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PYE CT 72

 
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Re: PYE CT 72

Post by PYE625 » Sun Jul 12, 2015 12:44 pm

nuvistor wrote:just looked in the library and a suggestion is check R35 and R38 on the IF panel.

edit. the resistor across the line coil did give trouble and caused striations down the left of the picture.


Indeed there are striations down the left, noticeable very much on a blank raster :bba

 
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Re: PYE CT 72

Post by nuvistor » Sun Jul 12, 2015 12:46 pm

although nothing to do with this fault, if the LT transformer is original i.e no thermal fuse, it would be worthwhile fusing the LT supply to the rectifier, couple of fuses. If the bridge goes s/c it usually takes the TX with it.

 
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Re: PYE CT 72

Post by PYE625 » Sun Jul 12, 2015 12:54 pm

PYE625 wrote:
nuvistor wrote:just looked in the library and a suggestion is check R35 and R38 on the IF panel.

edit. the resistor across the line coil did give trouble and caused striations down the left of the picture.


Indeed there are striations down the left, noticeable very much on a blank raster :bba


And that resistor was reading about 600 ohms....so I replaced it with a 3watt wire-wound 1k5.

 
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Re: PYE CT 72

Post by PYE625 » Sun Jul 12, 2015 12:54 pm

nuvistor wrote:although nothing to do with this fault, if the LT transformer is original i.e no thermal fuse, it would be worthwhile fusing the LT supply to the rectifier, couple of fuses. If the bridge goes s/c it usually takes the TX with it.


I will look into that too.....many thanks for the tip :aad

 
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Re: PYE CT 72

Post by malcscott » Sun Jul 12, 2015 2:07 pm

100k in series with line hold control.

 
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Re: PYE CT 72

Post by Refugee » Sun Jul 12, 2015 2:45 pm

Under the line scan chassis I spy a horrid yellow and blue capacitor that may well have caused the line hold problem. It is just above the transformers.

 
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Re: PYE CT 72

Post by PYE625 » Sun Jul 12, 2015 3:41 pm

malcscott wrote:100k in series with line hold control.


This resistor was completely O/C.
Replacing it cured the fault. All other components checked out ok.

Many thanks for the advice. :)

 
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Re: PYE CT 72

Post by malcscott » Sun Jul 12, 2015 3:47 pm

Common fault back in the day. The sync seperator base bias resistor (4m7) on the i/f deck will give poor sync if high res. I would be proud to own such a set like this. I have the Ekco CT107 version from 1970. Been dry stored since 1983. I am tempted to dig it out, Malc.

 
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Re: PYE CT 72

Post by PYE625 » Sun Jul 12, 2015 3:56 pm

Go-on Malc....dig it out and give it a spin !!

 
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Re: PYE CT 72

Post by Katie Bush » Sun Jul 12, 2015 7:57 pm

What's that "Dubious Dubilier" up to in there? (blue and white), and dare I ask, what is that black Callins looking thing? (next to it).

It shouldn't take too long to sort out, but I'd guess there'll be a few more little hiccups along the way.

Marion

>>>EDIT<<<

Ah rats... I looked at the pictures and dived straight in! Ignore me, I'm having "that sort of day" today...... :ccb

Marion

 
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Re: PYE CT 72

Post by Tazman1966 » Sun Jul 12, 2015 7:58 pm

What a corker of a set! A lovely result. While you have the set open, for some long term preventive maintenance I would change the A1 decoupling capacitor on the underside of the line timebase - it's C224 and is 0.1uF at 1kV. I've yet to come across a Pye hybrid either now or in the "olden days" where this doesn't go short circuit at some point taking out the associated resistor (100k) which then falls to some amazingly low value and to all intents and purposes shorts out the boost line - the PL/PY don't appove of this, lol! Also if the boost cap mounted on the LOPT is one of the half blue, half white ones, I'd change it too. and then....

ENJOY this wonderful set. Time to evict the Philips G8 from the dining room and install my Pye CT205 (697 chassis)!

Edit: Just spotted that C224 in your set is one of those half blue/half yellow Dubillier caps - change it :)

 
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Re: PYE CT 72

Post by PYE625 » Sun Jul 12, 2015 8:46 pm

Thanks for the advice, I will replace the cap's suggested as I sure don't want any catastrophic failures !!

Just removed C224 and discovered a split in the case, so jolly good advice. :aad

The only other thing I was thinking about is how do I know if the EHT level is correct without an EHT meter? :aaq
Is there anything else to measure as an approximate safe guide?

Thanks,
Andrew.

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 9:33 am

To answer my own question above, I'm wondering is application of ohm's law is a solution....

25kv EHT across the 5.6meg focus resistance should give 4.46 mA at the earthy end if my calculation is correct.
All I need to do is to measure the exact resistance of the focus resistor and measure the current at the ground terminal.

