TV62 picture tear at top

[Duke_Nukem]

A few years back I bought a second TV62 as a donor for my better TV62. It was minus a lopt, a couple of resistors and a few valves. But I eventually took pity on it. All the wiring on the power chassis was crumbling so a major stripdown _had_ to be done including removing the tag strip. I hate mass changes but no real option. Amazingly though I seemed to have done it right (talk about a miracle) as it now performs really well - except for the tearing at the top of the frame, possibly only affecting the odd field ?

In the back of my mind was some sets not liking the aurora's equalising pulses so switched that off. And it got worse. Now, maybe it had been off and we'd just turned it on, whatever, the effect was still there. A Bush TV56 (which has same sync circuit) was pressed into service and that worked just fine.

The TV62's sync is excellent, very stable to the point that you cant get the frame to roll. Mikey405 had the excellent idea of adjusting the frame hold to see if the fold at the top moved with the picture (=> sync is source of error) or stays at the top (=> maybe frame getting into line osc). It moves with the picture, as can be seen in the third attached picture.

Caps in sync circuit replaced, resistors checked (and some changed anyway), valves changed, disciminator diodes replaced, what I assume is interlace filter diode (?) replaced, ECC83 changed to ECC82, you name it, nothing changes the effect. Run out of ideas.

Had the set running for two hours, during which it was absolutely solid except for this tearing, the magnitude of the tear not changing.

C5 in sync circuit is where voltage is derived that feeds to control or line osc. You can see some frame on it (alternating large and small blips) but isn't this to be expected ?

TTFN,
Jon

[Lloyd 1985]

This looks familiar! My own TV62 had some odd problem similar to yours, the raster curved inwards at the top at both sides, and the top of the picture wiggled around. I had a good look round the circuit and the component layout, and found some odd components on top of the blocking oscillator transformer that were not on the diagram. Turned out the transformer was open circuit, and the extra parts had been added to get it goin again. I unwound the transformer and bodged the breaks then crudely rewound it, to my surprise it worked! Perfect picture. I'm sure I posted about it over on VRAT last year or maybe the year before.

Regards,
Lloyd

[Peter.N.]

Does it have a bypass capacitor on the video output valve cathode? I had a similar sync fault only much worse on a '50s Philips may years ago and that was the problem, it was o/c.

Peter

[Boater Sam]

I have seen elsewhere that frame blocking transformer breakdowns are common on the early series Bush models. I think it is documented on Radios-TV forum.

[FERNSEH]

Check the waveform of the sync pulse train at the anode of the sync separator.
It could be possible that the frame sync pulses are reduced in amplitude compared with the line sync. Because the frame sync takes the form of a serrated 400 microsecond broad pulse it follows that the line sync will also be reduced during the period of the frame sync.
There is a 33Kohm resistor (R33) which is connected between the screen grid and cathode of the video amplifier V7 (PCF80) on the receiver chassis. This resistor was known to go low value and cause problems with the sync.
Also check R34 the cathode bias resistor of V7. (220ohms)

Check the HT smoothing capacitor C33.
The coupling capacitor between the video amplifier to the grid of the sync separator is located on the receiver chassis. C39 0.1mfd. Likely to be a TCC waxie.

DFWB.

[Duke_Nukem]

OK, taken a bit of a look about:

Originally Posted by Lloyd 1985

..found some odd components on top of the blocking oscillator transformer that were not on the diagram. Turned out the transformer was open circuit

Everything was original - though I changed the two caps on the transformer. All windings seem to measure OK, and compare with transformers on a couple of scrap chassis I have. In the search for the problem, also changed both diodes but no change.

Originally Posted by Peter.N.

Does it have a bypass capacitor on the video output valve cathode?

There's no bypass, the cathode is biased via a 33K to HT and a 220R to deck.

Originally Posted by Boater Sam

I have seen elsewhere that frame blocking transformer breakdowns are common on the early series Bush models.

I think Bush had now seen the error of their ways, it uses a multivibrator circuit.

Originally Posted by FERNSEH

Check the waveform of the sync pulse train at the anode of the sync separator.
It could be possible that the frame sync pulses are reduced in amplitude compared with the line sync. Because the frame sync takes the form of a serrated 400 microsecond broad pulse it follows that the line sync will also be reduced during the period of the frame sync.

Can't scope at the moment, will compare with TV56 when I next get the chance.

Originally Posted by FERNSEH

There is a 33Kohm resistor (R33) which is connected between the screen grid and cathode of the video amplifier V7 (PCF80) on the receiver chassis. This resistor was known to go low value and cause problems with the sync.
Also check R34 the cathode bias resistor of V7. (220ohms)

Good plan, checked them but they are well within tolerance

Originally Posted by FERNSEH

Check the HT smoothing capacitor C33.

Tried bridging it with some more capacitance but no effect. I did notice that after running for an hour there was slight frame breathing but the amount of "kink" at the top of teh picture remained steady.

Originally Posted by FERNSEH

The coupling capacitor between the video amplifier to the grid of the sync separator is located on the receiver chassis. C39 0.1mfd. Likely to be a TCC waxie.

Although little has been done to the receiver, all three caps between video, sync and agc line have been replaced (two waxies on rear panel plus sneaky 'unts on the top deck) and associated resistors tested.

The receiver seems to be working well, the forth row of frequency bands on the test card are resolved if I turn the brightness down (yet to fiddle with the focus connection).

TTFN,
Jon

[dominicbeesley]

I'd be tempted to check those diodes. I had a similar issue in my VT4 and it turned out to be a leaky diode and a leaky cap.