dac90a - to replace or leave alone??

[Yorkie]

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

The original cap is unlikely to be faulty, but the only way of checking would be on a capascitor reformer to see what the leakage is when it's been reformed, or to replace it with a brand new one, (not 'new old stock'), as sold by the BVWS. The correct value is -as you no doubt know, 16 + 32 uF, but the tolerance of smoothing caps (or more correctly, the smoothing and reservoir caps) was very wide when the DCA90A was rolling off the assembly line.

As to the working voltage of the cap(s), in AC only sets, this can be quite a bit higher than the mains voltage, depending on the mains transformer characteristics, but in AC/DC sets such as the DAC90A, will tend to be at, or below, the mains voltage. I think that typically, in the DAC 90A it will be about 175 - 180V, but you can check it to satisfy yourself Rob, so I wouldn't worry too much about the fact that the one that you have is 250V.

As to the 8UF cap connected to the 16uF side of the original cap, I can only assume that at some point, someone has assumed that adding extra capacitance in parallel, bringing it up to 24 uF, it might reduce the hum. The 8UF doesn't belong there, and if it were me, I'd take it out. I don't like the idea of paralleling electrolytic caps anyway - especially of diffrent values as you can't be sure how they interrelate to each other. It's not like paralleling say 2 x 100 pf caps to get 200pf in a tuned stage in the front end of a set, not subjected to high voltages.

Just a little tip about replacing the waxy caps in DAC90As:

Like most people I guess, I'd become used to either unsoldering the leads of the old caps, or clipping them off and soldering the new ones to the same tags. Then I realised that all the tags are 'twinned' and staring us in the face are spare unused tags, flat against the tag strip. All we need to do is to snip off the old caps, carefully bend those spare tags upright, scrape them clean, and solder the new caps to those 'new' tags. The attached pics might make it clearer. I've shown one of the way I used to replace the caps until I cottoned on to the spare tags, and another pic of another chassis in which I bent up the tags. I've also added a pic of the 0.1uF X Class cap that should really be used for the mains suppressor cap rather than a polyester one, unless you remove the supressor cap completely as it seems to make not one jot of difference, and many sets don't have them. Finally, I've added a pic of a cream DAC90A that I bought for £7.00 at Hornsea radio rally last year, originally looking very scruffy, but otherwise sound. I'm not a great fan of DAC90As, but at £7.00 I couldn't pass it by.

Hope that helps a bit.

Best of luck with the restoration Rob.

David

[Yorkie]

Me again!

I forgot to mention in my earlier post that if you look at the first pic, you'll see a brand new BVWS 16 + 32UF cap that I fitted, thinknig that it might reduce thev residual hum a bit, having already fitted a new UL31. Didn't make a scrap of difference - dogs have fleas, DAC90As have hum! (I kept the original for a spare - it shows negligible leakage on the cap reformer).

David.

[Aerodyne]

I suspect that the additional 8uF was added in an attempt to kill the hum. That probably means that your original main electrolytic is OK anfd the hum is sourced elsewhere, most likely in a leaky k/h UL41. Check your electrolytic using a test meter, preferably an analogue type, set to high resistance. Clip to negative (or can), touch the positive probe on each section in turn and watch the needle or numbers rise rapidly into the thousands of ohms. A smooth, quite fast rise says the cap is OK - but of course that doesn't check for voltage breakdown point, though as already said, these DAC90A caps rarely seem to fail. I'd replace it if it reads OK and try a replacement UL41. A few other tips: Also check that wiring to the volume control hasn't been disturbed. If VC is a replacement with a plastic case, that could make the hum worse. Ensure a metal case is truly earthing. Gently reposition the mains lead to the switch and any newly fitted (not by you) AF capacitors, listening for a level change in the hum. Some modern capacitors are more sensitive to AC fields than original types were.
-Tony

[Bushbaby]

Thanks David and Tony,
Both of you have supplied some very useful and helpful advice.

David,
That ivory DAC90a looks a gem and worth every penny of £7!

Regards

Rob

[Aerodyne]

£7? I'll say!
-Tony

[Yorkie]

Yes, I guess it was a lucky find - it was a small amateur radio rally where many punters aren't really into old radios. No-one seemed to show any interest in it - the rally had been open for a couple of hours when I spotted it. I only casually asked how much the guy wanted, expecting the usual drivel about 'these fetch £100 on e-bay' (which begs the question 'why brinig it to a rally then?'). I never say 'how much is this' always 'what's the least you'll take for this?' I never get to car boot sales early, because if you arrive about 11ish and stuff still hasn't sold, when it's been on the table since 6am, there's either no interest in it or they've been asking too much for it, and by 11am, punteres are already thinining out. If I saw a DAC90A at a boot sale, I'd say 'a tenner for your radio?' with a tenner in my hand. I'd go to £15.00 at a pinch but I'm not all that keen on them and I've got several. There's just no way I'd shell out silly money for such a commonplace radio, but I can understand that anyone who wants one and doesn't have one will pay more to get their hands on what many consider to be an affordable classic radio. AC/DC receivers aren't the ideal beginners' radio, but so much has been writtern about the DAC90A, and they're a straightforward radio to restore with sensible precautions, so I can understand their popularity.

David

[Bushbaby]

Hi All,
Replacement for C17 appears to be a problem - is seems that the lowest value cap that I can do for this is a 1000pF 350v (lower than the 500v waxy) silver mica cap. Will this make any difference?

Thanks

Rob

[Yorkie]

Hi Rob,

Firstly, the two caps you kindly sent me arrived today, are fitted, and have got my little toaster set back on song, so many thanks for that! Secondly, the working voltage rating of 350V was fairly standard for waxy caps, and only reflects their maximum working voltage - it is not a good indication as to what the actual voltage is at a particular point in a radio that the cap in that position has to contend with. To illustrate that, I've just checked the voltage on C17 on a DAC90a in my workshop, and the actual voltage is only 65V at that point. There are few points on a DAC90A where the voltage is much above 100V, eg, anode of V4 (UL41) is 190V, and anode of V5 (UY41 rectifier) is 222V AC. Hence, 350VW caps are very conservativly rated in that set. As I've said before, the crucial one is C22 (0.1uF) across the mains, which should either be dispensed with, or replaced by a Class X cap. (The one held in place by a clip on top of the chassis, front left).

That said, I do have a new 750 VW 390pF mica cap for C17, which I'll happily pop in the post to you if it helps. One good turn deserves another, so shout if you'd like it!

David

[Bushbaby]

Hi All,
I appear to be making heavy weather of this DAC90a!!

Is a 275 volt x1 y2 cap 0.1uF ok to use for suppression?

Thanks

Rob

[Yorkie]

Yes if that's it's AC rating, as I'm sure it will be, because they're designed to go across the mains. They're fittd to such things as eletric drills, strimmers, lawnmowers etc etc, across the mains switch. They're designed to be self healing and fail safe.


David