Yet another DAC 90A rebuild.

[medge799]

Good grief Nicko, you put my miserable efforts to shame! Can't wait to see the final product...

Ta,
Mark

[vinrads]

Now that's what I call a total restoration job, like Mark I cannot wait to see the finished job.

[Ian - G4JQT]

Hi Nicko,

I've done just that to three DAC90As. Rewarding isn't it? But however did you get the valve holders looking so good? And the variable cap? One of mine was badly corroded and no amount of caustic soda really sorted it out cosmetically.

And 'passivation'? In a nut shell (or a url) how do you do it at home? A (very) quick Google didn't really throw much light on the matter.

Cheers,

Ian

[Nicko]

Originally Posted by Ian - G4JQT

Hi Nicko,

I've done just that to three DAC90As. Rewarding isn't it? But however did you get the valve holders looking so good? And the variable cap? One of mine was badly corroded and no amount of caustic soda really sorted it out cosmetically.

And 'passivation'? In a nut shell (or a url) how do you do it at home? A (very) quick Google didn't really throw much light on the matter.

This is only my second ever radio rebuild - I have a huge amount to learn and I know this is not one for the purists as I am replacing all the damaged components and completely re-wiring. However from above the chassis it'll look much the same except that the mains decoupling capacitor will no longer be there (I've changed it to being an X2 and on the back of the switch for safety).

I wasn't originally intending to take the valve holders out, but the chassis was heavily pitted by corrosion and I couldn't get a good "brushed" finish on it properly with them in place, so I drilled the rivets and removed them. This allowed me to make the long, straight, strokes with a sanding block plus various grades of wet&dry (lubricated with 3in1 or water) to take back the surface and remove the worst of the pitting - there is still a small amount of pitting left

Everything was de-soldered using a Metcal DS1 de-soldering gun on a MX-500 base station - this removes the bulk (but not all) the solder on the joint - the windings at the end of the components were then unpicked with a stainless-steel dental spike whilst keeping the remaining solder liquid with an MX-RM3E using a power tip (in my other hand) - I HATE having to unpick knotted & soldered joints Each tag then had all the remaining solder removed. This leave all tags undamaged and completely clean.

The pile of components were then placed in batches in a heated (55C) 100W ultrasonic tank using Tikopur R33 at about 3% - I did a thread on this before: https://www.vintage-radio.net/forum/...ad.php?t=68929. Tikopur R33 has a pH of 9.9 so it's quite alkaline - after cleaning you need to wash with deionised water then to passify the surface you wash with dilute nitric acid & hydrogen peroxide mixed, followed by more rinsing with de-ionised water - this removes impurities from the surface of the aluminium and allows a clean oxide layer to develop - see: http://en.wikipedia.org/wiki/Passivation_(chemistry). This is one of the processes used in anodising - I have the chemicals for this for use in other hobbies !! (too many hobbies, too little time...)

Items that needed it were then phosphor-bronze & glass-fibre-pen brushed to remove remaining marks, polished with 0000 wire wool, and put back in the tank.

I have found that with care (and experience) valve sockets (ceramics etc.) and most items can be cleaned up well this way, however ultrasonic cleaning is very aggressive and items left in too long or on too high a power can be irrevocably damaged - you need to experiment for each type of component. Ceramic valve bases clean very well this way - I also use "inter-dental" brushes to clean each socket pin individually (they are an excellent tool for this). Note that if plated items are left in too long the tank will strip the silver plating - you have been warned! As for the variable cap - it was a bit fiddly but came up like new - I re-greased the bearings using some waterproof mountain-bike PTFE-loaded bearing grease - useful stuff.

[Nickthedentist]

This is fascinating, and very impressive, Nicko, but very unlike the "restoration" that the majority of vintage radio fans would do.

I think I'm right in saying that most of us would aim for a radio like this to work perfectly and look like it's spent its 55 years in everyday use, being lovingly dusted every week. Some would aim for total authenticity with replacement parts being disguised to look like the old ones, others (like you and me) would be happy to use modern parts in areas that aren't visible such as under the chassis. Dust and grime would be removed, but minor wear and tear, and the dulling of metal chassis etc. that comes with age would be accepted and left as-is.

What you are aiming for is probably more like what some vintage car fanatics do, i.e. restoring it to look like the day it left the factory.

I look forward to seeing the end results.

Nick.

[Sideband]

Originally Posted by Nickthedentist

T
What you are aiming for is probably more like what some vintage car fanatics do, i.e. restoring it to look like the day it left the factory.

Yes it's called 'over restoration'! This sort of thing is fine with a DAC90A as there are still zillions of them about. Like you say, most people clean all the loose dirt and dust off and treat all the bad rust but anything with just normal wear and 'patina' is left to look original.

Anyway looking at the photo's it make you realise just how many component parts there are in a simple radio.


Rich

[Nicko]

I somehow thought this might happen

I deliberately didn't use the word "restoration" - I used "rebuild" (ibid.). This is pretty much a "commodity" radio and was in shocking condition when I got it - every cap was leaky, way out of spec or had exploded (literally), every resistor was way high, the chassis and other components were heavily corroded as was much of the wiring, the output transformer primary was o/c. Someone had previously had a go at the chassis. Mice had made a home there. In short, it was a real mess.

There is a choice at this point - do you do a minimal restoration which will keep it going for a bit longer until something else fails, or make it work reliably for the next 60 years?

I chose the latter - I restore other items, notably old tube amps, sympathetically, but in this case I decided on a rebuild. It's never going to be sold - far too much effort has gone into it - it is a learning exercise for me - I have absolutely no pretensions about this being in any way a sympathetic rebuild - that's just the way it is ! Anyway, it's got kaylee20's mods as well and I did the transformer rewind "sympathetically". What's more, every mechanical and electrical component that can be re-used, is being used...

The use of words/phrases like "fanatic" or "over restoration" are in themselves pejorative, they're just someone else's view - what about those who carefully re-stuff old wax capacitors? Fine by me, if that's what you want to do; others might consider it just a touch obsessive, no? I suspect that one man's "patina" is another man's "deep corrosion" !

Anyway, I'm having a bundle of fun with this - it'll be lovely in the house, used every day (via a local upower AM xmtr) and SWMBO loves it

Cheers

[HamishBoxer]

Looks mint to me.

[Nickthedentist]

Originally Posted by Nicko

The use of words/phrases like "fanatic" or "over restoration" are in themselves pejorative...

I certainly didn't mean this in that way. I consider myself to be a radio fanatic!

As you've said, yours was a mess and there are a dozen of these listed on eBay all the time, so what you're doing is a fun, rewarding exercise, if not everyone's cup of tea.

I would be interested to hear how long the whole re-build takes.

Nick.

[Sideband]

No it wasn't meant in any derogatory way at all! In the case of your particular radio as you say, it was a choice and you are right in it being a commodity radio. I do restuff capacitors sometimes, like in an original 1934 Philips radio that I have. In that case I decided that everything should be kept to look original. Later sets and certainly from 1950's onwards, I'm not so critical. Exceptions would be round Ekcos if I was ever lucky enough to obtain one.

It will be interesting to see the end result. Hope you get the result you want.

Cheers,
Rich.

PS Just a thought; what if you have any odd bits left over?

[Nicko]

Originally Posted by Sideband

No it wasn't meant in any derogatory way at all!

Originally Posted by Nickthedentist

I certainly didn't mean this in that way. I consider myself to be a radio fanatic!

Absolutely no offence taken guys! I was just making a point about variable interpretations.

Progressing slowly and steadily, waiting for a few bit and bobs still and doing some research.

Too many projects, too little time.

Cheers.