Barn Find PL400

dcmeigs

dcmeigs

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#43
laatsch55 said:
Is your excessive offset MINUS or PLUS voltage?
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When it latches up it’s positive. About 6 volts on a 150w DBT. The mains voltage is greatly reduced. If I start it without a load (speakers or dummy load) it is just a few millivolts.

It will play for at least 30 minutes without issue if I plug in the speakers after it Is up and running. That why I said the delay in the protection relay might make the problem moot. It did have an offset earlier but the new Q1 and Q2 corrected that. Matching ksc1845s.
 
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dcmeigs

dcmeigs

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#44
laatsch55 said:
https://forums.phxaudiotape.com/thr...-up-tendency-in-phase-linear-amplifiers.2307/

Joe explains it very well here...
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Thanks for that link. Very helpful. That appears to describe my observations to a tee, however I have installed the diodes as recommended. I posted a photo a few posts back. My reservoir caps are Kemet 9100uf and only reach about 10 v on the positive side before it latches and conducts enough to stabilize at that state under a DBT. It will pop a 5 amp fuse in this condition if it’s started off the DBT.

I couldn’t tell from Joe‘s essay but I don’t see a mention of blown fuses. Is that not a feature of the typical latchup?
 
laatsch55

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#45
No, fuses are typically ( not always) due to a shorted output. I've not had fuses blown during any matchup. Does one side have the KSC 1845's and the other side not?
 
dcmeigs

dcmeigs

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#47
laatsch55 said:
Have you disconnected one end of the zoebel network?
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No, I overlooked that. A shorted cap would explain a lot. I will definitely do so today. I imagine a volt meter across the resistors during latchup would be revealing. Thanks.

Q1 and Q2 have been replaced with KSC1845s in both channels.

Edit: Yesterday I removed all the OTs and tested them and replaced some mica while I was at it. All the collector to negative buss resistancereadings are in the Meg ohm range. Given that it plays once started without load, I feel like I have the bad OT possibility surrounded. Today I want to start it under load with the flukes on the C6 R10 R15 node to be sure the diodes are controlling the voltage as intended.
 
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Gepetto

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#48
dcmeigs said:
Thanks for that link. Very helpful. That appears to describe my observations to a tee, however I have installed the diodes as recommended. I posted a photo a few posts back. My reservoir caps are Kemet 9100uf and only reach about 10 v on the positive side before it latches and conducts enough to stabilize at that state under a DBT. It will pop a 5 amp fuse in this condition if it’s started off the DBT.

I couldn’t tell from Joe‘s essay but I don’t see a mention of blown fuses. Is that not a feature of the typical latchup?
Click to expand...
If it latches up with an 8 ohm load, it will blow the fuse. If it latches up without a fuse, it will just latch up.
 
dcmeigs

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#50
laatsch55 said:
Have you disconnected one end of the zoebel network?
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I didn’t disconnect the Zoebel but I measured voltages across the resistors while latched up and got no voltage on the mv scale with good flukes. The C6 voltages while locked up are both .575 so the diodes are good.

I’ve attempted to isolate the problem to a single channel by starting it with the load on just one side without success. It will latch with a load on either single channel.

I have some fused speaker wires. I’m going to start it, connect some expendable speakers and let it play for a few hours to see how it behaves. I’ll watch the wattage on the AC mains for an anomily.
 
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Have you CAREFULLY measured the collector of Q2 and Q1 when you find it in this condition? I suspect you may have upset the stability of the amp when you subbed the KSC1845 devices instead of the original TIS97 devices used? Q2 C should be much lower than Q1 C.
 
dcmeigs

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#52
Gepetto

In the latched condition they are slightly negative.

Left
Q1 is -.501 and
Q2 is -.571

Right

Q1 -.496 and
Q2 -.560

Operating unlatched the collectors are both at about 20v. I don’t see Q1 much lower than Q2. About 50mv difference.
Edit: I’d expect the parallel reversed diodes on the collectors to keep the collector voltages very close.

What transistor has been used successfully as a TIS97 substitute? I don’t have an TIS97 but I do have some matched 2sk470s.
 
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Gepetto

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dcmeigs said:
Gepetto

In the latched condition they are slightly negative.

Left
Q1 is -.501 and
Q2 is -.571

Right

Q1 -.496 and
Q2 -.560

Operating unlatched the collectors are both at about 20v. I don’t see Q1 much lower than Q2. About 50mv difference.
Edit: I’d expect the parallel reversed diodes on the collectors to keep the collector voltages very close.

What transistor has been used successfully as a TIS97 substitute? I don’t have an TIS97 but I do have some matched 2sk470s.
Click to expand...
Did you properly swizzle the leads of the KSC1845 since they do not match that of the TIS97 devices? Both TIS97 and KSC1845 have non standard pinouts
 
dcmeigs

dcmeigs

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#54
Gepetto said:
Did you properly swizzle the leads of the KSC1845 since they do not match that of the TIS97 devices? Both TIS97 and KSC1845 have non standard pinouts
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Yes I did Swizzle them. Properly? That’s worth a second look. I get back to you.
 
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TIS97 Flat facing you leads pointing down

C-B-E

KSC1845 Flat facing you leads pointing down

E-C-B

Both are oddballs

Standard TO-92 is E-B-C
 
dcmeigs

dcmeigs

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Yes, they are done correctly. Doesn’t hurt to double check.

Edit: I’ve been listening to it for hours now on some Baby Advents. is spooky how quiet it is with the 9600uf filter caps. Measures about 2mv ripple at most. I’m using 1 amp fuses for speaker protection so I’m keeping it down below 8 watts, and the speakers can’t handle much more anyway, but I’ve experienced no problems once running. I mention this only because it helps rule out some obvious mistakes.
 

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dcmeigs

dcmeigs

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#57
We’re all thinking that the KSC1845 were to blame so I put the TIS97s back in. It still latches up.

Snake bit, haha.
 
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