TMB272 preservation

[djrm]

I've examined the blanked off extra socket, I'm pretty sure it is just being used as spare tags. All my TV photos are here: TMB272 photos the last one shows the hidden sockets.

Yesterday I tried the TV again and it seems to have developed a new fault - I can't get sound or video when switched into TV mode, but it continues to work in radio mode.

The instability is still there so the set isnt quite dead, I was about to look at the receved signals using the 'scope but there wasnt any.

I have a schematic but no service data, if anybody can help me find the sevice data I'd be tankful.

I'm about to try injecting a singnal into the IF amplifier ...

[Murphyv310]

Hi.
From your pictures I see you have a lot of the original wax capacitors in the set. I'd be inclined to replace all of them that are HT, Anode feed and screen feed decoupling. Also any that are coupling capacitors in the audio and frame stages.
There is also a 0.1uf capacitor hidden in the tuner and this is an infamous cap for causing instability in virtually any Ekco set of the era.
There are also many other caps (eg brown Hunts) of between 1000pf & 3000pf that also leak badly and they too should be replaced.
There is also a 60pf cap C92 in the line timebase that effects the AGC, another stock fault.
Give the radio/TV switch a good clean and if you have contact grease use it after cleaning.
These are really good sets.
   
   

[ppppenguin]

I've had a reply from my friend who is pretty sure he has a TMB272 that's been modified to use as a monitor. As far as he can remember it wasn't done by the BBC but by an early amateur TV enthusiast using BBC information. He says he'll try to find it.

[Murphyv310]

The TMB will make a very good monitor, I'd suspect a video level control would need to be incorporated as the existing contrast control works via the AGC.

[ppppenguin]

Here's a view of the back of my friend's set showing the video input on a B&L socket. I suspect the BBC would have more likely used a UHF socket (SO239, F&E) or possibly a MUSA. Neither of these would have been readily available to an amateur in the 1960s unless they could "borrow" them from work. BNC wasn't commonly used for video in the 1950s.

[peter scott]

What does the B9A connect to?

Peter

[Murphyv310]

Tell him to adjust the mains tapping to 240-250. Its getting a bit overrun in the UK now on the 220-230v setting. He might get a surprise if he measures the heater volts.

[djrm]

Hello Trevor, and others. Thankyou for your suggestions. I'm making a little progress now, changing one of the IF screen decouplers has stopped the unstable oscillation. The waxy capacators with high voltage had already been changed previousley, they are mostly polystyrene now. The remining ones were melting so Ive changed them too now. The 60pF checks out ok too. I found the top feed resistor to the brightness/contrast controls had drifted high so thats been changed nw too. I wonder what the boost voltage is meant to be on these sets, I have around 250 VDC which seems a bit low. I wonder if the EHT cap is ok, I cant measure any leakage.

Still little sensitivity on sound or video, the tuner capacators have also been changed already but it could be that they need changing again considering its age. I'm wondering what the AGC voltage should be for maximum sensitivity, I presume somewhere around 0V DC and that more negative is less gain. Can anybody tell me the valves used in the tuner, perhaps ECC84 / ECF80

Waving an FM signal near the earial works on the radion but an AM sgnal at 41.5 MHz is hardly picked up at all. I could try injecting an IF signal but have no idea what llevels should be suitable, I suppose if there is some gain then it would be a good sign.

Another odd problem has occured, I think the local oscillator doesnt work on the band 3 channels, its difficlt to be sure as my frequency meter only goes up to 111 MHz.

I have 181 VDC HT1, I was looing for the HT rectifier but cant find it - is it hidden in the mains transformer housing? The HT is a bit higher in FM radio mode, not unexpected really.

Perhaps I need to get the valves checked out, I'll start by swapping them around a bit to see if this helps at all. I've bumped the mains input selection to 220V, it was a bit low before and now its a bit high - 13 VAC on the heaters.

Testing will be much easier when I have Franks Hedghog built though this is going to take quite a bit of work in itself. I'd like to have a baseband input but Ive nothing to feed into it yet.

The whole thing is beginning to look like a can of worms, no wonder it was cheap.
Anyway I shall do a bit more tomorrw and then I'l have to leave it for a bit.
I look forward  to having it working nicely one day ...

[djrm]

I havnt thought about it recentley but it must be over fourty years ago now. I used to service TVs for a living. first as a part time self employed basis and then for a TV refurbishing company in Bradford. I learnt most of what I needed from a good book called The PAL TV System or similar, I still have it somewhere. Back then there were very few dual standard set around most people has switched to colour and transistorised sets were becoming the norm. I stared with PYE and Bush and Philips hybrid sets, moving on to Thorn and Bush solid state. Old sets had very complicated high current convergence settings to deal with. The old G6 had a huge slide out drawer under the front of the set to give aggess to all the controls. My favorite at the time was the Bush/ Murphy ??? model. Since then I moved onto computers, learning by expanding and making peripherals for 8 and then 32 bit systems. I'm now nearing retirement working only part time as a  system designer on microprocessor control ststems. Ive restored a couple of old military radio recevers, A Racal RA17 I found at a car boot sale and an Admiralty Murphy B40. Both of these I could never afford in my youth but wanted to own, The only other valve equipment I have is an old russian oscilloscope and a naval PA amplifier which my Dad used t use for audio ampsfeeding a lowther horn speaker.
   

[Murphyv310]

Hi.
There are two selenium rectifiers sandwiched between the back panel and the main rear chassis. They are reliable and mine are the original. Even with a bit low HT the set should work well. The tuner valves are ECC84 & ECF80, the 6F1 IF valves can get tired too. Remember in these sets if the line stage is not performing correctly it will effect the AGC. The 60pf cap tends to break down under load and is difficult to read on a meter for faults.
Don't give up, I've done many basket cases with good success.