dac90a - to replace or leave alone??

[Bushbaby]

Hi All,
I've finally got around to working on my ivory Dac90a that have the extra 8uf Cap attached to the 32 + 16 mains smoother. I've tested the dual cap with the ESR meter and it indicates good to both caps. The set suffered from main hum when it was last switched (with the extra 8uF attached).

Do I put the dual cap back or do I replace??

Help

Regards

Rob

[Steve_P]

Remove it, whip out the capacitor across the On-Off switch and then go for it. If it hums loudly and the volume control does not affect this, then the chances are it's faulty. If it hums and the stations sound like they are under water, then the same applies.

The capacitor may be OK on the ESR meter but may be breaking down under load.

There are a load of paper caps underneath the set, Change all these. Especially 'that cap'.

The UL41 may be past it's best - there may be a heater-cathode leak on any of the valves, but the UL41 is the favourite.

Cheers,

Steve P.

[Bushbaby]

Thanks,
All of the waxies have been replaced with yellow aerial caps from Just Radios. As each one was replaced the set was tested and the sound did improve with each one. At the eand of this process, the set was woking well but you could definitely hear a background mains hum which is why I turned my attention to the smoothing cap (a Hunts cap from 1952 - as indicated on its side).

Steve - you may well be correct that this is breaking down under load.

Regards

Rob

[Yorkie]

DAC90As are noted for background hum Rob, and generally, it's the UL41 that's on its way out, partly due to heater/cathode leakage. I have a DAC90A in which I'd replaced the UL41 as the output was low and the set hummed, then although the smother/reservoir caps had reformed nicely and showed little leakage, the set still hummed, so at last year's NVCF, I bought a new dual cap from the BVWS stall. It made little difference, so I've kept the original cap for a spare. Neither does it sem to make any difference whether or not C22 - the 0.1 cap which is a mains suppressor is removed or replaced. As it's directly across the mains, it should be replaced not with a yellow polyester cap, but with an X Class cap, which - if they fail, they fail safe, and are self healing.

So much has been written about DAC90As that there's little more that can be said. On mod that many have made, which is said to improve the tone by giving it more treble, perhaps more suited to today's listening, is to replace the tone corrector cap, C17 - a .003uF, with a 390pF.

Hope that helps a bit Rob.

Best of luck with the DAC90A - eveery home should have one!

David

[tiv48]

i have also never had a faulty smoothing block in one of these sets but they are over 50 years old so who knows.
ul 41 valves are well known inter electrode shorts so it would be worth trying another one .
rob t

[Bushbaby]

Hi All,
Ok I'm stuck!
I'm wondering whether to:
a) put the original cap back in and retry (I do have a spare UL41 too) but I'm concerned why it had a 8uF cap connected to the 16uF side?(see photo)
b) use a triple cap that I have that 32/32/16 but no fourth connector (cap has -ive can indication)
c) use a 16/16 500v can (RS) but I only have a 15uF 250v electrolytic to put in parallel with one of the 16uF side - I'm concerned that 250v is too low.

Help please??

Regards

Rob

[attachment=1560][attachment=1561]

[tiv48]

hi rob i have a spare used one if you want it .came from the same set as dropper that was working fine but got dropped and scraped
rob t

[Bushbaby]

Hi Rob
Yes please let me know costs + P&P please.

Thanks

Rob

[tiv48]

will dig out and post tu/wen dont worry about cost you may have somthing i need one day. any thing under £2 not worth bothering with
rob t

[Bushbaby]

Hi Rob,
many thanks indeed.

Regards

Rob