CTV [Sticky] 1974 HMV 2726 Coloumaster; Thorn 4000

I had a rummage in my "Not New" draws, I found three candidates.

1. A .022uF Dubilier "Oi stop booing at the back!" 😉 rated at 1500VDC
2. Two sausages which are close, .025uF 1600VDC
3. A Blue one, .022uF @ 1500VDC.

Two things entities have appeared on my shoulders.

The one on the left is saying, "go on...you know you want to see a raster.. just do it". 🤠 🤬

The one on the right shoulder is advising caution, "Crusty be patient, wait for your order. You've waited all this time, wait a little longer, you know it makes sense". 😇 

Hmmmm . Tea break. 😀 

I would err on the side of caution here, no matter how tempting.

But I'll not stop you 😉

The "a blue one", go for it lol

One thing I neglected to point out, and I think unless you're John & Jim, you would not be aware of it.

I've often said Thorn always seemed to have the service engineer in mind when they designed sets. The modular design etc, but how about this on the 4K, a built-in discharge stick for EHT. Not something I've seen on any other set, it resides in a slot on top of the Tube yoke unit, neat, don't you think? Saves looking for a screwdriver and croc clip.

That is, I have to say, a really nice touch. 

Posted by: @jayceebee

I think it would be wise to keep it to the original component until you get it fully working. I'm fairly certain the original selenium diode will have a higher voltage drop and may give strange results with a silicon alternative, might need a series resistor of a couple of K Ohms to get the EHT regulation right.

Yes, I'll stick with the 'Stinky' AEG992 until the TV is working, then we can address the issues around the substitution. I've resolved the replacement, and have ordered 16, just need to know if it needs a series resistor or not.

The AEG992 spec was 1300/1450V, 2.7mA. The replacement diode I've ordered is, CG3 1400V 3A Sinterglass (VISHAY). Among other things, it appears to be designed for horizontal deflection circuits.

Posted by: @crustytv

I've resolved the replacement, and have ordered 16, just need to know if it needs a series resistor or not.

It may do, if only to limit the peak current whilst charging C336 - everything else has large values of resistance around it.

The order arrived, I've installed a new 2kV .022uF for C336 and a new AEG992 for W308. I've a few chores, later this afternoon, another attempt at getting the HMV up and running. Fingers crossed.

Well the HMV is up and for now, it appears to be staying up. I have sound hiss, I have neons, and the touch tuning channels all function. No raster though. I adjusted contrast and brightness, this made no difference. I then fed in Test Card F, set the tuner to channel one, opened the tuning draw and tuned in to the signal, the hiss went as I tuned onto the test signal as I was not feeding in any sound.

So, it's up, it's running, I just need to start fault-finding the no raster following the Thorn guide. As they suggest, I'll try shorting 6&7 of IC157.

Before you do the short test please make sure the frame stage is running. We don’t want a spot burn.

All the waveforms appear to be present, if not a little odd, but likely me getting used to the 200MHz scope. 1. R411, 2. R412 3. Anode W403.

Shorting 6&7 of IC157, did not produce the white line.

Now I have to check the CRT cathodes, though how, I've no idea. They decided to totally obscured the base giving nos access, I'll be back.

Posted by: @crustytv

All the waveforms appear to be present, if not a little odd, but likely me getting used to the 200MHz scope. 1. R411, 2. R412 3. Anode W403.

Hi Chris,

has your scope got persistence settings turned on? That might make waveforms look a bit like that.

 Regards 

Lloyd 

Thankfully, the base cover could be unclipped.

Cathodes are R-166V, G-160V, B-170V

Next check CRT grid and A1

No grid volts at PL22/2 and 939V A1 at PL23/3. Now to trace why there is nothing at PL22/2

Step by step you're getting closer to getting a raster on that old beast of a 4k. 

R357 is open, time for some supper, then I'll replace it, hopefully a raster soon.

Nice bit of diagnosis. Had something similar on my Telefunken 743 - the control grids were connected to +12V which had disappeared. Once sorted, had pictures.

Posted by: @crustytv

All the waveforms appear to be present, if not a little odd, but likely me getting used to the 200MHz scope.

I'm not 100% sure but I suspect the HF in the waveform may well be the N/S pincushion correction component. Line pulses are applied early in the field generation process

I hope Sir Jules is looking down, as here it is, not quite ready for your horse racing Sir, but we're getting there.

Decidedly green, sound is motor boating and no lock, but I've not tried adjusting yet. Stoopid old crusty is a bit emotional, jeez it's only a telly, but after such a long search to find 4K's. Still no counting chickens, a long way to go and things can still go pear shaped. Remember, I've virtually no specific 4K spares if anything rare should decide to pop.

Looks like I have a decoder fault too, as sad as it sounds, good, I really like decoding a decoder fault. Famous last words, a!

Still a bit to do but my congratulations go to you 👍 Well done 🍾.