Night of the living AC50 MK iii

[Helmholtz]

That is the measurements WITH the coil installed. WITHOUT the coil, OT plate --> ground shows OL. I've never run into a faulty choke before, but the more I work on this the more I suspect it. Plate and grid voltage at the OT is right on the nose when it's NOT installed.

Your measurements indicate a short between choke winding and its core. What resistance do you measure between disconnected choke leads to ground?

[Mr_bibbles]

Just bypass the choke with like a 470-1K, 5-10w power resistor for now.

Ok! with a 1k resistor, i have 1k resistance from each OT plate to ground. Voltage (on the limiter) jumps up to 128VDC on BOTH plates. At C27, we have 128v, but C28 has 1.6v still All measurements have been made with no tubes.

So to delve further- C27 is feeding the the plates through the OT transformer. C 28 is feeding the screens and the preamp. So, with the drastic jump in voltage WITHOUT the choke AND the increased resistance to ground from plate should we assume that the choke is bad AND we have something pulling voltage down through either the screen or preamp connections?

And as a side note, r32, a 470 ohm resistor across the OT transformer's taps, is measuring 0.05 in circuit. Im hesitant to remove it since it's mounted in a precarious way but I know OT measurements can be very low through internal connections, so I thought I'd ask.

[Mr_bibbles]

Your measurements indicate a short between choke winding and its core. What resistance do you measure between disconnected choke leads to ground?

188 ohms lead to lead, OL to ground on each lead.

VOX used a rubber coated gromet to attach the + side of the filter caps- perhaps the rubber has melted and the cap terminal itself is shorting?? I'll remove all connections and test it. Here's a picture of someone else's amp so you can see what I mean:

https://lh5.googleusercontent.com/-n...0/IMG_5864.JPG

[Mr_bibbles]

Sooooooo.

The B+ lead from C28 positive terminal to Chassis is about... 32 ohms. With nothing connected. Kind of surprised I'm not dead. I'll mount up something that's not shorted directly to chassis and get back with my results.

[Helmholtz]

188 ohms lead to lead, OL to ground on each lead.

If this measurement was done with the choke still mounted to the chassis, it indicates no choke problem.

[Mr_bibbles]

Sooooooo.

The B+ lead from C28 positive terminal to Chassis is about... 32 ohms. With nothing connected. Kind of surprised I'm not dead. I'll mount up something that's not shorted directly to chassis and get back with my results.

That terminal's rubber that was isolating it from the chassis had indeed disintegrated, after installing an isolated terminal strip and remounting the Filter caps with long through-chassis leads, I'm now getting excellent readings across the board, installed tubes, have signal! Sounds pretty terrible, going to fully recap the thing and go from there.

Thanks for helping me with this y'all! Let this be a lesson to anyone working on the old MKiii's, beware the B+ layout! I'm going to go through and replace all of these eyelets with modern terminal strips.