PYE Studiocolour / KT3 brightness changing

[GadgetM]

Hi everyone,

I have picked up a PYE 20" set with (I believe) a Philips KT3 chassis, in a local flea market. Not sure if £40 was a good price... I guess it was compared to ebay prices.

Generally It works great for it's age, needed slight V-linearity adjustment and a minor tweak to the Green cut-off.

However it has an issue with brightness:
When you turn the set on, the picture is bright, really bright! the backs are light grey. Then over 20 minutes it gets darker, and darker, then settles at a good black level.

I don't much fancy doing a blanket re-cap if it's not needed.
I've checked the +129V test point and it's spot on. Nothing else varies as it warms up, so could this be localised to the video drive section? or a possibly varying G2?

Any ideas?

Thanks
Meurig

[jayceebee]

Can’t remember the reference number but it will almost certainly be the video HT decoupling capacitor, 4.7uf 250V working. Very common fault back in the day.

John.

[GadgetM]

Originally Posted by jayceebee

Can’t remember the reference number but it will almost certainly be the video HT decoupling capacitor, 4.7uf 250V working. Very common fault back in the day.

John.

Thanks, I'll see if I can track it down, and report back.

[GadgetM]

Sorry, but whereabouts will I find this? I can't see any 4.7uf caps. Must be there somewhere....

[jayceebee]

It's C582 according to the KT3 manual, It seems to suggest it's 47uF but I'm fairly sure it was 4.7uF. If I remember correctly it's on the main chassis right above the CRT base board with the chassis in the upright position.

John.

[GadgetM]

Originally Posted by jayceebee

It's C582 according to the KT3 manual, It seems to suggest it's 47uF but I'm fairly sure it was 4.7uF. If I remember correctly it's on the main chassis right above the CRT base board with the chassis in the upright position.

John.
Attachment 258539

Thanks John, Found it, it's indeed a 47. Tests 48uF 0.5ohm esr, looks good but that's not to say it is. I'll order one of those.
I've also changed a bunch of gray electrolytic caps in that area that had obviously leaked. the 47 is the only one I don't have spare.

Once I've sourced one I'll report back.

Cheers
M

[Welsh Anorak]

Besides the capacitors you've changed plus the one you're going to, Philips capacitors are generally reliable.
Does your set have a one or two chip decoder? If the former the socket can cause this sort of fault.
A can of freezer and a hairdryer might help as will a meter hung on either the G2 rail or the HT line to the RGB board. Chances are it's that capacitor though.

[Maarten]

Gray electrolytic caps are not made by Philips, though.

[GadgetM]

So far.... I've swapped most of the unbranded gray caps, all looked like they'd leaked to some degree, one was actively leaking all over the board, they definitely don't look original.

The 47uf cap tests fine, and I temporarily tried a replacement that was close-ish (33uf), same issue so seems that isn't the problem on this occasion. Even swapping the nasty gray caps has made no improvement.

So, next chance I get I'll take your advice Glyn, and check the G2 voltage for any variation as it warms up. Thankfully there are some voltages marked on the board. Hopefully applying the hairdryer will narrow down the location.

Once it's warmed up and settled, the picture is actually pretty good, very watchable indeed! Looks similar to my Fergie TX100, same mullard tube I think.

[Welsh Anorak]

If it's a Mullard A51-570X you're very lucky as these had a very short life and were the main reason these sets were scrapped.
The 22" TX100 used a 30AX A56-540X, and the 20" TX9 used a A51-580X which was a bit more reliable.
I've seen the odd KT3 with a 20" Matsushita tube. Back in the day if we saw one of these at the disposal houses we'd snap it up whatever the cabinet looked like. Nothing else fitted the KT3 - and believe me, we tried!

[GadgetM]

Originally Posted by Welsh Anorak

If it's a Mullard A51-570X you're very lucky as these had a very short life and were the main reason these sets were scrapped.
The 22" TX100 used a 30AX A56-540X, and the 20" TX9 used a A51-580X which was a bit more reliable.
I've seen the odd KT3 with a 20" Matsushita tube. Back in the day if we saw one of these at the disposal houses we'd snap it up whatever the cabinet looked like. Nothing else fitted the KT3 - and believe me, we tried!

