watt. It is most likely a half watt resistor.

George

[Mikelo]

Just to verify your prediction, I took out
R24 and it was fine. So I will replace
2SK389 and hope this was the culprit. The
support from Karen and Pass/DIY is terrific
because they will send a replacement. Truly
a great outfit.

[Mikelo]

Replaced the SK389 and the Pearl is up and runnning
again. Karen sent a pair, which was a good thing
because the voltages after R14 would have been quite
different in the two channels if I had only replaced
the defective one. This also goes to show that PASS
seems to send reasonably matched SK389 because the
first pair also had very close voltages after R14 before
one of the FETS shorted.

Also, for what it is worth, those FETS seem to contribute
to difference in sound.

Michael

[Panelhead]

Originally posted by Mikelo
Replaced the SK389 and the Pearl is up and runnning
again. Karen sent a pair, which was a good thing
because the voltages after R14 would have been quite
different in the two channels if I had only replaced
the defective one. This also goes to show that PASS
seems to send reasonably matched SK389 because the
first pair also had very close voltages after R14 before
one of the FETS shorted.

Also, for what it is worth, those FETS seem to contribute
to difference in sound.

Michael

Michael,
I learned a little about the 2SK389 trying to match up my two channels. The pair in the Q pack had a 30% difference in idle current. It took three to get the current draw matched to 1 ma.
Duals like the 389 are hard to match out of circuit. The socket required is not very common.

George

[Mikelo]

Michael,
I learned a little about the 2SK389 trying to match up my two channels. The pair in the Q pack had a 30% difference in idle current. It took three to get the current draw matched to 1 ma.
Duals like the 389 are hard to match out of circuit. The socket required is not very common.

George

I admire your persistence because desoldering and replacing these was a royal pain for me.
I am used to soldering tubed equipment, not transistors. Anyway,
I admit that the second pair I received was much closer than the original pair, but
I am just going by measured voltages over several days. What I have now is
13.31 and 13.20 volts at Q4 and the voltage at the output caps input (also R14 bottom)
is 15.57 and 16.05. Using Ohm's Law at R14, doesn't that get me well within 1mA?

Michael

[Panelhead]

Originally posted by Mikelo


I admire your persistence because desoldering and replacing these was a royal pain for me.
I am used to soldering tubed equipment, not transistors. Anyway,
I admit that the second pair I received was much closer than the original pair, but
I am just going by measured voltages over several days. What I have now is
13.31 and 13.20 volts at Q4 and the voltage at the output caps input (also R14 bottom)
is 15.57 and 16.05. Using Ohm's Law at R14, doesn't that get me well within 1mA?

Michael

You are fine. Also, the new pair is drawing a little more current than the one shown in the circuit diagram.
In my Pearl one was 18.7 the other something like 14.5. Dummy me swapped the one at 14.5 for another and got 14.5. Then another. Finally swapped the channel the read 18.7 and got both down around 15 volts at the bottom of R14.
I could not hear any difference even when one channel was drawing a lot more current than the other. But theoretically the higher current has two advantages.
1. Higher current usually sounds a little better.
2. The higher voltage drop across R14 should lower modulation when swinging a high output. The voltage drop across R14 is what allows the output to be developed.
If you ever swap a 389, try the method I used. Clip all seven leads of the chip. Pull the leads out of the through holes. Then use a solder sucker to clean the holes. This method allows a 389 to be swapped in 2 -3 minutes. But it destroys the 389 removed. I do not have the patience to try and remove one without a desoldering station. But my method is easier than that also.

George

[mattjk]

Originally posted by Panelhead


Try replacing the 2SK389. Looks like a short. This increased current draw is whats causing so much voltage drop across R14. I suspect R24 is fine, it is paralleled across a low reistance now.
R14 will not hold up long in this condition. It is dropping a little over a half watt. It is most likely a half watt resistor.

George

I had this same exact problem tonight on one board!!!!

Now I have to order more