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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Power transformer

Hello!

I have repair a Koch Jupiter 45 a month ago. The amp came with the transformer HT tap with only 10VAC ( it should be 250VAC ).

This amp has two 12ax7 on the pre amp section and two mosfets on the power section.

I have send an email requesting a quote for the transformer and since they didnt bother answer me i send it to a shop for a rewinding.

After one month playing with it, the same thing as happen again. this time 50VAC on the HT so i send it again for repair.

My question is;
Do you think the circuit has a fault that is pulling a large amont of current of the transformer?
Or it was just a bad winding?

Thanks
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
All sounds very odd to me.

Its very rare for a transformer to fail like you describe, and then for the rewound winding to fail again... something doesn't add up here.

Have you checked the voltage with the transformer out of the circuit, and also looked at the physical connections to it?

Does the transformer look like it has actually been rewound?
 
yesterday i have put the transformer on the amp. They didn't charge me and said it was two burned windings.


Right after i posted the message here, I put a smaller fuse on the HT tap ( it was 160ma and i put one of 100ma ), connected the amp to a load while monitoring the output and the voltage on the B+. At idle the voltage was around 315V with the signal dropped around 5V.



I let the amp about 1 hour on the load, everything looked ok, so i was about to turn off the amp...

Immediately after i pull the jack with the signal the B+ drop to 70v.

It happen again,
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Very difficult say without actually seeing it all.

Maybe the transformer is faulty and it hasn't been rewound properly, it all sounds a bit suspicious to me.

If the amp was working OK with normal voltage and then suddenly this voltage falls and fails because of (lets say) something external pulling the voltage down then that should not kill a transformer unless it was left for a long time so it overheated.

Also, why did the fuse not blow?

Based on what you say, my opinion would be that this this all sounds like a faulty transformer that hasn't been repaired properly.
 
The fuse didn't blow.



I was also checking the current on the variac meter.
At idle was pulling 0.26A and with the signal was at 0.5A, but always stable and the transformer was warm.


I call the shop and they said that i was connecting the wires the wrong way????
How can i connect the two AC secondary wires the wrong way???
 
I dont have schematics.


I put the probes after the rectifier.

The wire ( Red one connected to the filter caps ) is soldered direct on the pcb.


img_20120316_120304-jpg.210796
 
This kind of faults rarely has an overload origin. Mostly they are caused by faulty design and manufacturing of the transformer itself. The wire surely is very thin and if improperly manipulated, the varnish can deteriorate, and fail. Also, if bad winding techniques are used, stacking turns in a messy way may produce a high potential turn be near of one with lower one, and a high voltage gradient, which with time it cause a spark. The wire itself acts as a fuse, and blowns. As there exists capacitive coupling in the cut turn with the rest of the winding, you will measure some lower AC voltage but incapable of giving current because this capacitance is small (in the order of 100's of picofarads). It represents several KΩ of impedance in series with a smaller voltage created in the sane turns. I saw this fault several times in my life including one toroidal winded by myself.
 
1) unless a Koch Authorized Tech chimes in I very much doubt a schematic is available anywhere.
It is a very modern amp.

It´s a Hybrid amp , or "an SS amp with 2 12X7 added".

2) I see no transformer of any kind there, and it might even use an SMPS.
Supply seems to live in the leftmost board where I see a slider 220/110/100V selector, it might also point to an SMPS, please popst a picture of the transformer involved, I would not be surprised at a small size, ferrite core one.

3)
The amp came with the transformer HT tap with only 10VAC ( it should be 250VAC ).

This amp has two 12ax7 on the pre amp section and two mosfets on the power section.

i send it to a shop for a rewinding.

After one month playing with it, the same thing as happen again. this time 50VAC on the HT so i send it again for repair.

They didn't charge me and said it was two burned windings.
The World does not run that way.
If they charged nothing, they did nothing, period.
Even less rewinding a burnt one which is a long and tedious process.
They can "not charge" for something trivial, say resetting a connector, a loose fuseholder, ann improperly set switch, but not a complex job.

So it´s either "no problem found" which is very common or they simply don´t want to bother with it.

Waiting for transformer pictures and you show us where you measured.

Please also add the large capacitor values, none is readable.
 
And the voltage between both 250 Vac leads measures 10 or 50 Vac even if the tranny idles?
Best regards!


Yes.

The first time was 10VAC.



I send it to the shop for a rewinding. When it came i put it on the amp, everything was fine and the owner call me after a month saying it happen againg. The red lead was 50VAC this time.


I send it to the shop for a rewinding, again. Today while testing, as i described above, the transformer as fail.