ab763 fender still humming with no valves

[andrewking]

Definitely buzz in the speakers not a transformer buzz. Maybe not 50hz but higher frequency. Hard to tell with a buzzing noise

The components under the “dog house” what values are they meant to be. I’m having trouble locating this stuff on the schematic.

[andrewking]

Ahh Andy, I just looked on the web and I think I found the schematic you’ve been looking at is a different amp. Mine is a twin reverb. No gz34.

[floobydust]

This model is an export for 240VAC power? or are you using a 240-120V transformer to power it? The power transformer TR1 p/n I believe would be 125P34AX (022764) 1964-mid 1970's or 013830 for 70-80's 135W models (non USA).

Grasping at a few straws, the hum could be caused by a carbon-tracked tube socket. If the filament pin 2/7 shorted to plate? For the hum to be making it to the speaker through the feedback resistors, that would be several parts shorted, not likely. Unless there is a short-circuit in the wiring or phenolic plate rivets.

I would go after the fault causing the last burnt resistor feeding "D" the pre-amp section. That is a strange failure... There would have been a lot of smoke and stink when those two resistors roasted.

I'm wrong about this model, the two main filter caps are in series, so 2x100uF 350V for giving 50uF 700V which is ok.

Careful- don't run it then switch stand-by switch off, with no valves there is nothing to discharge the three large electrolytics "B","C","D" and a terrible shock hazard. Check DCV with a multimeter on the electrolytics before going in.

[andrewking]

FIXED
Hi all. Firstly, I am humbled by the fact people all over the planet have taken the time to constructively share their knowledge in such a positive way.
I started my electronics apprenticeship in 1986 and I have never seen a resistor go low in value. As per the diagram a filter cap was open circuit and the two cooked resistors went low in value and hence drew more current and got all toasty. I have never seen this. It’s a bit like saying my tire exploded because it pumped itself up too much…..doesn’t happen!
The proof of concept is dodgy as all shit but the correct value and shaped parts are on the way.
Thankyou again!

[wasedadoc]

Still no confirmation that the hum is from the speaker cone, not from the mains transformer.

[andrewking]

Definitely buzz in the speakers not a transformer buzz. Maybe not 50hz but higher frequency. Hard to tell with a buzzing noise

The components under the “dog house” what values are they meant to be. I’m having trouble locating this stuff on the schematic.