18″ PYE CT-200; Chelsea

PYE: CT200 Chelsea

Model/Chassis: 717

Year: 1975

System:625 Line

Original List Price : £175.00 ( 1975 Argos catalogue)

Valves: No

Transistors: Yes

CRT: A47 343X

18" PYE CT-200; Chelsea 1

Never worked on one of these before, in fact the only colour PYE I’ve worked on was a PYE CT203/1 with a hybrid 697 chassis. That was 9 years ago, it’ll make a change from my usual excursions through Thorns’ product catalogue

For the most part it seems to be in reasonable condition. I was shocked to see on the chroma panel a similar thick film unit for the RGB, as used on the 3000 video module. However unlike the 3000 thick film unit of which I have many, the PYE unit 428A/F does not consist of 3 x 11K/56K, it has 3 x 27K/4.7K. I hope it’s OK as unlike the 3000 the PYE chroma board does not have through any hole provision to revert to discrete components.

On the line/field timebase and PSU panel, there’s also a creative lash-up for R531 consisting of 3 series resistors. As I say, overall it looks to be in fairly good condition.

I’m not throwing caution to the wind so no hairy arsed shoving it onto the mains. First I’m going to service the PSU, remove and if possible reform the reservoir caps 535/536 600uF/250uf. Remove and replace the green grenade C501 which is across the line. And replace another green grenade C656, the flyback tuning cap. Oh, and replace the series resistor lash-up. I’ll likely check the line output transistor which is a BU205. I’ve already tested the CRT, and it’s a cracker. So no time wasted in doing the preparatory work.

The 717 chassis is identical to the 715, it’s just cabinet presentation that differs.

The Circuit Boards From The Brochure

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This Sets Boards

The boards once cleaned reveal they are in remarkable condition. The only work I can see that’s been carried out is on the Chroma panel, unsurprisingly the thick film unit. I wonder if this is a low hour set, and if so why? The CRT tested VERY good, am I going to find its low hour due to some obscure fault, I always seem to attract the sets with problems.

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Mentioned many times before, but worth going over for anyone new looking in. Some may bother with this, some may not, each to their own. I always reform the main reservoir caps, I don’t like brute forcing them, especially if they’re faulty.

The combined multi-can reservoir caps 535/536 600uF/250uf have been removed and is currently reforming. This allows me to supply rated voltage from 0-500 if required in controlled steps. The neon indicator is struck all the time there is leakage, when it extinguishes, leakage has ceased, you can then move onto the next voltage step all the way up to max rated +10%. I connect two meters so I can monitor current and voltage attained. Good caps reform very quickly, some take a few hours and some fail, resulting in replacement.⁠ This one is taking its time.

First section, before and after reform18" PYE CT-200; Chelsea 17 18" PYE CT-200; Chelsea 18

Second section, after reforming

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Next, the two green grenades. C501 a 330n 1000VDC across the line, it’ll be replaced with an X2 .15uF 275VAC. The other C656 a 2n7 1500VDC will be replaced with a 2n7 film 1600VDC.

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Mon 19th July Update

Reinstalled the newly reformed C535/C536. Replaced the across the line cap, flyback tuning and  the lash-up for R531, of which I shall say no more!

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Below the 3 x 150K bodge for R531 (470K).


Can we please stop with the bollocks that this was a factory fit! I’ve spoken to an ex TV engineer of old who serviced these TVs on a daily basis, and has many versions of this chassis in his collection. He confirmed R531 was a single 470K. Any further comments posted will be like the last, deleted.

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There that’s neater! I also uprated it to a 1W

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Further Update:

I had the meter hooked on F541 to chassis, to monitor H.T. which should be 155V

Powered up, nothing!

F526 (1.6A) fuse blown.

Possible causes now need investigation, these are as follows:-

  • Zener diode D518 o/c
  • o/c junction in T516
  • C521 o/c or has decreased in capacitance
  • R515 incorrectly adjusted

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So after finding none of the above faulty, I just replaced it and then found I had no AC into the board. This was traced to an o/c section of cable from the switch output to timebase/PSU. Repaired the cable now I had A/C into the board, F526 was still OK, but still no life. I then noticed no CRT heaters, checked CRT and it was OK, checked Socket ‘D’ pins 1/3 that was OK so fault is for sure on the timebase. I have AC into the bridge and 322V DC out but the 155V rail only gets to 58V then drops rapidly down to mV. No CRT heaters because that’s derived from the supply DOH! Just trying to get my head around how it’s all supposed to be working, as I’ve not serviced this chassis before. I got really thrown by the BR101 (even though I do have some in stock), not seen one of these before a silicon controlled PNPN switch, configured as a DIAC. I had no idea what that was either until I googled about 10 mins ago. No idea how to test these either, don’t even know if it’s faulty, but it might be or the BT106, though that tested OK in circuit as did the zener and the BC147. Will tackle all this tomorrow.

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Tuesday Update

I think I’m going to get the scope out and see what that state of play is with the BT106 Thyristor, the BR101 PNPN switch and the BC147 transistor.


