Resurrected a Customized WOPL 400

Post 152 on pg 16 may have been some of your trouble. When you were having problems getting the power supply wired you could have caused this damage.
 
laatsch55 said:
Post 152 on pg 16 may have been some of your trouble. When you were having problems getting the power supply wired you could have caused this damage.
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Well, I'll check everything again. I don't remember the specifics of the problem in the post you referred to. So I'll go back and start over from that point for diagnosis. Thanks for taking the time to go through all that stuff.
 
You powered up with the power supply wired wrong once in there. You got it squared away but never got this far since then. Replace the pico fuses and the 1 ohms and I think you'll be OK. Order some TL 431 voltage regulators too, just in case the -15 volt regulator is fucked...
 
laatsch55 said:
You powered up with the power supply wired wrong once in there. You got it squared away but never got this far since then. Replace the pico fuses and the 1 ohms and I think you'll be OK. Order some TL 431 voltage regulators too, just in case the -15 volt regulator is fucked...
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Okay, thanks. I'll get my Mouser mind going and order up. But you think the problem occurred way back when, and that it occurred because of a mix-up, mistake, fuck-up? So just get everything right this time and try again?
 
scottonnob said:
I think Dennis' life is a little nuts now. So I don't mind waiting a bit longer. He's come through a bunch of other times. It'll work out.
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Almost through this - another move at work that has/is very stressful plus the Thomas Fire shit. Previous dumbass management and a team move has keep us afloat. I could write a novel about this. Fcuk!
 
My on again, off again project has hit yet another snag. I replaced the TL 431s on the board, plus all four of the pico fuses before, once again, attempting to breathe life into my WOPL 400. All went well, until it didn't. I went through the Variac and DBT process with no apparent problems. The DBT came on with full brightness, then dimmed down somewhat. I thought it was a little bright for idle, but when I looked on my meter, the unit was pulling no more than half an amp – 0.453. So I thought everything was okay. Then I plugged it directly into the AC — no noises, no smoke, no nothing for about 15 seconds. Then, just as I was turning away to get the DMM and start checking voltages, there was a pop and a puff of smoke. A resistor in the left channel blew, and I shut everything down immediately. It's R24L, 182 Ohm. I feel like an idiot, repeatedly having problems with this amp. But it's actually rewarding. With every problem, I learn something new. So I'm still plugging away, even though I need to again ask for some assistance from you much more experienced guys. Does anyone have any thoughts about this particular resistor blowing up?
 

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scottonnob said:
My on again, off again project has hit yet another snag. I replaced the TL 431s on the board, plus all four of the pico fuses before, once again, attempting to breathe life into my WOPL 400. All went well, until it didn't. I went through the Variac and DBT process with no apparent problems. The DBT came on with full brightness, then dimmed down somewhat. I thought it was a little bright for idle, but when I looked on my meter, the unit was pulling no more than half an amp – 0.453. So I thought everything was okay. Then I plugged it directly into the AC — no noises, no smoke, no nothing for about 15 seconds. Then, just as I was turning away to get the DMM and start checking voltages, there was a pop and a puff of smoke. A resistor in the left channel blew, and I shut everything down immediately. It's R24L, 182 Ohm. I feel like an idiot, repeatedly having problems with this amp. But it's actually rewarding. With every problem, I learn something new. So I'm still plugging away, even though I need to again ask for some assistance from you much more experienced guys. Does anyone have any thoughts about this particular resistor blowing up?
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Hi Scott
When you look at this resistor in the schematic, you will see that this 182 ohm resistor is shunted by one of the 0.33 ohm emitter resistors on the backplane board. If this resistor is not blown open on the backplane board then you should not be able to experience damage to this resistor. It feeds the base of the upper half protection transistor. A key to this is why your nearly 0.5A line current is occurring. That is nearly 60W sitting at idle which should not be the case.

When you were on the DBT, what was the bias on this left channel reading at and what was the output offset voltage? You should be able to put it back on the DBT and get that ascertained. The resistor you smoked is not critical to the proper operation of the amp, just the protection circuit.

Double check that you do not have an open solder joint on backplane resistor R35. Cannot see any of that in your photos. It would also help to get us some higher rez photos. These are somewhat grainy which makes it hard to blow up and see anything.
 
I don't see anything wrong on the backplane. Do you?
 

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I'm assuming this is a Rev E board? If so, my schematics show R24L as a series resistor in the positive side current limiting circuit. Overloading that circuit would require a short to ground in the output rail or a miswiring in the negative side drawing excess current through the positive side. This is a difficult area to troubleshoot because this circuit is benign in normal operation and only comes into play when the output is drawing excessive current. I would like to know what pin 8L measures to ground with the resistor open, if the resistance is high, I would suspect Q7L the SOA protection device on the positive side. First verify that it is a MPS 651.
 
Okay, additional information. I just saw that the red wire to the Bias-C connection is not connected. It must have broken off when I was replacing the pico fuses.
 

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Gepetto said:
Hi Scott
When you look at this resistor in the schematic, you will see that this 182 ohm resistor is shunted by one of the 0.33 ohm emitter resistors on the backplane board. If this resistor is not blown open on the backplane board then you should not be able to experience damage to this resistor. It feeds the base of the upper half protection transistor. A key to this is why your nearly 0.5A line current is occurring. That is nearly 60W sitting at idle which should not be the case.

When you were on the DBT, what was the bias on this left channel reading at and what was the output offset voltage? You should be able to put it back on the DBT and get that ascertained. The resistor you smoked is not critical to the proper operation of the amp, just the protection circuit.

Double check that you do not have an open solder joint on backplane resistor R35. Cannot see any of that in your photos. It would also help to get us some higher rez photos. These are somewhat grainy which makes it hard to blow up and see anything.
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Joe, does the 400 BP boards mount like the 700's? His ground lug set up could be causing board flex/short the way it is.
 
Plus, the Bias transistor holders are crooked which can cause the leads to short. Look that over good like I mentioned before
 
It's possible that bias wire shorted to something. Obviously, when I opened it up, the wire moved. So I have no idea what it might have come in contact with.
 
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