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Focus VDR/Metrosyl altering value when warming up....  

 
Alastair
(@alastair)
Reputable V-Ratter Registered

I have a rather odd fault on CVC2 where the focus will vary.

It (if focus is set while set is cold) will go out of focus when the set's been on for say 10 minutes. If set when hot, then next time set turned on from cold--will be wildly out of focus.
Once fully warmed up and focus set right, the picture is razor-sharp--even on bright bits....

This Metrosyl is supplied at its top end by full EHT from a dedicated lead from the tripler, to a 6.8M at the top of the stack and at bottom end via 2.2M to deck. The rod is prolly around 9-10" in length, and focus is tapped off approx 3" from bottom end...

Both these high-value fixed resistors and the series one (100K) at the CRT base have been replaced.

Any ideas as to what I can do about this, or any idea what could be used to replace it, I did think of summit like a TX10 pot as I recall, they were dropped from EHT too...?

--In the absence of the more usual (of the era) shunt-reg/EHT rec, I suspect this part also gives some form of EHT regulation in this set.....

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Topic starter Posted : 03/10/2015 4:29 pm
The_Teleman
(@the_teleman)
Active V-Ratter Registered

Hi Alastair E , this type of focus slider does indeed go faulty , I remember servicing these sets & on several occasions had to replace the vdr stick .
I can't think where you might get a replacement but having said that the PYE hybrid focus vdr might be easier to find & could be used in this position as a substitute .
The bad news is you could have a faulty crt too .
Just as a thought check the crt base as I've had these cause this fault too .
Let us all know how you get on

Chris

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Posted : 03/10/2015 4:54 pm
Alastair
(@alastair)
Reputable V-Ratter Registered

Hi Chris--Thanks for the info...

I don't think the CRT is at fault--The original did much the same--except the focus and grey-scale were pretty awful in comparison to this replacement, it still had the focus shift from cold to warmed up.

CRT Socket--Well, There's a possibility I haven't checked yet, and any spark-gaps there may be...

Right, looks like then--IF this rod is faulty--can it be repaired--doubt it. Therefore, replacement or modification for alternative arrangements...

Looking for ideas.....! :elc:

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Topic starter Posted : 03/10/2015 7:24 pm
Katie Bush
(@katie-bush)
Famed V-Ratter Moderator

I'm only thinking aloud here, but the good old Philips G6 used a long chain of high value resistors from (focus) EHT supply, to chassis, with the focus potential tapped off part way down.. This could supply the focus potential, but wouldn't really help to stabilise the EHT to the final anode.

Late 50's EKCO TVs used to use a rod arrangement as an EHT stabiliser.. If I recall correctly, it was sometimes quite effective to take the rod out, invert it, and refit it.. I'm not altogether sure how that worked, but it cost nothing and more often than not effected a long term cure.

Marion

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Posted : 03/10/2015 7:39 pm
Alastair
(@alastair)
Reputable V-Ratter Registered

Aye--Thats ringing a bell. My old G6 had an EY51 in the focus supply and loads of 5 odd meg resistors....

But cant think Where the high-voltage was sourced from, I don't think it was from the EHT or overwind--Was it doing summit silly with a supply from top-cap of the PL509....?

--That sounds like a plan--maybe there's enough high-voltage if rectified to supply a simple divider and pot--I could even use an EY51 if I can find one! :aak

I dont really want to mess with its original tripler, even though I have a universal type I could use, to get a focus supply, rather keep its original as-is....

EDIT--

Just found on the web, Schematic of G6 single-standard....

EY51 connected to PL509 top-cap then a chain of resistors to focus-pot! :thumb

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Topic starter Posted : 03/10/2015 8:41 pm
Rebel Rafter
(@rebel-rafter)
Active V-Ratter Registered

Hi, folks, RR here. It looks like a possible solution here might be to use a universal tripler and a flat pack type focus pot like the one on the Rank T20 for instance. If you want to use a TX10 type focus pot make sure you use the later tall narrow type, the earlier short fat ones were no good. You can still get the TX10 focus pots from a site called easywebstore and I think Donberg electronics in Ireland might have some still. The TX10 focus pot does indeed run direct from the main EHT feed and the pot has a resistor of something like 165 megohm in series with the top end, the pot is 25 meg and it has 60 meg in series with the lower end and doesn't return to earth, on the TX10 it returns to the 150 volt HT rail via 180k, (R732) It also has a lead from the low end of the pot that connects to the cathode of the first built in rectifier diode in the power transformer's EHT windings. This might not be needed in your set, try it at your own risk... RR.

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Posted : 03/10/2015 11:38 pm
Alastair
(@alastair)
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Looking at the Philips G6 schematic, the EY51 anode connects direct to PL509 top-cap. The Cathode connects to the top end of the divider chain and also to a 270pF C5018 Capacitor and via the system-switch, and a resistor R5051 to either one or the other of the two line-output transformer taps that also lead to the flywheel-line sync diodes...
--Whats That all about....?

--The output from the focus-pot is decoupled to deck via a 270pF cap.

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Topic starter Posted : 04/10/2015 2:37 pm