Cossor Telecheck Model 1320

[ChristianFletcher]

This is test gear but television related so posted here.

I don’t really have any experience with vintage 405 line televisions so this could be a really silly question, but have recently had two old 405 line pattern generators on the bench.

I’m a bit confused by the carrier wave output frequency on the Cossor 1320. The carrier control is calibrated on the dial from 7 to 70mhz. But the circuit diagram says the carrier oscillator is between 122 and 185 MHz. Checking the carrier on the spectrum analyser just confirming operation for 122 to 185mhz. So why is 7-70 printed on the scale.

I have to say the video pattern generation doesn’t look much like what I see in the old 405 line television books on the oscilloscope, so I’m not sure if this need more repair or is working normally. I recently got labgear signal pattern generator and it did at least have a passing resemblance to the video. ( I think)

Probably going to be posting lots of questions in the near future as along with the pattern generators I bought two 405 line televisions from goldborne. So I am looking forward to starting those soon. This is the Labgear i recommissioned if anyone is interested etc.

https://youtu.be/rfdXrJlDwIM


Thanks Regards Chris

[linescan87]

Hi Chris,

Can't answer your question, but Andy did a thread on one of these a while ago. https://www.vintage-radio.net/forum/...d.php?t=160965.

Hope that is useful,
John Joe.

[ChristianFletcher]

Thanks Joe I was told about it but couldn’t find it. Thanks for the link much appreciated

Regards Chris

[ChristianFletcher]

Frame and Line ?

Carrier set at 50mhz but measured as 163mhz

[dazzlevision]

The frequency range on your circuit was for Band 3 TV channels (ITV), whereas the dial on the equipment covers BBC Band 1 channels. I expect there was a later version that covered Band 3 or was switchable between the two.

[ChristianFletcher]

Hi Dazzlevision

Ok thanks for that information for the bands. But it seems very strange to my that a professional test gear manufacturer would build this and then not calibrated the printing on the dial. I wonder if it’s been modified in the past from band 1 to 3 by its previous owner. I guess I need to check out the components values in more detail, although I see no signs of previous work at first sight.

Best regards Chris

[dazzlevision]

These photo might help.

[ChristianFletcher]

Thanks for the pictures it’s nice to have the context and some history

[beery]

Hi Chris,
the 1320 version of the Telecheck is band 1 only.
The way it works is my having two oscillators which are mixed together just like in a Superhet radio. The lower product of the two frequencies mixed together should be in the range shown on the scale of 7-70MHz.

How it works...
The oscillator that is controlled by the variable frequency control on the front (1/2 of V5) has a range of 122-185MHz. This oscillator is amplitude modulated with the test pattern generator.
The other oscillator (1/2 of V4) has a nominal frequency of 115MHz, but is frequency modulated by the sweep generator V3.

The test patterns are rather crude, but the real value of the instrument is in its wobulator function which allows it to sweep across the whole bandwidth of the television signal to enable accurate alignment of a TV sets RF or IF stages.
You would need a scope with any XY mode to work with it. The output of the sweep generator (V4) is not a nice linear sawtooth as you might expect, but it didn't need to be, however it does mean that you can't simply trigger a scope's internal timebase with the output of the sweep generator, a true XY capable analogue scope is needed here.

I suspect one of the two oscillators, the buffer stage (1/2 V4) or the mixer (1/2 V5) is not functioning.

Cheers
Andy

[ChristianFletcher]

Hello Andy

If I look at the output of the signal generator on a spectrum analyser I can see only a 122-185 VFO signals I’m definitely not getting any high or lower heterodyne frequencies. Do you think that indicate 115 MHz oscillator isn’t running. So I’m just seeing the 122-185 oscillator. My reading of the circuit was the 115 is only used when in FM mode and it was out of circuit for line and frame mode. You have given me food for thought.

I plan to put the scope back on tonight and also take a look at the multivibrator. I read about the FM sweep for alignment but that a step above my pay grade.

Thanks for the useful comments any help much appreciated

Many thanks

Chris

[ChristianFletcher]

Andy you could be Genius!

I noticed tonight that looking at the spectrum analyser I did get a low output at the set dial frequency lasted about a minute before dying away. So I think I have a problem with the 115 oscillator. Wiggling the valve base brought it back briefly but was very inconsistent I’m think more my hand was capacitively coupling and with the valve making it work briefly. Looks like may be time to start swapping out those hunts capacitors

The picture on the left shows the correct heterodyne at 70mhx then the second and dominant F2. Picture on right lost my heterodyne.

[ChristianFletcher]

Well everyone, things that make you go ermmmmm

It’s putting out the correct frequency now within 1.5mhz of the dial frequency which I’m taking as being good enough. Well what did I do to fix it you may ask?

Absolutely Nothing!!

Went around it check the components with a meter and found it was ok. Thought it could be thermal related so was about to selectively spray the various resistor and capacitor in the 115 oscillator with freezer spay and low and behold the spectrum analyser shows it working. Been running now for nearly an hour and I cannot make it fail. Given everything a good poke and it just works. I’m guessing it’s waiting to be put back into service before it fails again.

Thanks for the advice I will let you know if it happens again.

Best regards Chris