Mid 1950s Bush DAC90A AC/DC 5 valve MW/LW radio

[ColinB]

Hi Howard,

I've encountered a DAC90A do this, and never did get to the bottom of it. My current one doesn't have this happen, though it had to be extensively restored (all new caps, new set of valves, new speaker and output transformer, several new resistors, conversion to UL84) - it does sound good though, but I probably spent more on parts than on the set itself

Another thought, have you cleaned the valve pins / holders? I've known this range to suffer with the pins becoming tarnished. Wire wool is useful to clean the pins, and a spray of contact cleaner and tightening any loose sockets with a fine screwdriver works well.

I seem to recall someone else mentioning a crackling DAC90A being due to an bad contact in the mains dropper, causing a tiny arc?

No definitive answer I'm afraid, but maybe some food for thought?

Best regards,

Colin

[howard]

Originally Posted by Richard_Newman

Hi Howard......... One way to isolate would be to earth the control grid of the UL41 and see if the noise stops. If it does, then it's before the UL41. If not then it's output stage or power supply. Unlikely to be power supply I would think. Rich

Hello Richard,

That was done and the investigation was then concentrated around the audio stage.

Originally Posted by ColinB

Hi Howard,

Another thought, have you cleaned the valve pins / holders? I've known this range to suffer with the pins becoming tarnished. Wire wool is useful to clean the pins, and a spray of contact cleaner and tightening any loose sockets with a fine screwdriver works well. I seem to recall someone else mentioning a crackling DAC90A being due to an bad contact in the mains dropper, causing a tiny arc? Best regards, Colin

Thanks Colin,

One of the first things I do when I acquire a set is to take out all the valves and clean pins and bases. The bases in this set are in excellent condition. I replaced the wiring to the dropper but I rechecked the contacts and all is well.

Originally Posted by Richard_Newman

..........
I've not stripped the old transformer but I suspect the failure is either 'green spot' or just wire break-down. It's possible that when the transformer starts to fail, the break allows the ends of the wire to just touch with some possible carbonising so there is some continuity which can be heard as the faint crackle. Eventualy of course, the winding fails completely. Rich

You're right Richard,

A worn output transformer is the most likely cause of the crackling. I may well try and find a replacement and swap it to see if that sorts it out. It doesn't spoil the sound quality at normal volume but it's worrying that any day it might just expire.

Howard

[Sideband]

Howard. You may get some idea of the condition of the transformer by measuring the primary resistance. I found that mine varied from over 1K to 700 ohm. It should be about 600-700 ohms. Yesterday I stripped the transformer...not with the intention of rewinding it. I got down to the tenth layer before I found the break in the primary then found two. It didn't appear to be green-spot...wouldn't have expected it that deep into the transformer anyway so it was probably just wire failure due to excess current.


Rich.

[howard]

Hello Richard,

I've checked the output transformer. After the set had been idle for a couple of days the primary resistance was found to be 15k ohms (sic) and even higher the next day. I ran the set for a few minutes, switched off and retested it and it was then 1200 ohms. The Avo meter is still attached and the resistance is slowly climbing back up again.

Methinks I need a new output transformer !

Howard

[Sideband]

That's your problem then . I suggest you either contact Ed Dinning and see if he has any of his replacements available or see if Past Times Radio have any. I got my last one from there (I think the Bush AC11 uses the same transformer).

Ed supplies a complete new core and winding that fits the original clamp (although it's very tight and you will need a vice).


Rich.

[howard]

Hello again,

I managed to get another original DAC90A output transformer for under £5. The primary resistance on this one measured 602 ohms which is considerably better than the original one. It wasn't too difficult to swap it over in my DAC90A, the fiddliest bit was re-fitting the two bolts which hold the transformer in place, but fortunately I had a socket which fitted the nut.

Crackling is all gone now, so I now declare this DAC90A restoration a success. Now to find a white DAC90A ..........

Howard

[SGT.Pepper]

Hi Howard, brilliant restoration! The Bush DAC90A, as well as being a fantastic-looking radio, was what started my interest in old stuff! Before I saw one, I didn't think much of "retro" items. It was in 1998, when I was five years old, when I saw one in a shop by the sea, which sold mainly books. However there was one shelf which had old radios and record players on it, with a rather dusty DAC90A in one corner. I absolutely loved it, but as it was priced a £110, I couldn't afford it with my £2 pocket money.

That radio inspired me to buy a 1940s HMV set in a newspaper ad, thus starting my vintage radio collection. I went to the same shop the next year, and while all the other radios had gone, the DAC90A was still there, now £85. I still couldn't afford it. I somehow wound up at that shop at least once every year, each time it was still there, but at a slightly lower price. Every single time I didn't have enough money.

Fast forward to today, about five hours ago. I went to the shop again. The Bush was still there, now looking rather tatty, priced at £45. I asked if it worked, and apparently it did when the owner first got it. I knocked him down to £35, and finally bought it! I've no idea if it works yet, there's no lead on it. Apparently it's from 1957 and the serial number is 73/182410. Here's a picture of it just after I bought it.

[Nickthedentist]

Originally Posted by SGT.Pepper

Fast forward to today, about five hours ago. I went to the shop again. The Bush was still there, now looking rather tatty, priced at £45. I asked if it worked, and apparently it did when the owner first got it. I knocked him down to £35, and finally bought it! I've no idea if it works yet, there's no lead on it.

What a wonderful story, and picture!

I'm glad the vendor didn't insist on plugging it in.

N.

[Dave Moll]

As Nick implies above, just plugging it in without a thorough check out first could have spelt disaster for it.

[SGT.Pepper]

And that's precisely why I fished out the multimeter and the variac! It's been working fine for about an hour now. It has a great sound, I think the muffling is just part of its charm!