Looks like the Line output transistor may be responsible for pulling the supply down, the BT106 is OK

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Managed to get into the workshop before it gets like a sauna. Replaced the BU205, and as a precaution, replaced the BT106 thyristor and BR101 DIAC. Still no change. New BU205 s/c on B-E so something killed the Line output transistor straight away. Suspect the LOPT.18" PYE CT-200; Chelsea 29


Well, as a precaution, I removed the LOPT, and it tested OK. Upon further investigation, the BU205 is also OK, turns out there is a short somewhere in the cct that is shorting B/E. Just a matter of finding it now.

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Blue wire removed from base Bu205, tests OK now.

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Meter across the Base & Emitter connections, reveals a short somewhere further back in the cct

Also, it’s not due to a false reading through Line drive TX, I removed that and the LOPT is out but the short remains.


This has to be a Red Herring!

I Individually removed and checked the surrounding components, all were OK, so put back in, but the short persisted. I’ve just removed them all, now the short is gone.

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I suspect I’m just wrong, but I thought reading 27R was not right. What makes me believe I’m wrong is I put R652 back and the 27R reading was back and guess what R652 is 27R. Therefore, I’ve been chasing a bogus short. Hey-ho a twonking I will go.

I’ve put everything back, but replaced the Mullard tropical, as they were quite baked. So to recap, the BU205 has been replaced, along with the Thyristor and DIAC. Disconnected the tripler, powered on and the 155V rail only makes 59V before plummeting. See short vid.

So after all this, I’m still no further discovering what is pulling down the 155V rail.


As Dr Zachary Smith would often say…. “Oh the Pain, the Pain”…

After replacing everything that looked baked in the PSU, I’m still no further, so out with the scope. This reveals the BT106 is totally stalled.

Not sure why the waveform at F526 has that horrible kink in it. I suspect the poor waveform from the anode of the Thyristor is likely due to it not being driven. Why is another matter entirely, and I’ll keep on plugging away.

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It’s alive, but the doubler died. The fault was the 15K across the anodes of the DIAC and Thyristor, gone high.


A new 5-wire PYE Doubler arrived, and fitted, however this was also faulty. It only managed to get to 15kV before failing. Before you all start screaming that there is a fault with the TV that is killing the doublers, let me put your minds at rest.

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I have in stock two 3-wire low focus voltage PYE CT200 doublers, so I decided to connect one of these. The theory being, if the TV is fine and the 5-wire doublers were just faulty, the 3-wire doubler will produce 20kV and I will have an unfocussed raster. If the TV is killing doublers then it will kill the 3-wire doubler.

Before doing this, I checked the focus unit on the base of the CRT with a Meggar for leakage, it was perfectly OK. I then connected up the 3-wire doubler and powered up. Low and behold I had 20kV and an unfocussed raster, and it remained stable for ages. Nothing wrong wit the TV, just had the misfortune to have two bad 5-wire doublers.

Photo below of  PYE TV running with the 3-wire doubler. Now all I have to do is figure out how I provide 5kV of focus voltage.

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Not that you can as it won’t photo too well, this is TCF rolling

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After a chat with our resident MacGyver John Colman, regarding how I’m using the 3-wire doubler and how to get 3-5kV focus, John as usual came up with a cunning plan. Use an 8500 rectum-fryer 😉 tray but just energise the focus side of it.
So the plan; splice the 8500 final anode EHT lead into EHT cap of the PYE Doubler. The long white focus lead from the 8500 tray to be connected to the PYE CRT focus connection point and finally the short 8500 tray lopt lead and everything else to be insulated.
This has now been put together, what do you all reckon, will it work or will it go bang!🤪
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It worked, I have focus, but there are many other issues to sort, but to my amazement, I have colour!

I think I have some tuner/IF issues which is not surprising as the IF panel seems to have suffered from poor storage, the tuner can is very rusty, and I suspect the whole board needs a good service.

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Well That’s it…. I’m done with this TV, the doubler just died. I’ve had enough and will use this set for spares.

THE END Or is it………….

Update, another 5-wire doubler has been sourced. Although stated as “New old Stock”, I believe it’s a pull, as evidenced by dirt and grime on the final anode lead. Still nothing to lose.

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Installed, and this one is holding up, Ssshhhhh don’t want to jinx it!

As you can see still presenting a possible IF fault.

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Time to remove the IF and reflow the entire board as it seems to have suffered from moisture at some point and the Tuner can is totally rusty. Though inside it seems, OK, note I said seems.

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After reflowing as you might have already guessed, it made no difference. Next I read the excellent article from 1979 by Mike Phelan. Now this then exposed a huge discrepancy with my board. The mains source of problems on the old IF panel was the amplifier module next to the tuner. This was double-sided print, had printed coils, the rivets linking the sides were always a problem and the caps equally not connected that well. The advice being to spend a great deal of time fixing and reworking the amp module.

Imagine my total surprise when I find the module is nothing like the one described, nor does it match what I have in the service manual. Upon further investigation and input from Tas on our Facebook group we discover this board was the LEDCO upgrade that was offered, it uses SAWF!!! remarkable for such an early sets as I thought SAWF came much later.

