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PYE CT 72

 
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Re: PYE CT 72

Post by PYE625 » Wed Jul 15, 2015 10:07 pm

Katie Bush wrote:Was that the big brute, next to the blue/white (blue/yellow) Dubilier under TB/power chassis?


Hi, this one was from the frame board, but there was another in the line stage too.
I'm still waiting for the 0.47uf 1000v to arrive, the last one to be fitted and then it should be all systems go!

Like the electrolytics, the Dubilier types were generally measuring much higher on a capacitance meter than their rated value .
I found all but one was in fact slightly leaky when tested at 500vdc (half their rated voltage).
I have been fitting LCR types as replacements and naturally, these have no leakage whatsoever.

Granted, the Dubilier plastic types are not as leaky as old wax capacitors, but in high voltage stages they are not trustworthy.

 
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Re: PYE CT 72

Post by PYE625 » Thu Jul 16, 2015 6:15 pm

Hi,
Now up and safely running again after the cap change.

Checking EHT level.
I made up a string of 10x 33meg hv 1watt resistors in series and inserted them in PVC tubing.
I started with the set switched off of course and a careful routing of the resistor network from the anode cap of the tube.
I used an AVO 8 to measure the current at the earthy end of the string to ground.
I set the EHT preset to minimum and slowly increased it until I had a reading of 76 micro-amps on the AVO.
This should correspond to approximately the correct voltage of 25kv EHT.
It couldn't have been too far out as the position of the preset was pretty much as it was before.

Last pic is of the resistor string.
Attachments
rsz_img_1945.jpg
rsz_img_1947.jpg
rsz_img_1951.jpg
Last edited by PYE625 on Thu Jul 16, 2015 6:48 pm, edited 1 time in total.

 
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Re: PYE CT 72

Post by malcscott » Thu Jul 16, 2015 6:25 pm

Nice work !

 
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Re: PYE CT 72

Post by nuvistor » Thu Jul 16, 2015 8:38 pm

Great to see it working and to know the suspect parts are replaced, hope it carries on giving many years service.

 
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Re: PYE CT 72

Post by PYE625 » Sat Jul 18, 2015 8:34 pm

Well, I have had it running now for a few hours on and off and no further issues except a minor, perhaps normal, one?...

Even though the EHT is set pretty close to the right level, when the set has warmed up after a good 20 mins or so, the width has shrunk in about a cm either side. The width control is set to maximum and the raster only just fills the screen.
Ok, I'm being picky I guess, but I would have thought there should be plenty of over-scan available.

Cheers,

 
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Re: PYE CT 72

Post by nuvistor » Sat Jul 18, 2015 9:09 pm

I don't remember major picture size changes when these sets warmed up. There was always enough width, the frame output with the BD124's was marginal but always enough to slightly overscan.
Check the EHT after it has warmed up, if it drifts high it will reduce the width and height somewhat.

Frank

 
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Re: PYE CT 72

Post by malcscott » Sun Jul 19, 2015 12:43 am

Check the ht volts, smoothing caps could be suspect, Malc.

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 20, 2015 7:33 am

Thanks for the tips.
EHT and HT all check out fine, no drift , so at a loss as to what it can be. :aaq

The above picture with TCF was taken after the set was on for about 10 mins, as you can see, it only just fills the width of screen.

 
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Re: PYE CT 72

Post by crustytv » Mon Jul 20, 2015 7:46 am

Not sure if you would class this as poor EHT regulation but lets just imagine it is, then my stock faults for the 691 one suggest the following:- Tripler, LOPT, the beam limiter needs setting as per the manual and EHT should be set for 25kV at black level. There's nothing else listed.

Although this is not intially lack of width, it might be worth checking the following:- PL509, PY500, R223, R224. VDR2 RV17. Screen voltage on pin 6 of V2, R231

 
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Re: PYE CT 72

Post by nuvistor » Mon Jul 20, 2015 7:56 am

There are a few components worth checking if you have not already replaced them.
C219 170pf
C221 200mf
C225 200mf

and any component connected to the LOPTX that could damp the TX, shift control cct, line lin etc.

Have you tried both output and boost valves, if not those are the easiest to change.

Frank

 
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Re: PYE CT 72

Post by malcscott » Mon Jul 20, 2015 7:57 am

Remove the loptx front metal cover. There is a 180pf 8kv disc cap on the loptx, replace that. Also adjust the width coil on loptx, it may improve things, Malc.

