1964 Rediffusion Mk8

[Colourstar]

OK, some progress to report. I'm pleased to say that Graham's Telebox is in fact working, so we're in business.

After replacing three very ropey 0.047uf caps on the timebase panel and a 250uf electrolytic (30PL14 cathode bypass) the frame has opened up albeit with some issues still to resolve. Both line and frame holds are right at one end of their travel, so no doubt the associated series resistors are out of spec.

The LOPT doesn't appear to run warm but unfortunately the picture loses brightness after a few minutes running, the width comes in and the picture centre darkens. Possibly the age-old problem of dampness after years of storage. The photo shows the current state of affairs. The foldover on the left due I think to the line hold being right at the limit of adjustment. The set is of course running at 625.

teve

[FERNSEH]

Hi Steve,
I hope the problem is not the line OP transformer.
The line output stage circuit is drawn in a rather difficult to understand manner. I'd reckon the boost capacitor is C189.
The fact that the picture is distorted on the left side indicates that it is the reclaimed part of the waveform is missing.
The attached circuit diagrams might help.

DFWB.

[Colourstar]

Hi David, Yes I reckoned c189 was the boost too. It was a TCC Supamold type and was evicted as a matter of course.

Having looked again at the symptoms, the picture starts to slowly balloon and fade out after the set has been on for only a couple of minutes. The width in fact stays steady although is insufficient and with the foldover on the left. R25 is in series with the line hold control but is still well within tolerance (470k). The control remains hard over to one end to lock a picture. The line osc valve is an ECC82 and I know these can age, moving the lock. I'll have to see if I have a new one somewhere, but did a quick swap with the other ECC82 on the board to no discernible affect.

Difficult to progress further with the short window of time before the picture vanishes.

Steve

[FERNSEH]

Hi Steve,
it's a long shot, but I'm sure there is a spare LOPTx in the shop somewhere. The search is on.

DFWB.

[Colourstar]

Now that would be impressive!

I've whipped the LOPT out (done in seconds as it unplugs and the assembly unclips) and it appears to be a later version than the one on the Trader Sheet I'm looking at. That one shows an EY86. Mine uses a DY87 (Rediffusion branded original) and has a different component layout.

If the problem is due to damp ingress, I wonder if a spell in the airing cupboard would revive it.

Steve

[FERNSEH]

Hi Steve,
that's unlike the one I have. Mine has edge connectors instead of plugs and sockets.
A spell in the airing cupboard might well drive out any moisture.

DFWB.

[whyperion]

Found some of the original paperwork for this, which I will scan when I find the time.
Looks like it was bought outright in 1963, (Dec) (about £49inc tax), and no service contract option was taken up. In 1969 they appear to be renting a later model(at£47 a year) (still B&W), so it is possible that a fault developed in 1969 and rather than repair , something new was sourced.

[MALC SCOTT]

When i worked at Rediffusion these faulty loptx were sent to the Rochdale factory to be "triplerised" The faulty overwind was removed and a 3 stick (i think) tripler fitted. This practice went on untill the Mk11 chassis. The eht regulation was never as good with the tripler but the customers never complained, Malc.

[Colourstar]

Originally Posted by whyperion

Found some of the original paperwork for this, which I will scan when I find the time.

Great news! I'd love to see this.

Originally Posted by MALC SCOTT

When i worked at Rediffusion these faulty loptx were sent to the Rochdale factory to be "triplerised" The faulty overwind was removed and a 3 stick (i think) tripler fitted. Malc.

The 'tripler trick' may yet be the way to go on this set.

Steve