Fixing a Ferguson 8500 Colourstar

[coredump]

I've been having a look at a friend's Ferguson Colourstar. It's the improved version (8500 chassis) and we have some circuit diagrams for it. First of all, the line hold coil (L405) needed slight adjustment to get a stable picture. But after a moment or two of rejoicing at that result, the thermal cutout tripped. Darn.

Now, we'd seen signs of leaking electrolyte from the power supply smoothing capacitors (700uF and 400uF), so we were prepared with modern replacements (820uF and 470uF, 400V, salvaged from big switch-mode PSUs). But even with new caps, it still trips the cutout in just a couple of seconds.

Any suggestions? I'm beginning to suspect that there's more than one fault in this set, given its age. It's been unused now for a few months, but was in use before that. It'll probably need a bit of setting-up, too, to get a good picture.

[paulsherwin]

I have a Colourstar which still sees occasional use. It went out of use as a main set in the mid 80s because the thermal cutout would trip occasionally for no obvious reason. About 10 years ago this stopped happening of its own accord. I'm not convinced these components are very reliable, and it may be worth replacing it with a conventional fuse. Maybe some of the ex TV repairmen on the forum would like to comment?

Best regards, Paul

[Nickthedentist]

There was loads of info published in Television magazine about these Thorn 8K/9K chassis. In the unlikely event of no-one on this forum being able to give you the answers you need, send me a PM and I'll dig out my back copies (all issues from 1975 onwards) and let you know what they suggest.

[oldeurope]

I am not sure understanding the text.
Please let me see a foto of the thermal cutout.
I have an idea what is wrong!

Darius

[AC/HL]

It's a bit distant now, but I seem to remember that these cut-outs did weaken with age. Only a few sets were fitted with them, and they fell out of use so it's probably safe to replace it with a fuse, if only until the cause is investigated. If the set then runs OK you can make the change permanent.
Remember, though, that it could be a designated safety component so care should be taken with any repairs. 3.15A antisurge was a common fuse rating.
Also, sets of this vintage should never be left unattended for long.

[Hybrid tellies]

If the cutout is blowing straight out it is more likely you have got a fault.

First suspicion is the power supply. The main thyristor could be short cct, or leaky, firing the crowbar thyristor. Also mounted on the power supply board is the MJE340 class A sound output transistor which can also go short cct. I would check out these devices first before attempting to link the cutout out.

There could also be a a fault on the thyristor control cct.While you are about it you could also check the BU208A line output transistor for short cct but I cannot remember if this operated the cutout or not.

At least with the 8500 the power supply is reasonably accessible on the left hand side of the chassis. The earlier 8000 it was stuck right down the bottom making access difficult.

You should count your blessings that you are looking at this rather than the slightly later 9k chassis with it's cyclops cct or the earlier 3k which used the worlds first chopper switch mode power supply.

Let us know what you find.

[Mike Phelan]

These cutouts on the 8500, like the ones on the 3000/3500, did go faulty.
These chassis (not one of my favourites - give me a 3500 or 9000 any day) were plagued by intermittent destruction of the BU208, which may be your problem here.
The tripler could also be the cause - start by taking the anode cap off the CRT, but park the lead clear of everything
Then, if cutout still trips, disconnect collector of BU208. Sometimes a s/c tripler will destroy it; in your case, the duff reservoir electrolytic may have done the job.
We always used to run these up on a variac after replacing the BU, and if it was OK, kept fingers crossed during a week's soak test!

[Aerodyne]

It is certainly the case that the triplers on the 8k and 8.5k series failed regularly: taken apart, many showed burned contacts and sometimes even a hole right through. I carried spares on field work and if any doubt existed, replaced them as a matter of course. Its a good while back now, but I remember the line output transistor failures were quite regular, also the power regulator transistor (was it a chopper? I can't remember now) caused problems, among which was a 'sidewards' flutter when the width was set correctly. And of course the sound output transistor often shorted!
What memories. 20-odd calls per day, 50 mile radius of York... how did we do it?
-Tony
I should read existing posts with more care, and I would have known that the regulator I mentioned was a thyristor... but the fault I mentioned was caused by it!

[coredump]

Had another look at the Colourstar last night (my friend has been moving house since the last episode). We have a supply of 3.15A anti-surge fuses, but they're blowing immediately.

