Problem with transformer or EZ80 - Telefunken D656W

[electronic_93]

Hi all!!! I'm new here, so I'm sorry for the way I'm gonna write my problem.

My great-grandfather has bought a vintage radio "Telefunken D656W" (since 1955) and this radio has moved into generations and now I own it! I'm a new graduate electronic and I like to work with these stuff. One day, my grandfather had realised that there was a problem with the radio and he put it at the warehouse. It has stayed there for years. Few days ago I've decided to try to fix it. I've searched on internet and on forums and after research, I've found that there was a problem with the filter capacitor! (cause there was hum on the speaker).

It was a two section 50uF filter capacitors (50uF + 50uF) and I've replaced it with two capacitors of 50uF each (same volts, etc.). Then the radio has started to work fine again!!! I was so happy for it. It was working for hours. Then, after few hours, I switched on the radio again and suddenly there was sprarks from somewhere in the chassis! I closed immediately the radio. I re-opened it and it was working fine again.

The next day, I switched on the radio, but only the flament lights were on. No sound, no hum, no anything. The tubes were getting power as I could see. I opened the chassis to check it and I've realised these:

1. When the EZ80 is fited and the radio is ON, after a few seconds the EZ80 sparks inside a few times and the transformer gets SO hot that it is smoking after a minute.
2. When the EZ80 is OUT but the other tubes are ON, the transformer is OK (cool enough).
3. I was measuring the voltages on the filter capacitors. When the radio was working fine, the voltage there was about 130-140 volts DC as I remember. Now the voltage is about 50 volts.
4. I've made a measurement on Pin 3 (Cathode) of the EZ80 tube to the chassis (check image). I disconnected the DC output bus wire from the cathode (pin 3) of the rectifier tube, then powered up the radio and after a few seconds I had 50 Volts DC on Pin 3. Then, suddenly the 50 volts started to drop gradually (not so fast, but it drops) and when it went to 30 volts, I decided to close the radio. I think that on this point I should have 240-250 volts, right?

I don't know where is the problem. But I think it's on the transformer or on the EZ80 tube. Please If you can help me.

[Station X]

Sounds like a classic case of rectifier failure.

I would also suggest you change the 0.01uF capacitor feeding the grid of the EL41, unless it's a "Mullard Mustard" or similar reliable type.

[Radio Tech]

May I suggest, disconnect the HT from the set after the smoothing caps, if the fault still exists then work back from there, and yes also do as Graham has suggested, you may even have other capacitors showing signs of leakage.

Hope this helps

Ken

[electronic_93]

Originally Posted by Station X

Sounds like a classic case of rectifier failure.

I would also suggest you change the 0.01uF capacitor feeding the grid of the EL41, unless it's a "Mullard Mustard" or similar reliable type.

Thank you for your reply!!! You mean the one I have put in red circle?

[electronic_93]

Originally Posted by Radio Tech

May I suggest, disconnect the HT from the set after the smoothing caps, if the fault still exists then work back from there, and yes also do as Graham has suggested, you may even have other capacitors showing signs of leakage.

Thank Ken for your reply too! Sorry, but can you show me on the schematic I've uploaded which part of the HT to disconnect? I didn't get it before. Sorry again

[Station X]

Originally Posted by electronic_93

Thank you for your reply!!! You mean the one I have put in red circle?

Yes. That's the one.

[paulsherwin]

The cap you have ringed in red is the one that must be changed.

The HT +ve supply will come from the rectifier cathode to an electrolytic capacitor. There will be a power resistor or choke from there to another electrolytic, which is likely to be in the same can as the first one. From there it will go to the humbucking connection on the output transformer, which you can see on your circuit diagram in the middle of the primary coil.

You could disconnect the HT at any of these points, but I think you almost certainly have a bad rectifier valve.

[electronic_93]

Originally Posted by paulsherwin

The cap you have ringed in red is the one that must be changed.

The HT +ve supply will come from the rectifier cathode to an electrolytic capacitor. There will be a power resistor or choke from there to another electrolytic, which is likely to be in the same can as the first one. From there it will go to the humbucking connection on the output transformer, which you can see on your circuit diagram in the middle of the primary coil.

You could disconnect the HT at any of these points, but I think you almost certainly have a bad rectifier valve.

