siemens: to much current

Hello dear friends,

I have quite unusual problem with this apparatus.

Just to mention, I have quite experience with old radio restoration and reparation.

Problem with this radio at start was completely destroyed power supply section, do to previously reparation atempts.

It was all misconnected, but with diagram I downloaded from our Radiomusem site, it was quite easy to correct all misconnections, but!!! after I resoldered and checked all connections, after switching on, on referent point where it should be -9.1V, I measured -20V, and output tube anode start to glow, all other voltages were too low, and minus g1 was -3.6V (see picture). So I started to check again resistors and capacitors arrangement, replaced UL41, tried another output transformer etc..., but nothing happened, too much current flows, and quite simple problem is now headache....

Does anybody have an idea?

Thanks in advance, Peter.

• Shematics (165 KB)

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Hello Perica.

It looks as if the bias is too low on the UL41.  3.6 is too low.

This is derived from the series 110 and 23 ohm resistors in the negative supply.

This low voltage would occur if the 0.25uF filter capactor to the junction of the those two resistors were leaky or shorted.  I suggest that you check this first.

Regards,

Roy

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Hello Perica,

there is a big chance that the 10 nF capacitor between anode UAF42 and control grid g1 of the UL41 is leaky. What happens, if you disconnect this capacitor ?

Best Regards,
Jacob 

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Hello Perica,

Presumably you have checked or replaced the UL41 with a known good one?

The UL41 often develops internal leaks to the grid from the high voltage electrodes.

Only the UL41, the 10nF that Jacob suggests or the 0.25uF can be the cause.  Hopefully your new wiring is correct!!!!!

Good luck,

Roy

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Hello Roy,

thanks for suggestions, but I already replaced all capacitors and all resistors with signs of overheating, tried another UL41(also used one), filter capacitors, output transformer, and nothing changed...

This is so simple radio, simple bias circuit, but still doesnt work as it should be, I mean it works, but with so distorted sound, and UL41 anode overheating...

Regards, Perica.

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Hello Jakob,

thanks for suggestion, but, as I wrote to Roy, all capacitors were replaced before first start, and the problem persists...

Regards, Perica.

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And yes, new wiring is correct,

I checked few more times...

Thanks Roy!

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Hello Perica,

anyway there is an undesired voltage drop across the 100 + 800 kΩ grid resistors.

As you replaced the capacitors and the UL41 itself, there is the chance that the UL41 socket has a leak between the g1 and the g2 connections.

Or both of your UL41s are bad, as this type has the inclination to develop internal leaks, as Roy already told.

Best Regards,
Jacob

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Hello Perica.

Since you have eliminated most components, I suggest:

1. Short circuit the 800k Grid resistor on the UL41.  If the high current still exists, measure the grid voltage with the short still in place.

IIf the high current still exists repeat with the other UL41.

2. If the current falls to normal levels the fault must be leakage to the grid either IN the UL41 or, as Jacob suggests, the tube holder.   Check this visually for any blacking or this tracks from the g1 pin.  If in any doubt replace the socket.

3. If the current is still high, check that the output transformer does not have a leakage to ground.

Let us know!

Regards,

Roy

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Hello Jakob,

on the end it was the socket....a thin burned canal in the pertinax between g1 pin and chassi...

Thanks so much for the idea...

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Hello Roy,
I tried everything You suggested, and  on the end it was the socket...
Thanks again for all practical advices.
Sincerely, Perica!

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Hello Daniel,
I checked, and all switch contacts are in good order, on the end UL41 socket was burned.
Thanks, Perica.

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Hello Perica,

there was little left what could be the cause !

Using pertinax sockets on tubes becoming that hot as E/UL41s or E/UL84s was never a good idea !

Regards, Jacob 

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