Tube preamp PT problem, troubleshooting fun inside.

[Baguette]

Hello,

Just got an old H&K Tubeman preamp (the old early 90' one).
It came in with a blown PT (the thing converts the 12VAC wallwart into 250V).
So I get a good replacement, install the new one and the thing works. But the sound is not that great.
I grab my DMM and read that the supposed 250VAC at the secondary is actually around 120VAC. Primary voltage reads a good 12VAC.
Rectified voltage (B+) is around 180VDC, without no tube in the unit.

Why are my voltage so low?

The thing came in with a blown PT, so some components in the power supply might have failed too?
If so, which ones are likely to have failed, and to cause my low voltage problem?
The big PS caps are expoxied to the board and will be a PITA to replace, and the whole thing is PCB of course, so I don't want to waste my time and pull my hair replacing good components.

Here is part of the schemo:



Thanks a lot for your help!

[craneclimber]

First thing to do, if you haven't already, is disconnect the secondary from the rest of the circuit and check the voltage to confirm you have 250VAC.

[Shrapnel]

Also check the diodes in the rectifier circuit to see if any are blown. That would overload the transformer and cause low voltage measurements. what is the condition of those filter caps? obvious bulges, leaks?

[Baguette]

First thing to do, if you haven't already, is disconnect the secondary from the rest of the circuit and check the voltage to confirm you have 250VAC.

Guess what?
Removed the bridge rectifier and measured AC volts at the PT secondary.
I read a low 140VAC.
Primary is still good at 12VAC.

WTF????

The transformer is a nice Made In Germany unit, looks like a little cube.
It's supposed to convert the 230VAC Euro line voltage into 12VAC. So I guessed it would work just the same if I reversed the polarity (in order to convert the 12VAC wallwart into 230VAC high voltage to feed the tube).
Well apparently it's not so.

What's happening here?
Could I have a mislabeled transformer (I just can not believe this but who knows?)?
Or is it a mistake reversing a 230VAC > 12VAC transformer to get a 12VAC > 230VAC transformer?
Or could the transformer have a centertap somewhere inside the casing that I should connect or disconnect? (I'm wondering because the voltage I get is about half what I'm expecting).

Thanks lots for your help!

[sluckey]

The transformer is either bad or just not the right replacement.

[Baguette]

Answering mpy own questions here.

1/ no center tap thingie into this box. It's all epoxied and there's just 4 pins sticking out of the plastic.

2/ might have a lead here: the little PT is supposed to be a 230VAC > 12VAC unit, 200mA. Wouldn't this ratio be true @ 200mA current draw only?

If so, I guess the circuit does not pump up much current (much less than 200mA, it's just a single 12AX7 after all).

Could it be the cause of my low voltage at the secondary?

[sluckey]

PT is supposed to be a 230VAC > 12VAC unit, 200mA. Wouldn't this ratio be true @ 200mA current draw only?

Yes. It will also be true @ 0mA such as when the secondary is not connected to anything. Your PT is either bad or mislabled.

[Baguette]

Thanks Steve,
I'll get another one, and a 230V > 6V unit too, in case the wrong ratio happens on all their transformers.