PL400 II Resurrection

'Were it not'. Autocorrect snafu, gimme a break, it was 5.30 am when I posted it, and I was already heading out the door. And my phone's native tongue is Flemish.

After checking some dodgy wiring on the power transistor side of things yesterday night the bios voltage went up do around 0.05V. Let's say I have an 'adjustment range' now between 0.04 and 0.075V.

I wouldn't read to much into the difference between the bias transistor C-E and Q6/Q9, because the bias transistor measurement was a bit of and afterthought and not taken at the same time. Will do if required.
Incidentally, R21 and R30 are fine, even that botch repair I made on 30 is doing OK.

I'm trying to get a handle on how the left OK channel dropped by a factor 10 when I fixed the right one. How did I create a voltage devider (yeah making a sink, but where ?). I didn't touch left ! I think it happened at the same time that base wire worked itself loose.

I don't understand the sexy nurse babe thing but it sounds interesting ! AFTER I get this amp fixed :)

Peter
 
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Gepetto said:
Peter can only get it to 37.5mV instead of 375mV. Both channels are now stable but bias is low by a factor of 10.

He needs one of Navo's sexy nurse babes to fix this.
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I agree Joe. We're sending a nurse your way Peter to help out. She'll increase your bias by an order of magnitude. She also understands human anatomy quite well. Cheers.

nurse_38.jpg
 
NavLinear said:
I agree Joe. We're sending a nurse your way Peter to help out. She'll increase your bias by an order of magnitude. She also understands human anatomy quite well. Cheers.

View attachment 36970
Click to expand...
Lord knows where all my solder is going to end up when she shows up....
But does this order of magnitude bias increase mean this is in fact a class A nurse ?
 
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This thing needs to get of my workbench this weekend, or it gets buried.

Now, when the R channel base wire came off, r channel bias on the Test point went to 20V, and the left, previously at .375 as it should be, collapsed.

Reconnecting the base wire dit not bring the bias back up on the L chan, but did drop the R chan bias to the same order as the now collapsed L channel.

Since I'm measuring on what is essentially the base return (why is it called the 'bootstrap' ?) can anyone please tell me what the value on pin 9 is supposed to be ?

Additionally would it be useful to disconnect pin 5 on the display in case there's a short there ?

Edit : disconnecting the meters did not change matters. The low output on BOTH pin 9's I find suspicious. There's only around 0.9V there.
 
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How com R21 is 22 Ohm on the schematic, and 56 on the BOM ? What should it be ?

Because given there's 80V on 11L , R34 is 10Ohm, R31 is 10K, C18 is 47 uF, R20 is 3.32K all measured OK, the only reason base voltage can be too low if R21 is indeed too high.

Gepetto ?
 
TapeHeadOne said:
How com R21 is 22 Ohm on the schematic, and 56 on the BOM ? What should it be ?

Because given there's 80V on 11L , R34 is 10Ohm, R31 is 10K, C18 is 47 uF, R20 is 3.32K all measured OK, the only reason base voltage can be too low if R21 is indeed too high.

Gepetto ?
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The BOM rules as far as values are concerned Peter, the schematic is a reference doc. R21 should be 56 as the BOM shows.
 
Need some reference voltages from working systems. Without them I’m just faffing about. Anyone took notes when installing their boards ?

Cheers Peter
 
TapeHeadOne said:
This thing needs to get of my workbench this weekend, or it gets buried.

Now, when the R channel base wire came off, r channel bias on the Test point went to 20V, and the left, previously at .375 as it should be, collapsed.

Reconnecting the base wire dit not bring the bias back up on the L chan, but did drop the R chan bias to the same order as the now collapsed L channel.

Since I'm measuring on what is essentially the base return (why is it called the 'bootstrap' ?) can anyone please tell me what the value on pin 9 is supposed to be ?

Additionally would it be useful to disconnect pin 5 on the display in case there's a short there ?

Edit : disconnecting the meters did not change matters. The low output on BOTH pin 9's I find suspicious. There's only around 0.9V there.
Click to expand...

When the right channel base wire came off (which is a very bad thing to have happen) your bias goes to a very large and damaging voltage as you indicated above. Did you blow one or both of the +/-80V rail fuses when that occurred?
 
No (and there's only 2A in there), but it ate the 2 MPS 651s. Yes. BOTH channels. Everything else seems to have survived. Didn't have 651s available so waiting for parts.

I'll continue with it when I have time or feel so inclined, which won't be soon. It's taken up far too much of my time.
 
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My backlog seems to have cleared up somewhat, so this weekend I will once more try to resuscitate the beast . Pete
 
I have now restarted in earnest. I have replaced the 2 bias transistors that now had broken legs from the many manipulations.
I replaced a suspect wire, reassembled the amp and switched it on. Remarkably, no fuses blew, and no smoke was immediately obvious.
Upon measuring the Left and right bias a prescriped, the initial value was 50 mV, and turning up the pot meters enabled ramping up to about 70 mV.
This is obviously not right, but now at least values on both vhannels are equal.
I will continue by repeating the measurements suggested in the previous 7 pages, putting them into an excel and see where they lead. Sugeestions, as always are most welcome.
 
What bias transistors did you use? The pin out can change when replacing from original.
 
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