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

[GerryJ]

Hello, I've got a early 70s YBA-1A Mk2, years ago I had it gone thru by a tech, who also installed a proper 3 prong power cord, with green going to a chassis ground. I recently moved, noticed a hum on the amp, which went away with a flick of the 'ground' switch in the back (it was apparently left intact by the tech). I checked the wiring of the house with a 3 prong circuit tester, it's intact and correct. When the ground switch is in the 'quiet' position, I checked with a voltage detector, no voltage anywhere except in the AC accessary plug on the back (which is 2 prong btw). However, with ground switch in 'noisy' position, the voltage detector starts singing 6 inches from the chassis!
So I'm confused. The chassis ground of the 3 prong cord appears to be wired up correctly, but if it was then the ground switch shouldn't be doing anything, but it obviously is! The house wiring (house built in 2000) seems correct.
Any insights are much appreciated!

[GerryJ]

Here's the ground & power switches

[GerryJ]

Here is the power cord grounding...

[sluckey]

Remove the yellow cap from the ground switch.

[GerryJ]

thanks!- will give it a try.

[xtian]

If those brown paper caps are original, it's way past due for a cap job, which may be contributing to the noise you're hearing.

[pdf64]

… The chassis ground of the 3 prong cord appears to be wired up correctly …

Using a dedicated lug and steel (preferably high tensile) fastener, with a toothed washer between the lug and chassis?
‘Dedicated’ means that the lug / fastener isn’t used for anything else. Especially a transformer.
What resistance do you measure between the plug earth pin and any point on the chassis?

[GerryJ]

Thanks, the added 3rd (green) ground is wrapped & soldered in to a dedicated crimp-hole in the chassis, pretty solid. The resistance as desribed above is 3 ohm, so I think it's a robust chassis ground. Will try to remove the cap tomorrow & see what happens. Not sure why the tech left it in, he went 'off the grid' ~ 20 years ago so I can't ask him. Looking up this topic, it's surprising how often techs have installed 3 prong outlets to these old amps, but have left the ground switches in.
xtian- yes, I believe 2 of the large brown paper caps on the circuit board are original - a 3rd was replaced when the amp was restored ~ 15 yrs ago. They all seem to be working----including the two large aluminum cannister caps which are definitely original. Kind of inclined not to replace those right now, as the amp is very quiet if the ground switch is correct, and I'm hoping it will stay as quiet after removing the 'death cap' from the ground switch to ground.

[GerryJ]

Remove the yellow cap from the ground switch.

Thanks - I've seen some posts where they say (after the 3 prong cord has been installed with green to chassis ground), that you should remove the ground switch entirely. Others say to just remove the 'death cap' , but that the ground switch and it's wiring otherwise can stay in, as "it doesn't do anything anymore." I'm confused about what the ground switch now does (if you toggle back and forth) in an amp with the green 3rd ground, and no capacitor or connection to chassis anymore from the ground switch. If you flip the ground switch, isn't it then routing 120 V thru the white (neutral) path?

[martin manning]

I'm confused about what the ground switch now does (if you toggle back and forth) in an amp with the green 3rd ground, and no capacitor or connection to chassis anymore from the ground switch. If you flip the ground switch, isn't it then routing 120 V thru the white (neutral) path?

Here's a snip from a later YBA-1A with a 3-wire power cable and ground switch (UK model). The cap is just toggled from one line or the other to ground. You can see that removing the cap makes the switch non-functional, and it can remain in place.You could replace the cap with a Type XY, which is designed for this application. In the case where it goes from hot to ground there is some small current flow, a few milliamps, but it might be enough to trip a GFCI.

[GerryJ]

Thanks! That diagram was very helpful.
Well, I disconnected the 'death cap' from the ground switch, had amp connected to speaker and guitar (on a stand, not on me), kept it on standby mode, powered it up. Didn't touch anything, just used my little voltage detector.....which lit up and sung in a 2 foot radius around the amp, also lit up around one foot from the guitar! I just powered everything down and disconnected (safely).
To me this is an indication that the 3 prong 'ground' the tech put in years ago is NOT functioning (& perhaps never was)!
As a novice, I've reached my limit on this bad boy. This either will go to a qualified tech (if I can find one), but more likely just salvage out tubes & parts. At 50+ years old (the amp, lol) I see more headache & expense ahead.
Funny, I've still got 2 YBA-1s (the regular Bassmaster w/out fans), which haven't caused me any problems. Another YBA-1A I had briefly years ago by contrast red-plated tubes regularly, tech couldn't find a problem (tho it was the same guy...). Think I'm done with the 1As, lol!
Thanks to everyone helping me, it is much appreciated.

[LOUDthud]

Did you test the ground wire in the Mains cable for continuity ?

[GerryJ]

Did you test the ground wire in the Mains cable for continuity ?

thanks, yes I did. The following are intact circuits: ground prong to green ground wire anchored to chassis. Both hot (black) and neutral (white) prongs are in continuity with their terminals on the accessory plug, AC power switch, and the ground switch. Black and white wires and prongs are also in continuity with each other, which I guess is expected.
But what I don't understand is the ground switch. Here's a shot, ground switch in middle, with the circuit breaker on the left, and the AC power switch on the right. As you can see, I disconnected the 'death cap' from the ground switch middle terminal. But continuity is present between all 3 terminals on the ground switch, regardless if the switch is up or down. I would have thought you get continuity between the middle terminal and either top or bottom terminal, but not both at the same time.( I guess continuity between the top and bottom is expected.) Thanks!

[maxkracht]

You are reading the primary winding of the power transformer. It is essentially a long bit of wire and your meter thinks it is short. Nothing to worry about.

[LOUDthud]

Is there any continuity between either side of the Mains and the chassis ? Could be part of the Mains wiring or a short inside the power transformer. To prove it's a fault inside the transformer, you would have to disconnect both sides of the primary, then test from the chassis to those primary wires.