Pilot light on - everything else dead. Blackstar HT-60

[Helmholtz]

no it’s not stable. I will confirm the voltage when I get back home later. If I recall it was around 10mV. Should it be stable?

TP13 is the bias supply voltage. Should have a constant negative DCV wrt ground. Something like -70VDC.

[Delta362]

Also, don't rule out a flaky bias adjust or bias balance pot. But yes, as Helmholtz indicated, TP13 should be steady. You can measure it with the tubes removed and you don't even need a guitar cable inserted into the jack for this. That particular point does not care either way. What does care is the BIAS_CNTRL line. That is dependent on your input jack. If TP13 is fluctuating, you'll need to check the diode bridge BR2, C156, C157 and R212.

Regarding D36 and measuring it in a circuit, because it is parallel with R217, which is a 1 ohm resistor, you'll get a bad reading in the circuit. Typically you can read diodes in the circuit, but just be aware of any parallel resistance as that can impact your readings and lead you astray.

[Steve Blackdog]

Thanks for that. As you can tell I am a little lost at this point.

I managed to trace the input signal through to the fx return using my self assembled oscilloscope (£20) and signal generator (£10). I can also see that the signal is following the channel selection. Ie. Press the o/d select and the signal path changes. Still no sign of the o/d led working -perhaps I should just swap it to see.

i will see whether I can find any issue with the bias circuit.

[g1]

Originally you were blowing the HT fuse. Without bias that is what happens. And the power tubes can be ruined.
So the bias is what you have to make sure is right before you consider any other faults with the amp.

[Steve Blackdog]

Originally you were blowing the HT fuse. Without bias that is what happens. And the power tubes can be ruined.
So the bias is what you have to make sure is right before you consider any other faults with the amp.

Next step: I am going to do some homework on bias!

many thanks

[Steve Blackdog]

TP13 is the bias supply voltage. Should have a constant negative DCV wrt ground. Something like -70VDC.

TP13 now 11.5mV. Blackstar bias setting is 50mV (+). Adjustment pot makes no difference. 370dcv at the plates - should be nearer 570v.

I will try new tubes. One of mine was dodgy to start with. Now no sign of any life at all. Could this have a accounted for the ht fuse blowing?

I was about to swap out the 1uf 450v coupling(?) caps in the bias circuit (c123 &4), but the dog chewed the package with the new caps before I could get to it!

I will try new tubes and report back. Picture of old tube attached- this seemed to work before the HT fuse blew, although had the signs of red plating in its former life.

all the best, Folks

[g1]

Do not plug in power tubes til you have proper bias at TP13 and pin 5 of the power tube sockets. You will ruin them.

You are confusing the 50mV idle setting with the bias voltage. The 50mV is not what we are talking about. You need around -70V at TP13.
That is negative 70volts DC.

[Helmholtz]

TP13 now 11.5mV

Means the bias supply is dead. Do not insert power tubes.
What is the DCV at the other end of R 212 (V_L TP)?
Is F5 good?

[Steve Blackdog]

Thanks chaps.

So I will test without power tubes and report back.

[Steve Blackdog]

Means the bias supply is dead. Do not insert power tubes.
What is the DCV at the other end of R 212 (V_L TP)?
Is F5 good?

Aha! F5 is shot! 70v coming from the bias winding on the PT measured at pins 4&5 of The Molex Connector #6. what would cause F5 to blow? Could the dodgy el34 be responsible, or do I need to look further afield?

Many thank!

steve

[Helmholtz]

Ok, now with EL34s out and C122/123 connected, measure grid bias voltage at both EL34 sockets, pins 5, wrt ground.
Should be between minus 40VDC and minus 50VDC. Watch if steady.
See if the bias (level PR2 and balance PR1) pots work.
If grid voltages are more like minus 70VDC you may need to connect a guitar cable to the input and connect a speaker.
(I don't completely understand the bias control function, maybe someone familiar with the amp could help. But I assume power tubes are biased into cut-off with nothing connected to input and speaker jacks.)

[Steve Blackdog]

I have no replacement fuse (0.1A) so will take several days. I could use a 630mA fuse + lamp limiter, but am concerned that I might damage something. What d’ya reckon?

[Helmholtz]

I have no replacement fuse (0.1A) so will take several days. I could use a 630mA fuse + lamp limiter, but am concerned that I might damage something. What d’ya reckon?

I would wait for the correct fuse. 0.63A instead of 0.1A might be too much to protect the bias winding.

On the lamp limiter all voltages will be lower than normal.

[Delta362]

The bias circuit will give about -70V at pin 5 if either the input jack is not used or the speaker jack is not used (control circuitry in section D4/D5 of sheet 4). When they are both used, it will then drop that voltage somewhere between -40 to -50V.

The amp has both bias adjust and bias balance pots. This amp has a convenient test point (across TP1 to TP7) for the combined bias current. However, I don't see how they get the individual bias that way to know if they are balanced. In that case, I would use the octal socket and go back and forth between the bias adjust and balance pots to get a reasonable reading.

[Helmholtz]

The bias circuit will give about -70V at pin 5 if either the input jack is not used or the speaker jack is not used (control circuitry in section D4/D5 of sheet 4). When they are both used, it will then drop that voltage somewhere between -40 to -50V.

I think TR4 is used as a switch which lifts the shunting part of the bias divider network from ground when input or speaker jacks are not used, putting the full TP13 voltage on the EL34 grids.
Means that bias can only be adjusted with input and speaker jacks used.

Also the bias control signal controls the PI muting FET21.