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Old 27th May 2017, 5:05 pm   #1
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Default Westminster Radio Model C.T.A.5350. Restoration.

As mentioned in another thread I'm restoring a pair of these sets. They have serial numbers which are wide apart ie 03485 and 07299. The earlier version has a mixture of waxies and metal cased capacitors, but the later version has all waxies.

I decided to replace the tone correction capacitor C13 which is a waxie. I'd expected to find this wired across the output transformer which is mounted on the speaker, but in fact it's wired to the base of the output valve.

When I looked carefully I saw that it's wired between pins 2 (anode) and 3 (g2) of the 7C5 in both sets ie between anode and screen grid. I couldn't believe this and checked several times, but it's definitely true.

Pin 4 of the 7C5 is shown in datasheets as an internal connection, but the valve doesn't actually have this pin. Pin 4 is used as a termination point for C14, the output transformer primary, C13 and R15. So I'm thinking that the tone correction capacitor should in fact be wired between pins 2 and 4 which would make things correct as per the diagram.

Is it the diagram or the actual wiring which is wrong though? Were thousands of sets incorrectly wired?

If I'm to rewire as per the diagram, the easiest way would be to snip out C13 and put a new cap across the O/P TX primary.
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Old 27th May 2017, 5:20 pm   #2
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

In the actual data it shows Nc not Ic:

http://www.shinjo.info/frank/sheets/127/7/7C5.pdf

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Old 27th May 2017, 6:33 pm   #3
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

Wiring between anode and g2 is "odd" but should work OK since g2 is at signal ground- decoupled by its own capacitor. No need to rewire it, anyway.

Possibly whoever did the wiring schedule was thinking of g2 straight to HT which wasn't uncommon.
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Old 27th May 2017, 6:52 pm   #4
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

Thanks Herald.

It's a 0.002uF 1000VDC cap which tests OK on a capacitance meter, but leaks badly when tested with a Megger. I think it would be best to change it.

However "listening" tests with off air signals show that it doesn't matter where the cap is connected or indeed whether it's connected at all. I have plenty of other caps to replace, so I'll leave a decision on this until last.

I'd better read up on what a tone correction cap actually does. I'd always took it for granted that they were connected anode to ground, or across the O/P TX primary and that was that.
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Old 27th May 2017, 7:00 pm   #5
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

If it doesn't have any audible effect it may be there just to stop fast interference spikes from overstressing the TX primary.
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Old 27th May 2017, 7:09 pm   #6
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

The lack of apparent difference when listening might be down to my hearing.

I guess I could modify one set and not the other, then run a frequency check on signals injected at the gram socket.

Actually thinking about it, a record would have more high frequency content than an off air signal? Although this set was built in the 78 era.
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Old 27th May 2017, 10:00 pm   #7
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

78's generate fast transients from scratches and the like.
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Old 27th May 2017, 10:26 pm   #8
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

I don't have any 78's or anything to play them on.

Tone correction capacitor aside, this restoration was going well until the set went completely dead. A few checks showed that the tag of the valve holder carrying heater current to the AF valve had snapped off. A bit or gentle probing showed that a couple of other tags on the holder were hanging on by a thread.

So tomorrow it'll be a new valve holder if I have one. I see that NOS valve holders exactly like the originals are still available, but I wonder if they too have corroded?
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Old 30th May 2017, 10:32 am   #9
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

A quick update on this.

I'm waiting for some new valve holders to arrive. I couldn't do the usual trick of using unused pin sockets to replace faulty ones as it's not easy to remove the pins. The holders are of sandwich construction with the pin holders sandwiched between two pieces of paxolin. Drilling out the rivets doesn't allow the two halves to be separated as they're also held together by the centre spigot which serves as an eyelet.
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Old 10th Jun 2017, 10:05 pm   #10
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

This afternoon I finally got round to replacing the faulty valve holder. This got the set working again and it gave a good account of itself.

The problem I have now it that moving the tone control away from the fully ACW position makes the sound unbearably shrill. I'll investigate that on another day.
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Old 20th Jun 2017, 11:11 am   #11
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Default Re: Westminster Radio Model C.T.A.5350. Restoration.

Coming back to the shrillness. In post #4 I said that it made no difference whether or not the tone correction capacitor was fitted, or whether it was connected across the primary of the output transformer, or between the anode and screen of the output valve. I left the replacement fitted across the primary winding of the output transformer as that was the easiest place to fit it.

Removing the cap made no difference to the shrillness, but reconnecting it between anode and screen grid made a vast improvement. The shrillness is now only audible on my local "Smooth Radio" MW transmission. However this is true of most of my radios and can be cured by putting an attenuator in the aerial lead, or by just shortening the aerial.

A quick alignment check showed that tracking was perfect on MW and SW, but way out on LW, with BBC Radio 4 appearing around 1400 metres. All the RF trimmers and inductor cores were sealed with wax, so the phantom twiddler wasn't responsible. The LW LO trimmer was already fully screwed up and I had to add a fixed 5pF capacitor across IT to get the alignment correct.

The drive cord from the pulley on the tuning capacitor shaft to the scale pointer was original, but so tight it was distorting the "gantry" carrying the pointer. I assumed the cord must have shrunk. I renewed the cord with angler's backing line.

The white paint on the "reflector plate" behind the tuning scale was flaking off and the plate itself was rusting. I cleaned it up with wire wool and sprayed it with two coats of zinc primer followed by two coats of white. The pointer received similar treatment and was sprayed red.

At some time in it's life the cabinet had been fitted with spindly legs and painted orange. Only the threaded mounting plates for the legs remained, so I removed them. This left sixteen holes in the cabinet, but as they're underneath I didn't bother to fill them. Quite a lot of orange pint still remained, mainly on the speaker louvres. I spent several hours carefully scraping it off. The felt washers behind the knobs were heavily impregnated with paint, but I was able to remove most of it by carefully trimming their edges. Unfortunately some paint had penetrated the rather nice back lit Westminster badge which appears to be of laminated construction. I was unable to remove this.

The cabinet and chassis were reunited, using washers cut from rubber sheet between the two. The set sounds very nice with its large diameter speaker.

All I have to do now is make a new back using that from a second set as a pattern.
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