Bandmaster AB763 Trouble *RESOLVED*

[seedlings]

A friend asked if I would put new caps and tube sockets in his 1963 Bandmaster.  It was his father's, and hasn't been used in quite some time.  He said he put new tubes in and it sounded 'weak'.  Before I fired it up personally, I looked inside to find some leaking caps, and decided not to turn it on.

I replaced all the caps with Sprague Atom electros and Mallory for the smaller caps.  The tube sockets didn't hold the tubes securely, so I replaced them all one-at-a-time.

After firing it back up (and hours messing around trying to fix) this is how it sounds:
Bandmaster problem

It sounds nice up to about '6', and then starts this motorboating deal.  In efforts to fix the problem, I have
-replaced each tube with known good tubes
-re-flowed solder joints I repaired
-chopstick tap-tapped each component and joint
-added 10K resistors to the 6l6 grids (thinking blocking distortion)
-removed the 820R NFB (removed it sounds like bassy white noise, and tried a 10K pot in series, but the original 820R was better, so it's back in)
-measured the OT, and the primary one side is about 82 ohms to CT, the other side around 71, and 154 across both.  Resistance is hundreds of K to Mohms from primary to secondary or secondary to CT, and now I don't remember... the secondary was like 30ohms or something like that.

Here are the voltages:
12ax7 Normal IN
1) 261
2) 0
3) 2.2
6) 268
7) 0
8) 2.18

12ax7 Vibrato IN
1) 273
2) 0
3) 2.1
6) 281
7) 0
8) 2.2

12ax7 Vibrato circuit
1) 450
2) 42
3) 20
6) 404
7) 41
8) 4v (voltage wandering around)

12at7 PI
1) 213 (motorboat noise when probed depending on which ground is used)
2) 81.6
3) 129
6) 230
7) 83
8) 129

6l6 (top, off PI pin 1)
1) 47.6 (motorboat noise when probed depending on which ground is used)
3) 462
4) 463
5) 47.4 (motorboat noise when probed depending on which ground is used)
6) 463
8) 0

6l6 (bottom, off PI pin 6)
1) 47.6
3) 462
4) 463
5) 47.4
6) 463
8) 0

Heater voltage is good.  The sound is the same using either the Vibrato or Normal channel.  The Vibrato works properly.  Using one 8 ohm Eminence Red White and Blues.

Schematic:
http://elektrotanya.com/PREVIEWS/63463243/23432455/fender/fender_bandmaster-ab763.pdf_1.png

If you have any suggestions at all, I would very much appreciate advice.

Thanks, thanks and more thanks,
CHAD

[Tiny_Daddy]

It may be the reverb transformer that is arcing. Try disconnecting power from that transformer.

[bigsbybender]

It may be the reverb transformer that is arcing. Try disconnecting power from that transformer.

AB763 Bandmaster doesn't have Reverb.

[seedlings]

PROBLEM RESOLVED!

There are tons of grounds in this amp.  The one that goes to the 25uF bypass cap pin 6 of the vibrato circuit 12AX7 also grounds the 100R of the PI grids.  The end of the wire under the vibrato jacks had a bad solder joint.  I think it may have been repaired before because all the other grounds have a giant factory glob of solder, and this one looked like the solder had been wiped away.  So, re-soldered and it roars alive.

THIS GROUND:


Maybe someone else will benefit.

Thanks,
CHAD

[Willabe]

Another mystery solved and wisdom shared!


          Thanks for sharing seedlings,