Frame transformer for BRC 960

I fitted the transformer tonight. They look identical and was gratified to discover the resistances between the windings compared to the existing transformer.

However, I found the connections were reversed. Seeing they're look identical I decided to connect it up like the original one, mainly in the interests of the lead lengths in the set.

Wrong move as this was the resultant picture!!!

Unfortunately it also revealed the fault is still present, the only difference being upside down the raster started breaking up at the bottom instead of the top of course.

I'm extremely grateful to Chris for taking the trouble to send the transformer. The original was unfortunately damaged on removal but is repairable. I'll re-fit it once sorted out and will keep Chris' for spares.

Cheers

Brian

Focusdiode said
Wrong move as this was the resultant picture!!!

 Being dyslexic it looks right to me! 

The "C" is certainly the right way round suggesting it was back to front as well as being upside down!

The original transformer has now been refitted. It's possible I got the secondary wires to the scan coils transposed. Anyway it restored the correct scan and of course the original fault condition.

Via the height control incredible overscan is possible. In fact it's not set to maximum with this shot as frame lock is impossible when set to maximum.

The boost feed to the frame is just under 200V with no variations.

A possible fault condition to note is when one side of the secondary is not returned to chassis. There's shading on the left hand side which can be seen on the second photo.

Cheers

Brian

So, what next?

It's occurred to me the scan coils may be mounted upside down in this model, the result being I've probably tried shorting out the wrong side of the coils, ie opposite to the troublesome thermistor.

John Heatercathodeshort over on the UKVRR forum suggested checking the linearity sleeve.

I'll check this out later.

cheers

Brian

No further forward with the set I'm afraid. Shorting out the scan coil thermistor having established the coils are indeed mounted upside down (!) had no effect. Neither did running the set without the linearity sleeve apart from resulting in unacceptable line linearity. The frame continued to judder just as before!

The only things left are the small components. Two Suflex capacitors known to cause problems in other sets for instance. I understand silver mica types can be fitted in their place.

Interesting to note with the left hand shading caused when the frame transformer's chassis connection is omitted it's still the left side which is shaded despite the 981 transformer giving an upside down display.

cheers

Brian

Focusdiode said

Interesting to note with the left hand shading caused when the frame transformer's chassis connection is omitted it's still the left side which is shaded despite the 981 transformer giving an upside down display.

That's because all you've done is to invert the field scan, it's now scanning from bottom to top. To see the C and shading reversed you would have to swap the line scan connections ie scan right to left.

The shading when the earth connection is removed is due to it upsetting the frame and also line blanking fed to the CRT grid. 

Does the setting of the height have any influence on the interference?

John.

Hi John,

Thanks for the explanation. I only got to see the symptoms of shading when the chassis lead was pulled off on fitting the transformer. One would have thought a problem in the line timebase would have been suspected.

Reducing the height to minimum makes no difference at all to the symptoms.

Cheers

Brian

Hi Brian,

Could you just go over the exact fault(s) again now you have the correct scanning sense?  

Hi Andrew,

The fault initially was unlocked frame hold with interference on vision and cracking on sound. When I first encountered the fault I thought the casuse was external interference. Checking on another set proved the set was at fault.

The EHT doubler was found very loosely fitted on the LOPT. Fitting it correctly and the retaining strap cured the problem or so it seemed.

....Only for the fault to return soon after!

I finished off replacing the resistors in the frame output stage which resulted in a more stable lock but judders with the top part of the picture breaking up affecting the linearity.

As soon as the raster appears the fault is evident before the signals fade in after another 45 seconds.

The two remaining EHT sticks have also been replaced. The tray now has three BY1840(I think) diodes in it.

The photos look normal as it's a difficult fault to photograph. I managed these stills from a video I took.

The PCL805 has of course been checked by substitution!

Cheers

Brian

Hi Brian,

So we have what is in effect two separate faults. The frame issue, and then the possible EHT arcing/brushing causing the interference?  

I would tackle the EHT fault first. It is interesting that when you re-fitted the doubler, the fault cleared for a while. An intermittent connection would seem the obvious suspect here, and at risk of stating the obvious, have you checked to see if there is a snug fit into the LOPT nipple and that there is a reliable connection to the doubler? Also, you could measure from the top cap of V11 to the nipple and see if you have a stable 75 ohms or so. 

Hi Brian, if you can hear the interference after the initial warm up have you tried running with the EHT tray completely disconnected? If it's still there then it rules out both the tray and a possible CRT problem. It's very rare but I have seen the odd CRT produce effects similar to what you describe but it usually comes with slight width changes also in the affected part of the scan

John.

Edit. Post crossed with Andrew

Jayceebee said
have you tried running with the EHT tray completely disconnected? If it's still there then it rules out both the tray and a possible CRT problem. 

An excellent idea  

As for the frame fault, one thing that springs to mind is tracking across the PCB (or within it) especially around the valve-holder if the board is baked with use. It could of course be the valve-holder itself.

But lets do one thing at a time lol   

LLJ covered this chassis and its brethren in ** "article 156 which can be found in the 1968-1969 folder here". 

A scant read did not suggest he came up against this fault or similar, though a more thorough read might be in order which others might do.

** members with library access

Thanks very much John and Andrew for your helpful suggestions. I've already checked the connection to the LOPT and replaced the remaining stick rectifiers but have yet to try running the set with the tray unfitted.

Funnily enough the picture becomes very ragged momentarily which altogether looks to be an entirely different problem. This usually occurs a few seconds after the signals appear.

I've tried tapping valves in situ to no avail.

The PCL85 holder is a conventionally wired type so PCB problems don't apply to this model. The valve holder looks to be in good condition and wobbling the valve in its holder has bo effect on the frame scan or fault.

A CRT problem? That wouldn't surprise me!

Next I'll try running the set with the tray removed.

cheers and thanks for your helpful contributions.

Brian

Ooops....I've been looking at the 950 data  

I have been reading the LLJ article and no mention is made of your faults other than "heavily shaded raster", but this is for modified sets with a BY101 (going short circuit) in series with a 100 ohm dropper for the heater chain. I think your set has the capacitor dropper ?

So ignore my waffle.

Absolutely not Andrew. With the exception of the peculiar heater chain feature the set is otherwise virtually identical to the standard Thorn 950 chassis. In fact the 960 manual refers to the 950 chassis for further information.

cheers

Brian

It's an interesting set indeed, but the EHT problem is fascinating. Once we establish if it clears with the doubler removed as mentioned previously, the wood for the trees may start to be seen.

Oh, the aquadag earth bonding is secure isn't it?

I have to admit to never seeing a 960 in the flesh but remember being intrigued when I read about the capacitor dropper, even my experience of the 950 is weak as they were pretty much at the end of useful life when I started with DER.