Band Master AB763 trouble

[xtian]

I have a 60s blackface Band-Master on the bench. Had bad hum. Tried to eliminate other possibilities, and then decided on new filter caps. Just finished replacing all in the doghouse, also 1k and 4k7 dropping resistors. Hum persists.

Bias is at -48v. New tubes everywhere. Voltages all look like those on schematic, plus 10% for my 120vac wall voltage. 458v on plates.

Pulled PI tube; hum stops. OK. Put PI tube back, but grounded it's input (at junction of 220K mixers and 500p coupling cap). Still bad hum. Tried several different tubes in PI spot, no change.

OK, then, maybe bad cap in PI? They are still blue molded originals. Which is most likely? Lift ends and measure DC?

[xtian]

schemo:

http://vintagefenderamprepair.com/wp-co ... ematic.pdf

Layout:

http://vintagefenderamprepair.com/wp-co ... layout.pdf

[dartanion]

Check all your ground points. Also, start at V1 and remove preamp tubes one at a time. This should help you track it down as the fault could be anywhere before the PI or the PI itself. Also, photos help us help you.

[xtian]

Check all your ground points. Also, start at V1 and remove preamp tubes one at a time. This should help you track it down as the fault could be anywhere before the PI or the PI itself. Also, photos help us help you.

I have isolated the issue to the PI already, I think, because there is no hum with PI pulled, and full hum with PI installed but PI's input grounded. To confirm, I removed all the other preamp tubes, as you suggested. Same results.

Grounds are all good. Also, chopsticking reveals nothing.

[xtian]

I lifted the power-tube-grid-side of the two 0.1uF caps that couple the PI to the power tubes, and measured voltage, with NO PI tube installed. Both caps the same: fluctuating voltage between 0.5 and -0.5 vdc. Could this much ripple be in the power supply at this point? The only thing in the PSU I haven't replaced yet is the two 220K balancing resistors on the reservoir caps.

But, I guess this much ripple isn't a problem, because earlier I noticed the amp was quiet with the PI pulled (same as in the test just run).

OK, I have soldered the caps back into place, and I'm using my scope. I see that in standby, the power tube grids show a 60-cycle hum. Very obviously the bias voltage supply, textbook shape.

But when running (not in standby) and with the PI tube installed, the trace shows a second wave superimposed, at about twice the amplitude and twice the frequency.

Any clues from all this?

[xtian]

Any ideas peeps?

[JazzGuitarGimp]

The first two things I'd look at:

- If the choke has suffered a meltdown due to a shorted screen or some other malfunction, you might be getting more ripple on the screen / PI node. I would measure the DCR of the choke and compare against published specification.

- Try clipping another electrolytic across the existing cap at the screen node.

Good luck!

[dartanion]

Could be a leaky PI coupling cap. I have seen this in a few amps. Frustrating to find, but a simple fix.

Check the voltage on the power tube grids with the power tubes pulled and PI tube in place. A leaky coupling cap will pass DC current and mess up the bias of the power tubes. You said with the caps lifted that you are measuring some voltage, which shouldn't be happening. I would simply replace the coupling caps and see if that fixes the issue as it certainly sounds like the issue.

[xtian]

Would it shock you to discover this amp has no heater CT and no artificial CT either? (Thanks, Richie, for suggestion.) I will try adding artificial CT next.

But, hey, do you see this often in 60s Fenders?

[ToneMerc]

Would it shock you to discover this amp has no heater CT and no artificial CT either? (Thanks, Richie, for suggestion.) I will try adding artificial CT next.

But, hey, do you see this often in 60s Fenders?

It's centertapped I bet , show an overhead picture of the PT. Are there two solid yellows terminated to ground?

TM

[xtian]

Fixed! I added an artificial CT, and it solved the hum! Richie scores!

Photo attached. You can see my 100R resistors on the lamp socket. Transformer has an HT CT, but no heater CT.


Well, on the the next problem: when I increase volume past a certain threshold, I get nasty, crackly distortion. Rats, now I have to do my "real" day job instead of working on this amp.

[xtian]

Awwwww, crap. This is the sound of a bad output transformer. I have a dummy load plugged in, no speaker. This is the sound the OT makes with a 1k sine wave at the input, as I turn the volume up and down. At first I thought it was a tube rattling, but no, this sound is coming from within the OT.

[ToneMerc]

Fixed! I added an artificial CT, and it solved the hum! Richie scores!

Photo attached. You can see my 100R resistors on the lamp socket. Transformer has an HT CT, but no heater CT.


Well, on the the next problem: when I increase volume past a certain threshold, I get nasty, crackly distortion. Rats, now I have to do my "real" day job instead of working on this amp.

Cool, someone must have removed them thinking they were doing the amp a favor, as I have never seen one without eiher the CT or the artificai CT. Hey, while you are at it, replace all those diodes in bias and HT string. That's a weak link in these older amps.

TM

[Richie]

I just took a {wag>wild ass guess} among the few other things. But since I couldn't see the PT in the first pic.. that's why I asked. This era, and so many models,plus you never know who or what has even been done before,you just have to look. The rest of the amp looks clean and mostly stock except for the electrolytic caps.

So like others posted, back up,and see what is working, meaning does the noise happen in both channels? If you lightly roll the preamp or small tube in the socket, and the amp makes a bunch of loud noise, the sockets are probably dirty,or could be tube pins. check for,loose grounds, could be control nuts,speaker jacks etc..

Make sure the input switching jack is making contact. Always clean those. Looks like the screen grid resistors were changed. Probably should have changed the 1.5k grid stopper too. But if you have bias on pin 5 they are probably ok. The Carbon Comp tend to get brittle from the tube heat. And lastly, if you need to retention the tube sockets, be sure to always drain the voltage from the amp.

[Richie]

Awwwww, crap. This is the sound of a bad output transformer. I have a dummy load plugged in, no speaker. This is the sound the OT makes with a 1k sine wave at the input, as I turn the volume up and down. At first I thought it was a tube rattling, but no, this sound is coming from within the OT.

did you read the resistance on the OT?