Anyway, the board in the module looks brand new, with very few parts, I’m not expecting this to be the source of my problem.

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I thought I might as well change the NSF tuner can, as I said it was very rusty and its a quick win to replace if its faulty

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Well, as you all probably expected this made no difference, but at least it is eliminated as a possible cause. Now its back to the drawing board to track down the cause of the poor signal. I’m wondering if I have multiple faults. If I tune off signal the picture goes black and white (well green as no greyscal done yet) but the definition is greatly improved. If I tune back on signal the colour comes in, but the picture then looks like you see in the photo below.

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Interesting that removing colour, the grainy signal appears to clear up. In an attempt to do a quick N dirty grey-scale, I also find the A1’s and RGB gain pots are pretty much seized, all need replacing. There’s a lot more in need of attention and is likely a combination of other problems not helping this, brightness, contrast and colour operation is horrid, crushing the signal. The thick film on the decoder is a top candidate as well, as these are notoriously awful.

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Well, I’m yet to see it used in anger, this is the first time it’s been up long enough without eating doublers and loosing sync for me to see a picture. First impressions with just the test-card, it gives a good account of itself. The funny thing is I just did a quick and dirty grey-scale. No attempt at convergence, which you can see is very good. Glad it didn’t need tweaking, as many of the pots are frozen solid. Perhaps that’s why the convergence is good, they are fused so cannot be twiddled.

As for the cct,  it’s been an eye-opener and I have to wonder how Thorn always gets bashed when the likes of this PYE far worse. I feel rather thankful that my IF amp module was not the original as, the double-sided board with its printed coils, poor rivets linking the layers and awfully soldered caps, sounded like a total nightmare. Rather than appalled I was totally confused, thinking I must have read it wrong finding the Beam-limiter shared the 12V supply,

I’ve not done a thing to the decoder, it’s running with all its original caps, including the green elctros. Of all the boards designed for the PYE, this looks the best and at a quick glance you could think it was a 3K.

The modular layout is OK, with reasonable access to all boards, but not like a Thorn. If had to choose, a budget colour TV 8K or CT200, Thorn would win every time. Even the Thorn cabinets have nice styling and quality.

I do think of all the Chelsea’s, this final one is probably the nicest looking one.  Time to move on to another set.

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3 years ago

Indeed, apparently this was the last of the production run and by all accounts not seen all that often. Its a very tidy example and looks a little less cheap the the former ones. However, their reputation for duff lopts and poor EHT regulation precedes them. Fingers crossed mine is the exception to that rule with regards to the lopt, though I’ll not hold my breath.

2 years ago

Goodness, that’s the first time I’ve seen one of those light green Plessey electrolytics in years. I remember buying some new for projects in the mid-70s! The fuse clips on the PSU/timebase board might benefit from a clean but otherwise it all looks to be in good shape.

Reply to  crustytv
2 years ago

Hi Chris, it’s a long time since I’ve seen one of these. Thyristor power supply shouldn’t give too much trouble. If you don’t already know, the EHT regulation on these is dreadful….the picture tends to shift around the screen when adjusting brightness or contrast and also between light/dark scenes. It’s just the way they are….a budget set. If you keep the brightness and contrast to reasonable levels then they give reasonable pictures. We had a nickname for them but I just can’t remember it now….

Reply to  crustytv
2 years ago

I wouldn’t say they were disliked….certainly not from the repair point-of-view. I remember at some point the thyristor was changed from a BT106 to something more meaty and I think it was normal to change the thyristor and diac together. The pictures were quite good apart from the EHT regulation. If I remember correctly, the CRT was ‘upside down’ with blue gun down. Some tubes had the EHT cap at the side, others were at the bottom. The Philips version was, I think, the 570 chassis. It’s all so long ago…..!

2 years ago

Did you see if the “glow switch” operated before the fuse blew?

Reply to  crustytv
2 years ago

The BT106 was known for breaking down on load….measures fine on a normal meter test. Try replacing it with one of the same or (I think) a BT121. The package is different so make sure you connect it correctly. You can use the existing heatsink. Change the diac as well…I don’t think you can test those on a meter, they measure O/C both ways but they have a breakdown voltage. I don’t need to mention the lockfit BC147….it might be OK but…..Not sure if these power supplies regulate off load but watch that glow switch. Basically that is nothing more than a GDT (gas discharge tube) that conducts when the PSU is in overvolts. When it strikes it effectively puts a short across the HT and blows the fuse.

2 years ago

It’s an odd way for a transistor to fail, but I’ve seen it before: is it anything special or just a common-or-garden BU205/8?

2 years ago

Pity about the doubler deaths, Chris: in the past you’ve built triplers from scratch, why not with this one?

7 months ago

We didn’t sell these sets, the picture couldn’t compete with the Japanese sets although the Hitachi and Toshiba cost more, customers were willing to pay the difference. We had 6 on a special offer from Pye and we returned them as not suitable. It was the EHT regulation that caused the problems, even customers noticed it when in the shop. Just why it was so poor I don’t know, it’s not as if the design engineers couldn’t see it. Perhaps it was the accountants and sales reps that wanted to have a sub £200 set and corners were cut.

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