Blackstar HT Stage 60 / Need help on strange noise issue

[FredPhil]

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

Indeed. So it seems we found the root cause for the bias issue...

Now could it be possible that it's also the root cause for the crackling noise? Or should I keep lookin for DC leakage?

[Helmholtz]

Indeed. So it seems we found the root cause for the bias issue...

Now could it be possible that it's also the root cause for the crackling noise? Or should I keep lookin for DC leakage?

Replace TR4 with correct type. Then we'll see.
If you're impatient you could use a piece of wire to jumper between TR4 collector and emitter eyelets.
With jumper in place and power tubes pulled measure socket grid pin voltage.
If around - 50V switch off and re-insert power tubes.
If the noise is still there, there's another/additional problem.

[FredPhil]

Replace TR4 with correct type. Then we'll see.
If you're impatient you could use a piece of wire to jumper between TR4 collector and emitter eyelets.
With jumper in place and power tubes pulled measure socket grid pin voltage.
If around - 50V switch off and re-insert power tubes.
If the noise is still there, there's another/additional problem.

I just did this, since I don't have the TR4 replacement yet, and the noise is still there after a few minutes with the amp on... This obviously happens when the amp begins to heat up, could it be a DC leakage somewhere, as Mick Bailey suggested?

[x-pro]

can you make an audio recording of the noise?
The MPSA93 can be replaced by 2N5401 or : https://alltransistors.com/ru/crsear...aps=TO92&cf=on

[FredPhil]

can you make an audio recording of the noise?

Here :
Crackling noise Blackstar.mp3

[x-pro]

It may take a lot of time to find it.....
It could be noise from bad contact of resistors, contacts in connector, bad soldering of components, transistor, chip.
You can try cooling or heating the components.
You wrote that you tested the preamplifier separately and it was fine.
So the number of components will be significantly reduced. All components through which sound passes must be heated or cooled.
It is better to cool them in a simple way - touch the cases of the elements with a cotton swab soaked in alcohol (acetone is better). But carefully. Elements under high voltage must not be touched.
Any significant reaction to temperature indicates element malfunction.

[FredPhil]

OK, bias issue solved! I replaced TR4 with the correct transistor type, and now the standby bias works as expected : standby ON means no bias current. Big thanks to you all and your precious advice for helping me on getting to the root cause.

The main issue (crackling noise) stays, though... So now I need to find out where is the flaw in the power section of the PCB that generates this noise just after the amp heats up. DC leakage? Ground issue? Malfunctioning component?

It is better to cool them in a simple way - touch the cases of the elements with a cotton swab soaked in alcohol (acetone is better). But carefully. Elements under high voltage must not be touched.
Any significant reaction to temperature indicates element malfunction.

I should try this on the small components, yes. Are there some components more subject to have this kind of issue when heating? Capacitors or transistors, maybe?

[g1]

We were always taught that active components (transistors, IC's) were more likely suspects than passive components (resistors, caps) when dealing with noise issues.

[FredPhil]

OK, I've found at least one flaw in the PCB.
When the amp starts to heat up, one of R143 pin (the one connected to V3-B, next to TR1/TR2) can easily move when I slighly push on it, like the solder melts... When the amp cools down, the pin seems firmly soldered.
No problem on R140 to 142, although they all are next to each other on the board (the 4 of them almost touching).
I tried to re-solder the pin but the issue remains. Standard solder (60% tin, 40% lead). Could it be a problem with the way the solders "grips" to the board?

[Helmholtz]

When the amp starts to heat up, one of R143 pin (the one connected to V3-B, next to TR1/TR2) can easily move when I slighly push on it, like the solder melts...

Check if the eyelet has separated from the board.

[FredPhil]

Check if the eyelet has separated from the board.

The eyelet looks OK

[g1]

Current through the resistor would be the same at both legs so maybe the heat is coming from something else nearby. Photos would help.

[Helmholtz]

How does the resistor look.
If it actually melts the solder (very unlikely) it should be discolored.
Is it a capped resistor?
If so, one of the caps might have come loose.

[g1]

Is it a capped resistor?
If so, one of the caps might have come loose.

Haven't seen this term before, does it refer to the resistor construction?

[Helmholtz]

Haven't seen this term before, does it refer to the resistor construction?

Maybe it's not the correct term, but are you getting what I mean (i.e. contact caps on each end)?
E.g. CC resistors are "capless".