ITT CVC32/1 22" CTV Wont start up

[yampy187]

Hi All

I'm Having a spot of trouble with the above set. It was working perfectly for a few months after rectifying various other faults described in this thread;

https://www.vintage-radio.net/forum/...d.php?p=608389

Its now refusing to start up. just makes a squealing noise from the mains transformer. I have tried re-setting up the SMPS as per the service manual from Radio and Television Servicing 1977/78. But to no avail. I have changed a couple of poorly looking electrolytics but haven't tried anything else yet.

Has anyone got any ideas? Any help would be much appreciated

Cheers

Jan

[MALC SCOTT]

Try without tripler connected.

[yampy187]

Hi Malc,

Dis-connected the tripler, still nothing

Cheers,

Jan

[Danny]

Check for shorts on the b+ rail IE the supply to the line output transistor you can do this the quick way by taking resistance readings from the line output transistor and deck if you are using a digital meter then use the diode setting and reverse the prods this should give a reading one way and high resistance the other way but do make sure the set is unpluged when you try this and check there isnt any voltage remaining in the b+ capacitor. if a short is pressent then disconnect the line output transistor and check it for being shorted if it checks ok then trace the b+ supply back cheking things as you go.Danny

[Studio263]

The traditional method is to pull out the line oscillator module first. If the power supply then starts up you are looking at things one the secondary side of the line output transformer (tripler, LT supplies etc) or possibly shorted turns in one of the secondary windings.

If the power supply is still unhappy then remove the scan coil plug, this disconnects the HT supply to the line output stage. If the power supply then starts up you are looking at stuff like the line output transistor being shorted or its tuning capacitor being up the creek. The line output transformer is also still suspect.

In fact the line output transformers were very weak in these, they are easily ruined by a faulty tripler (despite the sensitivity of the trip circuit). If you still can't get to the bottom of it check the soldering on the multi-way plug on the scan coil assembly (a regular trouble spot for burn-ups) and the large high value resistors on the power supply control module, which often seem to change value.

[yampy187]

Hi all,

Thanks again for the replies. I have just come back to this set. Now it is totally dead. No noises, nothing. Except for R78 and R81 are hot. I have pulled out the scan coil plug, removed the line osc panel, disconnected the tripler, no shorts on the B+ rail and the line o/p transistor is ok...

I'm really starting to get puzzled by this one...

Jan

[AidanLunn]

Check for any resistors that have gone high, possibly as a consequence of the original fault. You might also want to check that the diode bridge (if it has one) is all intact.

[Hybrid tellies]

Might be an idea to check the unstabalised DC line from the mains rectifier to the collector of the switch mode transistor. If this line is low it could be the main reservoir capacitor or some of the diodes of the full wave mains rectifier.
CAUTION Be careful here because you are working on the live side of the power supply and that main capacitor, if in good condition, can have over 300v across it and can hold its charge for a very long time if its unloaded..