DAC 90A Smoothing Capacitor replacement

[marconi_pete]

If you're going to cricklewood electronics, They do a 32uf - 32uf multi section capacitor that fits in the same physical space as the original smoothing can, complete with solder tags. Well worth it, i've been buying them for years now. They definitely come in handy and are of the correct voltage rating required for a Dac90A.

Regards,
Peter

[JasonKillbourn]

Wow, lots of replies with invaluable information, thank you all
Right, progress update as follows:
I've isolated pin 4 on the UL41, all the waxies and dodgy bits of wire have now been replaced, and the set is showing significant signs of improvement, as it seems louder, clearer, and the original bulbs have come back to life. However, there is still a little bit of a hum and, even though the LW/MW selection switch seems to be working now, I am just picking up patches of static and the vaguest hint of a station on LW, though that might partially be down to very poor reception here in my kitchen, I do suspect that there's still an issue with LW that needs ironing out.
I haven't done anything with the electrolytic smoothing can yet, nor did I manage to make it down to Cricklewood to get the 450V 33uF and 16uF caps. However, I actually have got two 400V 33uF caps sitting here, so I was wondering if I can use them to test things, as per Kalee's suggestion, by temporarily tacking them onto the can's terminals to see if the last hint of a hum goes away, or are they completely unsuitable?
Furthermore, in addition to the wiring side of things, I've also repaired a small split in the speaker with tissue and PVA, which seems to have worked nicely, so, once the hum and LW tuning are resolved, it looks like there only remains a dab of black paint to touch in a few small areas on the dial glass and a coat of white on the bulb reflector plate, and it'll be time to reassemble the radio

[stevehertz]

Originally Posted by Boom

It used to be the case that absolutely every repairman carried a neon screwdriver and the very first thing they did was to touch it against the chassis to see if it lit and therefore the chassis was live. This was especially the case in ye olden days because as well as having a two pin connector on the radio many mains wall sockets were also reversable two pin. You might reverse one connector making it safe and someone else might then reverse the other bringing the chassis alive again!

I don't know if such neon screwdrivers are still available.

Were'nt they blamed for giving unqualified bodgers a false sense of security?

[Station X]

Please start a new thread if you wish to discuss neon screwdrivers.

[paulsherwin]

They don't fail safe, but that's true of lots of testers. They're much better than no tester at all.

Contactless testing screwdrivers are available from market stalls or eBay for about £1.

[Ed_Dinning]

Hi Jason, there will always be a slight level of hum on this type of set.
If you have the set running for a while you can unplug it (safety) and see if the main can is getting hot. If not more than merely warm then the caps should be OK.
Note that as it has a frame aerial then turning the set may give much better pick up of any particular station.

Ed

[kalee20]

Originally Posted by JasonKillbourn

However, I actually have got two 400V 33uF caps sitting here, so I was wondering if I can use them to test things, as per Kalee's suggestion, by temporarily tacking them onto the can's terminals to see if the last hint of a hum goes away, or are they completely unsuitable?

Definitely, yes! Tack them in place and give it a whirl!

If they are at all old, check they're not getting hot after 1 minute and after 5 minutes and after 10 minutes. If they're OK then, they'll be OK for a long long time.

[marconi_pete]

On a Dac90A, the main smoothing can would originally have been a 32/32uf can, so see no reason why you couldn't tack in two 33uf caps @400v to test things.

Regards,
Peter

[Boater Sam]

An extra tacked across the smoothing cap is fine, I would not double up on the reservoir cap, you may exceed the value stated for the rectifier valve.

[marconi_pete]

Make sure the leads to the original smoothing can are disconnected from the set and connect up the two 33uf caps with the negative end of the caps joined together. The original smoothing can itself, can be left in place for aesthetics.

[JasonKillbourn]

Well, I've had the radio running all evening and the can didn't even get slightly warm, but the hum is pretty noticeable. In fact the only thing that did get hot was obviously the dropper and the stuff around it, including the two nearest valves, which I assume is what you'd expect. I also moved the radio to a worktop on the other side of the room and the reception improved a fair bit on MW, so it looks like my kitchen table is a poor reception area, however there was no noticeable improvement on LW, which is still just a mass of static.
Anyway, I'll probably have another tinker with it at some point tomorrow

[Ed_Dinning]

Hi Jason, as an initial trial just add a 33uF across the smoothing cap (electrically furthest from the rectifier and see if this reduces the hum.

