BRC 900-960 intermittent wobbling verticals

[Focus Diode]

Been working on an 11" Ferguson Personal 3629. It's now working extremely well but found it suffers from intermittent wobbling vertices. They are not ragged which would suggest problems in the line output stage and doesn't appear to be heat sensitive.

On recently switching on the 960 portable I found exactly the same intermittent fault! Both line standards are affected.

This would appear to be a common fault on this series. Any ideas what I've overlooked? Line and frame lock is solid in both model.

The photo shows the 3629 running on 625-lines with the fault not present.

Thanks in advance for any suggestions.

Brian.

[thermionic]

Brian.

Have you tried attenuating the signal input to the sets? Maybe this will change your symptoms before getting too involved in searching for a fault!



SimonT.

[dazzlevision]

Hello,

Faulty selenium rectifier sticks in the EHT doubler/tripler can cause a fine raggedness of verticals. The effect usually comes and goes if the brightness control is altered up/down. It's usually only the original "open" style (Thorn manufacture) of doubler/tripler that suffers from this effect. Later types are "potted" and may well use Silicon semiconductor rectifiers.

"Wobbling" verticals (i.e. a slower change with time) in sets with flywheel sync (such as yours) can be due to the line hold control requiring a squirt of Servisol or a faulty STC "SenTerCel" selenium diode block in the line flywheel phase discriminator circuit (on the small piggy back flywheel sync PCB). Replace these with two small fast silicon diodes (such as 1N4148, possibly with a 0.25W series resistor for each; typically 56k or thereabouts). I'd also give the EF80 flywheel sync PCB DC amplifier valve pins and socket a squirt of Servisol and the plug/unplug the valve a few times to overcome any tarnishing.

Many components in the flywheel sync stage could cause this problem, but the three I mentioned above are, in my experience, the most likely. Finally, take a close look at the "Erie" carbon composition plug-in PCB mount resistors on the flywheel sync PCB. If the metal end caps that fit over the resistive carbon rod crack, this will cause intermittent effects.

Regards,

Dazzlevision

[Hybrid tellies]

I had the same intermittent fault on my Decca Bradford colour set. I eventually traced it down to the line output valve, you could actually see an area of blue glow inside the glass envelope which would vary in size and after the set had been on a while would sometimes disappear completely and we would have nice straight verticals.
So it might be worth substituting the line output valve on both sets.

[Focus Diode]

Hi and many thanks to you all.

Simon- Yes I remember the problem with the Decca Bradford. Replacing the valves in the 960 cured the original problem. The Ferguson Personal has a PL500 fitted! Very rare to see one of these.

Dazzlevision- I'll certainly look into this. Think I've only changed the large capacitor at the back so far. Thankfully its the most accessible panel in the set.

Simon T- I don't think it's down to excessive signal input. The Personal's UHF tuner is valved. I experienced signal variations earlier due to nothing more than a dry jointed centre pin on the isolated coax socket. One of the isolating capacitors was also broken. I soldered in a 200pf 8kv disc type as a temporary measure.

Thank you all again. I'll report back in due course.

Cheers
Brian

[Welsh Anorak]

Even rarer to see a PL505!

[greg_simons]

Agree with dazzlevision about the multiplier sticks being faulty, seem to remember this fault from the far distant past, those things sizzing away with changes in beam current, try them first,

[Focus Diode]

Hi Greg,
They're innocent on this occasion as the setting of the brilliance control makes absolutely no difference. Thanks for your contribution with your thoughts anyway.

Ran the Ferguson up for sometime yesterday in an attempt to catch the symptoms on camera. It sat there performing like a monitor of course, typical!
When I next have a day off I'll look at the flywheel sync panel as Dazzlevision suggested.

Cheers
Brian

[Focus Diode]

I haven't got round to checking the flywheel sync panel yet but had the Ferguson set running last night to carry out some final adjustments.

I've had to compromise between the two standards as setting up on 405 results in unacceptably excessive width and lack of height on 625. Vice versa results in excessive height and lack of width on 405! I think the compromise here is satisfactory. Ideally I should've used the same DVD source but the DVD player lacks phono plugs to connect to the Aurora unfortunately.

Pictures on 405 are rock steady. On 625 there's the occasional wobbles and gain variations on touching the system switch knob. Not the tuner as tapping that has no effect. I'll be examining the system switch therefore as something isn't quite making, presumably the supply to the UHF valved tuner. The switch is also Bowden cabled, not the solenoid found in later models.

In the past a "cure" would've been to hard wire the switch to the 625 position. Definitely something I don't intend to do!

Cheers
Brian

[Hybrid tellies]

Originally Posted by Focus Diode

Simon- Yes I remember the problem with the Decca Bradford. Replacing the valves in the 960 cured the original problem. The Ferguson Personal has a PL500 fitted! Very rare to see one of these.

I think, but may be mistaken, that the PL504 was a direct replacement for the PL500.

[dazzlevision]

Simon, you are correct.

[Focus Diode]

Indeed. Don't think many sets will be working with PL500s in, the 504 must've replaced it very quickly, same goes for the PL505 soon to be replaced by the PL509.

The 500 in the Ferguson could be a replacement seeing it's Mullard and not Mazda. The PL500, EF80 and PFL200s are Mullards, the rest are Mazdas.

[HamishBoxer]

CRT earthing ok?