CTV 1979 PYE 66K1826/15P Hi-Fi ; K12 Chassis

Not seen one of these before but have heard the RGB thick film modules on the CRT base can be troublesome. The test card F picture appears to have no red to me and if this set has auto greyscale that could be upsetting things.

Edit Just found this from a couple of years ago and no red was caused by the TDA2771 not the red module apparently.

This fault is virtually identical to the one I encountered on the GEC McMichael, except that was washed out from the get go. As you know, that turned out to be the TDA3561A being at fault. If you look at the blog below, you can see the photos I took and the picture is identical. I remember initially I incorrectly thought that was a contrast problem, but voltage checks soon showed that its swing was OK.

https://www.radios-tv.co.uk/1983-22-gec-mcmichael-2290h/

Checking pin 19 of U405 (contrast), the swing should be 2.2V - 3.6V

At power on It climbs to 1.7 the plummets and remains on 60mV. At this voltage, operating contrast does not alter the swing.

I can only guess that the voltage is held at some preset level internally by the IC then once it carries out auto greyscale correctly the pin is allowed to rise to the voltage set by the customer controls. If it doesn't auto greyacale correctly then I presume the IC is pulling the pin to ground for protection, you never know the full ins and outs of these type of ICs. The fact the board is on a ceramic substrate leads me to believe it was a swap out item like the RGB modules.

Good grief! TDA2771 £54 Yikes!!!!

Luckily, I have just one left in stock, I hope there's not another fault that is taking out the IC as if I replace it, it's one helluva an expensive test. 😲 

Posted by: @jayceebee

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Edit Just found this from a couple of years ago and no red was caused by the TDA2771 not the red module apparently.

Well, I replaced the TDA2771 with one I had in stock and unless I have two with the same fault, it made no difference.

However, I did observe on minor difference at the first power up. The picture did start out bright (not as bright as it should be) but then it started to fade as before, it then came back up a little, then went down a little then settled on dim. I tried repeating this test to see if it happened again, it only did one more time.

@crustytv 

I have only ever seen one K12 chassis with extras in the form of a coffee table style projection set in one of the showrooms. Failed after a day!

Assuming the K12 circuit I am looking at is the right one, voltage measured on pin 19 of U405 appears to be derived from pin 17 of module U410. Contrast control voltage on pin 10 of U410 should be between 3.2V to 5V.

What is the voltage on pin 2 of module U410, and TS617/TS618 circuit?

Rich

Posted by: @marconi_mpt4

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What is the voltage on pin 2 of module U410, and TS617/TS618 circuit?

@marconi_mpt4

U410/2 = 2.27V no swing when customer control used

Despite looking for absolutely ages, I've failed to locate TS617/TS618. I'll keep looking.

Edit: Ah-Ha found them on the CRT base... Dope! (learning new to me, chassis)

OK readings

TS617 (BF423) B=198V, C=1.6V, E=199.3V

TS617 (BC548) B=654mV, C=8V, E=60mV

Omg I’m so sorry , I didn’t expect it to fail as soon as you got it home and it worked so well here ☹️

unfortunately I don’t have any parts for this chassis I don’t think I’ll have a look through I don’t even have any service information so I can’t really help much 

Ah yes, the execrable K12!

Along with the AWA-Thorn 4KA, one of the least liked (and least reliable) TVs in the marketplace in Australia.

I saw lots of K12s after Philips foisted them onto Pye in place of Pye's own reliable and in every way better, T30C chassis (that used a Toshiba CRT). It was the beginning of the end....

Philips locally developed the KL9 based on the KT3 chassis in order to kill off the K12. The KL9 was much simpler and more reliable.

One thing I did notice is yours has the thick film RGB modules. These failed regularly and so disappeared very early in the life of the K12 here, to be replaced by a conventional PCB with discrete components.

As did that huge diode split LOPT, to be replaced by a less expensive LOPT and tripler.

There is a special procedure for setting up the cut-offs to the mid-points of the compensatable range. It was a little motorised tool that linked into the chassis and tweaked the pots for you automagically. Doing it manually is very time consuming and error-prone.

I'll have a detailed read and see if that triggers any more memories, but I always tried to avoid the K12s, wimp that I am! Chris Johnson used to specialise in them so I didn't have to!

@crustytv 

The 'Contrast 1' diagram with 2.27V shown suggests something amiss with the contrast pot (R813) circuit or the regulated 12V supply from TS760 (BD135) on diagram B.

Might be safe to assume at the moment TS617/TS618 are probably working OK. Unfortunately circuit descriptions in the K12 manual I have are in Dutch.

Rich

Oops should have mentioned the contrast control seems to function now. I'll retake voltage readings now. However, when trying to adjust brightness and contrast you can never get anything decent.

Next power up and the contrast does not do anything again, which makes me think something elsewhere is causing this. I lowered the brightness a tad, and then I noticed the picture hunting. By that, I mean brightness seemed to be going up and down of its own accord some sort of autocorrection, certainly seemed like some auto circuit trying to do something, what I have no idea.

Video of the fault, ignore the noise on sound that's interference in the workshop

Beam limiter?

If that were the cause, why would that cause the contrast to have no voltage swing though?

I've downloaded the manual, I'll go and stare at it.

Could it be a fault in U405? Some sort of luminance madness.

Doesn't have to be an errant bit of silicon that's the problem though (and a £54 a go, I'd hope not).

Can anyone offer a second eye, I'm looking at the feed into the contrast control, and it shows a ref of B2 for in and A104 for out to F1 (U405). Believe me, I've looked for hours trying to find B2, The modules are labelled on the cct as such, U405 (F1), U440 (F3), U548 (B3), U590 (B1) etc.

Therefore, I expected B2 to be a module, nope, cannot find it, and it's driving me nuts! What I did find were a number of components that had B2 above them like R508, Q479, R475, S508, but they look like they're part of the convergence cct. I've never known such a disjointed crazy cct. I resorted to looking in R&TS 1980/1 page 161 as at least I can physically read rather than scroll, but that's just a scan of the original, so not much help either.

The Australian Pye and Kriesler versions of the service manuals for the K12 were much more helpful. They even went to the trouble of re-drawing the circuit so it's more readable.

I think the Pye K12 was called the T37.

I don't recall what Kriesler called theirs (when they were being polite 🤣 )

I've posted on the Oz site to see if anyone still has one of these.

When they are working properly they give the best picture you are ever likely to see on a 20AX tube.

Posted by: @crustytv

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Can anyone offer a second eye, I'm looking at the feed into the contrast control, and it shows a ref of B2 for in and A104 for out to F1 (U405). Believe me, I've looked for hours trying to find B2, The modules are labelled on the cct as such, U405 (F1), U440 (F3), U548 (B3), U590 (B1) etc.

Therefore, I expected B2 to be a module, nope, cannot find it, and it's driving me nuts! What I did find were a number of components that had B2 above them like R508, Q479, R475, S508, but they look like they're part of the convergence cct. 

As this set is a 30AX version you won’t find any convergence circuitry in this set. It’s a strange setup, why is the red channel treated differently and why grid drive, what’s the advantages there? 

As for manuals, Brian Desmond (@sideband did I remember that name correctly?) who used to come around on technical training courses to our locale took a lot of flack on the manuals. He did promise us that Philips were aware of criticism and promised they get better but they didn’t improve that much.

Sorry, I meant EW

No help on where the B2 feed is coming from to the contrast input of U410 then?

No matter, I think I will have to give up on this one! I cannot work out where things are on the data, let alone physically what I'm looking at. Very little tallies with what I have before me, and I'm certainly not skilled enough to work this TV out on my own.