DAC90A tuning problem

[Gordonc]

Hi guys,

The newbie here again.
I am currently attempting to restore my second Dac90a. It was in a real state when I removed the rear panel. The mains filter cap had given up the ghost in the usual spectacular manner, the dial lamp bulbs had blown and the tuning cord had snapped. Some of the wiring around the dropper had perished so needed replacing. Having repaired all of this, (the tuning cord was a real pain!), I then replaced all of the wax caps and fitted a new smoothing cap.

I powered the radio up via my lamp limiter and it worked, well almost! I could get some stations loud and clear but here's the problem - between 600 and 425m (MW) -1900m to 1550m (LW) I get absolutely zero! The tuning breaks up with a fierce crackle when the dial reaches about 425m and then nothing! Do I need a new tuning cap?

Any advice would be appreciated.

[ms660]

The symptoms you describe sounds like the rotor plates are touching the stator plates, check the shaft for any play and also check if the outer vanes of the rotor are of the split type, if so they may have received a small "knock" at sometime.

Lawrence.

[vinrads]

Hi It sounds like the tuning capacitor is shorting out. Turn it full open and look for bent vanes, a good tip is always fully mesh the tuning capacitor when working on the set. Mick.

Lawrence just beat me to it!

[ms660]

Yes, thats a good tip Mick (closing the vanes)

Lawrence.

[Ed_Dinning]

Hi Gordon, if it's irrepairable I may have a spare over at Newcastle you can have.

Ed

[pichacker]

Add to the other excellent comments... with the set unplugged use a multimeter set to the ohms range and connect it between chassis and each of the fixed vanes in turn. As you rotate the condenser you'll find which half is shorting and at what point. It may then make it easier to go in search of the fault.

I bet your rubber grommets are perished so replace these whilst you're in there

[Gordonc]

Hi all,

Many thanks for all of your excellent advice! I have now cured the problem! It was a shorting problem as you all rightly surmised. I discovered through close inspection and applying the meter as advised that a couple of the vanes were touching. I gently eased these apart and it worked. I now have reception along the full range of the dial!

Thanks guys!

Ed, many thanks for your very kind offer of a replacement tuning cap. It is very much appreciated!

Regards,

[ms660]

Just a note, bending the plates can affect the tracking, but if your reception is good across the bands then I would leave it at that.

Lawrence.

[ed731pdh]

As I have found out, fine dust, especially of the metallic kind, can cause absolute havoc on a tuning gang. High pressure air and a squirt out usually cures all.

[paul1956]

What kind of apparatus could forum members recommend, please, to blow out high pressure air?
I need to do this with a couple of old sets I've got.
Would some sort of hairdryer be sufficient, or is it more technical than that?
Thanks, Paul.

[Darren-UK]

Hairdryers are not much good; the pressure is too low and not focussed enough. A handy thing to have is a bicycle pump; the older type with the screw-in flexible tube. That'd give you a fairly powerful, focussed, jet and, of course, needs no power supply.

Alternatively, the likes of Maplin sell 'air brush' cans; literally compressed air in an aerosol type can.

Whatever route you take, goggles are advisable.

[Station X]

Beware of using high pressure air near a tuning capacitor. If you're not careful it''ll bend the vanes.

[Bruce]

With a caveat re the need to be careful not to blow fragile components to pieces through too much high pressure air, there is nothing like a compressor to remove dust - more air and unlike canned air you only pay for it once!

Here in Oz there are a lot of small Chinese made air compressors on the market at present and I'm sure this is the same in the UK. Many of these seem to have fairly poor head gaskets which "blow through" internally as they become oil saturated and allow seepage between intake and outlet, and so their effective pressure drops they are useless for their intended purpose of painting or operating air tools. As a result they end up being sold on auction sales or at car boot sales where they sell very cheaply. Even with their lowered output pressure these still remove dust from electronic kit very effectively and if you want even more pressure it is only a matter of removing a few bolts and hand cutting a gasket from a piece of gasket material.

Bruce

[Gordonc]

Hi guys,
Having taken on board all of your valuable advice, I have now sorted the tuning cap problem, and can tune to many stations now! However, the sound quality is very poor, with a lot of 'static' and voices sound like what I can best describe as sounding like Doctor Who Daleks!

Any advice or suggestions as to how I can cure this would be appreciated.

[Sideband]

As you have replaced the waxies and the smoothing cap, do you know what state the UL41 is in? These can suffer from internal leaking between electrodes and cause all manner of sound problems. You can sometimes improve things by isolating any connections to (I think) pin 4. This is a blank pin within the valve but is sometimes used as an 'anchor' for other parts.

Until you have established the cause, It would be better not to run the set for too long. If the output valve is causing the problem, the extra current drawn could cause the output transformer to fail.


Rich.

[kalee20]

Isolating Pin 4, and also replacing the wax capacitor feeding the UL41, should be done as a matter of course. Almost certainly, this will result in an improvement. Don't go buying and fitting a replacement UL41!

[Boom]

Check the loudspeak speech coil dust cover these have a habit of coming loose and causing Daleky audio. Easy job to reglue.

[ed731pdh]

Ground rules for using compress air, I usually use around 15 to 20psi and start blowing from around a foot away from the set, going in gently to get into all those odd nooks, whilst using a small paint brush to loosen up any stick bits.

[kalee20]

Before using 'blow' from a compressor, I use 'suck' from a vacuum cleaner. A small (1/2") paint brush works wonders to sweep dust, used in conjunction with the vacuum.

After that, use a compressor to shift anything that the paint brush can't reach.

This way round, you're not creating a dust storm, and possibly blowing lots of dust where you don't want it to end up!

[ed731pdh]

That's why I do it in the garden, with a bit of a wind blowing. Problem dust is relocated elsewhere.