New project. A 1957 bakelite GEC (Identified as a BT1155)

[matspar]

I had a quiet day today so thought I'd take the GEC to bits that I bought at the NVCF.
Well it didn't look this bad on the outside. Judging by the amount of rust, dead woodlice and spiders its been stuck in a shed for much of its life by the look at it! I've cleaned it all up and it all seems to be present and correct, with its original selenium rectifier and lots of nasty looking Hunts capacitors....

Must be quite a rare set. The bakelite case is very good and it did still have its back on, although that's a crumbling mess! Will it be worth doing? has anyone got a working one and some words of inspiration!!?

Matt

[mark pirate]

It does not look too bad, I have certainly restored sets in a worse state!
I am sure it will look a lot better for a good (careful) clean.

Mark

[thermionic]

Let's hope the GEC CRT is ok! Good Luck.
Following this with my fingers crossed for you.

SimonT

[Heatercathodeshort]

The chances are the CRT is OK. You will probably have to dry out the LOPT overwind by passing a current through it. It is a pitch covered type but other than that is very reliable.

I know of a slightly later model, the BT1156 that was picked up from Mike's get together at Solihull a few years back. It was minus it's valves and back.

To add insult to injury it was brought back south in the rear of my Ford Ranger, on it's face unprotected from the weather. It poured with rain all the way back and when Peter [The Organ] and myself unloaded it, about half a gallon of water poured out of the case!

Peter likes a challenge and took it home to play with. After a recap and a set of valves the little GEC performed as new and can be seen in the television room at Dulwich.

GEC manufactured high quality television receivers. The model you have is the BT1155 with 70 degree CRT, a slightly older model than the telly in the bath case that Peter dealt with!
Good luck with it. Should be an easy one.

[MonochromeMarc]

Hi Matt,

I have your set's last incarnation the GEC BT311 with it's 14" CME1402/GEC7205A tube (my set shown below).
I have always liked the the look of these GEC Bakelite sets even though they are plain and simple little beasts.

Good luck with your set.

Marc.

[matspar]

Thankyou all! I've had a day of it today and replaced all the hunts caps. They were in a shocking state! After that I was brave enough to plug it in. Nothing went bang or caught fire and I got a very healthy line whistle. Either the lopt or the U25 rectifier is a bit sick though. The heater in the U25 doesn't seem to be lighting up however if I change channels rapidly on the turret tuner there is a faint burst of lines and spots on the screen. I don't have another U25 to try. The one fitted has continuity across the heater legs, and with one leg disconnected there is continuity across the heater winding.

I'm going to wait until its gets a bit dark and fire it up again to see what I'm getting on the screen. Its quite bright here in my kitchen. Apart from this everything else looks ok, HT is ok, A1 is 315v and the HT rectifier seems fine and I reckon the crt is good. What do I do now?!!!

This is a good quality set, bit of a sod to work on though unless the crt is removed which I'm a bit reluctant to do at this stage!!

[mark pirate]

You will probably have to dry out the LOPT overwind by passing a current through it.

I would follow John's good advice, especially as the set looks to have been stored in damp conditions for a long time.

I am a great believer in making sure that any possible damp in the windings has been driven out before powering up, to give the LOPT the best chance of survival.

Mark

[matspar]

Thanks Mark, I had similar on a Pye VT2 a few years ago and John's expert advice saved the day! I ended up removing the overwind and fitting a tripler to the Pye however as I just couldn't get enough EHT. The GEC looks to have been damp stored for a hell of a long time so I'm pretty sure this is what's wrong. There's definitely EHT present as I can draw a nice healthy spark from the U25 anode, just not enough to light the crt

[FERNSEH]

Have you checked the CRT first anode volts? Measure at pin 10 of the CRT base, the A1 volts should be >300V. The ion trap position is critical. Unlike the Mullard tubes it does not lineup with pin 3. If the A1 volts are present and the EHT OK try shorting the CRT grid and cathode connections together. CRT will be at visual cut-off at about -44volts.
The Mullard MW36-24 or MW36-44 can be considered as a replacement for the Mazda sourced GEC 7204A = CRM144.

DFWB.

