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Old 18th Sep 2017, 7:41 pm   #54
Heatercathodeshort
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Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: And the next one! a GEC BT2147

A couple of weeks ago Matt brought over his Pye VT2 and a GEC BT2147 9" table model I will call the 'Bakelite Bitch'.

Matt was experiencing some problems with the VT2 and after a lot of testing and head scratching, it was returned to good working order and was signed off [we hope]

The second candidate proved much more challenging. the Bakelite Bitch had been stored if you could call it that in appalling damp conditions. At a guess I would say a simple garden shed or maybe an old leaky garage.

The chassis was corroded and it looked to be in a bad state. Matt had cleaned most of the grot from the chassis and successfully obtained first light by replacing the old selenium rectifier with a silicon diode and surge limiting resistor.
This gave a very good indication of the serviceability of the LOPT and CRT both in 100% condition. This was only the start of the epic as can be seen in the earlier posts.

Matt had done a good job recapping the line timebase but the original frame timebase was in a sorry state. It was decided to remove all tatty bits from repairs decades ago and rebuild the circuit as per the original GEC manual.
Matt discovered that the output transformer primary was O/C requiring a visit to a spider infested garage loft to see if I could find a transformer with a reasonable match. One particular one caught my eye from a Thorn 850 dual standard chassis 14 years it's junior, and it was certainly worth a try.
The frame timebase was completely rebuilt and tidied up. All solder splashes were removed and the H.T. was checked for shorts.

With a pair of EF91's plugged into the two vacant valve holders on the I.F. deck [to maintain heater continuity] the mains was applied. To my utter surprise what looked like a very promising shaped raster appeared on the tube face. It was out of focus slightly but with the seized focus magnet removed it was soon cleaned up and lubricated. It was now possible to focus and center the raster correctly.

It was now time to shove a signal down the I.F strip. Surprisingly the I.F. is 34/38 mc/s making it possible to wire in almost any tuner from the mid 1950's/60's. An old Cyldon Band 3 converter was dug up from the stores and given the once over. It had coil 'biscuits for channels 1 & 9 together with it's valves PCC84 and PCF80.

Power for the tuner was obtained from the I.F. unit. The heaters were wired to the B7G sockets originally intended for the external Band 3 tuner. H.T. was picked up from a tag strip and the I.F. output connected via a 1000pf capacitor to the I.F. input coil of the common sound and vision amplifier.

It was time to test the 'lash up' and upon powering up with the Cyldon switched to channel 1 I was presented with a test card. Both line and frame were off frequency due to useless slider presets. Temporary conventional pots were soon wired into place and after a few simple adjustments an incredibly linear picture was produced. That Thorn FOPT must be a perfect match!

The chassis is very stable and soak testing for over an hour proved that nothing drifted.

There is still a lot to do. I have not checked the sound but the circuit is so simple I might leave it to 'Interlace' my pet Orangutan to sort it if it's silent. He insists the tube needs an ion trap despite me pointing out that it is a straight gun triode with an aluminised screen hence no trap required.

The question is what to do about the lack of a front end? It might be possible to construct an RF stage followed by a mixer/oscillator using EF91 valves but for the time being I think the donor tuner will have to remain.

If anyone has one of these GEC receiver units either TRF or Superhet going spare I'm sure Matt will make very good use of it.

A replacement preset control panel will have to be constructed with a small sheet of Paxolin drilled to take conventional rotary pots.

Pictures show: Under chassis before start. Tuner lash up. Early raster. Picture on mirror. Dropper.
Regards, John.
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Last edited by Heatercathodeshort; 18th Sep 2017 at 7:47 pm.
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