Signal leaking with all controls at 0, other questions about Traynor YBA-3

[Bob Opera]

Hi,

I have a few questions about a non-master volume Traynor YBA-3 (schematic)


- signal leaks through with all controls at zero. What can cause this, other than a bad pot?

- There is a 10 uf 200V cap (the purple one in the picture below) connected to the 1k screen resistor and hovering over pin 1 of the power tube beneath it, as if it was once connected to it. i don't see this cap in the schematic and i've taken a look at other yba3s and they don't have it. What does it do? Should i connect it?



- That choke in the top left corner was also added on. What would be the purpose of adding it?


thank you

[Jazz P Bass]

With the amplifier off & the supply drained measure the resistance of V1B's grid to chassis ground when the volume control is set full off.
It should be a short.
If that is the case try grounding that point with thw amp on.
If that does not kill the signal then it is most probably "cross talk" among the leads.
I would also look at the power rails with a scope. The signal may be riding on the supply from a leaky capacitor.
The unterminated capacitor may be a screen filter. (or an attempt)

[Bob Opera]

Thanks for the reply Jazz P Bass

With the volume at full off, the grid (and center tab of the pot) read 1k to ground. Bad pot?

regarding the possible screen filter, i do see an indentation/crease in the solder gob on pin 1 of the power tube which leads me to believe it was once connected there but may have been lifted by the filament wire pair underneath it

[Jazz P Bass]

Thanks for the reply Jazz P Bass

With the volume at full off, the grid (and center tab of the pot) read 1k to ground. Bad pot?

Is one of the outside legs of the pot at ground?
If it is, then you need a new pot.
I meg Audio.

[Bob Opera]

Thanks again for your assistance

I disconnected the pot and cleaned it. This has made a difference and it now sweeps between 9 ohm and 0.95 meg (measured between the center and outside leg), so i think it's a good pot. The level of signal with the volume at zero is noticeably lower but still present. I am now reading 200 ohm between V1B's grid and chassic ground.

I should have mentioned that this amp has a number of modifications. Along with that choke and screen filter (?) capacitor, there's also an RCA input jack (which I disconnected) and a line out.
The two conductor cable connecting the center leg of the volume pot to pin 2 of V1 and the outside leg to the ground point on the board doesn't look original. You had mentioned cross-talk..

here's a picture of this area

[Jazz P Bass]

Take that wire off of the pot.
I do not have a clue what the mod is.
But that is why the grid is 200 ohms from ground.

[Bob Opera]

Should i remove both connections (middle leg to grid, outside leg to ground) of that wire?


EDIT: this is the only connection to pin 2. with this wire removed there is loud signal and ground hum and an open connection between pin 2 and ground

thanks for your time

[52 Bill]

Have you checked the physical ground at the pot? Is it clean and tight? How about at the input jack?

The shielded wire from the volume pot to the tube should only be grounded at one end, either at the back of the pot or at the tag board.

[Bob Opera]

I have disconnected the shielded wire from the back of the pot and have verified the physical ground at the pot and input jack (< 1 ohm to chassis on my DMM)

[jmaf]

I have disconnected the shielded wire from the back of the pot and have verified the physical ground at the pot and input jack (< 1 ohm to chassis on my DMM)

It seems to me like the ground is taken at random points soldered to the chassis? At the first picture, top right, above the aluminum electrolytic, is that a solder pool on the chassis? Also below, in between power tubes, that seems to be the power tube ground?

If so, the "leaking" may be due to lack of a star ground scheme, the ground at the volume pot may not be exactly zero so you still have signal going through. Just an idea.

[pdf64]

Have you tried a different tube (in V1)?
Some tubes can leak significant signal between their sections; if there's a vol control between the sections (as with the schematic linked), then signal can bypass the vol control. Pete.

[Bob Opera]

jmaf - that is indeed the case - here are the guts of another YBA-3

pdf64 - i did try a different tube, no difference

[jmaf]

Bob, that'd be simple to test...solder a temporary ground wire from the pot straight to the main caps ground and see if it quiets down...if it does, then it might be worth modding the amp to accommodate star grounding.

[Bob Opera]

am away from home but will do that when i get back. Can you please confirm that the goal is to see a short between the V1B grid and chassis ground as that's the reason why it's not quiet?

[jmaf]

am away from home but will do that when i get back. Can you please confirm that the goal is to see a short between the V1B grid and chassis ground as that's the reason why it's not quiet?

Yes, confirmed. Assuming the pot is good, if you turn the pot all the way down, it shorts the grid to the ground...guitar signal is unable to enter V1b, so how can it appear further ahead or even among internal tube electrodes if it weren't even entering v1b? The only way I see this possible is if your ground at the potentiometer is not at the same potential as the v1b ground and overall circuit ground.

Edit: You can also bypass the potentiometer and short pin 2 to ground at the same place the pot is grounded and see if signal passes...that would eliminate the possibility of a bad pot.

About what Jazz P Bass said, I once had an amp that when you turned all volume down, it'd modulate one of the transformers! Yes, I heard the guitar INSIDE the transformer shell....volume at zero, dummy load attached.... It was late at night, tired, and for a moment I thought WTF I have a dummy load, how am I hearing the guitar??? It was in the transformer....so crosstalk, power supply modulation, bad ground, all possibilities.