Blackstar ht40 No Sound

[Perkinsman]

New tubes, the filaments light up, speakers connected but no sound. I don't know these amps, any experts out there?

[Mick Bailey]

Any other outputs -FX send, Line out/phones?

[alexradium]

the standby mode is dependent from the input and output jacks,and a PIC controller does all the procedure.

[Mick Bailey]

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.

[Perkinsman]

Update: turns out the owner had disassembled the top cab screws & didn’t support the head. It dropped & cracked a power tube. I removed the tube socket (very tight) & replaced the tubes but no sound, just filaments lighting. What would be the likely suspects?

[Mick Bailey]

Assuming you removed the tube base from the socket, not the socket itself. Again - Is there an output from the phones socket or FX send? Determine if the fault is in the preamp or power amp section. If the preamp works, check if you have correct DC voltages on the output tubes.

[nosaj]

Update: turns out the owner had disassembled the top cab screws & didn’t support the head. It dropped & cracked a power tube. I removed the tube socket (very tight) & replaced the tubes but no sound, just filaments lighting. What would be the likely suspects?

Follow Micks advice and report back here. Anything at this point is an assumption...Just the Facts, man

nosaj

[Perkinsman]

No headphone or line out but there is an effects return/send but I’m not sure how to test. I ran an external test speaker into the Fx send & guitar into the input but still nothing. I tested voltages on both preamp tubes & power tubes & while I’m getting ac to light filaments, I get very low dc voltages in mv range.

[Perkinsman]

Also tried attaching the speaker to the speaker output & the guitar into the Fx send, nothing.

[nosaj]

Also tried attaching the speaker to the speaker output & the guitar into the Fx send, nothing.

First plug guitar into instrument jack and headphones in the FX send get sound? then
try this Plug guitar into FX return and play do you get sound at speaker?

Nosaj

[Perkinsman]

First plug guitar into instrument jack and headphones in the FX send get sound? then
try this Plug guitar into FX return and play do you get sound at speaker?

Nosaj

Nothing either way

[nosaj]

Nothing either way

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

[Perkinsman]

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

[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.

Ok, back at it, I wanna get dirty. F1+13vac, F2+13.5vac F5+28vac, TP9+3.3vdc, TP11+12.79, TP12 -12.9
V3&V4 grids -76.6

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

[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.

If the mosfets are TR1&2, they both measure 0vdc at the gate, source/drain both -45vdc