Ed

[Radio Tech]

I don't know why Bush ever used 32uf when perhaps they could have used 50uf that's the max for the UY41 rectifier. Still stranger things happen, if it was me I might have used a couple of 47uF caps for the smoothing. Just a thought.

Ken

[julie_m]

Capacitors have traditionally been expensive, in terms of Farads per pound, is why!

[JasonKillbourn]

OK, I am hoping I've worked this out OK, but I've had a double check of the schematic on the trade sheet and the original 16uF smoothing cap positive appears to be the red terminal on the bottom of the can, the 32uF reservoir positive is the other terminal with three yellow and red striped wires, and the black terminal goes to the chassis. So, I am assuming my first tentative test should be to tag the positive from one of the new 33uF 450V electrolytic caps onto the top of the red terminal, and negative to the black, and see how the radio likes that, via the lamp limiter?

[Boater Sam]

I would have expected the red terminal to be the reservoir cap, 32uF, connected to the rectifier cathode, being the "outer" layer in the can, and the other, usually yellow terminal, 16uF, to be the smoothing cap.

If you study the circuit diagram you should find the order to be, rectifier cathode, reservoir cap (red 32uF), 12k ohm resistor, smoothing cap (yellow 16uF)
[ C20, R5, C21 on the Trader sheet. ]

So another 32uF across the existing one is a no-no as it is then 64uF which exceeds the 50uF limit for a UY41 rectifier valve.

[kalee20]

Sam's right. However...

In the DAC90a the current is less than half the maximum for the UY41 rectifier valve, so it has an easy life anyway.

Increasing the reservoir capacitance will indeed increase the peak current through the valve (bigger peaks of shorter duration), but not massively. Manufacturers put a limit on the reservoir capacitor for other reasons - limit the switch-on surge if the valve is hot; limit current surges if the mains fluctuates, etc.

Adding extra capacitance for 10 minutes evaluation really won't hurt (I've done it!) though don't repeatedly switch off and straight back on, or operate heavy machinery from the same supply mains, while you have your extra reservoir. But if you temporarily throw lots of uF's at the radio, and the hum hardly changes, you know the HT rail is then blameless and you can look elsewhere.

[JasonKillbourn]

Well, in the end I decided to play it safe and just desoldered the can and connected the the two 33uF 400v caps (both negatives to the chassis, one positive to the red wire, and the other to the wires that were on the yellow terminal). To be fair, it does sound a bit better, as I am listening to Absolute radio this morning, the reception seems strong (perhaps down to good atmospheric conditions), and, compared to how it performed the other week, it has more volume and the tone is much improved, but the hum is still noticeably in evidence, so I do suspect you're right and it's time to look elsewhere.
Would the smart money be on the UL41 at this stage? I don't think I've really got the equipment here to flash the valve, though someone did mention using the ignition on a gas cooker, which I do have, but I am not quite sure how to perform such an operation?
Also, I did isolate pin 4, but reading back through the replies, there was a mention of isolating pin 3 as well? I ask, as I hadn't seen pin 3 mentioned anywhere else.
Anyway, seeing as I was given the radio for free, and have only spent about a tenner so far, I might splash out, give myself an early Xmas present and buy a replacement UL41, while they're still available, as I imagine I'll be keeping this radio, especially seeing as my kids have fallen in love with it

[kalee20]

Hmm.

£10 is not much to splash out on a hobby. But it's still fun to try to find out, beforehand.

If you had a known-good UL41 you would do a swap as a trial, because it's easy, quick, even if the likelihood of success isn't definite. But you haven't.

You could short pin 3 of the UBC41 to chassis. If the hum largely goes, then the fault is not in the UL41 and I'd say, get another UBC41.

You might try YouTube to see if anyone 's put up a video of their DAC90a, something to compare yours against!

[julie_m]

Don't chuck away the old UL41; even if you don't try any method for recovering it; even a valve that "works but hums" is enough to prove whether or not another valve of the same type is dead. But do label it carefully when you stash it away; otherwise you run the risk of forgetting what was the matter with it, fitting it in a set and then starting looking for the cause of the hum .....