[matspar]

Hi David,

The A1 is showing 315v and the ion trap seems undisturbed. I did wonder about this as the magnet is aimed on the left hand side of the crt neck (pointing to 9 o clock) I found a picture online of another BT1155 and it is in the same position. I've just had the set running again and I'm beginning to wonder if its something else. When changing channels there is a healthy crack from the speaker and at the same time there is white lines and disturbance on the screen, yet no raster. Also getting a tiny square and then nice white dot in the centre of the screen on switch off. The U25 eht rectifier does have its heater lit but its very faint. Only just visible in a darkened room. Does this type usually glow brightly? thinking possibly not as only a 2v heater?
I haven't yet shorted the grid and cathode (forgot earlier but I'll do this next time!) I'm lucky enough to have a good mullard MW36-44 in stock if needed but my gut feeling is that the GEC tube is good.

Matt

[TonyDuell]

I seem to remember reading (I have never tried it) that an old trick was to wire a low-current 2V (or 2.5V) bulb in parallel with the U25 filament, or a rather higher current one in place of it temporarily. If you do the former, be careful as the bulb is at EHT voltage with respect to ground. And see if said bulb lights properly.

[Freya]

Sounds to me like you don't have a signal getting through and the line speed is low with no lock causing the dim U25. I suspect if you try the trick Fernseh has suggested, shorting the cathode to grid you will get a raster.

[matspar]

Good news! with the cathode and grid shorted momentarily I'm getting a nice bright raster so it looks like the lopt and crt are ok. I won't have time for an in-depth session until the weekend so will spend the next few nights studying the circuit. Any thoughts what the culprit could be chaps?!

Matt

[Heatercathodeshort]

Worth checking around the brilliance control. Check the slider voltage as you adjust it. The cathode voltage could be high due to a video output stage fault.
I know you have a bright raster but make sure the ion trap is adjusted for maximum brightness but remember with the tube biased full on [G/C shorted] the beam current may overload the EHT supply and give a false position of the magnet. John

[FERNSEH]

Information for the GEC BT1155 can be found in the 1957/58 R & T servicing book. A capacitor C96 (0.5mfd) is connected between the junction of the two resistors which are in series with control grid of the CRT. In some sets C96 can be 1mfd.
This capacitor is most likely to be leaky.

DFWB.

[matspar]

Absolutely spot on Sir! Would you believe that C96 was the only single cap I didn't replace as I didn't have anything suitable at the time. I'm working from the trader sheet where it's shown as a 1uf but it was actually a 0.5uf fitted. Just soldered in a replacement (total swine to get at and I had to remove the turret tuner to gain access!) and yes a nice bright raster is now present!

Plenty of width available, can't get the full height yet and the linearity looks awful so Saturdays job will be going through all the resistors. I've not yet put a signal on the set so will be interesting to see what happens!

Thankyou to everyone for your kind assistance so far, I'll give another progress report over the weekend!

Matt

[MonochromeMarc]

Looking good Mat

Marc.

[mark pirate]

Good progress, tube looks nice and bright. I would connect up the Aurora and see if test card C appears!

Mark

[FERNSEH]

On switch off the capacitor C96 maintains it's charge for a few seconds thus keeping the grid volts of the CRT high. At the same time the CRT cathode volts fall rapidly. The EHT capacitor will maintain it's charge so it is possible that an intense spot will appear on the screen and cause a burn mark. So by keeping the CRT grid volts high at the instant of switching off the receiver the CR tube will conduct heavily and rapidly discharge the EHT capacitor.

DFWB.

[matspar]

Thanks for all your advice David, noticed that you restored a pair of similar GEC's a few years back and this has made very interesting reading.

Today has been a success. Brought the Aurora out to play and ran the GEC up and it soon became obvious that the line speed, linearity, and the height were all up the creek. Test card C had the appearance of an Audi badge so delved into the local area and found two resistors (330k & 100k) which were silly high. The track on the linearity pot was o/c and the height pot was missing its wiper.

They are the type which are mounted on a small piece of paxolin and adjusted with a small slotted screwdriver. I fitted a good used 500k wirewound pot in place of the linearity. And the wiper from this pot I managed to detach and then solder on to the height one, thus making one good one out of the pair.
After all this I have a nice crisp and stable test card with plenty of brightness. I had to tweak the width and the picture shift to get everything lined up. The CRT looks very good with excellent focus.

Still plenty to do though, the contrast control has no effect on the picture, and the sound is very low, only just about audible. I'll investigate this next time. Who would like to have a guess what this could be!? Here's some pics of today's efforts!

Matt