Blackstar HT Stage 60 / Need help on strange noise issue

[FredPhil]

Thanks a lot for so much advice, I'm doing these checks as soon as I can, I hope tomorrow.

[FredPhil]

Sorry, double post

[FredPhil]

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?

Sorry I don't get your notation. I understand I have to check the voltage on b-e TR4 when in standby mode, but what value should I get?

Also look at the state of C100, which filters the Vref.
It is to the right of IC11 in the schematic.

I don't see C100 in the schematics next to IC11; are you talking bout C10 or C108?

[x-pro]

С108.

[x-pro]

Sorry I don't get your notation. I understand I have to check the voltage on b-e TR4 when in standby mode, but what value should I get?

It is necessary to measure the voltage of b-e TR4, with standby mode on

[Helmholtz]

I'm not familiar with the amp and the schematic is incomplete and a PITA to read with all these "CON"s.
Can't find the standby switch.
But here's what I think:
In standby mode B+ is left on and the tubes are (supposed to be) switched into cutoff (zero idle current).
Cutoff requires a large negative voltage at the grids.
When TR4 is on (fully conducting), the upper end of PR2 is connected to ground thus pulling down the grid voltages for normal operation.
When TR4 is open the grid bias jumps to its max. negative value, given by V BIAS.

So if cathode current doesn't drop to zero in standby, TR4 doesn't switch off.
Reasons could be a shorted transistor or a problem with the BIAS_ON switching.

For normal operation the TR4 base to emitter voltage should be around - 0.6V.
For cutoff (i.e. standby) the TR4 base to emitter voltage should be around zero.

[FredPhil]

I measured the voltage on TR4 when on standby and here's what I found :
- b-e = 240mV
- c-e = 880mV

With Standby OFF :
- b-e = 2.14V
- c-e = 2.78V

Can't find the standby switch.

It's located between SP5 and SP6 (on "FRONT PCB" schema, page 3, lower left), then PIN7 on IC11

[x-pro]

I measured the voltage on TR4
With Standby OFF :
- b-e = 2.14V
- c-e = 2.78V

Did you measure directly at the transistor pins? True, it depends on the polarity of the measurement.

[FredPhil]

That doesn't make any sense. Did you measure directly at the transistor pins?

Yes, base of TR4 pins, power ON, standby off

[Helmholtz]

I measured the voltage on TR4 when on standby and here's what I found :
- b-e = 240mV
- c-e = 880mV

With Standby OFF :
- b-e = 2.14V
- c-e = 2.78V

Polarity of voltages matters.

It's located between SP5 and SP6 (on "FRONT PCB" schema, page 3, lower left), then PIN7 on IC11[

Yes, but the actual switch is not shown.

[FredPhil]

Polarity of voltages matters.

Red probe of the voltmeter on e for all measures...

Yes, but the actual switch is not shown.

Do you need some precisions about it? It's kind of a standard power 2-way switch directly plugged between SP5 and SP6. But if the switch were faulty, I would get guitar sound with standby ON, wouldn't I?

[Helmholtz]

I suspect TR4 is at least partially shot. Best remove and test outside the amp.
With TR4 removed pull the power tubes and measure socket grid pin voltages.
Also measure V BIAS and the BIAS_ON voltage with and without standby.

[x-pro]

This is what you got from the measurements.
That's not right.

[FredPhil]

Thanks for the advice, I need some time to remove TR4, will do it tomorrow.

Also measure V BIAS and the BIAS_ON voltage with and without standby.

V BIAS is -53V, standby ON or OFF
BIAS_ON is 11.6V when on standby, -2.6V when standby is off

[x-pro]

The emitter of transistor TR4 is accurately probed at TP14, TP15 ?