Thorn 960 "Ignition" like interference

[Welsh Anorak]

Hi
I've found an 'open plan' NOS doubler (two sticks and a wire link) on the shelf. The EHT lead is only around 8" long though. Is this for your TV? If so it's yours for the price of postage.
Glyn

[Focus Diode]

Hi Glyn,

You have a PM.

Cheers,
Brian

[Focus Diode]

A further analysis of the fault symptoms reveals the interference is apparent before the signals fade in which rules out problems in the DC side of the heater chain. Ironically I've yet to experience any problems with that side of the set apart from having to replace the PCL86.

Seems they're two faults present. Still suspicious of the doubler and 180pF disc capacitor across the LOPT tags- the capacitor does run extremely hot.

Tomorrow's the day I'll be sorting it out, hopefully it'll give at least a week before the next fault reveals itself! Thank goodness it's my set and not one I've fixed for a customer!

Glyn- thanks for the EHT tray. Please PM me with your address and I'll reimburse you.

Cheers
Brian

[Focus Diode]

Unfortunately our efforts were not met with success. In fact the frame timebase is worse than ever. It runs much too fast. When one ended it slows to near locking. This can be achieved with the frame controls set the result is a small picture with bottom cramping..

What had I done to achieve these state of affairs? R103 2M2 crumbled in my hands so was replaced. For good measure seeing they're were original and the type prone to changing value R109 1K and R104 15K were also replaced.. R102 and R107, both 330K and R208 3M9 were changed earlier. I also replaced C81 and C82, both 0.02uF with 0.022uF types

A 2Meg height control also went in. Not very satisfactory and wished I adapted to 1M rotary control fed via a 1M5 feed resistor from the boost rail. I'll do that another time as it's probably impossible to obtain the correct replacement nowadays.

With the results above I couldn't understand why this resulted. It's not impossible, yet highly improbable for the two capacitors to be incorrectly marked. I take 223 to mean 0.022uf as 224 would mean 0.22uF. I double checked the connections and resistor values which are all correct as are the connections. Totally puzzling! There's about 300V from the boost feed to the height control.

We gave up after sometime trying to sort this out to no avail on this occasion. Best to sleep on it and have a fresh mind the next time. Think the set might need some DFWB expertise.

That's the update for now.
Cheers
Brian

[Focus Diode]

When stuck with a puzzling problem like this it's best to put it to one side and get back to it having slept on it.

Think I know what could be wrong: I think I soldered one end of C82 to Pin 2 and not Pin 1 of the PCL805! It's not surprising the frame wouldn't lock! Hopefully have it sorted when I get back to it this evening.

Brian

[FERNSEH]

Hi Brian,
The frame timebase circuit is unbelievably complex. But getting down to basics we find that the ramp forming capacitor is C81. In an attempt to improve linearity of the scan the "bottom end" of this capacitor is returned to the cathode of the output valve. A parabolic waveform is developed across C100 and this is added to the ramp waveform. Actually, increasing the value of C100 (100mfd) can make the frame linearity worse.
The frame sync pulse is supplied to the grid of the output valve by a differentiating network C85, C86 and R108. Thorn designers always favoured this method of frame synchronising rather than the usual frame sync integrator. Interlace is always excellent.
The ramp capacitor C81 is discharged through triode V8A at the point of flyback, the leading edge of the pulse is differentiated by C79 and R87 and coupled to the grid by C80. Most designers get away with one flyback feedback capacitor.
Those components C83, C84 and R105 need some explaination, it would appear that the flyback "spike" is fedback to the grid of the output valve, but why? Seems that C87 and C107 are there to function as some sort of speed up circuit. Precedence to sharp leading or trailing edges of the waveform, can't offer an answer.
Thankfully, the feedback frame linearity network is conventional. VDR Z2 limits the amplitude of the flyback pulse. With all these speed up circuits present the flyback pulse could be in the order of 1KV. That needs to be clipped to protect the transformer and valve.

DFWB.

[Focus Diode]

Thanks David l. It would be impossible to introduce the fault on a conventional 950. The 960 is wired in the frame, audio and line output stages. All too easy to make a connecting error.

The PCL805 is rather burdened with so many components connected to Pin 1.This image was taken with C82 removed and prior to fitting new resistors and a temporary height control. Tonight I'll put it right and tidy up the area as best I can.

Cheers
Brian

[Focus Diode]

I'll try and upload the photo. Doesn't always work when using the phone.

