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Old 11th Dec 2018, 2:46 pm   #1
mark2collection
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Join Date: Jul 2010
Location: Royal Berkshire, UK.
Posts: 470
Default Bush DAC90A, Write-off to Worker in no easy steps.

Ordinarily, I wouldn't write up about the repair of such a popular set. As the festivities beckon, making our way to 2019, the supply of Bush DAC90A's, repair advice, general interest, seemingly never fading.

Why would my story of 'yet another' be worthy of a write-up? Whilst most points may have been covered in the past, this may prove inspiring, or at least, an interesting tale.

It all began earlier this year, as spring 'sprung', the Daffodils showing promise of warmer days ahead; we made our way to an open-air antique fair. You know the ones, vast open field, cold, raining sideways, with dealers offering their 'wares' under some of the worlds flimsiest temporary structures!

Nestled under one such structure, sat on a raised part of a table, was indeed, the Bush radio I now write about, looking in fantastic cosmetic condition, externally. With a thumbnail-sized white label tied around a vent-slot in the back cover and slightly smudged ink, I enquired whether the price tag read £5 or 15, as I was unsure if it was supposed to be a 'one' digit, or a Pound sign. 'Fifteen pounds' was the reply. I went on to ask 'what can you tell me about the set?'

'You would have spotted the mains connector with one pin having been ripped out & the remains of some mains cabling poked through the hole, snipped level with the back cover' … the chap went on to say he'd owned the set for 10 years as a display only item, the current antique seller said the previous owner was 'into' radios and had removed two valves plus a transformer. Looking through the back cover under the gloomy clouds, hopelessly failing staying warm, never mind dry, I pointed out the dreadful condition of the rubber wiring and the fact the steel parts of the chassis looking very badly corroded, plus I could indeed see vacant valve sockets, where a rectifier and output device should live. Both I.F.T's looked original, could just about make out a gap where the output transformer should've been, under the gloom.
The seller was quick to point out he knew how much 'these go for' on eBay. Hmmm, since the only items of any value in these sets (if those items were in good condition) are now missing, 'will you accept £10?'

Needless to say, the deal was done at £10, and I was happy, the sellers’ wife put the set in a carrier bag to 'keep it dry' too.

Once home, set on the bench, back off and oh-my … it wasn't a pretty lot. Clearly the set had shared a home with smokers, possibly had a life in the kitchen too. It was grubby, sticky and had a bouquet of its own. Out came the chassis, the rubber insulation falling away as the ‘speaker leads were extended. The output transformer had, by the looks of things, been unceremoniously removed by wire cutters and a pair of Stilsons! Looking underneath the chassis, it clearly hadn't been touched since leaving the factory, things were looking up. My thinking was, if someone went to the trouble of removing the rectifier, output valve and output transformer, the radios’ biggest downfall must be the rotten wiring, as the sleeving on the wiring around the dropper was also a crumbly mess, the same could have been said for the internal aerial wiring. Looking deeper, all the rubber grommets where shot too, plus the dial glass channels, this set was decomposing!

Checking the coils, including the 'speaker proved these were good, the wave-change switch clearly hadn't seen any action for a good number of years, the contacts were green. Amazingly though, all the resistors were within specification, the waxies looked ok on my cap-meter, all reading roughly in the right-order of uF's, my DMM telling a different story on the ohms setting, the waxies ranged from 2-Megs up to around 7-Megs. The HT electrolytic(s) read about 4 Megs.

The loudspeaker was removed from the cabinet, sure enough the bellows had detached from the basket. Cleaning debris away as best I could, then using some contact adhesive, I applied some glue to the surfaces of the bellows & basket via a long cotton bud, the type with wooden stems, then laid the 'speaker facing upwards, carefully placing a reel of solder dead-centre, ensuring the voice coil made no contact with the magnet, and left to dry.

Taking further pity and a chance, over the course of the following months during summer, I gathered some spares from various events, on-line auctions & even made some parts with a view to returning the radio into a fully functioning set again.

I later removed the lamp diffuser panel only to discover the two lamps were also missing. I went on to remove the steel-work for the internal aerial, dial lamp reflector and associated mountings, pointer runner, dial cord pullers, wave-change switch, tuning capacitor and volume control. The chassis steel-work, nuts and bolts plus dial lamp reflector, went into the ultrasonic bath for an hour at 30 degrees, thinking it'll remove some of 'the rust'. Turns out they were not so much rusty, but caked in so much muck, it had in many ways, preserved the metalwork underneath. While these parts were being cleaned, I continued with cleaning the muck off the chassis, replacing all the rubber grommets, especially the ones for the mains and HT/output wiring, terrible mess. Once the metalwork was out of the ultrasonic bath, the paint for the dial lamp reflector had, in a number of places, come off. Oooops, oh well, it was off-brown to begin with, so I removed the remaining paint, made the surfaces clean/smooth, repainting the reflector white, aerosol Hammerite, smooth, about 3 coats. Whilst the paint was drying, I made a new decal to replace the missing 'Calibration Key'. This was printed, laminated & then double-sided tape applied to the rear. Once the paint had dried, the decal was trimmed, then fitted roughly where the original would have been (you could see the outline of the original).
The diffuser itself was also very yucky, both sides were cleaned with anti-static foaming cleanser, the smooth side came up lovely, the coarse side required a few goes with some T-Cut. Just before re-fitting the now clean metalwork to the shiny chassis, I fitted my spare output transformer that Ed Dining had previously rewound, about a year or two previous, merrily replacing the duff/too short wiring with PTFE sleeved wiring, of identical colours to the original, where possible. I also removed the dropper plus associated mountings.

