Blackstar HT Stage 60 / Need help on strange noise issue

[g1]

Base and collector voltages should read negative.

He mentioned having the red probe on the emitter, and that may be confusing things.

[Mick Bailey]

One of the main sources of noise (amongst other things) with these amps is DC leakage around the PI mosfets and it's worth checking as its so common, resulting in a high and unstable voltage on the gates. The problem is usually the board, not the mosfets, though these can become damaged and just replacing them without checking just damages the new ones, leaving the same mysterious fault.

Edit: I forgot to mention - the voltage can leak over a much greater area then just around the mosfets; anywhere a high voltage is nearby.

[x-pro]

There's one problem to solve first.

[FredPhil]

Yes, but we need only control grid (g1) voltages.

Voltages on g1 are:
- -98V when standby OFF
- -91V on standby

(TR4 and power tubes removed)

[FredPhil]

One of the main sources of noise (amongst other things) with these amps is DC leakage around the PI mosfets and it's worth checking as its so common, resulting in a high and unstable voltage on the gates. The problem is usually the board, not the mosfets, though these can become damaged and just replacing them without checking just damages the new ones, leaving the same mysterious fault.

Edit: I forgot to mention - the voltage can leak over a much greater area then just around the mosfets; anywhere a high voltage is nearby.

Thanks for the advice, Mick. Indeed I read that somewhere else, and it seems that's the next step to my analysis... Thing is, this might be quite complicated to check...

[x-pro]

[Helmholtz]

Voltages on g1 are:
- -98V when standby OFF
- -91V on standby

Fine, so this is roughly the grid voltage we expect to see in standby mode when TR4 is switched off by the BIAS_ON signal.
Now what does the BIAS_ON signal measure in both modes?

How does TR4 test?

[x-pro]

What if you connect the switch to the k-e points and play with it, but with the output tubes. Will the bias current on D27 change?
C156 C157 their condition to match the rating ?

[FredPhil]

Now what does the BIAS_ON signal measure in both modes?

-2.44V in standby OFF
11.8V in standby

I should take BIAS_ON between R232 and C163, right?

How does TR4 test?

hfe reads 174. But wait, according to schematics, it should be a mpsa93, with a hfe=40. This is actually a BC184, with a hfe=130...

[x-pro]

Now what does the BIAS_ON signal measure in both modes?

pulsing current if C156 and C157 became smaller?

[x-pro]

This is actually a BC184, with a hfe=130...

n-p-n ?

[FredPhil]

n-p-n ?

Well yes... This amp has been repaired (not by me) a few years ago, but could it be that the person made such a mistake? How could it work properly anyway?

[g1]

There are other BC184 on that board, so make sure this is TR4 and that it is connected to the bias trimpot like the schematic shows.

[FredPhil]

There are other BC184 on that board, so make sure this is TR4 and that it is connected to the bias trimpot like the schematic shows.

Yeah, no problem about that, I checked something like 7 times before unsoldering it ;-)
But you gave me an idea, I'm checking if the person who repaired it inverted a BC184 and a MPSA93 located somewhere else on the board...

[Helmholtz]

-2.44V in standby OFF
11.8V in standby

Looks ok.
The negative voltage should turn TR4 on, the positive voltage will turn it off.

Of course an npn transistor like a BC184 will not work for TR4.