[Heatercathodeshort]

Thanks Graham.

Thanks for sending your receiver and timebase panels from your GEC BT302 received yesterday [Saturday]

First to tackle was the timebase panel. On examination it was discovered that a number of dry joints were evident and a couple of the new capacitors were of the incorrect value. The use of non leaded solder will aggravate this with vintage receivers.

I thought the best approach was to remove all black GEC capacitors from the board, together with the replacements that you had fitted and with the aid of the GEC service manual, rebuild the board with everything hopefully in it's correct positions.

With the board cleaned up it was a simple matter to fit high grade modern capacitors knowing that in this department at least, everything should be correct.

With the old capacitors removed and the printed circuit cleaned up it was time to fit the new components. It didn't take long to do this and I was very tempted to let INTERLACE my pet Orangutan do the job but he scurried from the workshop having read the saga of the history of this BT302.

With the new capacitors fitted I decided to fit a new main frame linearity preset. This passes a heavy current and often failed. The GEC mod was to fit a 120K resistor across the track on the reverse side of the board.

All new solder joints were sealed with varnish to give a neat and tidy appearance.

With everything carefully checked I fitted the panel to my chassis using my set of valves and powered it up.

The line note was soon heard and a few seconds later a raster appeared on the screen. It was lacking height but this was soon sorted out by means of adjustment to the height and linearity controls. It was noted that both the line hold and frame hold controls were at the ends of their travel to obtain correct lock. Sync was very solid and a look at the circuit suggested a check on a couple of resistors should clean the problem up and it did. R90 a 360K 5% connected from the slider of the line hold to the grid of the oscillator read 390K. Due to it's odd value this is obviously a critical component and with a couple of resistors in series brought the line hold to the center of it's track. In a similar situation, R105 390K read high and a replacement brought the frame hold to mid travel.

Next to examine was the receiver unit/IF panel. Upon removal it appeared to be completely original and when fitted produced a fair picture without adjustment but where was the sound? Not a click or trace of hum from the speaker and my cynical approach led suspicion on the audio output transfromer. A quick check on the anode of the output valve read zero volts but a check on the HT side of the transformer also showed ZERO!

A quick check on the circuit revealed the supply to the output stage is derived via a 470 ohm 2 watt resistor and a 32uf, part of a twin can electrolytic. A meter short confirmed a very heavy leak at this point but a check revealed that the 32uf was innocent. The actual short was caused by the tone correction capacitor C90 .01uf. It is connected from the anode of the output valve to chassis and was completely S/C. A replacement brought the sound back with a roar and all was well. Fortunately the components used by GEC are of very high quality. The black moulded capacitors had a very long service life and even today rarely shut down a chassis completely. The audio output transformer appears unaffected by the overload but the 470ohm supply resistor looked a bit stressed and was retired from duty.

I decided to replace the remaining black moulded capacitors. It seemed a shame to risk further breakdowns after having travelled such a long road. I also replaced the screen grid decoupling capacitor to the vision IF amplifier V5 Z329 Mazda 30F5. These tend to crack and go O/C resulting in the stage bursting into oscillation producing a bright white screen with the video amplifier glowing red!

With the IF board refitted it was only necessary to adjust the two beehive trimmers, C56 and C58 the sound rejectors to obtain a very steady and reliable picture and sound. These are critical adjustments with all GEC chassis from the BT2147 to the end of genuine GEC television production at Coventry.

I will give the chassis a long soak test before returning them to Darren. I hope this clears up this long running saga.

Components replaced. TIMEBASE BOARD: C113, C112, C110, C111, C114,C115,C118,C117, C116, 120, C126, C123, C119, C124, C108, C107. VR106 Frame lin control preset 250K + 120k mod resistor.
[16]IF/RECEIVER:C61, C60,C91, C44,C45,C67,C74,C77, C79,C85,C90 and R41 [burnt].

I would add that many of these capacitors are probably 'serviceable' and I would probably have left a number in place had it been my own receiver but Darren is a long way from Horsham and the less chance of a breakdown is well worth the trouble of replacing them.

Well it's over to you Darren. I've post them off early week weather permitting!

Pictures show the two PCBs and the results. Regards, John.