Blackstar ht40 No Sound

[Randall]

"I’m not sure which ones are mosfets, can you please tell me their number. "

TR1 and TR2, IRF830.

[Perkinsman]

In that case it's time to get dirty. Here is the schematichttp://dealers.korgusa.com/svcfiles/...0schematic.pdf
We will start at the power supply Page 3 check F1 and F2 and F5.
Then at TP9 you should see 3.3v
TP11 +12.6v and TP12 -12.6v


Then go back to Micks post #4.

nosaj

It should output to phones in standby to verify the preamp side. If the amp is stuck in standby there will be a larger than usual negative voltage on the output tube grids of around -90v, The MOSFET PI on these amps can fail due to leakage between pins. If one side is gone the output is low-volume and distorted. If both, then no sound. When they begin to fail it sounds like bad output tubes. Check the gate voltages on both MOSFETS. If bad you'll see tens of volts.

Thanks nosaj...I’ve got to wait until later today or tomorrow to get back on it.

"I’m not sure which ones are mosfets, can you please tell me their number. "

TR1 and TR2, IRF830.

Mick referred to 2, I’ll assume he meant tr1&2.....So if they show 0vdc, are they both bad?

[nosaj]

Mick referred to 2, I’ll assume he meant tr1&2.....So if they show 0vdc, are they both bad?

He said if bad you will see 10's of volts on the gates. You report 0 vdc so I'd say good.
Do you have an audio probe? If not here is how to build onehttp://diy-fever.com/misc/audio-probe/

Now when you check out the audio probe the side that says amp plug that into like an mp3 speaker.

Steps plug an audio source into the input jack playing music you.

Take audio probe connected to your mp3 speaker clip one side to the chassis and the other wire is your probe.
To start off with I would probe IC1-B pin5 for audio. If none your issue is between the input and IC1-B pin5 If yes to audio we move forward.
Just to note IC1-B and IC1-A are the same chip just 2 IC's on one chip.
nosaj

Also what is at TP3?

[Perkinsman]

TP3=0v..............I don't have an mp3 speaker...is there anyway to use my signal generator?

[nosaj]

TP3=0v..............I don't have an mp3 speaker...is there anyway to use my signal generator?

Is that AC or DC for TP3?

No the mp3 speaker was for the probe.(because it is a speaker and an amplifier in one small package is why I suggested one. You need a second amplifier of some sorts to hear where the signal is or isn't in the amp.
Check this link maybe it explains better just scroll down to listing amplifier.
http://el34world.com/Hoffman/tools.htm
nosaj

P.s hopefully someone chimes in on the TP3 test point being 0v I think we should see something there.

[Perkinsman]

I used DC for TP3, is that wrong? I measured in mV when I measured AC. I'll work on the audio probe tomorrow. Btw, what is the significance of the TP's you had me measure?

[nosaj]

I used DC for TP3, is that wrong? I measured in mV when I measured AC. I'll work on the audio probe tomorrow. Btw, what is the significance of the TP's you had me measure?

No DC is what we should be seeing.
The test points I suggested show that the transformer is working properly Your 12 +- rails and 3.3v rails are good. I am not so sure about the bias supply, but judging byt tr1 and tr2 your getting voltages there . So I'm thinking the power supply section is working properly. Next step will be to audio probe the amp.

nosaj

[Perkinsman]

In an earlier post Mick said to expect -90 at the grid, I only had -76, 15% high. Doesn't the Gate at 0v indicate a problem with tr1 and tr2? Doesn't a gate need to show voltage in order to allow current from drain to source?

[Mick Bailey]

I said around -90. I wouldn't be overly concerned right now that it's -76. The intention of the standby is that it puts a more negative voltage on the grids which stops the tubes from conducting. Also, standby takes the drive off the PI MOSFETS. So, the amp could be stuck in standby or the bias pot is set to the extreme. For now I wouldn't get hung up on the MOSFETS other than it's a good sign there's zero volts on the gates. I'm back to basics for the time being - do you have plate and screen voltages on the power tubes? If so, what are they?

[Perkinsman]

-.6vdv at plate & screen voltage on each power tube.

[nosaj]

-.6vdv at plate & screen voltage on each power tube.

What do you have at SP1 and SP2 The HT AC.
nosaj

[Perkinsman]

Sp1=+12mv, Sp2=+147mv...I’m assembling an audio probe but not sure what to plug into. Can I use a 1/4” to 3.5mm adapter & plug into my phone for a sound source or is that dangerous for the phone?

[nosaj]

Sp1=+12mv, Sp2=+147mv...I’m assembling an audio probe but not sure what to plug into. Can I use a 1/4” to 3.5mm adapter & plug into my phone for a sound source or is that dangerous for the phone?

I don't believe you can use a phone like that. You will need something like an amplifier. Another guitar amp or just one of those little cheap music speakers that charge off usb.

No voltage at SP1 or SP2 on the AC setting to me that say you have no high voltage so therefore no output tube amplifications. I did not see another fuse on the schematic for the HT section. But I'm leaning towards a replacement Power transformer, if anyone else here concurs.

nosaj

[Perkinsman]

I just looked closely at the 3 fuses & they’re all good but the middle fuse holder has a broken “U arm” that holds the fuse securely in. It’s still making contact with the remaining “U arm” but measures 13.5vac on one side of the fuse & 1.2vac on the other side. The other two fuses measure the same vac on both ends of the fuse. Could that be the issue?

[Helmholtz]

Most signal tracing can be done with a cheap crystal/piezo earphone like this (no amp required): https://www.amazon.com/Philmore-Crys...7828351&sr=8-3 .

Just make sure that the phone never "sees" more than a couple of volts (AC + DC).

These things are very sensitive and behave like a ~2nF capacitor. A suitable voltage divider and maybe clamping diodes/zeners may be required.