CTV A Brace Of Co-op C2001s
I picked these identical sets from Wales well over two years ago now, one came from Neil in Newport and the other from Alastair in Swansea. They are fitted with versions of the Rank A823 chassis used in many Bush/Murphy models. We’ll call them Set A and Set B, I’m sorry but don’t recall which came from where. Both cabinets are in poor condition, Set A due to being knocked about somewhat and set B mainly due to water damage.
Each chassis was removed from their respective cabinets and inspected. Set A dates from January 1973 sporting an A823A and set B from August 1975 with the A823B, the latter has many changes such as resolderable spring off fusible resistors for BEAB requirements. All the mods for the troublesome HT flutter are included and a series diode/resistor network to protect the thyristor.
Set B looks the most promising, some very mild rust spots on the chassis, at first it look electrically very good although It has seen some action. The PSU having seen some attention in that the 25V rail four bridge diodes have been replaced with a four diode package as seen on the earliest versions and replaced with 4 x BY126 due to the BY164 being unreliable. Also previously replaced are two of the cans, the PSU mounted 2 x 2200uF and also one of the large 600uF 300V units on the main chassis. The SL901B on the decoder has also been replaced in it’s service life
Set A has a lot of rust especially on the plate and clips that hold the smoothers, the 2 x 2200uf on the PSU looks iffy with slight leakage and C9 has released it’s now crusty electrolyte all over the mounting plate. The convergence board has seen action, two of the pots are badly damaged and rust , one has been replaced with an alternative type with it’s control knob now on the wrong side of the PCB. The line linearity coil has also suffered either corrosion, impact damage or both. It’s going to be a challenge to get replacements for these.
So Set B is now on the bench and unfortunately a rather nasty surprise when I inspected the PSU out of the set. It’s very toasted as it runs rather warm but the surprise was on the print side, some very poor quality repair work has been carried out, in fact it’s a mess. The VA1019 varistor one of the troublesome heat sources and is also very badly pitted due to arcing, no way am I powering this board in this state, doing so risks damage to the rare Plessey IC’s on the decoder.
Next step is to reform the electrolytics and see what we have. While this is happening I’ll connect up the B&K and check the CRTs for emission.
Something I've had in mind for sometime is to get rid of the unreliable VA1104 varistor (not VA1019 in original post) and add some form of soft start function for the RBM PSU. GEC did this in the later 2110 chassis and I believe it was incorporated into the Ledco replacement for the A823 chassis, I've not seen one or it's circuit. Below is a comparison between the RBM and GEC versions using a BT106 thyristor, as you can see the two circuits apart from the soft start components in the red box are very similar. I shall attempt to construct something and see if I can get it to work.
Hi John, The set you had from me should work 😮 well at least it did the few times i switched it on 🙄. and apart from replacing the cap on the if/audio panel supply to the audio amp i never did anything else to it. and i did think like the 20"bush version i still have and another 26" A823 from the same seller that the original owner wasn't that competent at repairs going by the mess that you've now found.
The company called LEDCO made a replacement for burnt out power units in the RBM A823 TV receivers. Like JayCeeBee never bought any myself but I believe the VA1104 varistor was indeed replaced with a soft start system in the emitter circuit of the voltage control transistor. The diode in series with the thyristor anode was employed with good effect.
Nevertheless, the use single thyristor to control the HT supply voltage is a horrible circuit solution. Half-wave rectification is bad enough but with a single thyristor to control the latter part of the positive part of the sine wave is even worse. A short duration high current bite out of the mains supply was not a good idea!
Connect a 1ohm resistor in series with the mains neutral to the TV set under test. Connect an oscilloscope across the resistor to see how the thyristor regulator is conducting for a short duration over the positive part of the mains waveform.
Hi David, I'm sure that my co-op A 823 has a soft start power supply fitted . It belonged to a Rank engineer who fitted it with various modifications including an RGB pre amp .
Looks as if you have your work cut out there! All part of the fun though!
I find it interesting to see the various repairs a set was subject to during it's life. - The Field engineer's dodges that got a picture and him out of the house when the right part or diagnosis wasn't available... Sometimes I leave slightly "Bodgeitt and Scarper" repairs if they are safe and not too bad, all part of the sets history. I have a Thorn 1400 which must win the 'world most bodged dropper' award, it is very untidy and even sports a spring off resistor hanging on the daisy chain! I am waiting for a replacement to turn up as they do from time to time on ebay, and then it will be replaced. I shall keep the bodged one though for the hall of fame...
It will be interesting to see how the A823s turn out, are you planning on restoring both or making one good one out of the two?
@neil1974 Hi Neil, good to hear from you. I dare say both sets would have produced a picture of sorts if I had just plugged them in but I didn’t want to take the risk. In a perverted sort of way I’m glad that one of the PSUs is in rather poor condition. A few times I have mentioned these two sets in other posts and I’ve always said one will be restored to original and one will be subject to experimentation to see if the PSU and picture quality can be improved upon.
