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Old 21st Feb 2006, 9:53 am   #1
Station X
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Default DAC90A. No LW; silvered-mica cap problems.

It's OK I've fixed the problem, but I can't explain how.

The DAC90A I'm restoring worked fine on MW, receiving stations every 9 KHz at the right place on the tuning scale. Switching to LW resulted in a few local MW staions being received weakly, but at the wrong place on the MW tuning scale. I deduced that the local oscillator wasn't working correctly on LW.

The wave change switch had been damaged as described in this thread:-

https://www.vintage-radio.net/forum/...ght=wavechange

So I dismantled it, cleaned it, repaired it and reassembled it. Still no LW.

Next I used an ohmmeter to confirm that the switch was working correctly. This showed that the resistance across the aerial contacts of the switch increased as the LW coil was switched in series with the MW coil. On LW a change in frequency of the Local Oscillator is accomplished by switching a 515 pF capacitor with parallel trimmer across the LO coil. I connected the meter across the 515 pF capacitor and it showed an open circuit on MW, which is correct. Switching to LW caused the resistance to drop to near zero as the capacitor was shunted by the LO coil.

I then used another receiver to listen for the LO working on LW. With the DAC90A tuned to the LF end of the scale I could hear the LO beating with a station near 1000 KHz. This is about correct for MW. ie 1000 - 465 = 535 KHz. This just confirmed what I already knew. The LO wasn't switching to LW.

A check with a frequency counter showed that when switching to LW the LO frequency only shifted by about 20KHz.

I bridged the LW contacts of the wavechange switch with croc clips. Still no LW.

Next I connected a 470 pF and 47 pF capacitor in parallel with the 515 pf capacitor. This brought the LO frequency down, but not enough to receive LW stations.

I connected a capacitance meter across the capacitors and measured 1072 pF which is about right when you allow for the trimmer and stray capacitance.

I couldn't think of what to do next, as I had conflicting information. ie way too much capacitance, but close to the right LO frequecy. I disconnected the extra capacitors and reconnected the capacitance meter. 557 pF as expected.

You've guessed it. LW then worked. I removed the croc clips and it still worked. Operated the wavechange switch several times and it still worked.

I can only conclude that the 515 pF silvered mica capacitor had gone open circuit, then cured itself. However no amount of freezing, frying, poking and proding would make it go faulty again. I'm going to soak test the set for a few days and see what happens. The only problem at the moment is that Radio 4 is at the wrong place on the dial even with the LO trimmer at max capacitance. It looks like I will need to up the value of the 515 pF capacitor a bit.

Any ideas anyone? Stephanie seems to be the only one of us who has problems with silvered mica capacitors.
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Old 21st Feb 2006, 10:32 am   #2
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Default Re: DAC90A. No Longwave.

Interesting story. You've certainly done a good diagnostic job. These faults have always been down to wavechange switching or coil problems in my experience, and I've never come across a mica cap displaying these symptoms. The only other possible explanation I can think of is a dry joint somewhere, but I imagine you've checked for that.

Congratulations on getting everything working anyway

Best regards, Paul
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Old 28th Feb 2006, 9:04 pm   #3
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Default Re: DAC90A. No Longwave.

Not exactly in the same place I know, but my DAC90A had an o/c 110pF silver mica capacitor in one of the IF transformer tins so it does happen. I've kept it in my 'evidence' tin along with other werdos (I thought it was worth hanging on to in case people didn't believe me).

Regards,


Andy
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Old 28th Feb 2006, 9:46 pm   #4
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Default Re: DAC90A. No Longwave.

The capacitor concerned was one of those dipped in wax rather than one with a hard outer coating as is common today. I think that the leads are attached to the plates by a riveted on solder tag. I wondered whether this had perhaps corroded, but made contact again when moved/heated during fault finding.

The set is still working fine on LW, but I had to put an additional capacitor of 27pF I think in parallel with the 515 pF to get the calibration right on LW. I decided not to replace the 515 pF with an increased value capacitor, as I want to see if it fails again. I'll know where to look first if LW fails again.
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Old 28th Feb 2006, 10:09 pm   #5
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Default Re: DAC90A. No Longwave.

Quote:
Originally Posted by G4ILN
The capacitor concerned was one of those dipped in wax rather than one with a hard outer coating as is common today. I think that the leads are attached to the plates by a riveted on solder tag. I wondered whether this had perhaps corroded, but made contact again when moved/heated during fault finding.
I reckon you are right. I've had this before and it can be caused by oxide build-up around the rivet. Heating/movement etc can break down the oxide layer and it works again. May go for days/months/years .

Good bit of diagnostics...well done.


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Old 1st Mar 2006, 11:15 am   #6
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Default Re: DAC90A. No Longwave.

I've had silvered mica caps of this form exhibit intermittency at the rivets. They can be tricky to pin down. Dry joints on coil connections can also give trouble where one band fails to function.
Re the cap; if in doubt, replace.
BTW the wavechange switches are flimsy items - as no doubt you've realised!
-Tony
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Old 1st Mar 2006, 8:56 pm   #7
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Default Re: DAC90A. No Longwave.

Yes that's exactly the type that failed on me!

Nice to know I'm not alone.

Cheers,


Andy
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