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Ekco T221, 1954.

 
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Re: Ekco T221, 1954.

Post by Terrykc » Sat Jul 23, 2016 5:34 pm

This probably a stupid questio but why does the CRT have to come out to change the scan coils - or is there something else that I've missed?

 
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Re: Ekco T221, 1954.

Post by Derren » Sat Jul 23, 2016 6:09 pm

Something I was going to ask myself, presumably the angle of the tube or how it is suspended? I was also wondering how common it might be for scan coils to fail as this was a very reliable model was it not. To be expected eventually at this age I suppose, unless something else may be causing it.

 
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Re: Ekco T221, 1954.

Post by Cathovisor » Sat Jul 23, 2016 7:15 pm

Terrykc wrote:This probably a stupid questio but why does the CRT have to come out to change the scan coils - or is there something else that I've missed?

Well, the tube comes out if you have to do this on a Pye B18T or a Bush TV1, it has to be said...

 
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Re: Ekco T221, 1954.

Post by Terrykc » Sat Jul 23, 2016 8:12 pm

Whether I've ever had dealings with this particular model, I'm not sure, but I would imagine it isn't much different to the later models with similar presentation.

It should be possible to remove the chassis easily, in that case, leaving plenty of access to the scan coil assembly and fixings.

If it isn't, perhaps Till can oblige with a picture to highlight the problems?

 
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Re: Ekco T221, 1954.

Post by Cathovisor » Sat Jul 23, 2016 8:35 pm

Terrykc wrote:Whether I've ever had dealings with this particular model, I'm not sure, but I would imagine it isn't much different to the later models with similar presentation.

It should be possible to remove the chassis easily, in that case, leaving plenty of access to the scan coil assembly and fixings.

If it isn't, perhaps Till can oblige with a picture to highlight the problems?

I have the feeling the CRT might be attached to the chassis...

 
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Re: Ekco T221, 1954.

Post by Till Eulenspiegel » Sat Jul 23, 2016 9:00 pm

The CR tube has been taken out but in fact it does turn out that it is possible to remove the focus and scan coil assemblies without having to do so. The four brackets which support the CRT neck components are attached to the front tube clamping band by four 4BA screws. I assumed that the brackets were riveted to the clamping band.
Anyway, while the tube and it's associated components are out of the cabinet it's an opportunity to clean them up. Also, a right side support bracket was found to be loose, the screws which attach it to cabinet are missing. We do know that the CRT is good and, whether it is or not it is the original Mazda CRM141. It is possible it is a service replacement. The EHT connector is the cavity type known as ct8. Early tubes used a 7mm diameter cap type connector.

Till Eulenspiegel.

 
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Re: Ekco T221, 1954.

Post by Till Eulenspiegel » Mon Jul 25, 2016 8:12 pm

All that effort has proven worthwhile. The replacement scan coils are perfect, just one adjustment to do, the picture is upside down. Height and width to spare and the linearity is spot on as well.
The Mazda CRM141 displays a really bright picture. Because of that displaced CRT mounting bracket it was a bit of a fight to remove the CRT assembly. However, refitting the CRT into the cabinet was easy, those brackets are now correctly recessed into the cabinet sides.
The set was still in use in 1963. The RF amplifier valve was replaced that year with a Mullard PCC84 which has the date code B3D2, April 1963. It's likely the other tuner valve was also replaced in the sixties, it's a 30C1 (PCF80). Later Mazda valves had a Brimar look about them.
The restoration of this set is not complete, the sound is very weak.

Till Eulenspiegel.
Last edited by Till Eulenspiegel on Mon Jul 25, 2016 8:27 pm, edited 1 time in total.

 
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Re: Ekco T221, 1954.

Post by Derren » Mon Jul 25, 2016 8:26 pm

Hi Till,

All sounds good thanks. Yes you are right about the sound, it worked well initially, but faded over the last few months. I think the Visconol capacitor was in a poor state too, perhaps should ideally be removed but am pretty sure it had already been disconnected, I thought it bizarre the way it was mounted/suspended instead of on the chassis. Capable of a super picture this tube.

