FYI i have an LCR meter with leakage current and hFE measurements, as well as a scope and function generator if these tools may be of use to T/S.
I removed all the 2n6491 Leaving only the 4 2n6488 on one channel for output BJTs. Have a stable supply voltage. VCE of the 6491 measures to 45V which seems way to high. Noticed that the MPSU07 and MPSU57 are getting hot in this section. There is a pair of these for each ouput polarity on each section, of the 4 pairs of MPSU07/57 this is the only ones getting hot.
I just ordered 5 of each of MJE15031 and MJE15030 to replace these. Sounds like I'll have to jimmy rig the pins but I'll make it work.
I re installed just the top bank of outputs. So Q9-Q15. Then due to the quick heat, I hooked up the meter across the REs and only turned on the power supply for 1s each measurement. Even doing these 1 second measurements I still let the smoke out of Q13 while taking the NPN measurements. I replaced it and repeated using more air break between measurements for the PNPs. Note that all of the PNPs get very hot very fast, but Q13 seems to get the hottest.
Results:
RE09 - 0.128
RE10 - 0.074
RE11 - 0.110
RE12 - 0.110
RE13 - 0.120 * burned out
RE14 - 0.080
RE15 - 0.080
RE16 - 0.120
Results:
RE09 - 0.128
RE10 - 0.074
RE11 - 0.110
RE12 - 0.110
RE13 - 0.120 * burned out
RE14 - 0.080
RE15 - 0.080
RE16 - 0.120
Did you confirm that none of the output transistors were shorted or leaky before you reinstalled them?
What is the DC voltage from the left-most diode leg to the right-most diode leg for the 4 diodes between the output transistors?
Were the lower output transistors overheating as well?
If you clamp all of the transistors down, you don't have to work quite so quickly.
What is the DC voltage from the left-most diode leg to the right-most diode leg for the 4 diodes between the output transistors?
Were the lower output transistors overheating as well?
If you clamp all of the transistors down, you don't have to work quite so quickly.
lower outputs are off the board. I checked leakage current before installing the PNPs and they were all <0.04uA leakage current. (tested with BK-815). After the smoke came out of Q13 I replaced it with a known good transistor and tested the burned out one which read OL for leakage current.
I'll check diodes now...
D1 = .63
D2 = .63
Then Q13 shorted out again. This game is costing me $1.24 per measurement. lol. I even clamped it down this time.
Actually I haven't removed Q13 yet so I'm not certain it was the shorted one but I measure a short between pin1-3
I'll check diodes now...
D1 = .63
D2 = .63
Then Q13 shorted out again. This game is costing me $1.24 per measurement. lol. I even clamped it down this time.
Actually I haven't removed Q13 yet so I'm not certain it was the shorted one but I measure a short between pin1-3
with the burned PNP they read
D1 = .57
D2 = .57
D3 = .47
D4 = .47
Using diode check function on DMM I get .55V for all 4 diodes while in circuit.
D1 = .57
D2 = .57
D3 = .47
D4 = .47
Using diode check function on DMM I get .55V for all 4 diodes while in circuit.
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What do you mean by this (be specific)?
I checked leakage current before installing the PNPs and they were all <0.04uA leakage current.
I checked leakage current before installing the PNPs and they were all <0.04uA leakage current.
Don't use that. Set the meter to the highest ohms setting and check the collector to emitter resistance. Then reverse the probes and check again. The meter should read the same as when the probes are not in contact with anything but air.
Yes.
When you refer to the outputs, don't use 'PNP' or 'NPN'. Use the part number printed on the face of the components.
When you refer to the outputs, don't use 'PNP' or 'NPN'. Use the part number printed on the face of the components.
OK.
Q13 - 0 0 *this is the one that smoked before, then I replaced, then smoked again
Q14 - OL OL
Q15 - OL 20.6M
Q16 - OL 21.6M
Q13 - 0 0 *this is the one that smoked before, then I replaced, then smoked again
Q14 - OL OL
Q15 - OL 20.6M
Q16 - OL 21.6M
Q16 and 15 appear to be leaky but I don't know if it's enough to cause any problems. They will need to be replaced before the amp is returned to service.
What about the other 4 outputs in that channel?
What about the other 4 outputs in that channel?
I didn't remove the 2n6488 because they weren't getting hot. I'll pull them now and check for leakage.
I just measured 3 more 2N6488 that I bought from digikey. These are brand new never installed parts. All of them measure OL in one direction and 8.3Mohm in the other direction. Did I get a bad batch? Is my meter on the fritz?
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