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Old 2nd Jul 2020, 8:23 pm   #17
Heatercathodeshort
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Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: Peto Scott 12" console model TV122 1948

Part 3. The I.F. panel.

As mentioned this consists of a sub unit mounted to the rear of the timebase chassis, connected to it by a Philips octal plug and socket with single connections from the sync separator and the feed to the sound I.F. amplifier that occupies a small section of the timebase chassis.

It is a superhet circuit with I.F. frequencies of 13.25mc/s vision and 9.75 mc/s sound. Two EF42 R.F amplifiers in a similar arrangement to the Murphy V200 series are employed providing high gain in weak signal areas. These are followed by another EF42 as mixer. The local oscillator is an EF50. It is of high quality construction and forms an important section of any receiver, particularly when working in low signal areas.

It is interesting to note that adjustment of the local oscillator is a customer preset control, a miniature open vaned capacitor complete with Bakelite knob mounted together with the other usual pots on the rear of the chassis. It is of course a critical adjustment [very] and I think they were somewhat nervous about possible frequency drift but after hours of use I have not experienced this.

Oscillator drift must have been a problem back in '48 [The year I was born..] and probably led to the adoption of the TRF circuit so popular at that time. The PYE and EKCO ranges come to mind. What seemed a good idea at the time led to difficulties with the arrival of Band 3.

The BUSH TV1/2 certainly suffered from sound oscillator drift [the vision was TRF] and was eventually supplied with a TRF sound unit with EF91's replacing the original superhet unit with it's ECH35, EF39 and EB34 valve line up. This modification completely resolving the problem no doubt at considerable cost to Bush. It was certainly not a problem with this Peto Scott.

A single stage of I.F. amplification completes the vision amplifier. A double diode B7G based EB91 demodulates the vision signal, the other half of the EB91 being used as a non adjustable peak white interference limiter directly connected to the control grid of the MW31-14C. The tube is grid modulated as were quite a number of receivers of this vintage. The video output valve is yet another EF42.

The sound take off is from the anode of the mixer passed to the two stage sound I.F. amplifier comprising another two EF42 valves. A single EB91 functions as vision detector and interference limiter. The high level audio is passed to a Mullard EL33 without pre amplification, giving about three watts of very good quality sound from an ELAC 8" speaker.

It was a relief to discover that other than four waxie .01uf decoupling capacitors in the sound I.F. amplifier, the remaining were of the silver mica type and proved to be completely serviceable. Replacing these is not a task I like. It's similar to picking flies out of your soup in a sleazy cafe..

The sync separator is another little red canned EF50 in a conventional circuit. A single 25uf 25v electrolytic capacitor in the cathode bypass was suspect. Upon checking it was completely O/C and was replaced by a modern mini type using the case of the old unit as a shell similar to a hermit crab.

Once again it was time to put the old Griffin power supply to use. With the scope connected to the CRT grid feed and with an aerial connected, I applied some power.

To my surprise a vision signal was present and could be tuned by the local oscillator trimmer, improving it enormously. I disconnected the aerial plug just to prove it was in fact a real signal, not just the end of a 'Whats My line' programme of 1954.These old 405 line signals take some killing off!

Not a lot more could be carried out with these 'cold' chassis and it was time to liven them up and see what my efforts had to offer. [I dread to think]

Pictures show. Rear view of combined unit showing oscillator tuning control and overall gain pot. Top view of unit cleaned. Underneath of combined unit. Close up of I.F. strip. 'Blue' EL38. I forgot to post this one on the timebase page.Don't get concerned if you see a blue glow inside your EL/PL38s! This is quite normal but this example is certainly displaying itself with an almost Peacock appearance. It test 100% on the AVO with 90 m/a anode current.

Part 4 Will relay the results and I will post some off screen pictures.
Thanks for reading so far! John.
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