Blackstar ht40 No Sound

[Enzo]

Sp1=+12mv, Sp2=+147mv

AC voltage has no polarity, so either you had your meter on DC to get the + sign or you added the plus sign which was not there on the meter.

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 fuse holder is broken and not working. Replace the fuse holder. The fuse may SEEM to be making contact with the broken part, but it is not, as shown by the meter reading. A good fuse will have the same voltage at each end.

SP1 and SP2. WAIT, SP3 is not a center tap. We want AC volts between SP1 and SP2, not each of them to SP3, not each to ground. Just SP1 to SP2. ie put one probe on each. We expect a few hundred volts of AC.

Likewise power OFF. Unplug amp from the wall outlet. Now look at SP1 & 2. I expect the wires to be the same color, but we want to pull those two wires off the posts, now they are hanging free in mid-air. measure RESISTANCE from one wire to the other. We are measuring resistance through the winding. A good one will have something lowish, like maybe 50-200 ohms. In any case we want to not see open or really high resistance, and for that matter we don't want to see zero. Those tests are to verify the transformer as bad or not.

A shorted HT winding ought to blow the mains fuse.

[Perkinsman]

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.

Not sure why Jazz suggested it has a CT...but if Sp1 & Sp2 is an open winding, could the loose fuse have caused it?...and why do I get 13v on one side of the F2 fuse & 1.2v on the other?

[Jazz P Bass]

Alright.
After poring over this goofy schematic I have come to a few realizations.
- I find it odd that the HT is not fused.
Well, it may be.
By F4. (SP12/ SP13)
HT-Club 40 F4.pdf

I will bet that you will find it open.

Next: the preamp tube heaters are powered by the +12 / -12 Vdc supplies.
They are fused by F1/ F2.
HT-Club 40 V1 & V2.pdf

So start by repairing the holder on F2.
(a simple alligator jumper will do for now)
(You may still have something pulling down the supply)

Lastly: The EL34 output tube heaters are supplied by SP14/ SP15.
I cannot find where these originate on the schematic.
HT-Club 40 SP14_ SP15.pdf

But it appears to be separate from the preamp supply.
That is why the EL34 heaters work but the preamp heaters do not .

[Jazz P Bass]

I went looking at other HT schematics and came upon the HT1.
On that schematic they show the tube heaters coming directly off of the LT windings.
That may be where SP14 & SP15 originate.
Blackstar_HT-1.pdf

[Enzo]

nd why do I get 13v on one side of the F2 fuse & 1.2v on the other?

Because the fuse is either open or not making contact in the holder. Depending on which thing your probe is touching.

I will bet that you will find it open.

I'll bet you are right. I still urge to pull the transformer wires off and verify continuity of the winding.

[HaroldBrooks]

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

nosaj

Nice nosaj, you know how to proceed with trouble shooting.

Enough conjecture and supposition. Need to measure the remaining elements, bisect the circuit, and find the faults !

[Perkinsman]

AC voltage has no polarity, so either you had your meter on DC to get the + sign or you added the plus sign which was not there on the meter.



The fuse holder is broken and not working. Replace the fuse holder. The fuse may SEEM to be making contact with the broken part, but it is not, as shown by the meter reading. A good fuse will have the same voltage at each end.

SP1 and SP2. WAIT, SP3 is not a center tap. We want AC volts between SP1 and SP2, not each of them to SP3, not each to ground. Just SP1 to SP2. ie put one probe on each. We expect a few hundred volts of AC.

Likewise power OFF. Unplug amp from the wall outlet. Now look at SP1 & 2. I expect the wires to be the same color, but we want to pull those two wires off the posts, now they are hanging free in mid-air. measure RESISTANCE from one wire to the other. We are measuring resistance through the winding. A good one will have something lowish, like maybe 50-200 ohms. In any case we want to not see open or really high resistance, and for that matter we don't want to see zero. Those tests are to verify the transformer as bad or not.

A shorted HT winding ought to blow the mains fuse.

Thanks Enzo, I didn't see this message earlier...and yes, I added the positive sign just because... I pulled both sp1 & sp2 and read OL, so I guess that confirms an open HT winding. Looks like a new PT is on the list as well as a fuse holder. Would the loose fuse have caused this? Btw, just to make I didn't screw up and put sp1 & sp2 on backwards, is there a way to confirm they are on the correct terminal?

[Randall]

" Would the loose fuse have caused this? "

Not likely.

"Btw, just to make I didn't screw up and put sp1 & sp2 on backwards, is there a way to confirm they are on the correct terminal? "

It doesn't matter.

[Perkinsman]

So is it the consensus that OL between sp1 & sp2 means the PT is history? What are the likely causes for an open HT winding? I want to make sure to correct any other problems so that this doesn't happen with the new PT.

[nickb]

So is it the consensus that OL between sp1 & sp2 means the PT is history? What are the likely causes for an open HT winding? I want to make sure to correct any other problems so that this doesn't happen with the new PT.

Jazz called it.. the HT fuse is likely open, not the PT.

The drop would have caused the tube to fail so blowing the HT fuse. It's anybody's guess when the LT fuse holder failed.

[Jazz P Bass]

So is it the consensus that OL between sp1 & sp2 means the PT is history? What are the likely causes for an open HT winding? I want to make sure to correct any other problems so that this doesn't happen with the new PT.

Have you located F4?

https://music-electronics-forum.com/...4&d=1577927060

[Enzo]

Remember, I did not ask you to measure between SP1 and SP2, I asked you to pull the transformer wires off the board and measure between THEM. We need to verify the actual wwinding of the actual transformer is open or not. I tend to believe what Jazz said, that fuse is likely open. I would hate for you to buy a transformer, install it, and find the blown fuse still there.

[Perkinsman]

[QUOTE=Jazz P Bass;546705]Have you located F4?

https://music-electronics-forum.com/...4&d=1577927060[/QUOTE


Oh jeez, I'm such an idiot....but I'm learning alot. Jazz told me several times to check this fuse and I thought I had but until now I hadn't actually bent down and read the word MAIN. This amp has 2 external fuses, the main and F4...and it's blown. I'll have to get a 500mA 250V tomorrow....but assuming Sp1 & Sp2 is the LT & measured OL, doesn't that still mean there is an open winding?

[Enzo]

No, it means an open fuse. At least if I read things right. Follow the wires from the transformer, I'll bet they do not go directly to SP1 and SP2. Until I see the wiring I am not assuming the SP1,2 are the actual transformer wires.

[Perkinsman]

Thanks Enzo. I can confirm that there are 2 red wires that come from the PT & plug into sp1 & sp2. There are also 2 black wires from the PT that plug into sp14 & sp15