DAC90A (yes another one)

[stitch1]

Well I’m not sure this really warrants a success story based on the criteria it’s definitely not unusual or challenging but may be mildly interesting, and I wanted to ask a couple of questions so posted here.

I’ve not deliberately avoided DAC90As but just not that interested because they are so plentiful but when trawling eBay as you do I spotted this cream one which looked complete, unmolested and grubby.
It was advertised as collection only and nowhere near me and had received no bids so I messaged the seller and asked if it could be posted. They obliged I paid £47 and thankfully it arrived safely.

On inspection it was all complete and original so the restoration (more an overdue service than a restoration and nothing like the quality others on here do) was straightforward. Even the bulbs were serviceable, so I changed componants where required which didn’t take too long, it took longer to clean the chassis and case.

The set did have excessive mains hum so I suspected the smoothing caps, but searching this site I found out about the UL41 spare tag issue, I rewired this avoiding using the spare pin and was surprised at the improvement so much so I’ve not replaced the smoothing caps.

So a couple of questions...
kaylee20s post on DAC90A mods mentions changing C17 (trader sheet 1161) from 0.003uf to 390pf to improve the treble. Personally I don’t find the sound muffled but that may be just my ears but, if I wanted to change this what should the working voltage be?

https://www.vintage-radio.net/forum/...ad.php?t=35784

The seller said that the grill was dented but in fact it is perfect but has a discolored section as can be seen on the photos (looks worse on the photo). It seems to have been over polished but I tried polishing the rest to make it the same but it’s no better. Has anyone had success painting these and if so what’s the best match paint for the gold/copper colour?

John

[Graham G3ZVT]

I have a soft spot for these sets because one of them used to entertain us at boarding school in 1961.

I guess I will never know if it was a 90 or 90A, it could have been either, but I have solved that dilemma by buying one of each at the same auction.

Just go for 1000v capacitors and stop worrying

I found that Duraglit silvo wadding brought the cabinets up like new with practically no effort. Mine are both mottled brown bakelite though.

[stitch1]

This set has 2 tone correctors, the usual one on the output tx and C17. I only have the ceramic disc capacitors for small values which I think are 10V, I’ll experiment with these and get a higher voltage one if the change is going to be permanent.

John

[Ian - G4JQT]

Yes, you'll need a say 250V rated cap for C17, but at least a 600V cap for C20 (why so high is all explained elsewhere on the forum).

But you could try a 10k pot in series with C20 to adjust its effect on frequency response. Measure the value that gives you the tone you want and replace with a 1 watt fixed resistor.

Such a set up is sometimes used as a tone control; a bit crude for the purists, but it works. Just be aware of HT around that part of the cct while fiddling with it!

Ian

[kalee20]

C17 sees less than 100V in normal use.

My own rule is, what happens in abnormal use? If the UBC41 failed then C17 could see full HT, so 250V is a sensible rating. (I actually used a 630V polystyrene from Farnell).

Failure of C17 will cause no damage, though. So if you have a 63V, or 25V, capacitor - try it! It will hopefully last long enough to assess any improvement!

[stitch1]

Thanks for the comments everyone ... I’m happy with sound as it is but may change it just to see if I can tell the difference.

John

[Graham G3ZVT]

I think it sometimes helps to cut the bass if your speaker cone suspension is passed its best.