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Bush DAC90

 
PYE625
(@pye625)
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Continuing the theme of Bush radios, I happened upon this one in an antique shop whilst on holiday. It was priced at £40, but after some sweet-talking to the good lady at the till, I got it for £35. I had no intention of buying another radio, but this caught my eye as it seemed to be in pretty good condition with no damage to the bakelite case. Opening it up and withdrawing the chassis today, it would seem not a bad set. The mains RF bypass capacitor has previously exploded and there has, at some point, been work done to replace a couple of capacitors. The metalised coating of the EF39 has seen better days as indeed has the top connection area on the paxolin strip at the mains dropper. Throw in a nice burn-hole in the card back panel, and we have a radio ripe for restoration. Pity it couldn't have been the AC90 version lol.

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Topic starter Posted : 15/09/2019 5:09 pm
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Lloyd
(@lloyd)
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looks a nice radio, I have 2 of these, both black, one has been fully re-capped, I think I even removed the asbestos on that one, and the other has been left as found, as nothing has ever been changed in it, and it's in really nice condition. The restored one unfortunately has a rip in the speaker cloth, nothing massive, but big enough to be irritating. I have the grille from an AC90, came in a box of random Bush bits from the NVCF a good few years back. I stuck it on the front of a DAC90A to make it look different!

Regards,

Lloyd

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Posted : 15/09/2019 6:21 pm
PYE625 liked
PYE625
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Cheers Lloyd, mine has a rip in the cloth so it might have to be replaced. Before that though, there are the electrical repairs to do.

The first thing I did was to test the output transformer and speaker. I used an audio generator connected to the primary and about 15 vrms at various frequencies were enough to prove it all worked. A leakage test was done too and all was fine. Next step was to test and re-form the main 32+16uf electrolytic. It tested at 33+18 uf and reformed very quickly and well. This surprised me a little as I was half expecting it to be duff. The top has a bit of a bulge to it.

I then removed the remains of the mains RF bypass capacitor, cleaned up the area, and restuffed it with a new component. Next step is to work out how to effectively repair the burnt paxolin area near the mains dropper top, as it is unsafe to leave as it is.

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Topic starter Posted : 15/09/2019 8:52 pm
PYE625
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As for the asbestos lining of the dropper sheild, I think that is best left well alone.

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Topic starter Posted : 15/09/2019 9:07 pm
PYE625
(@pye625)
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I have removed the dropper from the sheild. I trimmed back and filed the paxolin so it is flush with the cutout hole in the metal edge of the sheild. I plan to use a nut and bolt to affix some stout wire to the dropper. Then, I can solder the other end to a tag-strip piece mounted on the same screw that goes in the top area of the paxolin strip. The wire will pass through the cutout where the brass post was located.

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Topic starter Posted : 15/09/2019 9:36 pm
turretslug
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I always smile wryly when I hear of antique radios being opened up to find exploded (often mains filter) capacitors within- I imagine eager but uninformed punters finding it in the proverbial lofts, sheds etc. after many years of neglect and saying "Ooh, let's plug it in and see if it works....". After the warm-up interlude, a few muzzy and low-level MW stations come in (tired rectifier, leaky PSU electrolytics, leaky screen-grid decouplers and output grid-couplers, grossly escalated carbon-comps, the usual) and there's a slightly disappointed murmur of "Oh, well, no wonder valves went out of fashion"....

BANG!

Hasty unplugging, " ? - stick it on eBay, loft find, untested".

Hopefully, the mains filter cap lets go fairly swiftly in most cases before CL33s/UL41s etc. come to too much harm from leaky grid-couplers. That mains filter cap could be described as the restorer's friend.

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Posted : 15/09/2019 9:59 pm
crustytv and PYE625 liked
PYE625
(@pye625)
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My heater.... erm, "mains dropper"  repair consists of the below picture. Not much else to say about it really.

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Topic starter Posted : 15/09/2019 10:16 pm
ntscuser liked
PYE625
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I have replaced the wire from the dropper as this had burnt insulation. It is the feed to the heater chain and is at around 75vac. I have replaced a couple of audio coupling capacitors with re-stuffed wax types. No DC leakage can be tolorated here to the grid of the output valve. So, time for some power via a lamp limiter. It is working very well on both bands and the perfomance would seem very good. Effectively, all I have done is to repair the top of the dropper area and replace a couple of capacitors.

As for the remainder of the wax capacitors (last photo), I have left them alone for now. They may well be a bit leaky, but they appear not to affect perfomance nor are they getting warm. Extended use will ascertain if they can be left.

