TMB272 preservation

[djrm]

Greetings from North Yorkshire.
I've just aquired an od Ekco TMB272 405 line portable TV, I discoveded the forum looking for a way to put a picture on it as Frank's standards converter took my interest.

The TV looks to have some partial capacator updates at some time in its past. Most of the wax ones have been replaced but I have had to change more too. When I first tried it there was frame collapse but its scanning now.

It seems to be just about working but perhaps not very sensitive, there is no 'snow' showing on the screen - do 405 line TVs do this, its been a long time since I've seen one?

The chassis is a littlle rusty in places and the volume control potentiometer is only partly working. The case is in fair condition and the aerial in complete without fault.


There is a problem I have not yet fixed, when the contrast is turned up then the video disply flickers, I think its like audio motor boating but in the video stages. I have read that the AGC capacitors in the tuner can cause odd pproblems, bit I have not discovered how to get behind the turret tuner coils yet to check this.



Can sombody tell me the approximate voltage of the video waveform at the input to the video amplifier from the detector please?
There  is still work to do to checkk the vertical scan linearity, I've bodged it together for now but until I can see an image displayed I cant tell how well its working.

Kind regards, David.

[ppppenguin]

I managed to find my 1956/7 R&TVS. The entry for the TMB272 is little more than the circuit diagram. I thought I had Ekco's manual but can't find it.

Not sure which cap it is, but there's a very nasty looking electrolytic in the bottom left corner of one of your photos. The sort with a metal case and rubber bungs. To be changed on sight. If it's a decoupler in the right are then it could easily be responsible for motorboating.

If you don't build a Hedghog then you'll want an Aurora: http://www.tech-retro.com/Aurora_Design/...erter.html A brilliant piece of kit with many hundreds now in use. In the UK they are sometimes available from Crowthorne which saves the faff of paying VAT/duty: https://www.crowthornetubes.co.uk/ The Aurora has test card C preprogrammed which is more useful than the simple test card in Hedghog. This isn't to decry Hedghog, simply a statement of fact.

You only get "snow" in absence of signal if a set has AGC which cranks up the gain substantially. The TMB272 must have AGC (I don't have circuit in front of me) but I don't know if there's enough gain to give snow.

[peter scott]

..and if you fancy something cheaper than a standards convertor there is always this.
Not just 405 lines either.

Peter

[ppppenguin]

Indeed there is and it works well. But you'll need to build a modulator. Not too difficult but both Aurora and Hedghog do the entire job.

[djrm]

Hi Peter,
Yes, I am already looking at the VGA modeline tool. it seems to be mainly compatible with my old laptop. I need to make the cable adapter and work out how to feed the signal into the TV receiver, maybee disconnect the detector diode or feed it from a low impedance buffer. I'd like to build Frank's standard convertor partly because it has a VHF modulator which I do not otherwise have.

BTW, I had another look at the VHF turret tuner this morning, it looks as if the turret is held in with a couple of spring clips, I'll try taking it out when I get back home.

Kind regards, David

[ppppenguin]

It would be interesting to find out what mods the BBC did to the TBM272 to use it as a video monitor. Then you could do an authentic modification. Or for non-authentic video input I'm sure a video opamp such as the classic EL2020 would sort out level and polarity for up to a few volts p-p.

There are several published designs for VHF modulators, mostly using 1496 chips. These include my own (see article on my website). They aren't too hard to build but you're really interested in them or possibly on a very tight budget why bother when you can get an Aurora or possibly a Hedghog.

[Murphyv310]

Hi.
Iv'e at least 20 MC1496 chips here if you need a couple just for Post costs.

[djrm]

Jeffry, Trevor.
I am quite interested in the analogue modulator design and I shall probably build one day just out of interest.
However my immediate needs should be well met by the Hedghog design.

Its a good idea to try and find out how the official BBC monitor conversion was done. I did notice today a blanked off socket by the interference limiter adjustmnet sockets. I think it is either just being used as a tag strip or maybee it is an external speaker socket. I'm going to trace the cirucit later out of interest.

For the record, my set is serial No. 02425
Kind regards, David.

[ppppenguin]

I've asked a friend, not on GVR, who may have info as to how the BBC did it.

I see that Frank has agreed to send you PCBs for a Hedghog. He really is a most generous fellow.

[Murphyv310]

Hi David.
Frank is brilliant as is his Hedghog.
The TMB is a brilliant set and I have to say one of my favourite sets.
Mine has a 3.5mm jack socket added shrouded in a Cod liver oil tin for screening attached to the rear panel for audio output. It's been there for years and it will be left for originality.
These sets are fairly common but a lot are incomplete. Mine is near mint .