VOX Night Train 15 NT15 DC Voltage at input jack

[Zouto]

Hi, I got 0,48 Volts DC at a VOX Night Train 15 NT15´s input jack causing severe scratchy noises from any guitarˋs volume pot.
Changing tubes and cleaning the jack didn’t help. So what to do next? The input design with ferrite bead etc. seems a bit strange.
Thanks for help
Zouto
http://www.prowessamplifiers.com/sch...s/Vox_NT15.pdf

[Mick Bailey]

First thing I'd do is to remove the input tube and see if the voltage is still there.

[Zouto]

First thing I'd do is to remove the input tube and see if the voltage is still there.

Thanks Mick, with input tube removed the voltage rises/swings between 0.8 and 1.4 Volts DC.

[Helmholtz]

Could you verify the voltages B/T at the grid (pin7) of V4A? Are your input jack switching contacts clean and working? But most important: Is FB1 intact?

[Zouto]

Could you verify the voltages B/T at the grid (pin7) of V4A? Are your input jack switching contacts clean and working? But most important: Is FB1 intact?

Thanks Helmholtz, B: 1.87V, T: 1.6V, FB1: 0.7R, switching contacts are clean & working. I‘m not quite sure how to check FB1.

[Helmholtz]

Thanks Helmholtz, B: 1.87V, T: 1.6V, FB1: 0.7R, switching contacts are clean & working. I‘m not quite sure how to check FB1.

As a fuse FB1 should read as short, but 0.7R seems ok considering meter tolerance/off-set and contact resistances.

Does your input DCV change if you disconnect terminal 2 of the input jack? If so check C25 for leakage/short. Do you measure any DCV between the input jack's ground "Chassis Ring Terminal" and chassis/ amp ground?

[Zouto]

Meanwhile I removed some of the orange goop supposed to fix the filter caps on pcb. This reduced the input jack voltage to 0.098V (T) and 0.210 (B). This goop apparently causes some leakage somewhere on the pcb‘s top around the filter caps. Unfortunately it is widely spread and really hard to remove. The jackˋs ground to chassis ground is ok, to disconnect terminal 2, I have to unsolder the input jack. The input jack voltage rises when turning the amp on and drops slowly when turning off, like discharging a filter cap. So it seems to be sort of a goopy issue...

[Helmholtz]

like discharging a filter cap

Or any other electrolytic/high value cap especially in a high resistance environment. From the schematic symbol it looks as if C25 (completely unnecessarily) is an ecap and I suspect it. But of course conductive goop could be an explanation as well - especially if moisture is trapped.

[Zouto]

What do you think, remove C25 completely or bridge it? At the moment jack voltage increased to 0.420 V (T) and 0.890V (B). This amp suxxxx....

[Helmholtz]

What do you think, remove C25 completely or bridge it?

Disconnect at least one end of C25, check input DCV and measure leakage resistance across C25.

[Zouto]

Disconnect at least one end of C25, check input DCV and measure leakage resistance across C25.

C25 is ok, no leakage. After cleaning all TAG wire connectors, input jack voltage dropped to 0.041V B and 0.002V T, but all of my Guitars‘ volume pots still create some scratchies, while all the other crackling and hissing noises almost completely disappeared. Nevertheless I‘ll exchange the somewhat crappy input jack with a solid ReAn type.

[Zouto]

The pcb´s top surface was partly conductive, causing some leakage. Cleaning with isopropanol fixed it a bit, the ReAn jack did not fit, swapping C25 with a high quality polypropylene cap was a good idea. Input jack´s DC Voltage dropped to -0,003 VB and -0,014V F, scratchies got much weaker, fini.