I goes without saying that the mains dropper resistor in this GEC BT311 needs replacement. We know there is absolutely no chance of finding an exact replacement. However, I believe I've found a dropper resistor that might well serve as a replacement. It is slightly longer than the original component so hopefully no metal bashing is needed to fit it in position. The original values were: from left to right, 77, 38, 66, 66 ohms for the valve heaters and the HT sections are 17 and 32 ohms. The replacement dropper resistor values are not too far away from the original, being 60, 33, 70 and 63 ohms for the heaters and 20, 19 and 15 ohms for the HT supply. Last two HT sections will add up to 34ohms, so that's near enough for the last HT section of the original component. Also, the heaters section are not too far out either. The BT311 has and extra common IF amplifier stage so the slightly lower total heater dropper resistance will compensate for the additional valve in the heater chain.
The set is blowing the mains input fuse after the valves warm up so that's something else to investigate.
About the GEC BT311. Electrically identical to the 1958 model BT1354 except for the finish of the Bakelite cabinet. Also has "one click" channel tuning, for example channels 1 and 9 and 3 and 10 just one click apart.
Till Eulenspiegel.
By heck that's an ugly scar you've got there! - I bet that set will weigh a pound lighter with all those tagged on bits removed.
And I also espy some ugly looking 'orrible 'unts with splits in them... Does this set work at present?
Hi Marion, from the era of the bodge repairs. I guess the sections were fitted in the 1960s. It was the same for many other things, you just have to watch the Car SOS shows to see some wonderful bodges. Stressed body panels just pop riveted into position rendering the car totally unsafe in the event of a crash. Marc has the other GEC BT311, I think his example escaped the attention of the bodger-in-chief.
Till Eulenspiegel.
"Does this set work at present?" Follow up to my last post. Hi Marion, The GEC was chosen to serve as a monitor for my standards converter. The clock pulse fault was found and the TV was displaying good(ish) pictures. Then the fuse blowing antics started, but before any fault finding is started let's get rid of that horrible dropper resistor thing.
Till Eulenspiegel.
Marc has the other GEC BT311, I think his example escaped the attention of the bodger-in-chief.
Till Eulenspiegel.
Hi Till,
No the other Welsh twin also had an attack of the 'spherical mints' too and they didn't last very long either. I ended up replacing the whole kit and caboodle with a pair of suitably hefty wirewound resistors which have kept things going nicely ever since. I even treated the little BT311 to a replacement CRT earlier this year with grateful thanks to "Freya" for it's supply and boy does it give a cracking picture now.
This set is one of my favourites and gets regular outings, in fact it's such a favourite it even gets a regular flick with a duster too ?
Marc
BVWS member
RSGB call sign 2E0VTN
The replacement mains dropper is now fitted in the GC BT311. A piece of 5mm threaded rod is employed to position new component correctly. During the course of re-soldering the wires from the voltage selector panel a wiring mistake was found. It was found that the heaters voltage adjustment panel had a wiring mistake, the 220volt tap on the mains dropper had been connected to the 240 volt tag on the voltage adjustment panel. See second attachment. All this means that the valve and CRT heaters have been over run since the set was made. This hasn't done the CRT any favours, it's even dimmer now. The N308/30P4 line output valve needed replacement, a GDR made PL36/25E5 works fine as a replacement.
The pictures are supplied by the repaired standards converter.
Till Eulenspiegel.
That's better ! And the picture is looking pretty darn good too ?
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Andrew, the camera tends to make the picture look much better than it really is. However, the set hasn't been in use for a long time so it is possible that it might brighten up if allowed to be switched on over a longer period if time. These 90 degree 14" CRTs Mazda CME1402 and the GEC version type 7205 are very difficult tubes to find. The Mullard AW36-80 is equally so. This was the 14" CRT that offered 10 percent more picture area compared with the older straight sided 14" CRT.
Till Eulenspiegel.
Hi Till,
That dropper certainly looks at home in there. shame about the over run for all those years though, make no wonder it's CRT is a bit tired.
I have to admit that my sets CRT was getting to the point of unusable so I guess I was very lucky to get that CME1402 from Freya. I don't think there were all that many sets fitted with this tube and it's brethren hence the scarcity perhaps ?
Marc
BVWS member
RSGB call sign 2E0VTN
Measured heater volts of the CRT, this should be 12.6 for a Mazda CME1402 tube but is only 11.3V when measured with a Fluke DMM. Reset the voltage adjustments for 230V, this resulted in a slight improvement of picture brightness. CRT heater volts is still less than 12.6V. The heater chain which in this set includes the additional common IF amplifier valve V3 (W719/6F19) and including the CRT tots up to 170 volts and in fact the figure measured at the heater chain end of the thermistor is 168V.
This set employs the Z329/30F5 RF pentode which has a 7.5volt heater. It likely many of these valves will need replacement in the GEC BT311. The 30F5 is possibly a development of the Mazda 6F1.
Till Eulenspiegel.
