B52 AT100 Help R137 1.2k fried again?

[BlueEyedDevil]

Any help would be greatly appreciated! MY B52 AT100 amplifier, which I purchased from ebay (knowing it would not power on) is still broken after I tried to fix it. The resistor at R137 has been rplaced with the same value (1k2 ohm) resistor just 3watts instead of 2.
The reason for replacing the resistor was it was 'fried" when I bought the amp. The reverb has also been cut (by the previous owner??) and there are no connections to the reverb or ground in the amp from the reverb transformer. "my question is if I reconnect the reverb and return the amp to "STOCK" would this help any? or just be a big waist of time???" Here are some pix also attatched a schematic. ThanX in advance for any one's time!

Joshua Adam's MY B52 AT100 Amp inside Gut shots Photo Album - MySpace Photos

[Jazz P Bass]

B52 AT100 B+

R137 on the schematic is 1 watt.
Someone put in a 2 watt & it fried.
Sold the amp on EBay.
Now you placed a 3 watt resistor in there.
STOP>
Something is loading down your B+ voltage.
Pull all the tubes & start checking voltages.
Put a low value fuse in the fuse holder (1/2 amp)
(the amp should not pull more than 150 watts at idle)
Reinsert the preamp tubes one at a time.
Monitor the B+ Vdc.
Reinsert the output tubes one at a time.
My bet is one of the output tubes is bad.
I would leave the reverb disconnected for now.

[Mark Black]

Looks like R137 feeds "B+3", which is tied directly to (among other things) the reverb driver transformer. A shorted tube in V10 might cause excessive current draw, but a primary short to ground in that reverb driver transformer would be much worse and could account for the problem.

The power tube plates & screens get their power "upstream" of R137 (from B+1 & B+2 respectively), so I doubt if they are causing the resistor burn.

Rather than continue burning things up I would unplug & power the amp down with standby set "on", wait a while for the caps to discharge, and start checking with an ohmmeter for low-resistance-to-ground at the B+3 node. If a low resistance is found I would then start disconnecting things attached to that node until the culprit is found.

Good luck!

[Enzo]

I would detatch the reverb drive transformer from the B+ node that serves it and see if that makes the difference.

I had one of those through here that was arcing between the little push-on terminal for the B+ to reverb transformer and the surrounding ground traces. So come to think of it, unplugging the transformer wouldn;t help. But looking real close at the board near its connections might.

[BlueEyedDevil]

B52 Powers Up No sound. Blown R137

R137 on the schematic is 1 watt.
Someone put in a 2 watt & it fried.
Sold the amp on EBay.
Now you placed a 3 watt resistor in there.
STOP>
Something is loading down your B+ voltage.
Pull all the tubes & start checking voltages.
Put a low value fuse in the fuse holder (1/2 amp)
(the amp should not pull more than 150 watts at idle)
Reinsert the preamp tubes one at a time.
Monitor the B+ Vdc.
Reinsert the output tubes one at a time.
My bet is one of the output tubes is bad.
I would leave the reverb disconnected for now.

Looks like R137 feeds "B+3", which is tied directly to (among other things) the reverb driver transformer. A shorted tube in V10 might cause excessive current draw, but a primary short to ground in that reverb driver transformer would be much worse and could account for the problem.

The power tube plates & screens get their power "upstream" of R137 (from B+1 & B+2 respectively), so I doubt if they are causing the resistor burn.

Rather than continue burning things up I would unplug & power the amp down with standby set "on", wait a while for the caps to discharge, and start checking with an ohmmeter for low-resistance-to-ground at the B+3 node. If a low resistance is found I would then start disconnecting things attached to that node until the culprit is found.

Good luck!

I would detatch the reverb drive transformer from the B+ node that serves it and see if that makes the difference.

I had one of those through here that was arcing between the little push-on terminal for the B+ to reverb transformer and the surrounding ground traces. So come to think of it, unplugging the transformer wouldn;t help. But looking real close at the board near its connections might.

First of all, I would like to start by saying "THANK YOU for replying!" I did not expect such a quick well thought reply from so many members, seriously I am greatful for the help and concern!

Here are some pix of what's goin on under the hood. The reverb tranny is completely cut from connecting in any circuit and all leads wrapped together. The tubes all work great, I pulled form two other amps and was playing both these amps before i found you guys response, with tubes I used and the ones from the amps. So I am moving past the tubes for now...?

-The only loose "wire" i see is comming from a ground. Which I haven't had enuff time to see exactly where the ground was leading too? I have another working B52 At100 I have not taken down yet. (it's my working amp)
The loose lead grounds at P54 on the ground terminal board. There are no other missing leads from the power board so my assumption is the reverb tranny ground.

-I also noticed the power caps at C32 & C31 had some very small bubbles and I smelled the "burnt" smell down on that end of the amp. Maybe due to the overdraw what ever the prob is causing? Should I definitely replace these now?

-The 100uf 250v cap at C88 which lies next to the burnt resistor had scaring on it from the resistor's fireworx display as well. Should it be replaced?
-The B+3 at B7 on the preamp and tube board which connects to B32A on the power board also looks like it may be arcing to the resistors R108 and R109? Which you can see the burn at the terminal B7 in the pix. Could this be "because" of the the problem? Or could this "BE" the problem? what could I do to remidy this issue?

-One last thing was I noticed a Hole drilled under a resistor at R106 and some burning? by the B+3 at P8 on the preamp board back to the same B+3 P32 on the power board also P32A is right there (the burnt resistor 1.5k R137) Looks like all the tube ground resistors were raised to avoid the arc problem, because mine on my working B52 were all board flush mounted. My next step was to remove the preamp board???, however wanted to see what to do before I did this?

Should I still checking with an ohmmeter for low-resistance-to-ground at the B+3 node..?

thanX in advance from: Joshua!

[Mark Black]

The B+3 at B7 on the preamp and tube board which connects to B32A on the power board also looks like it may be arcing to the resistors R108 and R109? Which you can see the burn at the terminal B7 in the pix. Could this be "because" of the the problem? Or could this "BE" the problem? what could I do to remidy this issue?

You may have found your problem.

The arcing & carbonation at that terminal looks like it may have created a conductive path not to the resistors R108/109 but rather to the large area (which looks suspiciously like a ground plane) surrounding the terminal.

All of the carbonized PCB material needs to be removed (on both sides of the board if applicable), even if that means you end up with a hole next to the terminal - you can always fix a hole with a little epoxy. Kind of a judgement call as to when you've gotten rid of all the conductive carbonization and are down to non-conductive PCB material.

[blckmynnse8]

Out of curiosity, did you ever solve this problem?