It seems a "low hour" set, very clean inside and the tube has shed-loads of brightness available. Due to work I won't have a change to experiment more until tomorrow, but have spent a bit more time actually watching the set over breakfast. I have noticed something that may be a clue... when I turn it on, while the pic is bright, it's just the black level that's high, the contrast doesn't seem to increase with it, when warmed up, the blacks are much lower / greater contrast. Also I have noticed poor DC restoration, so wheen there are bright parts to the image, the blacks go beyond black, and when there's a plain black screen, the black level goes more gray.
Could this indicate something in the power suppy? (there's 2 gray caps in that I';ve not swapped out yet.)

[GadgetM]

A quick view, inside and out for anyone interested. * These were taken before I started re-capping, so you can see several dodgy gray caps inside.

[Welsh Anorak]

It's a long time since I saw one that clean! As you say, very low hours.
Just see which voltage line (G2 or RGB feed) is changing in sympathy with the fault. Bear in mind even a digital meter will appreciably lower the G2 voltage.
Might just be worth changing R1582 (1M) on the earthy end of the G2 control anyway - it used to give trouble.
Is it a series I or II? Easy to tell - series I has the 2 small chips in the decoder, series II the TDA 3560.

[Nickthedentist]

An absolute classic c.1980 telly in amazing condition. A bargain for £40 if you're into that kind of thing.

[GadgetM]

Originally Posted by Welsh Anorak

It's a long time since I saw one that clean! As you say, very low hours.
Just see which voltage line (G2 or RGB feed) is changing in sympathy with the fault. Bear in mind even a digital meter will appreciably lower the G2 voltage.
Might just be worth changing R1582 (1M) on the earthy end of the G2 control anyway - it used to give trouble.
Is it a series I or II? Easy to tell - series I has the 2 small chips in the decoder, series II the TDA 3560.

It's a 2 chip (series 1)

The voltage that's changing is the G2, 10v difference between cold and warm, On the high-voltage leg od the G2 pot (out of the focus block) 604V cold start, then drops to 594V warm.

Checked the 1M resistor, it's spot-on.

I've swapped the gray caps in the psu now, they were way off, but it's not done a thing to solve the problem. Also the 155v test point is reading 174, (or 184 when cold) so that varies too.

All other test points read more or less the correct voltages.
I'm out of my depth here (again)

[GadgetM]

Actually.... The 155v changes with the picture content, varying anywhere betwen 168 and 180, so it may not be changing with time, just with what's on the screen. The brighter the picture, the lower the voltage.
It's perfectly possible I'm expecting too much from a 40+year old set and I should just bung it up in my bedroom and enjoy it for what it is. The colours circuitry has had a line-up this evening (as per the service manual) and colour bars now look REALLY good.

[Welsh Anorak]

10v change in the G2 isn't serious and wouldn't really be noticeable. The other voltage change might well be the cause of the problem, but as it depends on the beam current a slight variation is to be expected.
It's very likely this TV hasn't been used for ages (luckily!) so my advice would be to give it a good run for a few weeks on and off and you might find the problem will gradually go away as the components reform - or you get used to it!

[GadgetM]

Originally Posted by Welsh Anorak

10v change in the G2 isn't serious and wouldn't really be noticeable. The other voltage change might well be the cause of the problem, but as it depends on the beam current a slight variation is to be expected.
It's very likely this TV hasn't been used for ages (luckily!) so my advice would be to give it a good run for a few weeks on and off and you might find the problem will gradually go away as the components reform - or you get used to it!

Thanks once again Glyn, really appreciate your advice. I learn a lot here!.

So.... here's some pics, the caps I replaced (all way off spec or dead). This is the picture the tv produces after 10 minutes warmup now.

[Heatercathodeshort]

The KT3 was one of the most reliable colour TV chassis. You are very lucky to have a good 20" tube. The 14" were indestructible but the 16" and 20" usually had a short life.
Looks really good, just as they did when new. J.

[GadgetM]

I think I got lucky. The tube does appear to be original, but the trippler definitely isn't, that looks fresh out of the box, and it's wires soldered in rather messily... I reflowed those connections early on.