Blackstar HT20C problem with balance BIAS.

[speed]

Hi,
I have a problem with the bias in Blackstar HT20 combo.
Exactly with balance bias. Left side is set max and there is no balance.
52mA and 44mA, pot bias is max down. EL34 tubes are new but old tubes have the same problem,

I saw when take of guitar cable left side down to 7,2mA and right side to 0,0mA, Biasmeter is ok, 100% I checked in other amps many times.
So,where is the problem?

[Delta362]

With both power tubes removed, and with the guitar cable plugged in, measure the voltage of pin 5 of each power tube as you adjust both the bias as well as the balance. What are the min/max voltages on each pin that you can achieve?

Edit: My bad. I was looking at the HT Studio 20 schematic.

[Jazz P Bass]

The proper 'Balance" procedure is to set the output that it just starts to clip.
The balance pot is used to set it that the top & bottom waveforms start to clip at the same time.
You have to use a scope.

[Helmholtz]

Is there a schematic?

[Jon Snell]

There are two controls to set up.
Firstly set PR1 to centre position.
Switch on and set PR2 to acheive, on pin 8 of one of the output valves, exactly 11.5volts DC with no signal passing.
Listen very carefully and adjust VR1 for minimum hum.

Check pin 8 again and re adjust PR2 if needed for exactly 11.5volts on pin 8.

That's all there is to it. Dead simple. No clipping adjustment here, that is the FET controller problem that they all suffer from in time.

[gui_tarzan]

Blackstar is a great example of why Right To Repair should cover everything we purchase.

[Jazz P Bass]

I was going by the Blackstar bias/ balance procedure.

[speed]

I think my problem not in set bias. After rearrange power tubes it's same. Problem is always in same lamp socket.

I checked voltage, no power tubes in amp
sockets tubes pin5 grid1 -36,6V and -42,3V and when put in cable guitar -8,7V and -13,5V
voltage other pins in both socket are the same. socket with problem is this one -8,7V, any idea next?

[Jon Snell]

Remove and test C122 & C123, whilst they are out, has your fault gone? If so replace them if not check/replace R166, R167, R170 & R171.

[speed]

I checked voltage, so problem is with R170 but multimetr showed 180K, is it to be replaced?

[Enzo]

It measures correct resistance, why do you think it should be changed?

Did you remove C122,123 and power up like Jon suggested? In a situation like this, checking a cap does not mean measuring its value. We are concerned they may leak at voltage. Positive DC leaking through such a cap will buck the negative bias voltage.

[speed]

Jon, Enzo shot at 10:-)
I remove C122 and problem is gone. I have to order capacitors, now.
thank you all for help

[speed]

tell me what good capacitors? of course I'm remember replacing both:-)
thx