Blackstar HT Stage 60 / Need help on strange noise issue

[FredPhil]

Hello all

I have 2 issues on my amp (which might be related) on which my investigations lead to nothing so far...

Issue 1 : after using it for 5/10 min, a cracking noise appears in the speakers, getting loud enough sometimes that I have to turn the amp off. If I only put it on standby, the noise remains (but the guitar sound disappears).

Analysis:

• I've changed the tubes, the issue remains
• checked with a different speaker, the issue remains
• It's a power stage issue : everything is good out of the loop send (no noise), but the crackling remains if I'm using another preamp plugged in the the loop send
• I have some little experience with this amp, so I changed TR1 and TR2 (phase inverter FET)
• I have redone the soldering on the power PCB (power tubes sockets)
• No component seem obviously damaged, no obvious dry solder
• The bias is OK (I have Blackstar's document for bias set-up), set to 50mV

Issue 2 : when I put the amp on standby during bias set-up, the bias stays at 50mV

According to Blackstar, voltage should drop to 0mV (described on their document, and confirmed by a Blackstar technician I contacted today).



Since when the crackling noise appears, it remains present even on Standby, I suspect there is a correlation between the 2 problems.

Does anyone has any idea why I still have current flowing even on stanby in the bias section? And could this undesired current be a cause for my cracklng noises problem?


Thanks a lot for your help, this is the only amp I have!

Fred

[FredPhil]

• It's a power stage issue : everything is good out of the loop send (no noise), but the crackling remains if I'm using another preamp plugged in the the loop send

I meant, a another preamp plugged in the the loop return, obviously

[Jon Snell]

Replace TR1 & TR2. Use only IRF830.

[FredPhil]

Replace TR1 & TR2. Use only IRF830.

I already did...

[Helmholtz]

Please post schematic.

[FredPhil]

Here is the schematic for the amp

[x-pro]

check D27 & R207, page 4

[x-pro]

Does anyone has any idea why I still have current flowing even on stanby in the bias section?

Because there's no standard standby mode here.
Here he just turns the signal line to mute.

No power transformer schematic and general wiring diagram of the devices

[FredPhil]

check D27 & R207, page 4

D27 is the diode used to measure the bias, and it's OK. R207 is the 1ohm resistor in parallel, and it seems OK as well. Anyway the bias measurement works fine.

Because there's no standard standby mode here.
Here he just turns the signal line to mute.

Well I thought so as well, but the technician from Blackstar confirmed their official bias set-up process, and said I should have 0mV on D27 when on standby.

[FredPhil]

If you're interested, here is an excerpt from Blackstar's specs:

HT 60 TEST SPECIFICATIONS


GENERAL TEST CONDITIONS:-

• Mains Input – as per country specification
• 8 Ohm resistive load connected to 8 Ohm Speaker Output
• All DC measurements taken with a True RMS meter – input resistance 1MΩ or greater
• All AC measurements taken with a Audio Precision System One Test Set – Level with no filters, or X-Talk tracking filter
• All AC levels and measurements are specified and recorded in dBV where 0dBV = 1.0 volt RMS

INITIAL SETTINGS:-

• Input shorted to Ground
• POWER switch to ‘ON’
• STANDBY switch to ‘ON’
• All front panel controls to Minimum (fully CCW)
• Power PCB PR2 set to CCW
• Mid PCB PR1 set to midway
• Back panel FX Loop switch in ‘OUT’ position
• Back panel Reverb switch in ‘OUT’ position
• Back panel Emulator Cab Type switch in ‘OUT’ position
• Channel 1 Selected, all voice switches in ‘OUT’ position

BIAS AND BIAS BALANCE SET-UP:-

1. Using a DC Millivolt Meter, with a full scale range of 200mV, connected across D27 adjust PR2 for a reading of 50mV
2. Connect the Hum Balance Test Jig to the Speaker Output (do not disconnect the resistive load)
3. Connect a DC Millivolt Meter, with a full scale range of 200mV, across SP1 and SP2 of the Hum Balance Test Jig
4. Adjust PR1 (Mid PCB) for a minimum reading
5. Repeat steps 1 through 4

BIAS CONTROL TESTS:-

With the DC Millivolt meter still across D27:-

1. Change the STANDBY switch to ‘STANDBY’ and check reading drops to 0mV
2. Change STANDBY switch back to ‘ON’.
3. Remove the Input Shorting Jack and check reading drops to 0mV
4. Replace the Input Shorting Jack
5. Remove the Speaker Jack and check reading drops to 0mV
6. Replace the Speaker Jack

[x-pro]

Then D15, TR4 and the elements around it are responsible for disabling Bias.
Check C126 for leakage, check transistor TR4

[FredPhil]

C126 is partially glued to the board but I don't see any leakage.
TR4 looks OK, do you think I should remove it to test it?
As for D15, it seems OK as well, but it's quite a small diode to remove by myself...

Do you think my 2 issues could be related? After all, the main problem is the noise, and not the bias current that stays at standby.

Thanks a lot for your help, x-pro!

[x-pro]

C126 is partially glued to the board but I don't see any leakage.
TR4 looks OK, do you think I should remove it to test it?
As for D15, it seems OK as well, but it's quite a small diode to remove by myself...

Do you think my 2 issues could be related?

Thanks a lot for your help, x-pro!

I'm not sure about the association of the defect.
Capacitor C126 is not tested for leakage, but for voltage seepage through itself. It means that after unsoldering it and checking it, it should not allow DC current to pass through it. It is possible not to unsolder it, but to check the voltage at TR4b-e when switching on the standby mode.
D15 can be checked directly on the board.

[Helmholtz]

Bias is controlled by IC11, pin23, through IC17-B (both page 3) which feeds the BIAS_ON input node of the power amp.

[x-pro]

Check the voltage at b-e TR4 directly at the transistor pins. It should not be (b-e)-(-0.6V-0). Is the polarity description clear?
Also look at the state of C100, which filters the Vref.
It is to the right of IC11 in the schematic.
Its absence can cause noise throughout the sound path.