Hmm I see. Something more: If i don't want to replace the EZ80 rectifier with other one and I want just to replace it with diodes (I read somewhere that I can replace the EZ80 with 1N4001 diodes and some resistors), do I have to replace all the tubes then with the diodes + resistors? Do you know something about that?

[Station X]

This set has the two diodes in the EZ80 wired in parallel, so you can replace it with a single diode and a series resistor. Plenty of threads on this if you do a search.

[DangerMan]

Originally Posted by Station X

This set has the two diodes in the EZ80 wired in parallel, so you can replace it with a single diode and a series resistor. Plenty of threads on this if you do a search.

However, a 1N4001 does not have suitable voltage rating... use at least a 1N4007.

[electronic_93]

Thank you guys!!!

[electronic_93]

Originally Posted by Station X

This set has the two diodes in the EZ80 wired in parallel, so you can replace it with a single diode and a series resistor. Plenty of threads on this if you do a search.

Thanks! I made a search but I see that people uses not the same resistor on this replacing. Can you please tell me what resistor in series with the diode should I use?

[Boater Sam]

Start with around 100 Ohms minimum, it needs to be a 7 or 10 Watt rating due to the ripple current into the reservoir capacitor (the first 50uf after the EZ80) then adjust the value to get the HT voltage back to what the sheet says it should be.
1N4007 diodes are suitable to replace the EZ80, as it is only half wave rectification, one diode will do, valve anode to diode anode, valve cathode to diode cathode which is the marked end. With the shorted valve out of course!

[paulsherwin]

The value of the resistor isn't critical. I normally use 200 ohms for a radio of this type. Anything between 100 and 200 ohms should be OK. I don't find I need to use as high a wattage value as Sam suggests but it certainly won't do any harm.

[Boater Sam]

If you do the Ohms law sum with say a 150 ohm resistor to calculate the wattage you then need to multiply it by a factor of at least 3 for the ripple current, 50uf is a hefty reservoir cap, the ripple current will be high. I tend to go on the safe side to prevent the OP saying that the resistor is getting hot and smoking!

[electronic_93]

Ok guys! Thank you very much!!! I will try to do that. And replace the old capacitors with new. I wish I have a good luck. Thank you again.

[Boater Sam]

Please don't get carried away changing capacitors unnecessarily, normally its only the waxed paper rolled foil caps that are suspect and any Hunts Mouldseal plastic horrors.

Changing any waxed flat silver mica caps in the aerial, oscillator and IF sections at the front end will wreck the alignment and be totally unnecessary as they practically never fail.

CCC (compulsive capacitor changing ) is a disease and it wrecks old radios all too often. There have been lots of threads here and on other forums where it has all gone irredeemably wrong.

Whatever you change, do them one at a time, testing the set works after each. That way you find any human errors before you get too involved.

[paulsherwin]

This is a German set so I would expect it to contain Wima capacitors, though the technology is similar. You shouldn't need to change any capacitors smaller than 0.001uF. The others don't need to be changed if the radio is working well, especially if you don't have a lot of expertise. However, you should certainly change the capacitor circled in red, as this has probably failed and will cause other damage.

[electronic_93]

Originally Posted by paulsherwin

This is a German set so I would expect it to contain Wima capacitors, though the technology is similar. You shouldn't need to change any capacitors smaller than 0.001uF. The others don't need to be changed if the radio is working well, especially if you don't have a lot of expertise. However, you should certainly change the capacitor circled in red, as this has probably failed and will cause other damage.

It has some "NennSpg", some "Hydra Weak" and some "Rauh Elite" capacitors as I can see.

Ok I will change only the 0.01 uF capacitor then. And I'm gonna replace the rectifier with the diode and the resistor.

If I have a problem again, I'm gonna reply. Big thanks!!!

[electronic_93]

Originally Posted by paulsherwin

This is a German set so I would expect it to contain Wima capacitors, though the technology is similar. You shouldn't need to change any capacitors smaller than 0.001uF. The others don't need to be changed if the radio is working well, especially if you don't have a lot of expertise. However, you should certainly change the capacitor circled in red, as this has probably failed and will cause other damage.

One question please: This capacitor (the 0.01uF one) is not electrolytic right? It's MKT?