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The B-2 Restoration

DBT will also load down your secondary power supply voltages..Including pinch-off voltage to the FET's..
 
Have made a little progress on this, but not smooth sailing unfortunately. All items in this post were done with VFETS not installed. I slowly brought it up on my variac/DBT. No DBT issues, but not all is good:

The good:
- voltage between B+ and B- (red and white on the VFET boards) is ~115V on both channels, as expected.
- voltage between B+ or B- and E (on VFET boards) is ~57.5V (half of B+ to B-) on both channels, as expected.
- +85 measured at 85V, as expected.
- no 2SC458 LGs (outhouse style) found on the power board.
- I have tested all components on the Right driver board (the one that was previously repaired) and all tested good.

The not good:
- Unit comes out of protection, and left relay clicks, but right does not.
- the -85V is running at -94V, with VR202 turned all the way down. I have confirmed that the trimmer is good. I am not sure if something failed during the reforming w/ variac, or was an issue previously. I took readings on +85 and -85 each time I increased voltage and both were solid up to about ~60V. When I took measurements at line voltage (100V) the -85 measured high, and could not be lowered with VR202).

I have not had a chance to do more diagnostics on the power board yet. If anyone has any initial thoughts on what to check, I would appreciate any guidance, either on the forum or privately).
 
bobdod said:
Some initial troubleshooting found TR205 (2SC1439-V) to be bad (base/emitter are shorted).
Click to expand...
you should not have any of the driver boards to this thing untill you have a rock solid PSU.
Your ~57v +/- is the unregulated rail and that will vary based on your mains...nothing on the PSU board that would affect this rail.

The regulated +/-85v rail must be fully functional BEFORE you introduce anything else into the mix.
 
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rottalpha said:
you should not have any of the driver boards to this thing untill you have a rock solid PSU.

The regulated +/-85v rail must be fully functional BEFORE you introduce anything else into the mix.
Click to expand...

Yes, driver boards are out and will continue to work on power board. Haven't had a lot of time to trace and troubleshoot from the aforementioned TR205 on the power board yet. Any components I should look at first?

I don't have a replacement for the 2SC1439 on hand to see if that resolves the issue. The only possible replacement that I found available is KSC3503DSTU, but not sure if this is viable.
 
I've not been able to solve it thus far. I replaced TR205 with a KSC1815 for the time being, to see if this solved the issue, but there was no change, and -85V was still running high and couldn't be adjust any lower than ~112V. The trimmer has very little effect turned all the way up or down (minimum is ~112V, maximum is ~112.5). I checked every resistor and diode, without finding anything, and I confirmed there are no shorted transistors. I noted measured voltages relative to the schematic, and noted them below. Any ideas on where to look?

powerboard.jpg
 
clinic-audio said:
?? D206 is a 13V Zener diode.(WZ130).....
30V ?
Click to expand...

Yes, I am measuring 30V across both D205 and D206. These are the diodes you are referring to, correct? Aren't these 30V zenners?
F96CF5F3-2BCE-4C88-B05C-A34A2618F15E.jpeg

Vint Age said:
Have you reflowed all the solder joints on the PS board yet?
Click to expand...

Have done most of the circuit in the above posted section of the schematic. I'll work on the rest and see where I get. I did find a loose wire on the power board, but it was a result of flipping the board up and down repeatedly. I've reflowed all of the wire points.
 
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bobdod said:
Yes, I am measuring 30V across both D205 and D206. These are the diodes you are referring to, correct?
View attachment 1992439



Have done most of the circuit in the above posted section of the schematic. I'll work on the rest and see where I get. I did find a loose wire on the power board, but it was a result of flipping the board up and down repeatedly. I've reflowed all of the wire points.
Click to expand...

Be carful , I did the same thing and am lucky I didn't fry the V-fets. Id remove all the wires from the board work on it, then re solder it back on and test voltages. Having a wire break and have the amp shut down while listening to the amp days after you "thought" you restored it will scare the **** out of you !!!

Athanasios
 
Nashou66 said:
Be carful , I did the same thing and am lucky I didn't fry the V-fets. Id remove all the wires from the board work on it, then re solder it back on and test voltages. Having a wire break and have the amp shut down while listening to the amp days after you "thought" you restored it will scare the **** out of you !!!

Athanasios
Click to expand...

Yeah, I am not anywhere near to running with vfets, and was fully expecting wires to break off while I am flipping the board over at this point. I've reflowed all of the wire points to keep this from happening while I try to find the issue. I am really stumped right now though. :dunno:
 
Decided to take a look at the protection circuit as I am not sure where to go next with the -85V issue. I've found voltages there seem ok. However, only the right relay clicks. I've swapped the two relays and ruled out that one of them may be bad. Not sure if this is a related issue to the -85v issue. Also, found the 12v to the meter lights measures a bit low at 11.3V, if that points to anything.
 
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