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Topic starter Posted : 17/09/2019 5:53 pm
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Lloyd
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Looks a perfectly good repair to me, those droppers do run stupidly hot, not really helped by being entombed in that asbestos coffin! 

Regards 

Lloyd 

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Posted : 17/09/2019 6:22 pm
PYE625
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B.U.S.H. = Best Universal Space Heater.

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Topic starter Posted : 17/09/2019 6:31 pm
Cathovisor
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Posted by: @lloyd

those droppers do run stupidly hot,

32W at full supply voltage.

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Posted : 17/09/2019 9:21 pm
PYE625
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It eqates to approx. 200 degrees c. on the dropper itself, and an air temperature of 115 degrees about half a centimetre back from the top of the sheild. Hardly surprising then that the card back in that area disintegrates with the heat over time. The remainder of the chassis runs pretty cool, fortunately.

Incidentally, I re-stuffed the capacitors previously mentioned because all of them, except the cathode bypass ones, begun to show molten wax at the wire entry points. They were, unsurprisingly, very leaky DC wise.

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Topic starter Posted : 18/09/2019 9:39 pm
turretslug
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That's eye-wateringly hot- this is one set that almost justifies an external mood-light "barretter" in place of the heater section of the dropper! A 60W bulb probably wouldn't be far off the mark.

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Posted : 20/09/2019 4:54 pm
PYE625
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Today, I replaced the rubber mounting grommets for the tuning gang. I have been wanting to do this because the single grommet opposite the tuning control spindle end had disintegrated completely and tilted the gang slightly back into the chassis. This caused the drive wire to keep slipping out of the groove on the drive wheel. It was a case of unsoldering three wires, undoing the nuts below chassis, then withdrawing the gang.

I also cleaned and re-lubricated the drive pulley. It feels much better and smoother in operation now. It was quite fiddly to refit the drive wire, but I found it useful to consult the service instructions. Luckily, I was able to keep the wire as a complete loop without needing to undo either end as they both are affixed to a small bracket held by a spring on the drive wheel. The wire was slightly springy and kept coiling up, but I managed with the aid of tweezers and pliers to refit it in the end.

In this picture, the tuning gand can be seen slightly tilting towards the chassis, causing the wire to ride out of the groove.

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Topic starter Posted : 21/09/2019 10:56 pm
PYE625
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The torn speaker cloth has been removed and I found some used cloth that will be ok as a replacement. I had run out of PVA glue, so bought some of this stuff pictured below. It sets within a few mins and seems ideal for gluing speaker cloth. There is enough time to slowly manipulate the cloth and stretch it tight as the glue starts to set.

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Topic starter Posted : 22/09/2019 12:28 pm
Nuvistor liked
PYE625
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After a clean of the bakelite case, the chassis has now been re fitted into it. I am enjoying Gardners' Question Time as I type. Oh, and I found a better EF39 as the original was flaking away it's metalized coating.

The only thing left now is how to repair the burn hole in the card back.....hmm.

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Topic starter Posted : 22/09/2019 2:30 pm
Norman-Raeburn
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Hi Andrew, There was someone over on UKVRR selling repro backs for these and the 90A made from MDF. I have one and it looks good, you would probably need to spray it to match the original. It wasn't expensive at less that £10.

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Posted : 22/09/2019 3:18 pm
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PYE625
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Thanks Norman, I'm hoping that I may have effected a repair though.

I used a scrap donor back panel and a Stanley knife to cut out a suitable sized section. I then cut and filed both the existing panel and the new piece. They fitted together quite well and I have used the same glue mentioned previously mixed with dust from the filing of the card to make a paste. It would seem pretty strong, so I'm hoping to use some black paint to help blend it in when the glue has hardened. As the radio will not be used for long periods, I'm hoping the bodge will be a success and the area not affected by the heat.

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Topic starter Posted : 22/09/2019 5:58 pm
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PYE625
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I think it is passable. I didn't use black paint, but a permanent marker pen gave pretty good results instead.

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Topic starter Posted : 22/09/2019 7:02 pm
PYE625
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Posted by: @pye625

I also cleaned and re-lubricated the drive pulley.

I probably should have described the dismantling of this in greater detail, plus with pictures, but it's sometimes hard to know what to include and what to leave out before it becomes boring for people to read. I'm guilty of condensing things sometimes.

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Topic starter Posted : 22/09/2019 9:44 pm