 
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Re: Ekco T221, 1954.

Post by Till Eulenspiegel » Mon Jul 25, 2016 8:33 pm

Hi Derren,
It's desirable but not essential that the EHT smoothing capacitor is present in the circuit. I'm considering restuffing the TCC visconol capacitor. The capacitor serves as a support for the EHT metrosil, a device to maintain a steady load across the EHT supply.

Till Eulenspiegel.

 
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Re: Ekco T221, 1954.

Post by Till Eulenspiegel » Mon Jul 25, 2016 9:17 pm

From the February 1959 Practical Television magazine: Replacing C. R. Tubes-2.
Details of the Ekco T231 cradle assembly. Note the position of the EHT smoothing capacitor.

Till Eulenspiegel.
Attachments
EkcoT221_4.jpg

 
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Re: Ekco T221, 1954.

Post by Derren » Tue Jul 26, 2016 12:42 am

Well I was thinking of where the Visconol was on the Dynatron TV27, on the chassis, that too was leaking and it had been long disconnected but I think you removed it. Probably the best idea but I do wonder if that is why I do sometimes get varying brightness on that set, never too much in it though and you can easily compensate with the controls. How on earth can you 'restuff' it? Isn't that rather dangerous :)

 
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Re: Ekco T221, 1954.

Post by Till Eulenspiegel » Tue Jul 26, 2016 11:51 pm

We're on the home straight now. I've finally worked out the wiring to the scanning coils. Although electrically identical the connections of the replacement scan coil assembly are totally different, hence the upside down picture. After that fault was sorted out the letter "C" on the test card was found to be the wrong way around. Because of the split coils arrangement in the T221 the connections of both line deflector coils had to be reversed.
The scanning coils fitted in very early production models were known to fail. Later coils sets have improved insulation between the line and frame coils. This can be seen in the replacement scan coils.
Other faults found: Frame hold was weak, now sorted. The interlace diode and frame pulse integrating capacitor needed replacement, frame lock is now firm. Excellent interlace secured as well.

Sound is a bit better but still could do with bit more. IF transformers peak up OK, the fault might well be in the AF stages. No audio amplifier in this set.

Not sure what to do about the mains dropper, yes, it looks awful but works as it should.

If you wish I'll just leave the Visconol X capacitor as it is. the set can get by with out it.

The CRT is very good, it's not often one sees a CRM141 so good.

Till Eulenspiegel.

 
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Re: Ekco T221, 1954.

Post by Till Eulenspiegel » Wed Jul 27, 2016 12:11 am

From my last post:
Sound is a bit better but still could do with bit more. IF transformers peak up OK, the fault might well be in the AF stages. No audio amplifier in this set.

At the eleventh hour the sound fault was solved.
The quiet sound fault has been solved. The 0.01microfarad coupling capacitor between the sound detector and noise limiter diode was found to be low capacity.

There was almost 50volts P - P across the 68Kohm load resistor and only a quarter of that figure was present at the cathode of the series noise limiter diode. Trader service sheet reference is C64.

Till Eulenspiegel

 
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Re: Ekco T221, 1954.

Post by Till Eulenspiegel » Mon Aug 01, 2016 10:03 am

Following the Glasnost rule I can report that the set broke down before it's owner was to collect it on Saturday.
The fault being that the replacement scan coils have failed again. As before, a low resistance reading between the line and frame coils.
It is now apparent what is causing the problem, the height compensation thermistor in series with the frame coils is positioned tight close to the left side line coil, not a good idea considering the high peak voltages present in the line coils.
Otherwise apart from that design flaw, the line and frame coils are well insulated from each other by means of plastic spacers and cellophane.
The only answer to the problem is to reposition the thermistor or remove it from the FTB circuit altogether and see what happens.

Till Eulenspiegel.

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