Majestic radio

[vinrads]

Having been in touch with Terybull to see where this radio was in Nottingham , I am pleased to say it now resides with me , I was only too pleased to give the Heart Foundation £50 for it. It now appears to be in bits, just could not help myself, you know how it is the latest purchase takes priority.

It turned out to be a good day to open the huge metal smoothing blocks, with it being so hot the things came apart easily.The big problem is the four gang tuning capacitor which has suffered from the deadly Mazak! problem and will req some major surgery. Watch this space!

Mick.

[terrybull]

Excellent news Mick.

[Herald1360]

Originally Posted by vinrads

Watch this space!

Mick.

Can we watch it through some pics?

If you haven't found it, the service info is in Nostalgiaair under Grigsby-Grunow 72.

It'll be interesting- an all triode directly heated (except for detector) TRF with p-p output and lots of audio transformers.

[vinrads]

Thank's for that Chris some pictures , the main thing is what to do regarding the tuning capacitor ,repairing is out of the question , my thoughts were to fit four single units and somehow connect the together , any thoughts welcome ,Mick.

[Boater Sam]

A pair of 2 gang?
Would it be impossible to pot the fixed vanes in epoxy using the moving vanes with spacers between as support, then connect them all together?
Or use conducting epoxy?

[vinrads]

This was my original idea , to fit a threaded rod through the fixed vanes but there is no room to do this , I like the idea of using epoxy resin , but it looks like the moving vanes are also deteriorating ,so I think it is well and truly beyond repair . Mick.

[TonyDuell]

Do you know anyone (model steam engine builder, for example) with access to a small milling machine? They could probably make the mounts for the fixed and moving vanes from a more stable metal.

[Freya]

My dear departed friend made some very similar alloy blocks with slits in using his mill with a slitting saw. Although not for the same application the block had alloy plates secured in the slits using a tiny punch.
He used a length of material cutting the slits along the entire length then cut into blocks, took about 20 mins from start to finish.

[PJL]

Another solution is to use epoxy repair putty. Get the vanes out. Cut spacers to set them to the correct position and clamp. Ideally the spacers would be cut to serve as a 'mould' for the new 'mazak'. Hand mould the replacement for the mazak onto the vanes. Repair putty is very easy to work so its best to make it bigger than required. Once done coat it with silver paint (expensive but it goes a long way).

The ones on the shaft don't seem too bad and a coat of varnish would stabalise them.

[vinrads]

Thanks for that you have just answered my question, how to stabilize the rest of the Mazak.

Mick.

[Herald1360]

Thanks for pics- looks like an interesting challenge.


Four-gang caps were used in things like HRO and AR88, though whether they'd fit?


The two two-gang idea could work fine if they're physically a bit smaller. Join the two shafts in tandem (tapped holes in shaft ends and a male-male spacer locked in with screwlock, perhaps) then mount the array on a suitable block or plate to pick up on original mounting holes and line shaft up with tuning drive mechanism.

[mark_in_manc]

Would it be possible to make ali shims (you might need a lot, in which case a bandsaw and an abrasive belt would be handy) to separate the vanes by the required amount, and then clamp the lot up, drill though and hold them together with a nut and bolt? Or even use penny washers to space the vanes, if there were enough depth to them to get hold of them like that?

[Ed_Dinning]

Hi Mick, I've used superglue fed into the cracks and holes in Mazak to successfully stabilise it when it was cracking up.

Ed

[vinrads]

After considering all other options , I am going to attempt to repair the original unit , I have some 8 BA rod on it's way and loads of washers and nuts .My plan is to drill a small hole where it wont interfere with the working of the capacitor in each vane fix them together using washers as spacers , not to difficult on these old units they tend to be fairly wide spaced . some thought is required as to the fixing of each capacitor to the main assembly , meantime I have the radio working loads of re wiring and four resistors o/c also one of the rf amp tubes o/c heater number 26 . A sub valve was used to chech that all was OK , and it was it now talk's and sings again , just need to get the tuning capacitor sorted . Mick.

[Boater Sam]

Fantastic!
Now that you have got so far, you just have to remanufacture the tuning condenser.

To get the spacing, could I suggest using the moving vanes to set the spacing and foam washers on the threaded rod as spacers on the fixed vanes? Otherwise your metal washers will need to be exactly the right thickness which may be tricky.
Tightening your 8BA rod will then close all the gaps equally, paper slips in between fixed and moving vanes to square them all up.
Then, when all adjusted and square, set the back ends of the fixed vanes in epoxy, Plastic Padding, Milliput or something similar.
Remembering that all must be electrically connected, it may be easier to connect them after assembly with a crossbar soldered on, aluminium solder is available.
Its an interesting problem that will need an inventive solution, real engineering.
Sam.

[vinrads]

Thanks for the encouraging words Sam. Mick

[Boater Sam]

A further idea, given my former profession, set them in molten lead, electrically connected, easily reversible.

[vinrads]

Do you know that was my thought earlier today , thank's Sam . Mick.

[vinrads]

Having decided to repair the tuning capacitor, I came up with the idea of fixing the plates together using 8BA threaded rod and spacers. I made a jig fitted it to my drill press so as to make sure all the 72 holes were aligned correctly, this done a piece of rod was cut over size a lock washer and nut fitted first then the plate ,then spacer I was very fortunate to find some nuts which were the correct size, having fitted all nine plates a lock washer and nut held the assembly together.

I think the hardest job was removing the plates from the mazak it looked crumbly but it took some time to remove all the plates the little tab left was the place I needed to drill, no room for error here, hence the jig. A brass plate was soldered to the nuts drilled and tapped to 4BA as a fixing for the assembly.

Several hours later I had all four units fitted and adjusted, to clear all shorts I did this using my mini 12v drill in series with the tuning capacitor, so I could hear when it shorted and also in the dark I could see a spark , very useful to pinpoint the exact place where the short was.

A quick alignment and the set was now working, with a new valve in the first RF stage the performance was very good. Some pictures of the chassis installed in the cabinet to follow.

Mick.

I wonder what my next challenge will be?

[Boater Sam]

Damn fine job Sir!
Patience and persistence will always prevail.
Sam.