Frame transformer for BRC 960

Taken from the Ferguson instruction manual. A black and white image of the 3635 version. Perhaps the first time the Thorn spike appeared on the front of one of their sets.

The Ultra Companion version is identical to the HMV 2633, only the branding differs.

Brian

Ran up the set earlier showing the top frame break up and creep is as bad as ever. Thoroughly cleaning the CRT and surrounded has paid off however as there's no longer the occasional ragged verticals nor ballooning. One fault to deal with is certainly better than three!

Looking at LL-J's servicing article mentions C79 (0.003uF) Part V8A grid coupling capacitor resulting in no frame lock. I've overlooked that one as I don't have any 1250V rated replacements to hand. I'm sure a 1KV type could be used for test purposes at least to rule the original out.

I'll give it a try later this evening.

Cheers

Brian

Unfortunately I'm still no further forward! Temporarily wiring in a 0.0047uF 250V AC capacitor made no difference, nor did substituting the 500pF Suflex capacitor.

What now? It's fortunate the fault reveals itself as soon as the raster appears which rules out the PFL200, EF183, EF184 and the PCL86.

Desperate times call for desperate measures. I'm not expecting results but progressively substituting the seven valves is the next step forward incase internal arcing in one of them is the cause.

Just to rule them out really as there's nothing left in the frame timebase to check!

Adjusting the linearity controls make no difference either....

Brian

Well that was a waste of time. Tried new PL504, PY800, 30FL14 and even another PCL805.

Absolutely no change so the old ones, remembering the PL504 is a French Mazda all went back in.

Re-assembling the set caused the system switch to jam again. Switch off quickly, manually set it, switch on and a perfectly stable raster appears!

I don't believe the system switch can be at fault as after all the frame side is identical for both systems. When the fault is present both 405 and 625 are similarly affected.

I can only guess there's a faulty component somewhere, the arcing sealing the problem. A classic intermittent fault in other words!

I know when the main smoother fails this can cause the sync to break up with the hum bar. There's no hum bar with this set however!

Hmmm.....

Hi Brian,

With reference to the Thorn RTS data in the library, I would be looking at components in the feedback circuitry of V8. Notably R111, R110 (top linearity pot), C88 and in particular R106 (2.2M). Any instability here could cause your symptom.

Hi Andrew,

All checked by substitution except the linearity controls themselves. They're a bit touchy and do need replacing which I will do in due course.

I don't think they're part of the problem however but I could be wrong.

Adjusting the controls with the fault not present certainly doesn't activate the fault condition nor stop it when it's present.

I'll also look at the system switch too. It certainly shouldn't jam as it does on occasions and in turn temporarily cure the fault! Unlike the conventional 950 they are difficult to get at.

Cheers Andrew for your help and advice. Very much appreciated.

Cheers

Brian

The switch does affect the height, the voltage is reduced on 625 to the height control by R66 620k on switch contact S2M.

I am using the 950 circuit from the library but that was quite a common technique due to a very often slightly higher boost voltage on 625.

Is there any chance the system switch could be tracking somewhere? Might be causing odd pulses to enter the field circuit and cause odd problems.....just a thought?

Of course you are right Frank! Perhaps the fault being identical on both systems suggested to me at any rate trouble in the frame timebase. Switching between the systems when the fault is present has no effect nor does the problem stop when present.

Nevertheless you could well have hit the nail on the head seeing system switch jamming seems to cure the fault for awhile!

Cheers and very many thanks.

Brian

sideband said
Is there any chance the system switch could be tracking somewhere? Might be causing odd pulses to enter the field circuit and cause odd problems.....just a thought?  

Could well be. As mentioned earlier access to this area is extremely difficult in situ so it's possible tracking could be occurring across the contacts.

I'll have to dismantle the set and remove the VHF tuner for access.

I think we could be close to a cure!

Many thanks for your help and input.

cheers

Brian

Switch contact S2M is interesting in the fact on 405 it affects the Line Osc waveform and 625 the height, typical of Thorn to squeeze every last saving in the circuit.

Indeed Frank. This can also explain why height on 625 is reduced on the 11" portable. It can be set correctly but results in excessive height on 405 so it's currently set to compromise. That set is mainly used on 625 as the performance is so good on that standard.

No doubt the resistor has changed value hence the lower height on 625. As the set is otherwise working perfectly I think I'll tackle the 960 first.

cheers

Brian

sideband said
Is there any chance the system switch could be tracking somewhere? Might be causing odd pulses to enter the field circuit and cause odd problems.....just a thought?  

This thought occurred to me also, as 50Hz AC is used to supply the switching solenoid this could possibly show as a disturbance in the frame stage. Probable causes could be the S2N contacts or S4 at the rear of the tuner.

Easy test, lock the system switch in one standard and o/c R147. If either of these are the cause the symptoms will disappear.

John.

Hi John,

This I must try. I think were now very close to solving the problem.

Who'd have thought the system switch/solenoid could have been the cause of the problem from the outset?!

Cheers and many thanks,

Brian

I'm not counting chickens yet Brian, just another thing to rule out. 

Well, with many thanks to everyone for their help, interest and contributions I now confidently believe the cause has been identified.

I'd removed the link to the system switch via R147 which gave a very clean blank raster. Temporarily re-connecting it still gave a clean raster (typical) but crackling on sound was apparent. Disconnecting it while the set was running immediately stopped the crackle. On connecting the aerial a rock steady Test Card "C" was displayed!

Well I've known system switches to give problems but nothing like this. Clearly arcing is developing either in the solenoid or the system switch contacts tracking. I've had problems with a set once which happily changed from one standard to another but only one way. For a permanent cure I had to fit a replacement system switch.

So that's that. For the time being until time allows for further investigation I'll use the set on 405 only.

Cheers and many thanks again.
Brian

In the Pye 11U series the switch on the signal panel used to go noisy but cleaned up ok, the one on the timebase panel usually had to be replaced if it was giving trouble, another item I had to keep on the van. At least the timebase switch was smaller and easier to replace.

Glad you have got to the source of the problem.

Hi Frank,

What had me fooled was the fault being identical on both systems. The switching worked between the standards ok apart from the couple of occasions when it jammed.

Perhaps the micro switch on the tuner was still partially energising the solenoid when it's supposed to briefly activate it on system change?

And funny how it jamming seemed to cure the problem!

cheers

Brian

Perhaps I shouldn't run this set when an apparent fault appears to have been cured.......

.......Back to square one I'm afraid with the top lines fluctuating with a crackle on sound when the audio stage comes in and white specks on the raster. I didn't need the Aurora to find the fault had returned!

So, the system switch solenoid and AC points can be dismissed as the cause of the problem as the supply has been disconnected. Perhaps the sliders themselves?

I haven't switched the set on for nearly a week. This might be significant as I'm sure the first time the fault re-occurred I hadn't switched on for a week or so.

Brian

It's obviously sulking, not turned on for nearly a week. 

Is the system switch visible, perhaps you could see some arcing on the contacts if viewed in a dark room.