[Focus Diode]

The set certainly needs some DFWB expertise! I sorted out my error and fitted a 1M pot fed via a 1M resistor. This gave a correctly sized picture, well actually the control has to be set at maximum. Although the original interference problem seems to have been rectified the frame still occasionally slips.

Not ideal and certainly far from perfect.

[Focus Diode]

What's also unusual is the top half of the picture varies in height intermittently, not due to the controls, rather like how a hum bar can break up the scanning lines yet no hum bar..... Wonder if the frame output transformer is breaking down? Nothing would surprise me with this set! Do have a spare if it comes to that.

Yes, I have tried a new PCL805!

Next- discover why the height just fills the screen at maximum setting of the height control and what's causing the above conditions.

[Focus Diode]

More on the fault symptoms. Replacing the height control solved the lack of height problem.

As can be seen from these screen shots the raster appears to be breaking up at the top. It judders with occasional height variations.

Any shading is from the camera. Absolutely no hum bars or hum on sound.

Not seen symptoms like this before. Perhaps it might jog a few memories? The fault is definitely present during the first warm up phase which rules out the DC side of things. Line lock is rock solid.

Cheers
Brian.

[Focus Diode]

I've checked the scan coils and the position of the line linearity sleeve for signs of a possible breakdown of the insulation between the latter and the frame side of the coils to no avail.

Anyone any ideas of what could be causing this?

Cheers
Brian

[The teleman]

There is a thermistor on the scan coils that give similar problems to yours .
I've had these give trouble on all makes & models , it may look ok but as a test you can short it out & the set will work fine
Good luck
Chris

[Focus Diode]

Hi Chris,
I was beginning to think along the same lines knowing these thermistors cause trouble. I'll give it a try.

Cheers and many thanks.
Brian

[Focus Diode]

Hi Chris and everyone,

Unfortunately shorting out the thermistor in the scan coils made absolutely no difference. Viewing on a blank channel, or before the signals fade in reveals the original ignition like interference persists.

I'm wondering if I've been barking up the wrong tree. Close examination reveals the original interference problem persists. Next I'll attend to the doubler and the disc ceramic on the LOPT tags

The good news is, Fernseh has agreed to have a look. I'll be taking it over to his on Tuesday.

Cheers
Brian

[System A]

Hi Brian
You've probably checked this already but is the aquadag earthing on the crt ok ? . Just a thought.
Regards.
Gary

[Focus Diode]

Hi Gary,

Seems to be ok but unusually doesn't have the usual components at the end of the spring which could be due to limited space with them being mounted in a different location. Thanks for the suggestion.

Giving it a more prolonged run yesterday showed up other problems like occasional very ragged verticals not tamed by reducing the brightness and variations in the overall picture size. On an earlier occasion the signals and line sync faded away briefly, presumably through power to the four valves in the DC heater chain being deprived of power. This hasn't happened since.

Still, on a positive front the work needed in the frame timebase was necessary. Despite the fault symptoms lock is now much more reliable and will hopefully prevent other problems showing up in the future in the area.

The funny thing is, there is absolutely no sinister noises coming from within the set, nor signs of arcing in a darkened room.

I'll now focus attention on the components in the doubler which should improve EHT regulation if nothing else.

I'm very grateful to David for offering to look at the set on Tuesday. Many thanks again.

Cheers
Brian

[Focus Diode]

David very kindly showed his expert skill with the set. After making many tests and checking waveforms we came to the conclusion the frame output transformer may be breaking down.

Fortunately I have a spare from the scrap 1400 panel the LOPT came from. In due course I'll be performing a transplant.

Cheers and many thanks again David.
Brian

[FERNSEH]

Hi Brian, we've agreed all that is left to replace is the C core frame output transformer.
There is however a positive going 1KV flyback pulse present at the anode of the PCL85. Perhaps this voltage is too high and causing a possible breakdown of insulation in the transformer? Before replacing the frame output transformer it might be worthwhile replacing the VDR (Z2) which is connected across the transformer primary.

DFWB.

[Focus Diode]

Hi David,

Yes, I'll do that. These transformers are generally reliable so that could well be the cause, if not perhaps a fault in the transformer is causing the high peak voltages? The poor set did undergo a period where it was severely overrun when the original dropper capacitor failed which might have contributed to the failure of the original line output transformer as well.

Come to think of it I've had to replace the height control twice now, the original and first replacement developed dud spots so that could be related. The PCL85/805 has survived though.

I remember reading about a Thorn 1500 with lack of width and high EHT, (Think it was a Television magazine Test Case September 1981). The cause turned out to be the line output transformer!

Cheers and many thanks again.
Brian