About a year ago I made some tentative steps in getting to grips with Kicad for this very project but didn’t get very far. The push was to come up with something for Chris’s 4000 thick films. I now have a fair grasp of it and the first job was to modify the existing PSU design and produce a new board with soft start, If it works out how intend it to I will pop a blank PCB to you in the post.
It’s interesting that the GEC implementation is very similar to the PYE 713 series, in fact the component values are identical. The PYE version also has an overvolts circuit in the thyristor control but this isn’t needed in the A823 as it has it’s own method, in this case over voltage kills the drive to the line stage.
Hi Till, I realise that this type of PSU was not well liked by the electricity suppliers, I’d considered going 100Hz G11 style but I couldn’t fit it on space of the original. When I’m retired and hopefully have a lot more time to play later this year I intend to consider adapting a switch mode design but that will be a steep learning curve. I realise all of this will likely offend some of the purists of the hobby but these are not exactly rare and it’s fun to experiment.
Hi Gary, any pictures of the decoder mods, really interested.
The two large 600uF 300V smoothers have successfully been reformed, feeding 25V to the LT rails from the bench PSU show the signals stages are working well and program sound is present. Scoping the collector of the line drive transistor also shows a healthy waveform.
Set A sports an A51-110X Vacuonics regun CRT who were evidently based at Dudley in the Midlands. This has emission on the high side of bad, not even into the yellow but I’m hopeful this will improve with use. Set B has what looks to be it’s original NEC 510DNB22 but I’m afraid it doesn’t look good, emission is extremely poor with each gun barely registering. Advancing the G2 setting couldn’t even get the green gun meter to the first gradation before measuring emission. The CRTs weren’t run for long and I won’t attempt anything to improve matters until both have been running for a few hours.
I’ve temporarily constructed a soft start circuit on proto board and fitted it into the emitter of 8TR1, doesn’t look pretty but I hope to prove it can be made to work on the A823. The VA1104 varistor has been removed and linked out, tomorrow I hope to try it out but first I’ll run it up gently on the variac to see that the HT is stable and controllable.
Advancing the G2 setting couldn’t even get the green gun meter to the first gradation before measuring emission
STEP 2: SET - CUTOFF
Rotate FUNCTION switch in SET CUTOFF position. Meter may read zero or slightly above ort below zero. Adjust each G2 control to ADD ONE DIVISION to reading of corresponding meter. Use Red G2 for black and white tubes. If max G2 does not raise pointer place G2 switch in HIGH position.
Oops, guilty as charged. 100 lines, must RTFM.🤣🤣
Well I tried the modified PSU this evening and the soft start is working a treat. First I disconnected the auto degauss so as not to stress the variac, disabled the line output stage then using the variac I slowly brought up the HT to around 170V. Adjusting the HT pot showed that the control circuit was working and firing the thyristor in the correct manner. The pot was wound up to the 200V which is the approximate setting for 25kV and left to run for a few minutes.
There was no flutter or voltage drift so it was now time to generate some scan and EHT. Again the variac was wound up gently and to my relief test card F was displayed as below. Reconnecting the auto degauss and removing the variac the PSU is working well.
As I had hoped after about 20 mins running the CRT pickup tremendously emission wise, a little tweaking and we have a half decent picture . I do have a wandering green black level which I thought might be an A1 issue but swapping the red and green outputs show this to be a decoder fault. Chroma is only just enough at maximum and has Hanover bars, there is also no fly back blanking action and quite a few convergence pots have little or no operation. All in all thoughI’m very pleased.
I’ll try this working chassis in the other set but I’m not too hopeful that the Japanese NEC CRT will get any where near the Vacuonics but you never know.
That tubes really improving John 😀.
@neil1974 Hi Neil, The CRT has come up a treat, emission is pretty good on all guns. I've spent a few hours working on it today, the best of the two PSUs has been repaired and restored to the original design. Needed a new set HT pot, 2x 2200uF can and the emitter pin had fallen off 8VT1.
The varying green fault mentioned earlier on the decoder was caused by a faulty clamp diode, it literally fell apart when being unsoldered. I also had to pinch an A1 pot from the other set as the blue would only work one ended. A little bit of setting up of the decoder and I must admit I'm more than pleased with the results. The picture attached doesn't really do it justice.
As for set 2 I'll try this chassis in it and see whether that NEC CRT comes up but I have my doubts. I managed to source some replacement convergence pots if it does and I have my redesigned Frankenbush PSU ready to build.
