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

[nosaj]

just for kicks put the 40watt in a regular lamp and note the brightness. Is it the same brightness as when in the dim bulb tester? This is just to give you a visual baseline.

nosaj

[Steve Blackdog]

I swapped the light bulb in the limiter to 100w, which makes a big difference. With both tubes in the light bulb is dim. The tubes heat up, so I can now move to the next test.

still no sound!

I’m still getting zero volts at the bias balance test points between tp 1 and tp7.


is there a chance that the problem relates to the switching circuit that protects the sensitive bits, so it goes into standby unless the input Jack tip breaks a circuit and there is a speaker Jack inserted?

I note that the bias control appears to be switched on when spkr-jack and Ip-jack are activated. How should I check this?

looking at the SPK-Ok output from the back board, it looks like I can bypass it by connecting a +12v to a 39k resistor to a 1n1418 to r241. Does that make sense?

I think the input is working as the bias voltage is affected when you insert the input jack

any suggestions will be much appreciated!

all the best

[Steve Blackdog]

These are the bits on the schematic

[Helmholtz]

I swapped the light bulb in the limiter to 100w, which makes a big difference. With both tubes in the light bulb is dim. The tubes heat up, so I can now move to the next test.

still no sound!

I’m still getting zero volts at the bias balance test points between tp 1 and tp7.

A higher wattage limiter bulb takes less of the mains voltage, so all amp voltages should be larger now. Please remeasure grid and plate voltages with guitar and speaker connected.

I assume that you have too much negative grid bias voltage now, not allowing the power tubes to conduct.
Tube bias can be adjusted via PR2, but makes no sense on the limiter.

As it looks fine on the limiter, you can connect the amp directly to mains now as next step.

[glebert]

Since the bias switching is done via a logic chip it is possible that the light bulb would mess with that operation.

Frankly the mute-with-no-input is nice, but I find it mostly useless. On the Blackstar amp I recently fixed I had a problem with this and just got rid of it completely rather than mess with fixing it. I could say the same about the protection looking at the speaker jack.

[Steve Blackdog]

Since the bias switching is done via a logic chip it is possible that the light bulb would mess with that operation.

Frankly the mute-with-no-input is nice, but I find it mostly useless. On the Blackstar amp I recently fixed I had a problem with this and just got rid of it completely rather than mess with fixing it. I could say the same about the protection looking at the speaker jack.

How did you bypass the mute with no input feature?

[glebert]

How did you bypass the mute with no input feature?

Mine was a different model and had a different mechanism for the mute, there was no logic chip. On this amp I would be looking to strap the output of IC17B to be high or low or what it needs to get the bias to turn on. I think it needs BIAS_ON to be a negative voltage to turn on TR4, right?

[Helmholtz]

Please don't jump to conclusions and proceed step-by-step (logically).

Right now there is no evidence, that there is a problem with the mute circuit.

If the grid bias is e.g. -70V or more, you can't have idle current and output.

[Steve Blackdog]

Good advice! I will take it slowly!

[Delta362]

With the LBL removed, all tubes inserted, speaker and guitar connected, what voltages do you read at pins 1, 3, 4, 5, and 8 of the two power tubes? Have you checked TR4 yet? Does adjusting PR2 change the voltage on pin 5 of each power tube socket?

[Steve Blackdog]

With the LBL removed, all tubes inserted, speaker and guitar connected, what voltages do you read at pins 1, 3, 4, 5, and 8 of the two power tubes? Have you checked TR4 yet? Does adjusting PR2 change the voltage on pin 5 of each power tube socket?

with the LBL removed I am getting 570v at 3 &4. I will check the others and let you know. Yes the PR2 pot makes a difference to the pin 5 voltage if I recall it came down to around (-)45v.

one thing I have realised is that when I got the loud pop before the silence before, it took out the heaters in my pre-amp tubes. I swapped one out for a 12AX7R from another amp and it glowed as expected. I will get another pair, but before I try the new ones I want who work out what would have caused them to blow.

with the LBL removed there is some signs of life, but clearly my original issue still exists.

[Delta362]

Sounds like you have the bias in at least a pretty good condition now that it is solid and adjustable.

What issue(s) is (are) remaining?

[g1]

one thing I have realised is that when I got the loud pop before the silence before, it took out the heaters in my pre-amp tubes. I swapped one out for a 12AX7R from another amp and it glowed as expected.

Some preamp tube heaters glow more than others. The ones you think are bad, try them out in your other amp. And check for sound, not for whether you can see the glow.

[Steve Blackdog]

Sounds like you have the bias in at least a pretty good condition now that it is solid and adjustable.

What issue(s) is (are) remaining?

Absence of sound! If I turn the tone as bright as it will go, turn presence and reverb up, I can get occasional spikes of sound, but otherwise it is so quiet as to be almost inaudible.

[Delta362]

Do you have a signal tracer? I made one with a chopstick, capacitor, resistor, a nail and shielded wire. I plug that into a small amp and can follow a signal thru the amp. Cheap and easy to make.

Do you have a waveform at TP3 for the clean channel and TP4 for OD? Does lack of sound occur on both clean and overdrive channels? It could be one or more of the J175 FETs is out preventing the signal from passing after the tone stacks.