Biasing Blakstar HT5R

[diydidi]

The problem is this:
The board around TR2 and TR3 is becoming conductive. The fix: Remove TR2 and TR3. Drill out the centre pin hole of each mosfet. Re-insert the fets and solder the outer pins back. The centre pins of the mosfets go through the enlarged holes and should not be touching the pcb.
Then take some wire and solder the centre terminals to their appropriate points on the two resistors.
Thank me later.

[xtian]

Thx, Enzo. That's the schematic I already have--and I'm saying it does not match my board.

[xtian]

Drill out the centre pin hole of each mosfet.

Took your advice. But, since I have new 3W resistors standing tall above the PCB, I just lifted the middle leg of the mosfet out of the PCB and connected it to the leg of the resistor. Didn't solve the issue, though--that resistor (R39) is still dropping 248vDC.

[g1]

Can you post your DC voltages on the Fet connected to R39 ?

[xtian]

Measurements are taken with a 60w bulb limiter in line. R39 gets hot to touch, but doesn't smoke with the bulb limiter in place.

TR2
D: 303vDC
G: 0
S: 0.6

TR3
D: 62
G: 0
S: 3

[diydidi]

You have to drill the pcb. Its conductive!!

[xtian]

The MOSFET's drain is no longer touching the PCB at all. How will drilling out the hole help?

[xtian]

@diydidi, now I will thank you for your advice! Drilling the PCB would not have helped in my case, because the damage was too widespread. I had to pull C46 out, invert it with leads up, and rebuild the circuit above the PCB. That worked!

So, thank you!

The amp is running smoothly and sounds good. Assuming the amp is now operating as designed, R38/39 are dropping 100v each, meaning almost half a watt each. This makes them too hot to touch within the first minute or two of operation. It's no surprise this slowly cooked the PCB, with the original 1-watt resistors right against the board!

[lowell]

Have one of these in where TR2 Rd was burnt. Replaced it AND TR2 with IRF830. This amp has STK830 in it. Also replaced C45 because if that was compromised it could have caused the problem too. TR2 Rd is still getting too hot and voltages are incorrect.

TR2
D: 42v
G: 0v
S: 3.3v

There is 10k from gate to ground. All components around the transistor test good and are indeed all connected. The Source transistor and diodes are good too. I'm posting now because I've checked everything I see fit to check. Came into the shop because it was crackling and new tubes did not help. So diydid... you seem quite convinced the board is conductive. Has this been something you've fixed more than once and is a common issue with em?

[xtian]

@lowell, is your PCB discolored from heat?

[lowell]

No it isn't

[xtian]

I wouldn't expect it of conducting until it turns darker.

[lowell]

Guess what...drilled holes between the pins. Fixed it. Havent listened for the crackle yet...but the overcurrent is fixed. Both drains have 280v now.

[lowell]

Completely fixed, crackle gone. I drilled the holes between pins on both PI FETs so it wouldn't come back with the other FET doing it. Never would've thought to do that. So it begs the question, what is wrong with the Blackstar pcb? Or, is it a case of bad pin-layout. IE the pins making a triangle shape on the board would be best.

[mikeydee77]

Just fixed an HT-5 with the same fault.

According to this thread which has good instructions on the repair on the Blackstar Forum it is a common problem linked with PCB made in 2012. On the one I am working on (from 2009) I could not see any sign of a short or scorching even with a x10 loupe but drilling out between the mosfets with a 0.7mm drill bit worked.