Bandmaster AB763 low output w/ weird distortion (scope shots attached)

[Leo_Gnardo]

I do believe that your OT is toast.

The A6A, voted most likely to fail Fender OT. (Sound "Taps"). Oh well, replacements don't cost an awful lot, as long as you don't match brands with the PT.

[waspclothes]

The A6A, voted most likely to fail Fender OT. (Sound "Taps"). Oh well, replacements don't cost an awful lot, as long as you don't match brands with the PT.

This is the first Fender failed OT I've encountered, that's why I was really hesitant to believe that was the case. It was 022848, looked to be original.

[The Dude]

This is the first Fender failed OT I've encountered, that's why I was really hesitant to believe that was the case. It was 022848, looked to be original.

That's actually refreshing. So many here jump straight to wanting to replace a transformer with little or no troubleshooting.

[Leo_Gnardo]

That's actually refreshing. So many here jump straight to wanting to replace a transformer with little or no troubleshooting.

I try to prove it's anything but the transformer, who wants that kind of bad news. OTOH if it's suspect, swapping in a known good one with 'gator clips is a fast test, and you don't even need the same exact transformer. If the power jumps up a bunch, you know what the bad news is.

[Tom Phillips]

I'd still like to know that you verified the screen DC voltage at pins #4 of the power tubes. The answer in post #11 didn't make sense.

[J M Fahey]

Feeding AC on one side, reading it on the other, by itself does not prove a shorted turn OT, which is the typical failure.

Input is 14.9 VRMS and output is .546 VRMS

Means turns ratio 27.3 to 1 or impedance ratio of 744:1 , meaning PP impedance with an 8 ohms load of 5600 ohms (almost perfect book example) or half that with 4 ohms speaker; low but still usable.

Proper shorted turn detection is by RG Keen´s neon bulb tester or plain replacement; unless you have a replacement OT on the bench the first one is faster and cheaper
https://www.premierguitar.com/articl...ormer-tester-1

[alexradium]

are the coupling caps tested?
If its a vintage amp many times thats the culprit

[waspclothes]

are the coupling caps tested?
If its a vintage amp many times thats the culprit

Hi Alex, I tested for DC voltage on the "other side" of the coupling caps while in circuit and they were all blocking DC so I left them in place. Is there anything else I should be testing with coupling capacitors?

New O.T. has been ordered and is on it's way, I will post an update when I get it in.

[waspclothes]

Hi Alex, I tested for DC voltage on the "other side" of the coupling caps while in circuit and they were all blocking DC so I left them in place. Is there anything else I should be testing with coupling capacitors?

New O.T. has been ordered and is on it's way, I will post an update when I get it in.

Okay the new O.T. came, swapped it out and got the same low output problem. Very confused on that one.. then I noticed the screen grid resistor was 47K instead of 470R! These looked orginal to me, I was never expecting to see a value off by 100. I even measured their resistance early on and probably just saw "47..." on the multimeter and didn't pay enough attention! After swapping those out, the amp was working fine.. so I put the old O.T. back in. So, lots of work for 2 resistors!

Here's a scopeshot of the output at full clipping:



1 (yellow) - 6L6 1 grid
2 (cyan) - 6L6 2 grid
3 (magenta) - P.I. input
4 (blue) - output to 8 ohm load.

[Tom Phillips]

...So, lots of work for 2 resistors!...

Yep. See post #35 from 20 days ago. The screen voltage is much more in play than we realize until it's driven home by an experience such as the one you had.

[TimmyP1955]

Make sure that you are not overloading the input of the scope - easy to do if it is a PC type.

[Tone Meister]

Okay the new O.T. came, swapped it out and got the same low output problem. Very confused on that one.. then I noticed the screen grid resistor was 47K instead of 470R! These looked orginal to me, I was never expecting to see a value off by 100. I even measured their resistance early on and probably just saw "47..." on the multimeter and didn't pay enough attention! After swapping those out, the amp was working fine.. so I put the old O.T. back in. So, lots of work for 2 resistors!

Here's a scopeshot of the output at full clipping:

[ATTACH=CONFIG]43328[/ATTACH]

1 (yellow) - 6L6 1 grid
2 (cyan) - 6L6 2 grid
3 (magenta) - P.I. input
4 (blue) - output to 8 ohm load.

Posts #35 and #36 would have been very useful in solving this one