Repair advice needed on potential bad PT.

[tdlunsfo]

Good advice. I'll do better next time. So I have updates, i swapped in the PT from the PA100 and its doing the same thing. One thing I noticed is that when the tubes are pulled the Bplus is spot on at 440v. As soon as I put in any tube, and the filament pulls current, the Bplus takes a dive. Any ideas? Caps are in correct. Bias voltage looks good at around 45v. The output signal looks bad.. there is some nasty noise riding the waveform.

[nosaj]

Any continuity from filament to HV taps?
nosaj

[tdlunsfo]

With searching for resistance from the filament to HV in mind, I found 75ohms through the lamp to the B+ feed to the OT. Thinking I had a leak (somehow) through the lamp, I disconnected it and wired the filaments straight to the output tubes and the resistance disappears. Amp b+ still takes a dive, so turns out thats a red herring. I also went through and lifted the filter cap supplies one by one to see if any of those circuits where loading down. No change. I am struggling to figure out how to troubleshoot this thing. Any suggestions? Maybe lift one side of the choke to take out the preamp/PI circuit and see if I can power the output tube HV and tube filaments by themselves? I haven't changed the diodes yet..although they test fine. Conductive board? Operator error? Ghosts? Cats and Dogs living together, mass hysteria?

[Helmholtz]

What is a PA 100? And what means "it's doing the same thing"?

The output signal looks bad.. there is some nasty noise riding the waveform.

Can you post a scope pic of the output signal?

As soon as I put in any tube, and the filament pulls current, the Bplus takes a dive.

So your B+ drops when you insert one preamp tube only?
How much drop?

Please post schematic.

[tdlunsfo]

Fender made some PA amps late in the CBS era and one of them is a quad 6L6GC amp called the PA100. The same customer with this bandmaster also dropped this off. I just happened to notice the power transformer topology was almost identical with the Bandmaster PT (except a little bigger), so I desolder the leads on both PT's and jumpered the PA 100 PT over to the bandmaster circuit for a test. The B+ took a dive just like the PT on the bandmaster. The voltage drop is about 40v with a single 12AX7 and about 80v with a single 6L6GC. The drop seems to settle off at the 320v mark and drops down to 285v when the amp is dimed with all tubes installed. Schematic says B+ should be 440-460v. Obviously the bias is way off due to the strange voltages so I have some crossover distortion but I also have a pretty significant buzz in the waveform. I'll take some pics and post them. Standby.

[Helmholtz]

Thanks.

What is the heater voltage with tubes in?

[tdlunsfo]

I haven't seen the heater voltage change, always around 6.2vAC but will confirm that.

[tdlunsfo]

Confirmed 6.2vAC on the heaters with all the tubes in. So I can't believe I haven't measured this yet but I am showing 70vAC on the B+ line going to the OT.
Not sure on this one as I have all new F&T's in place. Leaky diodes?

[tdlunsfo]

Note the power tubes have been converted to the AB763 style.
Bandmaster-AC568-Schematic.pdf

[Helmholtz]

Confirmed 6.2vAC on the heaters with all the tubes in. So I can't believe I haven't measured this yet but I am showing 70vAC on the B+ line going to the OT.
Not sure on this one as I have all new F&T's in place. Leaky diodes?

Check the reservoir caps (2x70µ) and connections.
Is the HT CT well grounded?

[Jazz P Bass]

Lift the OT center tap (red wire?) & see if your voltages come up.

[tdlunsfo]

So I just found it. Check out the pic - Broken 2x70uF cap ground wire buried under the other wires. That was tough to see. Hemholtz, your suggestion had me look closer and I found it. Thank you for staying with me and not allowing me to buy a $120 transformer.