Traynor YBA-1A Ground switch/hum even with a 3 prong cord...?

[GerryJ]

No, no continuity between chassis & any mains (black & white wires from AC cord) measured on terminals of AC accessory outlet, circuit breaker, power, ground or standby switches, thanks.

[GerryJ]

Reviewing things, with the death cap connected, the amp acted like it's old 2 prong self- flip the ground switch one way (presumably white/neutral), no buzz and no chassis voltage. Flip it the other way (presumably black/hot), you get buzz and chassis voltage. Disconnect the cap, you just get buzz and chassis voltage with both ground switch positions. This happening despite the green ground wire having continuity from the prong to where it anchors into the chassis. There is likely some faulty wiring somewhere, but that's the limit for me, expertise wise. Thanks again.

[Phil_S]

Not being there in person to see everything in context is a genuine handicap. On the basis of what I'm reading, I'd be inclined to think about reworking everything from the mains power cord to the PT primary. This bit of wiring is uncomplicated and shouldn't be that much work. Maybe what I have to say is redundant. Here's how I'd wire this.

Mains white is neutral. Connect that directly to one side of the PT primary. Splice is OK, but I like to use a lug on a terminal strip. If you splice with a butt connector, don't solder, just crimp and cover with heatshink. (In US, maybe Canada, too, this is typically not switched. In other parts of the world, a DPST switch is used to make/break the neutral connection simultaneous with the hot connection.)

Mains green is ground. Bolt it to the chassis as described earlier. Do not solder it. (I think the added solder is the reason you have a reading of 3Ω.) If you are concerned about it breaking loose, add a second nut or some thread locking compound.

Mains black is hot. Connect to one side of the mains fuse. [Note 1] From the other side of the fuse, terminate at the on/off switch. (Get a new, good quality switch.) From the other switch terminal, connect to the other PT primary wire.

Disconnect anything else, like the ground switch. Omit the death cap.

I'm just guessing here, but I'm thinking the ground switch is part of the problem, and possibly all of the problem. You can leave it there, but disconnect it.

I'd also upgrade the service outlet to 3-prong or totally disconnect it.

Note 1: This is a good practice. Whenever the amp is plugged in whether on or off, the fuse always sits directly between the house wiring and the amp. If there's a fault, the fuse will blow. This protects the house first and the amp second.

[GerryJ]

Thank you so much for the logical guide, I really appreciate it! I'll give that a try; I may also bypass the accessory AC plug, as it's a 2-pronger. Thanks again.

[Phil_S]

I didn't want to say it before, but I will now. The tech who installed the 3-prong should have taken care of the rework of the mains power. It would have taken him all of 10 minutes.

[GerryJ]

Hello guys, just a follow-up- the amp started behaving normally (no voltage differences between chassis-ground-neutral sites), likely because in probing things (after unplugging AND discharging capacitors & other standard safety measures), I probably partially fixed a cold solder joint of green ground wire/3d prong to the chassis. I freed the ground from this glop, drilled a separate hole in chassis just for this wire & bolted it firmly, kept the 'death cap' disconnected, & the amp sounds great, and no stray voltage on the chassis (ie, safe). Aside from saving repair costs, it's difficult to even find a competent tube tech these days! I contacted 3 shops that advertised tube amp repair & servicing, 1 shop said it was more than their level, & the other 2 never responded. You don't know how close that amp got to the landfill ! Thanks again to everyone's help!