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CTV [Sticky] 1974 HMV 2726 Coloumaster; Thorn 4000

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crustytv
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A short progress report on the 4Ks, including some background for the channel's new subscribers. 

I'm nearing first power application, just a few must do tasks first. Ok I know it's been a long time coming, but its not a race, is it?  Anyway, I first wanted to tackle the missing PCB and also the thick-film challenges. I'm glad I did, as it's been fantastic fun and a voyage of discovery that resulted in my ventures into CNC. Which by the way I'm really starting to enjoy and the possibilities it will bring to my TV repairs, no longer afraid of missing boards.

Nuff waffling

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Topic starter Posted : 18/01/2022 11:47 pm
jcdaze
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@crustytv I think that replica vertical defleflection board is an excellent piece of work. Also just to mention that I do have quite a few pages of modifications that were recommended for the 4000 and I will upload them soon but they are on yellow foolscap size paper which I am finding a pain to scan. 

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Posted : 19/01/2022 3:11 pm
crustytv
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Posts: 11262

Thanks Jim, that means a great deal coming from you. 👍 

OK, today I removed the Horizontal/SMPS module, it's not been out before so other than the missing TF301 and the odd lash-up on the rear, I'd not given it much of a look over.

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So what's what? Well, the tripler is a replacement, TF303 is missing, all three wires to VT308 removed, a huge ceramic on the rear and finally an additional resistor/diode/tant at the bottom, also on the rear. Other than that, it's really grubby, and some components look past their prime.

Let's address some of these findings.

TF303/VT308

As previously stated in the video above, my best guess regarding TF303 and VT308, is the previous repairer was fault-finding a width problem. They likely started at VT308 the EHT regulator, found that innocent and then discovered TF303 had failed. TF303 drives Vt308 to obtain width stabiliasation with varying EHT. I also tested VT308 and it is OK. I have a replacement TF303 in stock.

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The Huge ceramic on the rear

This large cap on the rear of the HMV horizontal board is nothing to worry about, it's a repair tacked on the back for C342. Probably a field engineer in a hurry to get home, or a thoroughly lazy amateur. It was easy to resolve by looking at the Line board Jim gave me, the original cap is still in place on that. A tubular ERO 1000pF 10kV. The one on the HMV is a ceramic 1000pF 12kV. As the board is out, I will rectify this by removing and replacing it with a more suitable type, on the preferred component side.

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The additional resistor/diode/tant

Now this was curious, although there are already similar resistor diode additions to the rear of both boards (factory mods?) the third is only present on the HMV board.

It is connected, as far as I can see, as follows :-

The diode anode, is to one side of C321, the cathode to one side of a 3.9K resistor, the other side of that said  resistor down to pin 10 of TF305. Where the diode and resistor meet is the negative of the 47uF 16V tant with the positive side of the tant, going to +ve side of C310.

Turns out this is indeed a Thorn mod, which was implemented on the HMV but not required on the spare board I have. These new component designations are, R329 (3.9K), C351 (47uF 16V), W317 (1S44). The reason for their addition, below.

The Mod

To prevent unwanted spurious effects during switch-on which causes VT301 failures. Only required with IC301 IC's dated earlier than 7525. However, there is no need to remove the mod if the IC is replaced and the date code is then subsequently later than 7525

The date of the TCEP100 in the HMV is 7620, it must have been changed. The IC date in the spare is 7633. Both ICs are later than 7525, therefore I could, If I wanted, remove the mod from the HMV, I shall however leave it.

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Conclusion

That clears up all the initial unknowns and findings, I shall now proceed with the required repair work, such as C342, adding TF303, rewiring VT308 and replacing those orange elctrolytics. A general clean and tidy up while I'm at it.

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Topic starter Posted : 19/01/2022 3:52 pm
Jayceebee liked
jcdaze
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@crustytv All good stuff, C342 was originally 3KV working voltage and it did cause problems and it was upped to 11 or 12 KV. 

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Posted : 19/01/2022 8:40 pm
crustytv
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With the module out, I'm free to spend quality time going over the board. It's amazing, when you think you've covered everything looking for signs of trouble, you find something you missed earlier. This is a significant find and ties in with the prior work carried out by, engineer unknown.

I spotted a rather large and odd looking diode, it was W308, and it looked bent with a dark spot on the glass. This diode is the first anode supply rectifier. I wondered if it was nothing, just a manufacturing fault, or maybe it had overheated.  If this diode had gone short circuit, it would result in no raster. 

See below; the glass envelope of the diode, W308, looks pinched in. I've only seen this on valves when they have red plated, and started to collapse in on themselves.

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I thought I would have a look at the one on the spare module I have, that was installed horizontally and looked perfectly formed, hmmmmm. First I tried testing the HMV diode in cct, it read short. To be certain I wasn't getting spurious in circuit readings, I tested the spare board, this just gave an unknown or no component detected result. Decided to stop faffing about and removed the HMV diode. Upon removal, I could now clearly see the diode looks to have buckled in the middle, clearly not right. Testing it out of circuit confirmed it was as tested, short circuit.

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Upon closer inspection, I found a split down the reverse side, clearly the package was not glass as I first suspect, but plastic. I parted the split away from the body to reveal what had occurred. The photos below clearly illustrate the external and internal damage, sustained by the diode.

w308
w308 1
w308 2
w308 3
w308 4
w308 5

I subsequently had an offline chat with John, and he suspected the EHT is possibly sampled from the A1 volts. John believes it highly probable, I've just located the source of the original EHT fault that was under investigation years ago. As I mentioned above, the diode going short would result in no raster. With the EHT regulator transistor disconnected, C342 and TF303, a picture (excuse the pun) starts to emerge of someone trying to fathom why nothing on screen. Maybe they just didn't spot W308 overheating.

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The diode is an AEG992, I checked my stores and I have them in stock.

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This is why I'm in no rush to gung-ho the power switch. Spending time going over and over the boards, is time well spent. Especially when dealing with such a rare set, I'm OK for general stock parts, but with limited or zero “set specific” spares to call upon, risks have to be mitigated.

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Topic starter Posted : 20/01/2022 12:30 am
WayneD and jcdaze liked
Jayceebee
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Excellent work and well spotted. Just to be clear, although I did think the EHT was sampled from the A1 I'm not entirely correct. Because it isn't derived from the tripler but from the EHT transistor collector, it is in fact a facsimile of the EHT voltage.

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Posted : 20/01/2022 12:52 am
Jayceebee
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That diode looks selenium to me. Heating those discs up wil certainly tell you.🤣🤣

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Posted : 20/01/2022 5:27 pm
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