We checked the PSU thyristor (a Motorola SCR957) and it seemed to be OK. Certainly not shorted or open. Also checked the audio output transistor, and that seemed OK too. We ran the set with the CRT anode cap disconnected and insulated, to see if there was excessive load on the LOPT, but the fuse still blew. We tried to check the line output transistor, but so far we havn't conclusively shown that it's OK.

Now, I was wondering if it would be possible to run the set with power cut off to just one section at a time, to narrow down where the excessive current drain is going? So, could we (say) disconnect power to the line timebase, run the set, see what happens, then reconnect and try again with the frame timebase disconnected? I'm wondering if a power smoothing capacitor has gone short somewhere.

Also, in the manual there's a procedure for installing a resistor in the line output stage to limit current during fault-finding. Might be worth trying? If the fuse holds, we'd know the trouble was in the line output stage. May be worth looking for a new transistor anyway (we couldn't find spare thyristors at all).

[ppppenguin]

The old trick of a 100W bulb in series with the mains (or the LOP supply once you have narrowed things down) stops the fuses blowing while allowing much of the circuitry to show signs of life.

Unless one the experienced telly menders says it doesn't really work with this chassis. At least it's difficult to do any more damage when there's a bulb doing some current limiting.

[Aerodyne]

For some unfathomable reason I called the thermal cut-out a 'tripler' on my earlier posting on this subject. Sorry about that: senility rules. Anyway, the cut-outs often cut out for no good reason and when in doubt, I replaced them. It should be possible to bridge with a fuse, I guess, as a last resort and no replacement cut-out to hand. Jeffrey's idea about the lamp is worth exploring too.

-Tony

[Hybrid tellies]

Well I think you have proved conclusively that you have a hard fault causing excessive current to flow.
Sounds like you are looking for a short cct possibly on the mains input side input side of things.

Good idea that using a 100w light bulb but as you are looking at such a definite fault I would do a few cold checks with a suitable meter first

As you have checked the thyristor and sound output transistor I would check the degauss thermistor and mains filter capacitor.
Both can break down on load and may check ok with a cold check.

You said you checked the main thyristor, what about the crowbar thyristoer?
I cannot remember how the LT voltage was derived on this chassis but I think it was a mains transformer feeding a bridge rectifier. The bridge rectifier is worth a check as these could also be breaking down or have gone completely short cct. If you are really unlucky it could be the mains transformer but I can never remember having to replace one of these.

The mains filter choke use to go noisy (it would emit a loud very annoying buzzing sound) or go open circuit. Unfortunately its over 20 years since I have worked on one of these chassis so memory is getting a little vague even though I had a spell at Radio Rentals repairing many of these and all the other TCE chassis of the day.

Let us know how you get on and if I think of anything else I will post it here for you.

Good luck Simon

[Mike Phelan]

I cannot remember if you checked the line output transistor? BDX32, IIRC?
This will operate the cutout, was a common fault on these sets, with more possible causes and official mods than you could shake a stick at. Rarely the LOPTx
Picture quality was very disappointing, even at 20"

[coredump]

Originally Posted by Hybrid tellies

Well I think you have proved conclusivly that you have a hard fault causing excessive current to flow.

I think so!

Originally Posted by Hybrid tellies

You said you checked the main Thyrister, what about the crow bar thyrister?

Um, there's no crowbar on this set. Maybe you're thinking of a later version?

Originally Posted by Hybrid tellies

I cannot remember how the LT voltage was derived on this chassis but I think it was a mains transformer feeding a bridge rectifier. The bridge rectifier is worth a check as these could also be breaking down or have gone completely short cct.

It's a transformer feeding a two-diode rectifier. We've checked those diodes and they seem to be OK. There's also an 800mA fuse in that part of the circuit, which does not blow.

Originally Posted by Hybrid tellies

Let us no how you get on and if I think of anything else I will post it here for you.
Good luck Simon

Thanks!

And to answer some of the other posts, the thermal cutout is now holding, but the 3.15A fuse is blowing instantly we switch on. So, there must be a short somewhere. We tried to test the line output transistor, but couldn't be certain of the results. Might be better to remove it from the circuit and test separately. Will test for trouble in the degaussing circuit, and trace right through the mains input side of things.

[Hybrid tellies]

I have just found some information from a servicing article in the January 1976 Television mag. (Written by a Barry F.Pamplin)

You are indeed right there is no crow bar protection on this set. I might be thinking of the later 8800 or 9800 chassis. Protection relies completely on the fuses and thermal cutout.