After various diversions I’ve eventually come back to these sets and have a progress update. I mentioned before about the lack of field flyback blanking and this was traced to a short on the beam limiter feed from the LOPT. It’s decoupled by two components, 6C9 a 5uF electrolytic and 6C14 0.1uF mustard cap. These two components are rather difficult to get at buried underneath the LOPT in the cramped line output/eht section and you will no doubt guess which one I snipped out, yes the electrolytic of course and the short remained. Checking the circuit there could be no other reason than the mustard and sure enough it was dead short, a first for me.
With this chassis now fully working I fitted it into the cabinet with the NEC CRT. Sure enough the emission was virtually non-existent and could only be viewed in darkened conditions. The B&K 467 was put to work, first with a clean and balance routine but very little improvement was seen. Rejuvenation was attempted and this did improve matters but for how long I don’t know, with subdued lighting it does give a quite watchable and pleasing picture but it has an irritating purity issue which cannot be cleared with any amount of degaussing or adjustment of the magnets on the neck. The attached screenshots look better than it is to the naked eye.
Now this is where I am possibly expect a little flack, the original PSU had seen many repairs and although working I had for a long time even before I obtained these two sets, harboured an idea to improve the PSU if I could. In high mileage sets like this the board can become blackened and conductive not to mention lifting print. Quite some time back I started to get to grips with Kicad for this very purpose and came up with a redesign so was in a way pleased to see the board condition which gave me the excuse for the exercise. Remember I do have a reasonable working PCB for the other chassis that is totally original.
To reduce heat the VA1104 surge varistor was deleted, this needed the addition of a soft start circuit to minimize the current inrush at switch on, I blatantly copied from the later GEC2110 and only recently found that the first versions didn’t have any inrush protection at all. The same soft start circuit I later found was also used in the PYE 731 chassis courtesy of member Chris The Teleman. I did have to make a slight change here to get it to work properly, a small testing oversight on my part when experimenting earlier, more on that later. The LT section was redesigned only to get rid of that big ugly 2x 2200uf can and heatsink for the 18V zener which supplies the decoder, this was achieved with modern smaller caps and using a simple three leg 7818 IC. The autodegauss VDR and thermistor were replaced with a much cooler running three legged posistor.
The Kicad generated Gerbers were sent off to JLCPCB in Hong Kong, were produced in less than 24Hrs then shipped arriving two weeks later and I have to say I am super impressed with the quality.
Very nice job with the psu. I remember the Ledco psu did give ht fluctuations, quite badly on some boards. Rediffusion w/shop Aycliffe took a batch of 50 for trial, Malc.
@malcscott I would dearly love to see the Ledco circuit if anyone has a copy, I don't think I've ever even seen a picture of a board. The regulation is as good as the original and with all the RBM changes implemented there is very little scene change induced flutter as was the bane of the original production.
I did make an error in testing when I added the soft start to the good board in that I didn't test it thoroughly. It was only after some lengthy soak testing although soft start did work I noticed that changes in mains voltage caused noticeable HT changes. When I tested with a variac regulation was appalling, I was convinced I had made a trace error but hadn't. Eventually I found it was the soft start addition that was the cause. A permanent cure was found by removing C705 but thankfully it has not affected the circuit operation I've found.
Over a period of 5 hours running there is a variance of around 4-5V on the 200V rail which is about the same as the working original. The PYE circuit has a VA1056S thermistor in the HT feedback and I might just add this if I can find one.
The board runs cool especially the thyristor no doubt helped by the big heatsink. The LT rectifier runs warmer than I would like but has provision for fitting a heatsink of it's own.
Your new power supply looks great! It’ll be interesting to see how it fares on test 🙂
I often wondered about making new PCB’s to replace damaged or missing ones, I should really have a go with the software, but computers and me aren’t usually friends…
@lloyd Definitely worth having a go Kicad. Took me almost a year off and on just playing before I really got to grips, I still wouldn't say I'm good at it but to be honest the hardest parts were the board cut outs for mounting screws, heatsink fixings etc and footprints for the old Pressac connectors although there is a wizard which helps.
That board looks fantastic, great work John, all your work studying Kicad has paid off.
Regards the VA1056S, I've had a look through all my thermistor stocks, unfortunately sods-law dictates not a VA1056 to be found.
Specs for the VA1056S state, resistance being 47K +/- 9.4, B-factor 3925K, max dissipation 600mW.
I found online Vishay NTCLE100E3; 47K, B-factor 4090K, max dissipation 500mW, colour code Yellow Violet Orange
I wonder if that might suffice? https://www.switchelectronics.co.uk/47k-ntc-thermistor
I've used this lot before when I was having trouble finding a replacement VA1040 (130R), dynamic trip, X601 for the 3000 PSU. I bought a batch of these modern types, and they work just fine. As luck would have it, only a few weeks later I managed to locate a whole raft of NOS VA1040, typical.
The tolerance on thermistor ratings is quite high, and is usually 20%. Any reading of around the figures quoted to around 40% higher would therefore be perfectly acceptable.