Another DAC90A restoration.

[AD360 Rob]

Good morning everyone,

Finally I am getting around to writing up my very first DAC90A restoration !

After an extended period of transistor set restos and a successful Bush SRP30 restoration I decided to see what all the fuss was about the DAC90A, it seems to be a very popular set with both restorers and users alike so I kept my eyes open for one on eBay (which is where most sets seem to come from these days) and after a few abortive bids I managed to obtain a complete looking set for under £40 inc postage.
When it arrived it did look a little sorry for its self, the case was filthy and dull as was the tuning scale and grille but the back was intact and didn't look as though it had been off recently. So, I got stuck in. Removed the back to be greeted by a dusty but unmolested chassis which was removed carefully. the speaker and dial glass came out next leaving the case ready for cleaning. After looking at the chassis (and after a good read up on the dac90a threads on here, I decided to change all of the wax paper capacitors before even thinking of powering up, I also checked a few other components out too such as the output transformer, dial lamps and main smoothing caps, all of which checked out ok. I ordered a load of the yellow high voltage caps that are popular for valve sets and whilst waiting for them to arrive, cleaned up the case and speaker grill, and very carefully cleaned the tuning scale. I was now left with a gleaming scale and grille and a dull but clean case. I polished up the case with Greygate polishing paste No5 which restored some colour, then 2 coats of colron finishing wax (I know some people prefer not to use a wax polish because of residue but I love the shine I get from it and as long as the set is well buffed, residue isn't an issue) Cabinet was reassembled with new dial glass rubbers, ready for the chassis.

First step was to give the chassis a good vacuuming and brush down to get rid of all the dust, dead spiders etc, I then removed the mains RF bypass capacitor (fully intact on this set), replaced/resleeved as necessary any wires with poor insulation, replaced all of the wax paper capacitors, cleaned the volume control and wavechange switch then turned my attention to the scale backlight which ws rather grubby, I removed the whole thing and gave it a good wash in lukewarm soapy water which improved things a lot but decided to give the light tray a quick spray which brought it up nicely, the cellulose diffuser was refitted and the whole assembly was bolted back in place.
So, I decided, we are just about ready for first sound ! A quick double check on my work and some judicious DC tests later proved all appeared to be well so I plugged it in and switched on. I was greeted by the normal operation of the dial lamps which settled down to a rather dim glimmer which was hard to see even in the dark but the set remained obstinately silent.
A bit of probing around later proved that the audio side was working and using the second radio trick proved I had a local oscillator so I started investigating around the I/F stages. Lo and behold, the UF41 wasn't doing anything, with all voltages except the heaters incorrect. A replacement was sourced and fitted, now I had stations coming through if a little quietly and muffled sounding too, also the speaker rattled loudly when turned up. The rattle was the usual Bush DAC90A thing of the corrugated centralising disc coming unglued so that was attended to with some PVA glue and a cocktail stick to get it into the right places. A few checks of the high value resistors revealed that they were all out of tolerance some by as much as 50% so replacements were fitted, this improved the volume slightly but it still sounded muffled. Closer examination of the circuit diagram revealed that I had mis read a capacitor value, C17 (manufacturers cct) I had read as 0.033uf and replaced with that value, in fact it's supposed to be 0.0033uf, I didn't have anything like that value but remembered reading about some mods to the DAC90A that involved this capacitor, the thread was found and a 390pf was fitted instead, I also took the opportunity to change the 75 ohm lamp ballast resistor for a 180 ohm to make the lamps a little more useable but not over run them especially on a hot start. (Thanks to Kalee for these mods). Now the radio sounded much better, plenty of volume and a nice balanced sound with virtually no hum whatsoever.
Before boxing it up I checked the grid volts on the UL41 as I knew this valve can be trouble prone. I measured almost 6 volts ! ! so switched offquickly. Another read of the various UL41 posts and DAC90A posts revealed that bush used pin 4 on the valvebase as an anchor point for other components and that this could give rise to this problem, so, all components were relocated away from that pin, this did make an improvement to the grid volts but still it was over 4 volts so a replacement UL41 was found in my box of valves and this was fitted, grid volts were now down to almost zero (40 mV) although the set sounded the same as before, virtually no hum and undistorted so, after a soak test the chassis was reinstalled and the set now is in daily use in my kitchen.

As a post script to this story, I did notice some instability especially at the higher frequency end of the MW band and an alarming tendency
for the set to be affected by almost anything being switched on or off. I'd done another DAC90A by this time and this set didn't seem so susceptible then I remembered in the later set, I had replaced the Mains RF bypass capacitor so out came the chassis again and this capacitor was installed. It has made all the difference, instability has gone (how, I don't know !) and the set is much less susceptible to things like the washing machine and light switches etc.
As you will have gathered, I now have a breeding pair of these sets and have just acquired the father of these, a DAC90 which will be the subject of another success story in time.

