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Old 1st Apr 2007, 10:14 pm   #1
Sean Williams
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Default TMB272 - 1st vision IF Oscillating

Hi all, well, title says it all really!

For some strange reason the vision IF is going off on one.....

I have tried changing decouplers, checking resistances, detuning transformers, changing valves, redressing wiring, adding shielding and screening, the only thing that stops the problem, is shorting the grid to deck!

Of course, this stops the picture too!

I have tamed the problem slightly by adding a grid stopper, and putting some R in series with the grid connection, but this also dramatically reduces the IF gain....

So, anyone come across this problem before?

Cheers
Sean
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Old 1st Apr 2007, 10:25 pm   #2
Focus Diode
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Default Re: TMB272 - 1st vision IF Oscillating

YES! I worked on one of these sets ten years ago. On replacing the Hunts capacitors in the sound channel this gave sound, but created a new horrendous sound of vision problem.

On TV ch1 I had to adjust the os coil to get sound and vision together. The whole IF strip seemed to go into oscillation giving black and white horizontal bars. Reducing the contrast restored some vision, but the whole IF strip looked to be badly misaligned, with multiple ghost images, akin to a colour set with a faulty luminance delay line.

Fearing what I had done I realised simple adjustment of a ch1 osc coil couldn't possibly misalign a whole IF stage. I then replaced the remaining Hunts and wax capacitors in the IF strip which gave normality yet with gross overloading. This was tamed by fitting a 1Kohm resistor in the aerial input to attenuate the signal from the modulator.

Hope this is of some help Simon.
Cheers,
Brian.

Last edited by Focus Diode; 1st Apr 2007 at 10:35 pm. Reason: More info on the fault condition and cures added
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Old 1st Apr 2007, 10:29 pm   #3
Sean Williams
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Default Re: TMB272 - 1st vision IF Oscillating

Hi Brian,

Thanks for that, I have (as far as I can see) changed all of the capacitors in the IF stages - I will have another look around this week....

Daft really, as the set was working just fine......

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Sean
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Old 2nd Apr 2007, 8:00 am   #4
Mike Phelan
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Default Re: TMB272 - 1st vision IF Oscillating

Sean
You may have missed a Hunts somewhere - most Ekcos at this time had this problem; we used to block-change them in the 1960s because of IF instability.
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Old 2nd Apr 2007, 12:24 pm   #5
Sean Williams
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Default Re: TMB272 - 1st vision IF Oscillating

Alan Partridge moment.....

It wasnt oscillating after all...

The crazy way the set gets it AGC was the problem - I can only assume that the AGC was not working when I first got the set going, and some frenetic valve wiggling bought it back to life..., Anyway, the cause of the problem appears to be a previous (before my time) repair - R25 on the trader sheet shows 330R - this is the resistor that the AGC voltage develops across - in My set this had been changed to 330K - thus -15 volts applied to the grid, when only 0.5v at cathode will result in one comprehensively shut down stage!

Normal service now resumes!

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Sean
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