Blackstar ht40 No Sound

[Perkinsman]

Open Power transformer?? How does that happen? Are there replacements on the market for this amp?

[Jazz P Bass]

"No voltage at SP1 or SP2 on the AC setting "
Did you test these with your meter set to read Volts ac?
Measure SP1 to SP3 & SP2 to SP3.
HT-Club 40 HT winding.pdf

[Perkinsman]

Yes, I used vac for the msmts. sp1 to sp3=148mvac, sp2 to sp3=126mvac.....what do these represent?

[nosaj]

Yes, I used vac for the msmts. sp1 to sp3=148mvac, sp2 to sp3=126mvac.....what do these represent?

To me it says shorted HT winding on the power transformer.

Possibly turned on with damaged tube.

You said
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.

nosaj

Replacement https://www.partsisparts.net/catalog...ower-mcpt01263

[nosaj]

To me it says shorted HT winding on the power transformer.

Possibly turned on with damaged tube.

You said
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.

nosaj

Replacement https://www.partsisparts.net/catalog...ower-mcpt01263

If you replace it I'd suggest building a dim bulb tester to check it after replacing the PT.
nosaj

[Perkinsman]

What else causes open heater windings? Will this have caused other problems in the amp that I need to look for once I replace the transformer? Will the new transformer have marked wires since they are the same color wires?

[Perkinsman]

Good idea. What other possible causes for open heater winding? Is it repairable or just needs to be replaced? Will it have caused other problems?

[Jazz P Bass]

I believe what failed is the HT winding.
That is the High Tension winding.
Some 400 volts.

[Mick Bailey]

Disconnect the HT winding and check the DC resistance to make sure. I find it surprising that the LT side should be fused and not the HT. I don't have an map by me to reference but I would be absolutely certain that the transformer is defective before replacing.

We're missing some of the history here - why did the owner remove the amp? Had it already failed? Unless the amp was powered up with the broken tube then that may just be coincidental. Either way, you have to make sure there's no underlying problem. If the transformer is replaced I monitor the current draw and bring the amp up on a variac. In this case I would be monitoring specifically the HT supply. Bear in mind that unless the amp is in operate mode (speaker and inputs plugged in) you won't get a true reading. With the correct HT voltage you would also need to monitor the gate voltage on the PI MOSFETS, or measure the voltage drop across R140/R141 to make sure this is not excessive.

[nickb]

Yes, I used vac for the msmts. sp1 to sp3=148mvac, sp2 to sp3=126mvac.....what do these represent?

It suggests that both windings are open circuit. That would be most unusual. Can you recheck and also check from SP1 to SP2. Check the filaments are glowing. If you get hundred of volts AC between those points have a good look at the connection from the transformer to SP3. Check those to SP1 and SP2 also.

You mentioned a fuse with different VAC readings at each end, I guess F1 or F2. That is a problem for sure. Either the holder and/or the fuse are bad.

PS: Just a thought. Sometimes things are labelled differently between the schematic we have and the amp you have in front of us. Be sure that SP1,2 and 3 are the HT windings in the example you have. That can cause all kinds of confusion.

[Perkinsman]

[QUOTE=Mick Bailey;546663]Disconnect the HT winding and check the DC resistance to make sure. I find it surprising that the LT side should be fused and not the HT. I don't have an map by me to reference but I would be absolutely certain that the transformer is defective before replacing.

Are the HT windings sp1 & 2? If so, do I measure resistance from ground to each? If so, they both show OL

[Perkinsman]

It suggests that both windings are open circuit. That would be most unusual. Can you recheck and also check from SP1 to SP2. Check the filaments are glowing. If you get hundred of volts AC between those points have a good look at the connection from the transformer to SP3. Check those to SP1 and SP2 also.

You mentioned a fuse with different VAC readings at each end, I guess F1 or F2. That is a problem for sure. Either the holder and/or the fuse are bad.



PS: Just a thought. Sometimes things are labelled differently between the schematic we have and the amp you have in front of us. Be sure that SP1,2 and 3 are the HT windings in the example you have. That can cause all kinds of confusion.

I double checked & the msmnts are correct....sp1 to 2 is 150mvac.....

F2 reads 13.4vac at one end & 1.2vac at the other end. The fuse is good but as i said one of the holders "U arms" broke off. It still makes contact but doesn't hold it in securely. Can this cause the symptoms?

The power tubes have filaments lighting but I don't see the preamp tubes lighting.

[Jazz P Bass]

That transformer is bad.

And you need a new fuse clip.

EDIT: ignore the CT info as the transformer HT does not utilize one.

[Perkinsman]

Could the loose fuse have caused this? If the transformer is open why do I get filaments lighting at the power tubes?

[Helmholtz]

Maybe I am missing something, but I don't see the PT in the schematic. What makes you think that the HT winding has a CT?
What I do see is that B- is grounded, meaning that there can't be a grounded CT.

If there is no ACV between SP1 and SP2, the (single) HT winding is open. This scenario is much more likely than 2 windings being open.