Hope you al enjoyed reading this

Rob

[Nickthedentist]

Your first DAC90A, eh, Rob? I note you now have two, and if you're anything like me, you'll end up with loads of them! I think I've got six now. I paid about £50 for my first one just over 10 years ago (one of my very first eBay purchases), but they've got cheaper since then. I even picked up a lovely white (rarer though not necessarily more attractive) one in an "antique centre" (not known for their bargains) for £40 recently.

They're a treat to work on, aren't they? Those caps are a doddle to change. And the sound is exceptional for such a small set, with excellent sensitivity from the frame aerial - no wires all over the room needed here.

I assume the speaker was OK? The central/inner suspension can become unglued after damp storage leading to very rough sound, but is easily fixed, as a quick forum search will explain.

You haven't mentioned it, but I trust you've checked that the chassis is connected to mains neutral rather than live, just in case someone decides to poke their fingers in the holes underneath, or by some fluke, the speaker grille connects with chassis.

Happy listening,

Nick

[AD360 Rob]

Morning Nick,

Hehe, yes, I have the DAC90A bug now, as you say, they are a joy to work on and the end result is well worth the effort, they do sound great and I get an immense amount of satisfaction in bringing back to life a set that may well have been dormant for decades. The speaker was suffering from the usual problem with the suspension "bellows" having become unglued so I got out the pva and cocktail sticks to cure that.
I did make sure to determine which pin of the input plug was connected to chassis and I have marked the set and original connector so it is really obvious which way round is the safe way ! Of course, if I were to pass any of these on to friends or family, I would replace the original connector with a fixed mains lead, earth the speaker grille and insert a couple of closed grommets into the access holes underneath which hopefully should avoid prying fingers getting where they shouldn't.

Waiting patiently for the DAC90 to turn up !

Rob

[threeseven]

Nice job and another saved. DAC90A's are addictive. I actually started with a DAC90 in perfect original condition, beginners luck. Then a non working but good cosmetically DAC90A found it's way to me and I thought, that will do I have both variants now. Then I found the lure of the different DAC90A types impossible to resist so now I have 6 like Nick!
The jewels are a brown mid type and cream late type, both in their original boxes.
It's easy to see why they were so popular in their time.

[Norman Raeburn]

Hi Folks, I love these radios, easy to restore and work well. A great set to start on as a straight forward restoration. I have a few of them and no difficult to find parts. I remember at the Scottish Meet a few people remarked on my "fetish" for these sets. Best regards, Norman

[crusher19860138]

Every DAC90 i have owned has had to have a new audio output transformer as the original had o/c primary windings.

The DAC90 seems to be harder on the output transformer than a DAC90A,i have not come across a DAC90A with o/c windings yet.

[mark pirate]

A great result!
I have restored many DAC90A's, from nice dusty sets to real rough examples.
They are one of my favourite sets to work on, and rarely have any major problems, bar the usual UL41 and speaker bellows problems.

You need to get yourself a DAC10 to keep your DAC90A's company, a little more challenging to work on, but a nice stylish set.

Mark

[AD360 Rob]

Originally Posted by crusher19860138

Every DAC90 i have owned has had to have a new audio output transformer as the original had o/c primary windings.
The DAC90 seems to be harder on the output transformer than a DAC90A,i have not come across a DAC90A with o/c windings yet.

So far with Dac90As, I've a 50% success rate with transformers, my second set having the usual open circuit primary winding. Luckily I found a good one for sale online.
I'm hoping that my first DAC90 (when it arrives) will have a good transformer especially as the work colleague that use to rewind transformers has sadly passed away, all I know about that set is that it has a non original back cover and looks pretty scrofulous, I'm hoping that it is unmolested inside, time will tell.

Originally Posted by mark pirate

You need to get yourself a DAC10 to keep your DAC90A's company, a little more challenging to work on, but a nice stylish set.

Hi Mark, I'm thinking of getting a DAC10 at some point although they do seem a little more expensive than the 90a especially for one with a good un cracked white face. I'll keep looking though.

[Phil G4SPZ]

Nice work, Rob, and I love the way the Bakelite has come up. I agree that wax polish can really make a difference after using Greygate Polishing Paste, and I use it myself.

I have a 3-waveband AC91, which is less popular than the DAC90 and 90A but it does benefit from cooler running due to having an autotransformer, and a white Bakelite speaker grille - no need for earthing.

[AD360 Rob]

Hi Phil and thanks for your encouraging words.
I too love the deep shine I get with a good wax polish although my second DAC90A has a much lighter coloured Bakelite, it's almost a chocolate colour and doesn't shine as much, it appears to have faded perhaps due to UV exposure. Believe me I have polished it to within an inch of its life and still can't get it any better. Still looks good though.

I've just taken delivery of what I thought was a DAC90 but it turned out to be an AC90 in black too ! I'm looking forward to getting stuck into that set and a write up will appear on here when I've done it.

Rob