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

[Helmholtz]

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.

I assume that you could adjust the voltage between TP1 and TP7 to 50mV.
Remove the mute FET21 from circuit and test it. Temporarily replace the drain-source path of the FET with a wire between the nodes C114/R139 and R256/R144. Do you get sound?
Also measure TR1/TR2 drain voltages.

Do you have a scope?

[Steve Blackdog]

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.

thanks G1! You just saved me the price of another pair of valves. I put my Sovtek 12AX7LPS tubes in another amp - no glow but plenty of sound so nothing wrong with them after all.

So, any thoughts what I need to fix/check/test to get to the bottom of this?

thanks again!

[Steve Blackdog]

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.

I have a cheap diy signal generator and a cheap diy digital oscilloscope, so I should be able to trace the signal - I will give it a try an report back.

all the best!

[Steve Blackdog]

Yes, I have signal at TPS 3&4. I can trace it to TP17

[Helmholtz]

Yes, I have signal at TPS 3&4. I can trace it to TP17

Does that mean you have no signal after TP17?

[Steve Blackdog]

Does that mean you have no signal after TP17?

no. I just haven’t traced it further than that. The problem with the PCB is that the traces and components are arranged in such a haphazard manner that l need a magnifying glass to find each SMD component. I will keep going and see if I can find where it stops.

[Helmholtz]

Did you try injecting a signal into FX RET, guitar or other? That's a standard troubleshooting procedure to separate preamp and power amp problems.
To check the preamp you could connect the preamp send to the FX return (or input) of another amp.

[g1]

Did you try injecting a signal into FX RET, guitar or other? That's a standard troubleshooting procedure to separate preamp and power amp problems.

I think with this amp you would also have to enable the input jack switch to turn on the power tubes?

[Helmholtz]

I think with this amp you would also have to enable the input jack switch to turn on the power tubes?

Yes, good remark. I would use an input shorting plug as advised in the instructions. But connecting a guitar will do as well.

[Steve Blackdog]

Using a signal generator in input socket, I can get the tone through speaker both clean and od channel. Quiet but clearly audible.

using signal generator in fx return, I get the tone through the speaker, much louder, but still not loud.

playing guitar through fx return, It is clearly audible with master volume on full. But by no means loud. By contrast guitar through input socket is very quiet and hardly audible.

any ideas?

[glebert]

Do you have another amp with a line out that you can drive the FX return with? A guitar will have a weaker signal that the FX loop is using so it is usually not super loud plugging in there, but it should sound "full."

[Steve Blackdog]

Do you have another amp with a line out that you can drive the FX return with? A guitar will have a weaker signal that the FX loop is using so it is usually not super loud plugging in there, but it should sound "full."

It definitely sounded full.

The only amp I have access to that I could separate the pre amp is my WF-55 champ clone. Would you take the signal from the volume pot, using half the 12ax7?

here’s the schematic

[glebert]

It definitely sounded full.

The only amp I have access to that I could separate the pre amp is my WF-55 champ clone. Would you take the signal from the volume pot, using half the 12ax7?

here’s the schematic

It sounds to me like the power amp is working OK.

[Delta362]

I agree with glebert in that it does appear your power amp is working ok.

You've already ruled out the preamp tubes from swapping them into other amps. Are both the clean and dirty channel volumes low? I'd next look at FET18 and D43 to make sure you don't have a partial S_MUTE pulling your signal down.

What does the output of IC6A and IC6B (TP17) look like?

[Steve Blackdog]

Quick update. I found a wiring issue with my fx loop circuit - send a return appear to have been switched. I have simply joined fx send to return.

Bingo! Well almost.

I have much more sound, but still nothing like as loud as it should be. Anything over volume 3 the clean channel breaks up. The od channel is super distorted - much more than expected.

I will check the bias again. ( I have -50v at TP13. )

where next?