The trannies with the loose caps didn't happen to have a "9638" date code, did it? If so, you got the same fakes I did. The tops pop right off. They're *** grade 2N3055's covered in white goo inside. They won't take the load a PL400 puts to them, and they don't match well enough to parallel (that's how I found out the batch was fake - the bias currents were all over the map).
Bias out of whack
Sorry if I am posting in an obsolete thread, My problem is somewhat related so I thought I'd put it along with the other PL 400 post. I just recently brought one back from the dead. Had Dc latch up, replaced Q3 and Q4, also some related diodes, all outputs and drivers with GOOD ones, also replaced Q1 and Q2 because the were originals with known issues. Fired it up, DC offset is perfect, but Bias voltage is running upwards of 600mv on both channels and I cant get it to adjust down with R20 pot. I have the 14a board, best I can tell, layout is the same but my Bias transistors were board mounted, not chassis mounted. D3imlay suggested I move them for thermal stability, which I did. All is wired correctly, and they are the correct transistors. Now, the really weird thing? In location R19, it is supposed to be a 470 ohm resistor. My board has 1k resistors in that location. All looks factory. Anything I am missing about my bias that I should be looking for? and does anybody have schematics that show a 1K resistor in location R19? Waiting for Dean at Phasetek to get back to me. Seems like I have some odd board or something and could use the proper schematics to figure things out. Any suggestion or info would be appreciated!
Sorry if I am posting in an obsolete thread, My problem is somewhat related so I thought I'd put it along with the other PL 400 post. I just recently brought one back from the dead. Had Dc latch up, replaced Q3 and Q4, also some related diodes, all outputs and drivers with GOOD ones, also replaced Q1 and Q2 because the were originals with known issues. Fired it up, DC offset is perfect, but Bias voltage is running upwards of 600mv on both channels and I cant get it to adjust down with R20 pot. I have the 14a board, best I can tell, layout is the same but my Bias transistors were board mounted, not chassis mounted. D3imlay suggested I move them for thermal stability, which I did. All is wired correctly, and they are the correct transistors. Now, the really weird thing? In location R19, it is supposed to be a 470 ohm resistor. My board has 1k resistors in that location. All looks factory. Anything I am missing about my bias that I should be looking for? and does anybody have schematics that show a 1K resistor in location R19? Waiting for Dean at Phasetek to get back to me. Seems like I have some odd board or something and could use the proper schematics to figure things out. Any suggestion or info would be appreciated!
Thanks Netlist for the reply. Phasetek was hosted on AOL hometown but AOL shut that service down last fall. I read on another board somewhere that it was going to be hosted again somewhere else, but haven't seen that yet. I hope they get that back up and running. I had an email address for Dean luckily, so I shot all that info to him. He usually is a few weeks behind, certainly he's still a very busy guy. I read a post on AK about changing the value of R15 to get the bias down. It did work for that guy. Just can't see why I have those values in that location and it was supposedly fine all those years. Praying that somebody has seen this before, and or Dean can explain it.
I see the high bias problem every now and then. I assume it is dependent on the gain of Q6. When I see this I usually bump R15 to about 4K. I've also changed the pot to 10K. As part of the checkout I always run a hair dryer over the heatsinks to make sure the bias drops off and then returns to the same point after cooling.
D3! How nice of you to join in.. Thank you! Something happend at AK and can't get in anymore! I think I'll try that with R15. So if i heat up the heatsink, I should see a change in the bias, and when it cools it should return back sounds like? Sounds like a good test to see if the Q6 Trans. is functioning properly. You have any refrence as to why I have those 1K resistors in the R19 location? Just very odd I thought. Thanks a million!
I believe your amp was a very early production run. If I'm right about the current gain of the transistor then I'd say the original batch of transistors that were used for the design had a lower gain. Later, during production, the gain increased in the transistors that were purcahsed and they changed the value.
Go ahead and change the R15 to 4K. If you do the hair dryer first and it doesn't work, the bias could increase up to around 650mv at which time all of the transistors turn on and go to war with each other. I had this happen on a 700B one time and all of the outputs and drivers failed in the channel.
Go ahead and change the R15 to 4K. If you do the hair dryer first and it doesn't work, the bias could increase up to around 650mv at which time all of the transistors turn on and go to war with each other. I had this happen on a 700B one time and all of the outputs and drivers failed in the channel.
Well, I'm on the right track finally. I replaced R15 with a 3.9K resistor and Bias is now too low running at 5mv. Looks like I need to split the difference with about a 7.5K possibly? R20 didn't seem to adjust it much once again, maybe a few millivolts was all. Just was too scared to let it run very long, but looks like we are zeroing in on it D3imlay! Once it's at an acceptable range, I'll throw some heat at it with the hair dryer and see how things react. Thanks for your suggestions and insight! Wow what a learning experience this has been!
d3imlay said:
So I think I see where the problem lies, it just sank in this morning after a few days off from this. R15 is in series on the 400C, but R19 is the one in series with the pot on my 400 14a! So looks like the advice from that other guy is correct for the 400C, which I believe he had. Good Gravy! So I'll try replacing R19 (which is in series with the bias pot) with that 4K and see what happens.
Well, anybody have a rope?? Really getting to me now! Just as I thought I was on the right track.... Arrrg! I put R15 back, played with some values in the R19 position. I'm almost up to 10k and only able to drop it down to around 560mv. Could anything down stream of this area can be causing this or should I just keep raising the value of R19? Just hard to believe I have raised it 10 times from what it was originally and not much change as of yet. I might be needing your address D3imlay! I just cant help but think I am over looking something.
Yes both channels exhibit this behavior. Junctions of Q6 look great, I'll try the Q7 and Q10 test next. But? Remembering that I had those 1K resistors in location R19, and all the schematics call for 470 ohm in that location, I have been raising the value of R19 higher, perhaps I should be lowering it? I'm probably wrong on that thought, but seems like if it isn't turning on maybe because the impedance of R19 is too high? or maybe it doesn't work that way. Anyways I'll check all associated components around Q6...
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