The main problems are as follows. diode W704 (IN4007 or BY127) goes short which causes the main thyristor W703 to go short.
This in turn destroys the Sound output transistor VT701 (MJE340) taking diode W701 (IN4004) with it.
The main thyristor W703 can also go leaky causing the 180v HT line to rise to 280v causing the line output transistor to go short cct the heavy current flow then blowing the thermal cutout or the 3.15A fuse.

Other causes of high HT are VT702 (MJE520) the 25v stabiliser going collector emitter short also check VT706 and W705 for shorts.

There was also a problem with a bare wire link on the main dropper section located between R709 & R727 which could short out R727 which could cause the line output transistor to fail.

Finally and from experience you do have to disconnect legs to conclusively test the line output transistor.
Hope this is of some help, keep us updated.

[davegsm82]

@ coredump

You dont know me but we have a mutual friend (yes, the one with the mog and the MZ).

She's sent me the schematic of the PSU and I've been looking at the design of the circuit. This design uses the SCR as a mains chopper run from the discreet oscillator (transistors VT704-6) and from what she tells me the SCR reads 140ohm's-ish in either direction. Looks like the SCR is most definately bad, but sourcing a new one is going to prove tricky, does anyone here know of a replacement for a BRC4443 or a SCR957?

I have a couple of BU208's i can post off to her should they be required too.

Dave.

[coredump]

I have to confess to having neither a Mog nor an MZ, but I do have a reel-to-reel video recorder and a couple of LaserDisc machines!

As for the PSU thyristor, it was 140ohms between Gate and Cathode, which is normal for some kinds of thyristor:

http://repairfaq.ece.drexel.edu/sam/semitest.htm

But it was made in 1973, so probably worth trying a replacement, if we can get one.

[davegsm82]

Originally Posted by coredump

I have to confess to having neither a Mog nor an MZ, but I do have a reel-to-reel video recorder and a couple of LaserDisc machines!

As for the PSU thyristor, it was 140ohms between Gate and Cathode, which is normal for some kinds of thyristor:

http://repairfaq.ece.drexel.edu/sam/semitest.htm

But it was made in 1973, so probably worth trying a replacement, if we can get one.

eep... I see now, the way it was described to me i was assuming that the 140 ohms was from A-K not G-K. well, I went rummaging through my stocks at my part time workplace today, and happened upon a BT116, brand new in packet. Tis also an SCR, may well do the job should you wish me to send it. also found a BU208, used, but good condition and not leaky.

The fault definately lies at or beyond the SCR, I had our friend remove said SCR and fire up the set, *nothing*, as would be expected (allthough at the time i thought the SCR was part of a crowbar circuit, so I expected life, silly me).

on another note, I have been perusing your site and its nice to see somone else has a D31 and a D43! although mine arent rack mount tho, work well as portable heaters in my opinion. My interest is more in the older glassware driven malarky, hence the 'scopes.

Laters, Dave.

[Mike Phelan]

Originally Posted by davegsm82

I went rummaging through my stocks at my part time workplace today, and happened upon a BT116, brand new in packet. Tis also an SCR, may well do the job should you wish me to send it.

BT116 should be fine - BRC4443 was Thorn's own numbering - AFAIK, it was a BT116.
A similar circuit was used in the Philips G8 and Rank A823 chassis for HT supply and regulation, and worked very well. It should measure completely O/C from A-K. They rarely went s/c - more often slightly leaky, which caused excessive fluttering HT.

[pyoor]

eep... I see now, the way it was described to me i was assuming that the 140 ohms was from A-K not G-K.

Aye, sorry about that. Looked at e-mail yesterday in fit of thinking 'did I say what I meant to, or what I fear I said' and realised that the "swimming through treacle" effect had occured. Since I've not done anything significant with electronics since I was 15/16... well. You get the picture.

well, I went rummaging through my stocks at my part time workplace today, and happened upon a BT116, brand new in packet. Tis also an SCR, may well do the job should you wish me to send it. also found a BU208, used, but good condition and not leaky.

That would be great; although as discovered the thyristor is thought to be okay, but a spare would probably be handy as they seem a bit notorious. The line output transistor which is a possible suspect could be very handy!

The fault definately lies at or beyond the SCR, I had our friend remove said SCR and fire up the set, *nothing*, as would be expected (allthough at the time i thought the SCR was part of a crowbar circuit, so I expected life, silly me).

Gah! You had me foxed with that. But then I was probably giving you duff information to go on. Should'a sent you the cct diagram earlier.

Yes, nothing happens with the SCR removed.