Thorn 850 chassis - tuning problem

[Colourstar]

Hello all, I have here a nice Ferguson 3617 (with optional fruit bowl!) - an example of the Thorn 850 chassis.

On 405, the picture is excellent and the sound is good, but achieving both at the same time is a problem. At the point on the fine tuner where optimum picture is achieved, the sound is almost inaudible. Conversely, maximum sound appears at a point where the picture has long since broken up.

Any thoughts on a possible cause? Presumably ageing valves or components drifting out of tolerance are more likely than coils adjusting themselves (unless the Phantom Twiddler has paid a visit... )

Many thanks

Steve

[FERNSEH]

Hi Steve,
That's a beauty! Amazingly compact for a dual standard set.
I've got a feeling that the IFs have been twiddled. Don't touch a thing until the valves have been changed. I do remember that aging valves can throw the alignment out.
It's OK on 625?

DFWB.

[Colourstar]

Hi David

Just tried 625 and all you get is patterning (see pic) which doesn't change at all across the whole range of the UHF tuner. It doesn't seem to be seeing anything.

Being an early dual standard, it's not very 'user-friendly'. To change over you have to twist the unmarked knob on the left of the set to set the timebase, then rotate the VHF tuner on the other side of the set to the 'UHF' position and then twizzle the UHF tuner. To make it even more fun, the brightness and contrast thumbwheels are out of sight 'round the back, right next to the hold controls.

Ah well, it was an early effort and as we know the UK set makers were having an uncertain time of it at the start of the dual standard era.

Steve

[FERNSEH]

Hi Steve,
There might be a problem with instability in the IF amplifier. The two stage vision IF amplifier operates on a broad band principle on both line standards. On 405 bandpass filters are introduced to accomodate the narrower bandwidth requirements. On 405 lines switches SW1A and SW1B select coils L1, 3 and 4.
Best to not touch the IF alignment yet.
I'd imagine the set has the valve UHF tuner.

DFWB.

[Nuvistor]

Did the IF transformer formers distort due to heat on the 850 chassis and cause quite a few different problems?
I never worked on many a Thorn sets but something is in the back of my mind, still it could be another set I am thinking of.
Frank

[Colourstar]

Originally Posted by FERNSEH

I'd imagine the set has the valve UHF tuner.

Yes indeed it does.

Steve

[Colourstar]

Originally Posted by Nuvistor

Did the IF transformer formers distort due to heat on the 850 chassis and cause quite a few different problems?

Quite possibly. It's all very tightly packed in this model!

Steve

[Heatercathodeshort]

Nice looking receiver. I have never known an 850 to twiddle itself! It may have been twiddled as David has suggested but quite a few tests have to be performed to confirm this. Don't twiddle anything. The tuner was prone to instability on channel 1 [motor boating] that the field engineers used to clear by exchanging the channel 2 coil for channel 1, retuning the oscillator core on the channel 2 coil. Thorn were not too keen on this 'cure' and supplied 'loaded' channel 1 coils to damp down the neutralized triode PC97 RF amplifier in the tuner. I serviced hundreds of these as I expect David did. Regards, John.
Others just twiddled the IF's........

[Nuvistor]

Yes many twiddlers about, usually with a screwdriver and broken dust cores. I was lucky that most the customers I had would not touch the inside, customer made faults were very few.
There was just something someone who worked at one of the big rental companies said about distorted formers on Thorn sets. Long time ago perhaps the memory is wrong.

[BGmidsUK]

What a lovely little set. I'm surprised there is no label for the system switch, is there any evidence it once had one? I also wonder if the (rather stiff) switch caused problems for some people or even the sets, if not operated cleanly.


Brian

[dazzlevision]

Originally Posted by BGmidsUK

I'm surprised there is no label for the system switch, is there any evidence it once had one?

Hello,

I have the HMV equivalent of this Ferguson and it does have "405" and "625" marked on the system switch knob located on the LHS of the cabinet. This has always been the case on the many Thorn 850/800 dual standard (or convertible) sets that I have encountered. Is the knob on your Ferguson an original (they have a rather long shank and are retained in the cabinet hole by a serrated metal clip)?

Regards,

Dazzlevision

[FERNSEH]

I do remember having to order IF coil and transformer assemblies from the BRC service department. The most common reason for replacing these parts was that the coils had been ruined by the butchering that was called TV servicing in those days.

DFWB.

[Nuvistor]

Originally Posted by FERNSEH

The most common reason for replacing these parts was that the coils had been ruined by the butchering that was called TV servicing in those days.

Perhaps that was the truth of the matter but they did not want to admit it.

[Colourstar]

Hello all

Dazzlevision is correct- the 405 / 625 markings are embossed on the knob, so it is the original one. I'd be interested to see a picture of the HMV version of this set.

Attached are a couple of photos as a reminder of the 850 chassis layout. As you can see, getting the two valved tuners into the cabinet is something of a squeeze...

I'm sure this series of sets were a reliable backbone of the second hand trade in the early 70s.

I think perhaps the best way forward is to try some new valves (if I can find any) in the UHF tuner and see if that brings it to life.

Steve

[FERNSEH]

Hi Steve, It certainly is compact set. Way back in the 60s I converted to dual standard operation a similar looking convertible set. Actually branded as a Pilot. The two tuners were in the same positions as your Ferguson. The real difference was the fitment of the hand wired BRC 625 IF amplifier unit which was attached to the top deck of the chassis. Performance on UHF was hopeless and later on BRC supplied a plug-in transistor IF pre-amplifier. This little unit was plugged into socket SKT304 and the UHF tuner plugged in the pre-amp. After the fitment of the IF pre-amp the set became a very good set on 625 lines.
Although in a sense the set was a horrific creation.

DFWB.

[BGmidsUK]

I seem to recall our set having a label attached to the wooden case just above the system switch, with 405 on one end and 625 on the other and some sort of rivet in the middle.

This could of course be a figment of my imagination augmenting my forgettory. The photos of the set's chassis do seem to correspond with my recollections however, except that, of course, ours had the front-facing controls (tuners, power, brightness etc.) being a 19" (?) version.


Brian

[Restoration73]

This set has been listed here for a number of years !
http://www.electrojumble.org/CLUB_Docs/TV_01.pdf

[dazzlevision]

Originally Posted by Colourstar

I think perhaps the best way forward is to try some new valves (if I can find any) in the UHF tuner and see if that brings it to life.

Hello,

Yes, that would be the first step, as these two valves (PC88 & PC86) didn't last very long, especially in fringe UHF reception areas. I expect that these two valves are still around and available fairly cheaply (not much use to Audiophools!).

Check that the correct (longer) UHF biscuit is fitted in the correct position of the VHF tuner and that all the contact fingers are making good contact with the studs on the biscuit (best to clean all the contact fingers and studs). Try not to get contact cleaner spray on the biscuit lettering, as it may disappear if you wipe it away with a cloth! While you're at it, clean the system switch contacts on the IF panel.

There should be two small metal screening plates fitted onto the solder side of the IF panel. If a "bodger" has left these off, it might cause instability issues.

Check that the HT supply to the UHF tuner is present when switched to UHF/625.

Regards,

Dazzlevision

[FERNSEH]

Hi Steve,
I've got plenty new and boxed PC86 and PC88 valves if you need any.

DFWB.

[neil29]

I bought a set like the one in post 17 from Bristol a fair while back off ebay. and in fact it's probably the very same set.

Cheers
Neil.