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Teleton TA12

 
freya
(@freya)
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Its been a while since posting anything so her goes with this 12 inch portable from 1969 era.
To start, its not mine. A good friend of mine Steve visits Cornwall a few times a year to bring me goodies that I gleaned from Harpenden or NVCF etc, however on this occasion the car was empty so he took pity on me with his Teleton set to look at with a totally blank raster.
 On to the set, it produces a very nice blank raster with a good amount of sound but nothing at all from the UHF tuner when connected to a known good source.(not sure if its supposed to produce snow)
Initial connection with a modulated signal generator will produce bars on the screen when connected to the VHF input, but hardly anything except slight tuneable patterning from the UHF input, I can however get the signal though if connected from the UHF tuners output.
Chris was kind enough to scan what is available, it doesn't include the tuner schematic sadly.
Any help to get this progressed further would be appreciated and an addition to my knowledge on UHF. I have included a picture of the set, and the UHF and VHF tuners as well as the layout of the tuners.

20180621 082621
20180621 082804
20180624 171540
TA12
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Topic starter Posted : 24/06/2018 5:11 pm
Nuvistor
(@nuvistor)
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What frequency are you feeding into the VHF tuner from the UHF tuner output, IF?

From the photo of the UHF tuner there is an untuned RF stage, an RF amplifier transistor. Then a band pass circuit, I cannot see it but there must be an oscillator transistor, top left of the photo perhaps. The output is taken from the centre of the tuner. I presume, can you confirm, that the item in a white plastic case between sections is a diode, if so that’s the mixer.

The practice in many sets was to feed the IF output of the UHF tuner into the mixer circuit of the VHF tuner and use the VHF mixer as an extra IF amplifier.

The VHF Tuner appears to be an incremental type, be careful not to disturb any of those coils, for that matter any work in the UHF tuner will require care not to disturb the tuning of the Lecher lines.

Frank

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Posted : 24/06/2018 6:35 pm
Nuvistor
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The Hitachi CEP180 circuit in the data library has a circuit of a UHF tuner. It’s slightly different in that the aerial input is tuned and it has a seperate IF amplifier, the IF amp is instead of the one in the VHF tuner.

May be useful for you.

Frank

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Posted : 24/06/2018 7:04 pm
Jayceebee
(@jayceebee)
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The circuit above shows a supply +Bu out of the VHF to the UHF tuner which I would imagine is only present when the VHF tuner is set to U. I would expect it should be somewhere between 9 and 12V but I suspect you will have checked this already.

John.

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Posted : 24/06/2018 9:32 pm
freya
(@freya)
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Frank, I am feeding in 38Mc /s into the UHF, this will produce slight patterning. If the output from the UHF is fed the same signal it produces good clear bars. There is indeed a mixer diode inn the plastic tube. The TO72 transistor has been identified as an AF139 and tests good.
The B+ feed to the UHF is good at 10v

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Topic starter Posted : 25/06/2018 12:08 pm
Nuvistor
(@nuvistor)
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You should get little or preferably no 38Mhz signal through the UHF tuner, the 38Mhz signal fed into the VHF tuner producing a good picture is what is to be expected and working correctly.

Presume you have tried a UHF signal into the UHF tuner and  had no output.

If the RF amp was faulty I usually found it to have a snowy picture in a good signal area, is that AF139 the RF amplifier?

So either the osc is not working or the mixer is faulty, I have not had either but again I have not serviced a TV for nearly 40 years. Any ID on the oscillator transistor, very early diode mixers were temperamental but this is much younger than those.

The AF139 is germanium if my memory is correct, perhaps the same for the mixer.

Frank

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Posted : 25/06/2018 12:54 pm
freya
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I will take a delve inside next but its micro surgery for my eyes. Not tried a UHF straight into the VHF, hadn't thought of that.

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Topic starter Posted : 25/06/2018 2:38 pm
Nuvistor
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No not UHF into VHF tuner, UHF into UHF tuner ?

Frank

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Posted : 25/06/2018 2:40 pm
freya
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Bit confused by that, if that worked then there wouldn't be the  problem I'm trying to find ?, or am I missing something.

Anyway, the transistors both checkout ok in the UHF tuner. Is there a conclusive way of checking the UHF is actually working independently of the VHF tuner ?

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Topic starter Posted : 25/06/2018 3:05 pm
Nuvistor
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I wasn’t sure you had tested the set with a UHF signal.

Anyway, one way these can stop working is the earthing springs on the tuning gang, the grease hardens and can stop the springs earthing the gang. Worth cleaning them and making sure they are tensioned. Don’t spray cleaner into the tuner it may  cause other problems.

No conclusive way of testing the tuner out of the set without other equipment. You don’t need to, the fact that the VHF Tuner passes the IF signal is a good test bed.

Frank

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Posted : 25/06/2018 3:39 pm
Nuvistor
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If you have a radio that will tune those UHF frequencies, some scanners can, you could try and find the local oscillator, should be enough leakage with the cover off. If the oscillator is working then that points to the mixer diode.

Frank

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Posted : 25/06/2018 3:46 pm
freya
(@freya)
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Spent a fair bit of time on this and simply cannot get the oscillator to run, when I first opened the UHF tuner it did look someone had been in it before as a couple of the vanes on the tuning capacitor were bent, shorting each other out.

I have to conclude that it is certain that parts have been swapped out previously unsuccessfully which is where I came in. Without a component list its not going to be worth me fiddling around further, the biggest issues are the confined space the components are in and the 50 watt iron is only just able to melt the solder with a quite large bit.

Another suitable tuner may turn up.

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Topic starter Posted : 01/07/2018 9:14 am
crustytv
(@crustytv)
Vrat Founder Admin

Hi Stephen,

I still have the couple of years later Teleton TX12 in my small B&W collection, I wonder if the tuner used in this set is similar or perhaps the same as the one on your bench? Would they tool up for a different tuner or save cash and use one already in production for the TA12....

https://www.radios-tv.co.uk/monochrome-tvs-i-used-to-own/tx12/#main

tontx12uhf

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Posted : 01/07/2018 9:55 am
freya
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Thanks Chris,

It certainly looks to have similar component values, the transistors are different but that's to be expected on a later model.

 I will get some new transistors ordered and have a go in a few days when I'm not fed up with it.

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Topic starter Posted : 01/07/2018 10:24 am
crustytv
(@crustytv)
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Posted by: freya

 I will get some new transistors ordered and have a go in a few days when I'm not fed up with it.

I know that feeling all too well Stephen, the 9k6 sapped all my enthusiasm

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Posted : 01/07/2018 10:28 am