Sounds simple, but there is a problem. The focus voltage looks to be taken off a VDR in series with the 5.6meg.
I bet it could well screw up any readings. :aaq

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 9:47 am

ppppenguin wrote:I rather doubt that the EHT generator would be supplying anything like 4mA. Perhaps 1.5mA maximum.

I don't think there's any realistic alternative to an EHT meter. You can make your own EHT probe with a string of 33M VR37 resistors. Mine has always worked well.


Thanks, I did think my calculation was wrong as the focus resistor would be dissipating 111 watts !!
You are right, there is no substitute for actual measurement of the EHT.

Cheers,
Andrew.

 
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Re: PYE CT 72

Post by crustytv » Mon Jul 13, 2015 10:03 am

PYE625 wrote:You are right, there is no substitute for actual measurement of the EHT.

Cheers,
Andrew.

Actually that's not quite true, as there is another method which was provided by BRC/Thorn on their 3000/3500 chassis. Maybe your idea, not so far fetched.

With the increased appearance of transistors and the odd IC, taking measurements with an EHT meter became ever more fraught with the risk of flash-over. So BRC/Thorn provided a test point on the Focus unit where you could safely probe with an AVO 8 to perform set EHT. It was a case of removing the focus lead and connecting it to the earth tag on the CRT PCB /Spark protection. Setting your AVO 8 to the 250uA and connecting between chassis and focus panel test point. With the contrast to minimum and A1 switches to off, you adjusted the set EHT to 120uA

I will never forget the mistake I made with my PYE 697, I failed to clean the surrounding final anode of dirt and carbon deposits, I casually took a reading with the EHT probe only to be greeted with one almighty "Track N Crack". The subsequent discharge took out every transistor on the CDA and frame panels. It also left my wits in tatters as the EHT spark which leapt out seemed to be heading for my hand.

I now clean thoroughly and have made a large rubber shroud for the probe end all this is put in place before powering on to take a reading, thus far it always worked and I use it on 3000/3500 etc chassis despite the AVO option being available.

Anyway I'm left wondering if the procedure your were thinking of could be refined and used along the lines of the BRC/Thorn way.

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 10:57 am

Well 25kv is a naughty voltage to be measuring and flash-over is a big risk as you discovered, Chris.
There must be a safer way similar to how you describe the BRC method.

Time for the thinking caps to be worn lol. :aaq :aaq

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 12:23 pm

Well I've decided to make my own EHT probe using a short length of PVC tube containing 10x 33meg 1w high voltage resistors.
330meg with 25kv should give 0.75 mA at the earthy end of the string.
Is this a satisfactory level of current to be drawn from the EHT supply ? :aaq

 
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Re: PYE CT 72

Post by slidertogrid » Mon Jul 13, 2015 12:40 pm

Hi I have a bench EHT meter that you can borrow if you ever come over to Peterborough. It has a cable with a claw that fits under the anode cap. so definitely a case of only connecting when the set is off!
I call on a friend in Stanground from time to time so I could always meet you there if it's nearer or easier. Otherwise you are welcome to call at my house in Bretton. the kettle is always on! :bba
Regards, Rich.

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 12:47 pm

Thanks Rich !!

That is brilliant...when I get the set ready (just ordered some HV caps for the line stage), I will send you a PM.

Thanks,
Andrew.

Here are some more pic's of the line stage....
Attachments
rsz_img_1914.jpg
rsz_img_1913.jpg
rsz_img_1912.jpg

 
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Re: PYE CT 72

Post by Doz » Mon Jul 13, 2015 1:29 pm

What a corker!

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 1:31 pm

Thanks Doz :aad

Now, I can't help thinking how nice it would have been if Pye had kept the PD500/GY501 arrangement from the previous chassis :aah

 
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Re: PYE CT 72

Post by nuvistor » Mon Jul 13, 2015 1:36 pm

Is the EHT adjustable?
I don't have a circuit but I don't seem to be able to remember one. I remember the dual standards having adjustable 3rd harmonic tuning and the SS one being fixed 5th harmonic to give a flatter top to the pulse fed to the tripler for "better" regulation.
Too many years since I saw one, possibly the information just lost in the past for me.
Frank

 
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Re: PYE CT 72

Post by Cathovisor » Mon Jul 13, 2015 1:37 pm

Similarly I have a nice Brandenburg EHT meter that is active and only draws a microamp or so from the supply. I could make my spare available for loan.. :aaq

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 1:40 pm

Hi Frank,

There are set EHT and beam limiter presets under a label on the back of the line stage.
I have not altered them, but the EHT will probably need checking in due course.

 
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Re: PYE CT 72

Post by PYE625 » Mon Jul 13, 2015 1:42 pm

Cathovisor wrote:Similarly I have a nice Brandenburg EHT meter that is active and only draws a microamp or so from the supply. I could make my spare available for loan.. :aaq


Well, thats very generous of you Catho and much appreciated.
I may well call upon your kind offer at some point, Rich has already kindly offered the loan of his EHT meter.

Cheers,
Andrew.

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