Yet another DAC 90A rebuild.

[Nicko]

Got one at a boot fair - only my second attempt at a rebuild (last was a Vidor Lady Anne).

Decent nick - all heaters OK but main bypass cap has exploded and made a mess, so all caps and resistors out and replaced - basically I'm doing a kalee-style rebuild. Speaker is fine, case very clean and all knobs etc. ok-ish. Has original mains lead and plug/socket.

Going to completely strip it, then ultrasound clean chassis and some of the bits (variable cap etc.).

Advice welcome (I need it!).

Cheers

[Nicko]

First question; the original back panel is warped, it's a sort of poor quality fibreboard.

Can this be flattened by any clever technique? Maybe dampening it and leaving it under a pile of books?

Thanks

[Darren-UK]

Have a look at this thread. You'll see moisture isn't recommended, although I personally have straightened warped backboards with steam followed by cooling with the board sandwiched between two offcut bits of planking, weighted with a brick. No problems were encountered.

[Nicko]

Yes, I put a clean tea-towel on the ironing board, put the backboard face down on it and misted it with the iron so it was damp, but no more than that.

Steam iron set to cotton (very hot) + steam, put another tea towel on top of the backboard and then very slowly (with lots of steam) went over the board softening and ironing it, occasionally misting the tea towel if it dried out, took about 5 minutes. This way the dampness is on the back, not on the visible face and so does not affect the exterior-facing finish or lettering.

Then placed tea towel on a flat table top, put the hot backboard face down onto it, another tea towel on top, then a hard flat placemat to ensure even pressure, then a sandstone flagstone! (over 10kg) Leave to cool (I also put a little bit of PVA into the "frayed" bits of the backboard round the screw cutouts to re-bond the fibreboard).

One hour later, perfectly flat and rigid backboard with no frayed edges. Very happy with that.

Replacement valves (spares), new scale bulbs, some new caps and other bits all arrived in the post today.

Case and knobs washed, T-Cut, Paste No. 5 + a finish of Liberon Black Bison Neutral (two very fine coats - first with 0000 wire wool). Lots of elbow grease, cotton rags and old toothbrushes.

I could get to like Bakelite. Never understood what people saw in it. Do now.

[ThePillenwerfer]

Sounds like it's coming on well.

- Joe

[Nicko]

Here's another question - are the bolts used in this radio BSF or Whitworth or what? and if so, what sort of sizes?

As this is only my second radio restoration (and the other one didn't involve much cosmetic/replacement of hardware work), I was just wondering.

Thanks

[Top Cap]

They will be BA. I do not have many of them myself but thankfully you can still buy from Internet. It's the awkward sized self tappers used to secure cabinet backs using spring clips that really give me a headache and they always seem to be missing.

[julie_m]

You could drill out the holes and re-tap with the next size up BSM. It's a bit naughty, but pragmatic: metric threads are used all around the world, but BA fasteners are in finite (and dwindling) supply.

It's the awkward sized self tappers

I have found some American screws fit these.

[Nicko]

Found my box of BA bolts etc., so ok with that now thanks.

However, the OP transformer primary is O/C - secondary is 0R9, but primary has gone AWOL.

Has anyone a replacement I can buy? - I would love to rewind this one, but just don't have the time (though I have a lovely Morris Coilmaster coil winder!).

Having said that, if I use thin Kapton tape, are there any winding instructions for this transformer anywhere? I remember reading the ratio was about 45:1?

The other thing is that the rubber support for the glass sclae has perished - I thought of using 3mm wide grommet strip, but is there anything better?

Thanks

[Boom]

Originally Posted by Nicko

The other thing is that the rubber support for the glass sclae has perished - I thought of using 3mm wide grommet strip, but is there anything better?

Thanks

The stuff used on greenhouse glass. Ideal and cheap from your local garden centre.

[mickjjo]

Mike Barker does DAC90A output transformers:-

https://www.vintage-radio.net/forum/...ad.php?t=85685

Rewinding data here:-

https://www.vintage-radio.net/forum/...3048&styleid=2

Regards, Mick.

[michamoo]

The other thing is tat the rubber support for the glass sclae has perished - I thought of using 3mm wide grommet strip, but is there anything better?

I just use the outer sheath of a piece coax cable cut to length and slit down one side for that job. Looks ok if you use black.

[Nicko]

Thanks for the help so far -

What I am now checking on is that this is a live chassis (my forst), so I really really don't want to blow up my test kit.

