Here we have a Ferguson 3749 fitted with the infamous 9800 chassis which has much in common with the 8800 and later versions of the 8500. I say infamous but I personally don’t see why there was much vitriol aimed at it, I always got on well with them although only 22” version came my way. In my opinion the pictures it produces are a big step up from the lacklustre performance of 8800, we didn’t rent them but employees bought for them for families and friends via staff sales. The 9800 was the last hurrah for delta gun based CRTs and I believe were sold as bargain basement, it’s essentially an 8800 but without the large mains transformer which fed the field stage, the 25v regulator and CRT heaters. These were supplied by the newly introduced Philips Diode Split LOPT which also provides an HT rail of 250v for the RGB output stages. As the 25v rail which also powers the control circuitry of the thyristor stage was now derived from the LOPT a kick start or as Thorn called it ”Inertia Start” was needed to get the thyristor going to supply the line stage to get our 25v rail. It seems to me this chicken and egg situation is where people had trouble?
Anyway, to the set in question. It came to me some years back from @Crustytv and has been on the to do list for some time. I believe Chris repaired a fault in the inertia start circuit but a few more intermittent faults have reared their heads while it has sat waiting. Intermittent colour, line drive, top R/G misconvergence and the classic thyristor bouncing HT flutter. Let's see how we get on.
John.
I'm looking forward to reading about this: I'm pretty sure my eldest aunt had one of these on rental. I remember that unusual bowed front around the touch sensors.
Your mention of "thyristor bouncing HT flutter" takes me back to a set I used to own that @crustytv has - a Telefunken 743. I never got to the bottom of that (a lack of experience and knowledge) but I do recall that when it was on, you could hear a knocking noise from the old Ferranti electricity meter that was in the house at the time.
Also often referred to as "Grandad's Telly" from only fools and horses and I've seen similar styled sets used on BBC studio shot programs. These had six sliders instead of the usual four, twelve touch pads for channel selection and carry slots in the side of the cabinet.
Couple of hours spent this afternoon and I thought the flutter may have been the result of very obvious cry joints but no. You mention the noise from your electricity meter, my workshop is mainly solar powered with a 280Ah LIFEPO4 battery and 2000W inverter but the latter makes an alarming noise when using any of these simple single thyristor PSU's, RBM 823, G8 etc, probably due to the high amount of current taken fairly late in the +ve half cycle of the mains.
John.
This set brings back memories, I had a few out on rental in the early days bought second hand from a local shop that obtained them when they bought another firm's rental accounts. This would have been around 1980/81 so I suppose they were getting on a bit by then. I still have a new convergence panel somewhere bought when Sendz were flogging off a lot of ex-BRC stuff. I can't remember having much trouble with then beyond the Mr Sheen being sprayed in the touch tuner...
I will follow with interest! 😎
Rich.
Well, the PSU is giving a little more trouble than I was expecting. The intermittent severe HT flutter turned out to be a heat sensitive BC157 in this case VT704 in the thyristor gate switch. There's no diac here as usually found instead it's a discrete PNP/NPN transistor arrangement of an SCS like the BR101/BRY39 which I have seen employed elsewhere. Even after replacing this the PSU can get unsettled after quick changes in beam current resulting from light to dark scene changes. Now this phenomenon which didn't seem to bother the 8500 was a problem with a lot of 8800's which was odd as apart from the addition of an overvolts crowbar had minimal differences. I remember there was a single resistor change which helped but in all but the earliest 9800's there were several changes which I will implement. The other fly in the PSU ointment is that very occasionally the HT will collapse or go low for a brief instant. Thorn handily put a diagram of the PSU and supply rails on a sticker behind the decoder board for anyone without a manual.
The intermittent convergence R/G error became permanent after cleaning R540, a new pot resolved this as the wiper had gone o/c. Still the intermittent line drive and chroma to sort but the picture it gives is pretty reasonable. The blue gun in the CRT is slightly down and I believe Chris did have to give it a kick with the rejuvenator but I've seen much worse.
John.
the set in question. It came to me some years back from @Crustytv and has been on the to do list for some time.
I acquired this set back in 2018 and it sat on my to-do list for two years before i made it to my bench. I passed it onto you in the first lockdown (March/April 2020) when I was having a collection downsize. I remember it was weird as I left it and the 9600 (which is in an identical cabinet) outside for you to collect as we socially distanced ourselves, weird times!
For those interested, I blogged the original get it going repair here 1978 Ferguson 3749
p.s.
By the way all the guns were tired, this was mentioned to me at the time by the previous owner, though they did not say just how bad the tubes was. It was not just the blue but b&K came to the rescue.
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I'd forgotten all about the time I picked them up from your doorstep, to anyone wondering I wasn't breaking lockdown but still working during lockdown keeping corner shops and a well known supermarket chain trading collecting when I passed near. It's rather ironic but I never succumbed to COVID until I retired. Certainly the tube boost has lasted and it does give slightly creamy peak whites it's a very acceptable picture. Now although this chassis is very similar to the 8800 these always produced a much more punchy picture, I mentioned before that the video output HT rail now comes from the DS LOPT rather than the main HT rail. Instead of 190v it's now 250v and I wonder if this increased voltage overhead is responsible. Other than this the signal stages between the two chassis are identical. Regarding the DS LOPT does anyone know if there were any other sets with delta guns that employed them?
This afternoon I modified the ramp generator part of the PSU to the later design and values and this made an immense improvement to the HT stability under big beam current changes, also the occasional HT dropout seems to have gone but it's not had a long soak yet so a bit early to tell.
John.
Very interesting read John
even more relevant at the moment because I picked up one of these sets yesterday
I haven’t had much experience on these sets I only came across a handful of them back in the day so I may be calling on your expert advice when I get it on the bench
@the_teleman It will be interesting to compare notes. I would certainly recommend powering up the 25v rail from an external PSU before you do anything and watch out for any burning tants on the decoder.
I solved the intermittent line drive and chroma issues late yesterday evening, gentle tapping around the line oscillator area would provoke things. No dry joints could be seen and even after reflowing the area the problems persisted. I did notice when the colour disappeared the picture shifted slightly to the right. Gentle probing of the components side I found C521 was very touchy. This decouples 18V the line osc supply and a replacement cured both issues. I was surprised to find it was one of those Philips axial electrolytics with the blue sleeves, never had one do that before.
I would be interested what CRT is fitted to yours, this one has an A56-120X but the sets I saw had A56-121X types fitted as standard which I believed to be Black Matrix types?
John.
Correction to the above post. It was C412 not C521.
John.
