AR88 recapping - Micamolds - All or just some?

[Martin G7MRV]

Hit a small snag while creating an order for capacitors to replace the Micamolds in this AR88, in that two values 2n7 and 3n9 seem unavailable.

Two of these are C116 to the AF gain pot, and C111 to the grid of V10 the 1st AF amp. Im thinking for these the closest standard value will be fine?

But two others C24 and C30, are associated with tuned circuits. Im thinking the correct value might be important with these.

So this has got me thinking - is it worth at this stage hunting these down and replacing them? Or, should I concentrate on coupling and bypass caps only and wait and see how the others fair?


Martin G7MRV

[paulsherwin]

I've no experience with an AR88, but Micamold is just a brand not a method of construction. As a general rule of thumb, anything between 0.1uF and 0.001uF from that era is likely to have some sort of paper dielectric and so is suspect, while anything smaller will be ceramic or mica so unlikely to be bad.

As you say, the precise values of AF couplers and decouplers aren't critical.

[Martin G7MRV]

Yes that was my take on it, 1nF and up crappy paper. From the parts list most caps lower than that - marked in pF - are listed as tubular ceramic. Its just these two at 2n7 and 3n9 that are slightly worrying me. I might just leave those off the list for now while I deal with all the others and worry about them if theres an issue later. Looking in the set, I can see mostly the ceramic tubes in the RF sections but there are a few suspicious looking pink lozenges marked in what I think is pF, im not sure what brand or construction those are.

The 2n7s i'll use 3n3 instead

Also looking now at what to replace the smoothing 4uF caps with, looks like these also need >500v ratingto be safe

[paulsherwin]

2n7 and 3n9 may well be ceramic. I'm much more experienced with domestic designs where component cost was a major factor in selection. The AR88 was an expensive professional comms receiver with much less penny pinching.

Generally speaking, caps in tuned circuits don't have many volts across them so a bit of leakage won't have much effect.

[Martin G7MRV]

Well they are in the depths of the RF sections which im hoping not to have to delve too much into!

The order is now in with Mauser - still cost £50!!! Ive added in the caps I might need for the Wartime Civilian Receiver as well, although this may have had most done already. I wont know until I can get the volume knob off and the chassis out. Next job today is extracting the broken screw!

Could have saved a quid each on the 4uF electrolytics, if only i wanted the minmum order quatity of 620...

[turretslug]

Originally Posted by Martin G7MRV

Two of these are C116 to the AF gain pot, and C111 to the grid of V10 the 1st AF amp. Im thinking for these the closest standard value will be fine?

But two others C24 and C30, are associated with tuned circuits. Im thinking the correct value might be important with these?

Yep, I used 3n3 for the two AF amp circuit replacements, but as C24 and C30 are oscillator padders, they need to be the listed value otherwise front-end ganging and tuning scale accuracy will suffer.

[paulsherwin]

If you change the caps padding tuned circuits then you will almost certainly need to do a full realignment. You may or may not find this a daunting prospect (and you may be planning to realign anyway, of course).

[Radio Wrangler]

There aren't any young AR88s out there, and with age, comes increased probability that they've been through the hands of some phantom twiddler.

They run high Q's in the RF tuned stages in a frantic attempt to get any image rejection available on the upper bands. The AR88s secret weapon, polystyrene coil formers were deployed in this pursuit. This all makes alighnment more and more critical.

I reckon a good careful alignment will probably be beneficial and certainly after any intrusion in the RF boxes. Some decouplers also wind up effectively in series with resonators and this can have an effect.

Micamolds? get rid of the whole lot or you'll keep having to dig in to change them as they go bad.

Look into high value resistors, and any buried in coil pack boxes. The G2 ones are notorious. They go high in value and you wonder where the gain has gone and blame valves. They can also go noisy.

A well-sorted AR88 is a lively and sensitive set.

If you can organise a sweeper/wobbulator/scope you can have a much easier time setting up the IF. Ignore the RCA manual. Crystals have aged a bit by now, set up the narrow crystal bandwidths because they have to suit the crystal, and then set up the wider LC bandwidths to centre where the crystal dictates. Now you can change bandwidth without having to retune and CW signals don't vanish as you switch the set narrower. Life is easier!

David

[Station X]

Originally Posted by Radio Wrangler

Crystals have aged a bit by now, set up the narrow crystal bandwidths because they have to suit the crystal, and then set up the wider LC bandwidths to centre where the crystal dictates.

David

Indeed. I believe there's an EMER describing how to do this.

[Station X]

Here it is.