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Old 28th Oct 2018, 9:10 pm   #21
Heatercathodeshort
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Location: Warnham, West Sussex. 10 miles south of DORKING.
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Default Re: Crazy Murphy 12" console V210C 1953.

As mentioned earlier the original CRM121A was very low emission but has a hard black getter.

It took less than five minutes to remove it and fit the CRM123 retrieved from the loft. Just one bolt on the CRT clamp, a slight tug and it almost fell out!
With the CRM123 in place the picture was much increased in brightness and contrast but the definition decreased slightly with a 'white after black' effect noticeble on the test card.

There was also a halo effect and I have no doubt this was due to the impaired vacuum of the CRM123. The getter is just a white milky stain around the base and to be honest i'm surprised it is as good as it is.

The chassis should take a Mullard tetrode tube such as the MW31-16/74. There is plenty of room for an ion trap and the boost voltage is more than adequate for the 1st anode. It would be a simple matter to substitute the 2v heater transformer for a 6.3v one or arrange to have it included in the heater chain. [100m/a + 200m/a series parallel.]

Peter was curious about the Murphy 'fringing' arrangement whereby the final vision IF transformers response is altered in favour of incresed gain but reduced bandwidth. Two links are cut on the transformer base removing two 2.2pf capacitors connected across the transformer windings. This peaks up the response giving higher gain but slightly reduced definition, giving a great improvement to the picture in fringe and difficult areas.

The picture shows the increase in gain obtainable with the links cut. The sensitivity control was not moved from it's position that gives a good picture with the links in place.

No real change in the bandwidth is noticeable either way but with the poor condition tube I didn't expect too much.

The chassis is incredibly stable and will work OK for ten hours without any adjustment whatsoever. The V210 was a very popular model for Murphy being released in early 1953 just before the Coronation.

The first picture shows the links in place, the second with them cut. John
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Old 28th Oct 2018, 11:25 pm   #22
peter_scott
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Default Re: Crazy Murphy 12" console V210C 1953.

Wow! That looks like a massive gain increase with only a couple of 2.2 pFs lopped off. It's a pity the CRT's not better but a really strange and interesting set. Thanks for doing the test John and for another great write-up.

Peter
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