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Old 4th Feb 2020, 3:23 pm   #1
Ken Taylor
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Default DAC90A Electrolytic capacitors

Hello all you DAC90A enthusiasts!

I am about to start work on one of these sets and I have read many of the postings of others and seen the impressive results obtained. I hope I can match them.

One question, I notice that most of the finished projects appear to have the original reservoir/smoothing capacitors in place (C19/21 in the Bush manual). Are they particularly reliable? I do not see any bulges in mine and will bring the mains up gradually with the variac. Date code on the can is Jul 53, they show a high but falling resistance with the ohmmeter but my capacitance meter does not read that high.

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Old 4th Feb 2020, 5:11 pm   #2
kalee20
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Default Re: DAC90A Electrolytic capacitors

Mine i9s still going strong.

However, bringing up with a Variac does not achieve very much. The rectifier will do nothing until the voltage is high enough to heat the cathode sufficiently, which will be about 80% of full. So you won't be able to bring up slowly over some hours.

Personally, I'd get a 1N4007 or similar and use this as a temporary rectifier to bring up HT on the electrolytic over a few hours, monitoring the DC voltage. You only need to clip two leads onto the can. Start at 50V and gradually increase to 250V. If the can is still cold at this, then it's probably OK.

Then you can disconnect your temporary rectifier leads and bring mains up as normal.
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Old 4th Feb 2020, 5:27 pm   #3
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Default Re: DAC90A Electrolytic capacitors

My DAC90A still has its original smoothing can.
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Old 4th Feb 2020, 6:43 pm   #4
Ken Taylor
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Default Re: DAC90A Electrolytic capacitors

That is a very good point - and I should have thought of it. Neat idea with the temporary diode, thanks for the suggestion.
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Old 4th Feb 2020, 6:46 pm   #5
HamishBoxer
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Default Re: DAC90A Electrolytic capacitors

I have as far as I remember only had to change one smoothing cap in all the many I have had through the workshop.

BVWS sell them though if needed.
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Old 4th Feb 2020, 7:02 pm   #6
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Default Re: DAC90A Electrolytic capacitors

I have repaired many of these radios and never had to change the smoothing capacitors. They really do seem to be reliable even after all these years.

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Old 4th Feb 2020, 10:00 pm   #7
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Default Re: DAC90A Electrolytic capacitors

Quote:
Originally Posted by HamishBoxer View Post
I have as far as I remember only had to change one smoothing cap in all the many I have had through the workshop.
Likewise....and the only one I had to change was in a particularly bad set that had been plugged in by an 'enthusiastic' person as he remembered his dad using it. He was lucky they didn't explode since the rubber wiring was so rotten, a wire from the dropper was touching an HT wire with the result that mains AC had be shoved up the HT line! Had it not been for the cloud of smoke that ensued from the dropper and a few resistors and of course the mains decoupler resulting in a very hasty unplugging, the caps would have exploded rather spectacularly! As it was when I got it, the bottom of the smoothing cap was bulging badly.

Amazingly enough, when it had been rewired and recapped, including the smoothers, it was one of the best DAC90A's that I had repaired.

In general, the smoothing caps have a relatively easy life since the HT is fairly low so hopefully all you need to do is a gentle reform.
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Old 4th Feb 2020, 11:37 pm   #8
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Default Re: DAC90A Electrolytic capacitors

I have a DAC90A in my 'work in progress' pile. Having, towards the end of last year, replaced all the waxies and some out of tolerance resistors, basically all I have to do is to retrieve an unused pin from a valve base to replace one on which the solder terminal broke off short. The reservoir and smoothing caps should be OK, though I was thinking of changing them purely because by now they're at least the best part of 70 years old, but, in the light of what others have said, I'll power it up via my lamp limiter, and check that it is working OK before changing the caps, though surely they'll have lost capacitance by now.
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Old 5th Feb 2020, 8:19 am   #9
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Default Re: DAC90A Electrolytic capacitors

I would use a 20k resistor in series with the supply to the cap. This will allow the capacitor to automatically reform at its own rate. The voltage drop across the resistor caused by the initial high ESR will slowly drop to near zero as the capacitor becomes fully charged.
If the final current (V drop/R of the series resistance) at the full working voltage of the cap is less than 0.5mA then I class it as more or less being reformed.
(Obviously the cap has to be isolated from the rest of the circuit.)

Mike
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Old 5th Feb 2020, 7:12 pm   #10
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Default Re: DAC90A Electrolytic capacitors

Quote:
Originally Posted by Station X View Post
My DAC90A still has its original smoothing can.
As does mine and the set is in use almost every day - thank goodness for BBC Radio 4 Long Wave.
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Old 6th Feb 2020, 12:16 am   #11
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Default Re: DAC90A Electrolytic capacitors

There has been no need to change old cans on sets here either, unless they are of a brand other than TCC or Plessey.
Paul Stenning has a circuit somewhere on this forum for making a capacitor reformer, it is a crude circuit but has proved invaluable when it comes to reforming cans.
The only time I have ever had to replace a can (capacitor, to clarify things) is when it either says Dubilier or Sprague on it!
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