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Old 14th May 2019, 12:15 am   #1
Heatercathodeshort's Avatar
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 7,520
Default Murphy V310 1957.

Against all promises to myself not to purchase any more junk I purchased this Murphy V310 at the NVCF.

It is an early version with the self oscillating line output stage that was prone to frequency drift as the chassis warmed up. Later models were fitted with a separate line oscillator employing a 6/30L2 double triode in a conventional circuit.

The cabinet of this example is finished in a light oak colour, rather nice contrasting with the dark Bakelite surround. These were produced in small numbers as this light colour was considered rather 'racey' even in 1957.

A quick removal of the shell cabinet revealed a very clean chassis that appears to be totally original. The oil filled LOPT is completely devoid of leaking oil very unusual for any Murphy oil filled LOPTs and appears to be the original.

I snipped the mains filter and plugged it in. The valves lit and after a short warm up the line timebase whistle could be clearly heard. A spark test at the anode of the U26 EHT rectifier showed plenty of AC, A further test at the tube anode produced nice loud cracks so at least we had good EHT.

Turning the brightness up produced a blank screen and a quick check to pin 10 of the Mazda CRM172 read only 35V instead of around 350V! A circuit check showed the A1 supply was feed from the boost rail via a 1m resistor decoupled by a .5uf waxie. Snipping the cap produced a dim raster that was completely off centre with severe corner cutting. After removing the plastic focus dome the ion trap was found to be miles off. It was covered in dust so must have slipped during years of storage. A quick adjustment produced a very welcome raster that was soon centered by the joy stick adjuster. The 17" CRM172 is without doubt one of Mazda's better tubes.

Now to apply a signal. Was the four position Murphy tuner fitted with Ch 1 & 9 coils? Unbelievably it was and a flick of the channel selector produced a brilliant but overloaded test card. The V310 does not have AGC and is fitted with separate gain controls for BBC and ITA. Turning down the BBC gain produced a surprisingly acceptable picture, poor frame linearity, cogging and poor video amplifier response but considering how long this receiver had been sleeping I was surprised at the results.

A quick cap change of the usual culprits cleared up all the faults. Frame timebase linearity feedback, A1 decoupler, boost cap not forgetting the .002uf cathode decoupling capacitor to the video amplifier. It is hidden away and goes O/C producing smearing with slight vertical displacement.

In all around an hours work and I must say it produces one of the best V310 pictures I have seen for a long time. It has been running for around 2 hours and so far no LOPT saturation.

The above is the basics and of course it will require a bit of cleaning and tweaking to finish it off. This has been a very easy project and I am feeling guilty having had such an easy result.....Regards, John.
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