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 20, 2015 9:01 am

Cheers fella's,
quite a bit to go on...will check the suggestions later :aad

Many thanks indeed.
Andrew.

 
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Re: PYE CT 72

Post by Doz » Mon Jul 20, 2015 3:08 pm

I'm a bit green with envy over this one!

Lovely set

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 20, 2015 3:12 pm

Hold on Doz, I'll reduce your green drive a little....

there, any better? :bba

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 20, 2015 6:35 pm

Hi, well there is no shortage of EHT and it seems to be pretty stable and well regulated, even with brightness variations. The beam limiter is working correctly too.
All the components check out ok and I think the problem is not to do with the EHT directly, but possibly to do with the line scan side of things.
I can increase the level of EHT beyond the recommended maximum and achieve more width, but of course, cannot treat that as a solution.

The slug of the width control is fully out.
Now, there is another inductor, L39 I think, that has a core that is fixed. If I insert a small screw driver into the coil, the inductance of course alters slightly and the width will increase. I am not sure of the purpose of this coil, but it goes from the wiper of the line shift control to the scan coils plus convergence circuitry. (L40 the width control is on the other side of the scan coils).
:aaq

 
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Re: PYE CT 72

Post by Tazman1966 » Mon Jul 20, 2015 7:07 pm

Give the Line Linearity core a tweak (making sure you've a square ended trimming tool). It might compromise the lin a bit but that'll probably do it.

Tas

 
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Re: PYE CT 72

Post by nuvistor » Mon Jul 20, 2015 7:26 pm

The red green symmetry is in series with the scan coils, are these components OK, it's a guess as the convergence looks ok but those convergence components work hard.

 
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Re: PYE CT 72

Post by PYE625 » Tue Jul 21, 2015 9:08 am

Thanks all,
Cheers for the advice :)

I had forgot just how warm these valve colour sets used to run....heat belts out from the line stage lol.
Certainly a lot of power used in the circuits.
This set is hardly going to be used for long periods, not like it would have been way back when it was a current model.

 
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Re: PYE CT 72

Post by malcscott » Tue Jul 21, 2015 9:35 am

No harm running this set for long periods, unlike the later version with the dreaded printed circuit psu/lop panel. We used our Ekco CT107 as our only set from 1971 until it was retired in 1985. The only modification i carried our was to fit stand off valve bases to the cda board, Malc.

 
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Re: PYE CT 72

Post by Doz » Tue Jul 21, 2015 1:15 pm

PYE625 wrote:Hold on Doz, I'll reduce your green drive a little....

there, any better? :bba


Perhaps a little more red & blue ? :aak

 
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Re: PYE CT 72

Post by Doz » Tue Jul 21, 2015 1:17 pm

PYE625 wrote:I can increase the level of EHT beyond the recommended maximum and achieve more width, but of course, cannot treat that as a solution.


Surely decrease the EHT to achieve more width?

 
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Re: PYE CT 72

Post by Cathovisor » Tue Jul 21, 2015 2:06 pm

Doz wrote:
PYE625 wrote:I can increase the level of EHT beyond the recommended maximum and achieve more width, but of course, cannot treat that as a solution.


Surely decrease the EHT to achieve more width?

Has to be said, I was thinking that too.... :ccf

 
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Re: PYE CT 72

Post by PYE625 » Tue Jul 21, 2015 3:35 pm

No, it is definitely and increase in EHT that increases the width....I tried it whilst measuring the EHT with the resistor string.

 
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Re: PYE CT 72

Post by Doz » Tue Jul 21, 2015 3:47 pm

PYE625 wrote:No, it is definitely and increase in EHT that increases the width....I tried it whilst measuring the EHT with the resistor string.


I suspect it's not only the EHT that's being altered then... More EHT=faster electrons=more difficult to scan=less width (and height).

I suspect more EHT=more scanning power as well (In this set), so an increase in EHT is also increasing the scanning power. Helps with variations in beam current to stabilise the picture size on top of the pops!

 
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Re: PYE CT 72

Post by PYE625 » Tue Jul 21, 2015 4:00 pm

:bba It increases the drive to the pl509, so i guess it would increase the line scanning power too.
The height does in fact decrease as the EHT is raised.

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