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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 23rd Jun 2006, 4:54 pm   #1
Pete_kaye
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Default DAC90A fading; UL41 problems.

I have faithfully restored a good one replacing all wax caps and some wiring near the dropper .It works very well but after about 2 hours the volume seems to drop on all stations ;not alot but noticeable to my ear.also the case seems warmer than others I have repaired .Does experience say to suspect failing output valve or is this normal? After an hour (off) it was OK again.
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Old 23rd Jun 2006, 6:21 pm   #2
paulsherwin
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Default Re: DAC90A fading after 2 hours

These do run warm, but it sounds as if something is wrong. All you can do is check circuit voltages after a couple of hours

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Old 23rd Jun 2006, 7:49 pm   #3
Ed_Dinning
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Default Re: DAC90A fading after 2 hours

Hi Peter, try monitoring the cathode voltage on the UL41 op valve. These often have internal leakage that only shows up when full warm. If you have it out of the case to do this and the effect does not occur then it is more likely to be a resistor. The DDT anode load is a good candidate for this.

HTH ED

Last edited by Darren-UK; 23rd Jul 2007 at 8:52 pm. Reason: General tidying.
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Old 23rd Jun 2006, 8:24 pm   #4
Sideband
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Default Re: DAC90A fading after 2 hours

Sounds like UL41 syndrome! It's probably going into grid current after some hours. Carry out the test that Ed suggested. Just measure the voltage across the cathode resistor. An increase in voltage suggests an increase in current. Dont worry about half a volt or so as long as it remains reasonably stable after half an hour or thereabouts. However several volts or higher suggests a fault with the valve. I once had one that was OK for about half an hour then slowly increased over ten minutes then suddenly very fast to well over 20 volts ! At that point I switched off!


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