The upper part of the Paxolin board was scorched, and so needed decarbonising plus careful Araldite reinforcement. All electrical connections were cleaned & the lot rewired in PTFE sleeved wiring, keeping to the original colour-code, where possible. With the chassis hardware refitted, time to see if the HT capacitor would reform.

Over the course of a day, drawing hardly any current, we reached 250 volts on my home-made variable DC bench supply, day two we ended up at 350 volts. Re-checking the value of the capacitor(s), it was within specification & read initially 25 Megs, but soon crept passed 40 Megs and beyond. The capacitor was cleaned up then refitted to the chassis. The date on the capacitor read 'May 1954'.

Having read a thread on wax capacitor re-stuffing (thank you John – Heatercathodeshort for the inspiration/guidance in your thread) I decided to give it a go. Best to use a cloth to grip the outer sleeve and keep any wax which forms on the vice, comes in very handy to plug the ends of the re-stuffed bodies, since my new capacitors were a bit smaller than the originals. I checked each waxie before removing, and indeed, each was more a resistor than a capacitor.

Replacement valves were sourced, again through the usual channels, days out, on-line etc, and whilst some parts were en route, a chap at work made a new mains connector mount on his lathe to replace the damaged original. Carefully removing the original and using its dimensions as a template, a new disc was made from Paxolin with a couple of mounting holes and a hole made for a Heyco cable grommet. I also bought some braided 2-core cable later to find in my stash, a modern, 3-pin safety plug, of the 'hot-condition' variety, which looks like black bakelite. Maybe I should've grounded the 'speaker grille, but I didn't.

Whilst cleaning all the valve pins, including my recently purchased replacements, I used cut-off guitar strings to clean out the socket pins. That's when I noticed one half of a pin missing from one of the heater legs, maybe this is why the set was taken out of service (?) Luckily I had a spare socket, simply de-soldering the duff leg inside the set & carefully removing a good leg from my spare socket, then fitting the good leg to the set … optionally, there would soon be a spare leg on the output valve socket, as I always follow Kalee20's advice by not using pin 4 as an anchor point.

With a clean, assembled chassis, waxies re-stuffed, HT cap reformed, new grommets throughout, lightly oiled tuning cap, new dial cord (the original was solid and didn't respond well to cleaning), new rubber wire for the 'speaker and aerial, PTFE wiring for the mains/HT and check after check to ensure all had gone back as per the diagram/my photos, it was time to apply power via my Variac, monitoring the current as we went, with the volume turned down.

Over the course of some 15 anxious minutes, we reached around 170 volts AC … nothing untoward to report and, out of the loudspeaker, I could hear the faint sound of 50Hz mains hum, I could see a very gentle glow from the dial lamps. Taking the set to 185 volts AC, selecting LW, turning the volume up a smidge & tuning until ….... Radio 4, we had sound!

Whilst listening to 'the radio' with its chassis and 'speaker on the bench, I went about cleaning the inside of the cabinet and later, switched the set off and gave the 'speaker basket a spruce-up. The date code for the loudspeaker was also May 1954.

The following day, the set was reassembled, captive mains lead fed through the back cover, plug refitted and a 1A fuse installed, the set then spent the following day running at full mains, with no issues to report, it's a great performer. Although the original mains RFI capacitor has been retained, it serves only a visual function, a modern X2 takes its place, electrically.

I'm glad I cleaned the lamp diffuser and repainted the dial reflector, the lamps are the brightest I've seen in an unmodified set. The outside of the cabinet was given a light dusting, as really, that's all it needed.

Given the current trend to turn old items into lamps, (this set would have been perfect fodder and would have taken all of 45 minutes to convert) this little radio has given me a day out in a muddy field, engage with like-mined souls on the forum whose skills and expertise made this project viable, it has made me attend various events/rallies in search of parts, meeting new people along the way, nattering, laughing and sharing experiences, plus making parts. After all this hard work, which was some months in the making, I now have a fine example of its type, rewarding me with a gentle hum, glow, warm aroma and hours of further entertainment, through listening enjoyment. The only real modifications being the tone corrector capacitor at 390pF & not using pin 4 of the UL41 as an anchor point.

The humble Bush DAC90a … no lamp conversion could ever come close, surely?

Thank you for your time reading all this, I hope it inspires and maybe, brought on a smile or two. Many thanks due to Kaylee20 & John (heatercathodeshort) for their write-ups which went some way into turning this set around, thank you to Ed Dining for his time in rewinding my transformer and all those folk who make this forum and other event possible, a very Merry Christmas to one and all.

Before, during and after photos, follow.

Mark
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