I was contemplating testing the radio on an isolation transformer and then earthing the chassis for safety and to allow my test kit to use the chassis for ground as well.

Is this a sensible approach? I would then inject signals through a 50pF 630V cap or something similar to protect my sig gen. 'scope likewize - I have a HV probe, but that''s really for 10kv+.

Tektronix P6139s (my "probe of choice") are supposedly good to 300V so will that be ok to protect the 'scope (Tek 2645B)?

Thanks

[ThePillenwerfer]

I think you may be meeting trouble before it's even set off.

Unless it's been messed with you probably won't need to put a sig gen on it and a 'scope is HIGHLY unlikely.

If you do need to realign it you can 'Inductively Couple' the generator to it, ie no direct connection.

- Joe

[Nicko]

Originally Posted by ThePillenwerfer

I think you may be meeting trouble before it's even set off.

Unless it's been messed with you probably won't need to put a sig gen on it and a 'scope is HIGHLY unlikely.

If you do need to realign it you can 'Inductively Couple' the generator to it, ie no direct connection.

Not that I was ever a Boy Scout, but I do like to "be prepared" !

Its a pretty sorry DAC90A internally (case is good) - all resistors & caps replacing (except in the IFTs), OPT shagged (rewinding that this w/e) - doing Kaylee's mods etc. - some bits needing ultrasonic bath (tuning cap) etc., so I am just preparing the ground...

I also have a fairly high "self preservation factor" - not worked on live chassis before and like to be (and feel) safe (one of the reasons I refused to work on colour TVs).

I have the isolation xfmr so was just wondering if that approach would be ok...

[Boom]

Originally Posted by Nicko

I was contemplating testing the radio on an isolation transformer and then earthing the chassis for safety and to allow my test kit to use the chassis for ground as well.

I'd be a bit careful. The DAC90A does NOT have the neutral of the mains lead connected to chassis. It goes via the dial lamps.

This might cause confusion or problems if you connect other equipment to the isolation transformer.

[kalee20]

Use your isolation transformer for testing, but don't earth the chassis.

If your isolation transformer is 'good', the secondary will be isolated completely. You'd have to make two connections of yourself to the radio to get a shock. If you earth the chassis, you'd only have to make one connection of yourself to the radio - the other would be then be up your feet from the floor (earth) / bench / whatever.

Using mains earth as a test equipment return is iffy, as the path is prone to pick up interference on the way. Test equipment 0V return should be as direct as possible.

[Nicko]

Originally Posted by Boom

I'd be a bit careful. The DAC90A does NOT have the neutral of the mains lead connected to chassis. It goes via the dial lamps.

This might cause confusion or problems if you connect other equipment to the isolation transformer.

Understood, but if the radio is the only thing on the xfrmr and the chassis is at GND, then the neutral (which it isn't really as its isolated) will float approx 7v (2 x 3.5v) BELOW GND (the chassis).

All the test kit is off ordinary mains where neutral is ground-referenced. Also, as the mains plug for the DAC90A is not keyed, when not on isolation the chassis could actually be LIVE, hence earthing the speaker grill... (or changing to a fixed mains lead...)

Originally Posted by kalee20

Use your isolation transformer for testing, but don't earth the chassis.

I was going to clip the test equipment probe earth clip to the chassis... I get your point about shocks, but that's the way with normal kit such as valve amps etc.

[Nicko]

Hi,

Completed strip-down today. Chassis was very corroded as were several parts, so did complete strip and ultrasonic cleaned everything (except the IFTs). Passivated the lot, then cleaned up with phosphor bronze brush, wet&dry, wire wool, Servisol, 3in1 etc. - had to "brushed aluminium" the chassis as the corrosion had led to extensive pitting - it came up pretty well considering. #2 son wanted me to anodise it, but that would be a touch obsessive (but not difficult). Variable cap cleaned well and was re-greased - measured very close to original specs on both segments. Backlight cleaned up and re-sprayed and another forum member was extremely kind and let me have a replacement diffuser.

Photos show what is essentially a DAC90A kit including the rewound transformer (covered in another thread) - a complete wiring plan is in the lower right corner of the first photo - Speaker, grille and cabinet also repaired and ready . The rebuild starts tomorrow!

(actually, I'm still short of three rubber grommets to reseat the variable cap - the originals are completely perished and its the only thing I couldn't find in the workshop...)