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Old 25th Feb 2015, 8:55 pm   #21
Heatercathodeshort
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Default Re: Thorn 850 chassis - tuning problem

Hello Steve,
This is an Ultra 6624 fitted with the Thorn 850 chassis. I have had this around for very many years but the Monday night before Christmas 2013 it took a 3ft fall onto a concrete floor. The cabinet fell into 4 pieces but other than that it has survived intact. I have repaired the case [easy] but it needs a good check over to return it to full glory. They do give an excellent picture when operating correctly.
[The Pye V4's test card puts it to shame and it's ten years older!]
Note the repositioned mains dropper on your compact version. It must have got a bit warm! I don't know how they crammed it all in.
The cabinet dates from the 405 line only version, the Ferguson 3600 series so I guess they had a lot of cabinets left over. Regards, John
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Old 25th Feb 2015, 8:59 pm   #22
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Default Re: Thorn 850 chassis - tuning problem

And here's the thread describing its resurrection.
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Old 25th Feb 2015, 9:09 pm   #23
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Default Re: Thorn 850 chassis - tuning problem

The coil forms on that series used to heat shrink, alignment often meant drilling or cracking the cores and poking the bits out with a pointed tool, the new cores were put in with silicon grease, if they would fit, if not it was stuffed on the pending spares shelf.

Lawrence.
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Old 25th Feb 2015, 9:18 pm   #24
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Default Re: Thorn 850 chassis - tuning problem

Some interesting info- thanks everyone.

The official Thorn service booklet shows two chassis layouts- 'Main chassis' and 'Main chassis - minimum size version'. John's Ultra is the former, my Ferguson is definitely the latter, with the re-positioned dropper.

Busy with voice-over work for a couple of nights now, but forum member Bobbyball is coming down at the weekend, so further investigations will commence then!

Steve
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Old 26th Feb 2015, 12:30 pm   #25
dazzlevision
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Default Re: Thorn 850 chassis - tuning problem

Quote:
Originally Posted by Restoration73 View Post
This set has been listed here for a number of years !
http://www.electrojumble.org/CLUB_Docs/TV_01.pdf
Hello,

That set has the Thorn 800 dual standard chassis. Easy to tell apart from the Thorn 850 dual standard, as there is a B7G socket on a metal bracket, next to the LHS of the IF panel, for connecting the UHF tuner directly to the IF strip. On the 850 D/Std chassis, the UHF tuner plugs into a B7G socket mounted on the VHF tuner, to provide additional IF gain.

Regards,

Dazzlevision
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Old 26th Feb 2015, 10:02 pm   #26
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Default Re: Thorn 850 chassis - tuning problem

It's easy to tell the difference between the 800 series chassis from the dual standard 850 series. The 800 has the 405 only signals board. The timebases are dual standard and required no modification. Conversion to full dual standard operation required the addition of two units. A special hand wired IF 625 amplifier chassis which was attached to the upper deck of the chassis. The other unit required for full conversion was the UHF tuner. It was an easy conversion procedure, the extra IF chassis plugged into the B9A socket on the upper chassis and the UHF tuner leads were plugged in the B7G socket in the 625 IF unit.
Performance on 625 was not very good, more IF amplifier gain was needed.
I mentioned in an earlier post a special transistor IF module was supplied to improve performance, this little unit really improved 625 reception. The result was a healthy snowstorm when not tuned into a UHF channel.
A special back cover was made to accommodate the extra 625 IF unit.

The fully dual standard 850 was much better. The output from the UHF tuner was routed through the VHF tuner. The UHF tuner connector plug was mounted on the VHF tuner. A special tuner biscuit was employed to allow the mixer valve to serve as an IF amplifier.

There was an earlier convertible chassis, the 700 series phase 2. A modification of the Ferguson 705T Senator and model 727T.
See post #15: https://www.vintage-radio.net/forum/...d.php?t=114296

DFWB.

Last edited by FERNSEH; 26th Feb 2015 at 10:10 pm.
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Old 27th Feb 2015, 9:57 am   #27
dazzlevision
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Default Re: Thorn 850 chassis - tuning problem

Hello,

Just to be clear, there were two versions of the Thorn 800 chassis. One was dual standard and the other convertible to dual standard. As has been said, the convertible version had a 405 only IF panel, but a 405/625 switchable timebase. The same applies to the 850 chassis. Thorn (Ferguson) issued four full service manuals to cover each of the four versions.

The convertible versions of both the 800 and 850 chassis had a B9A socket on the chassis for connecting the (hand wired construction) 625 converter chassis. On sets that hadn't been converted to dual standard operation, it could also be used for a connecting a flywheel sync add-on unit, for use in difficult reception areas.

Regards,

Dazzlevison
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Old 27th Feb 2015, 12:48 pm   #28
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Default Re: Thorn 850 chassis - tuning problem

Hi Dazzlevision,
OK I see what the dual standard 800 series set is all about.
The output from the UHF tuner does not pass through the VHF tuner. The attached circuit diagram shows how the tuners are selected at the input of the first IF amplifier valve. The 405 vision IF response curve is correct by traps L1,3 and 4. So, like the converted 800 sets the DS 800 would have certainly benefitted from the addition of the plug in transistor IF amplifier.

So now we know the 850 series is the version in which the UHF tuner output is routed through the VHF tuner for extra IF gain.

DFWB.
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Old 27th Feb 2015, 1:35 pm   #29
dazzlevision
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Default Re: Thorn 850 chassis - tuning problem

Hello,

Here is a picture of the Thorn converter IF amplifier unit that was part of the UHF/625 conversion kit for convertible Thorn 800 & 850 chassis sets.

It makes the converted set look like a "dogs breakfast"!

Regards,

Dazzlevision
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Old 27th Feb 2015, 4:03 pm   #30
dazzlevision
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Default Re: Thorn 850 chassis - tuning problem

Hello,

Here's the plug-in "flywheel sync" add-on unit. In a standard convertible 800/850, it screws to the chassis rail at the bottom, holding the timebase PCB in place (but not, of course, in the minimum size version!).

Regards,

Dazzlevision
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Old 27th Feb 2015, 5:57 pm   #31
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Default Re: Thorn 850 chassis - tuning problem

Hi Dazzlevision,
The add-on flywheel sync unit: Now that's an unit I've never ever seen. I knew it was made available for customers who lived in difficult reception areas. Learning all the time.

DFWB.
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Old 28th Feb 2015, 9:34 pm   #32
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Default Re: Thorn 850 chassis - tuning problem

Quote:
Originally Posted by dazzlevision View Post
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.
Here is a photo of where they should be.
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Old 1st Mar 2015, 9:27 am   #33
Colourstar
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Default Re: Thorn 850 chassis - tuning problem

Hello everyone

Just to bring you up to date, HT is present on the UHF tuner and a pair of NOS valves have been fitted but there is no change to the symptoms.

I guess the next step is to take the chassis out so the VHF tuner can be investigated, particularly to look at the UHF biscuit mentioned by Dazzlevision.

Steve
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Old 1st Mar 2015, 10:17 am   #34
dazzlevision
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Default Re: Thorn 850 chassis - tuning problem

Quote:
Originally Posted by Colourstar View Post
I guess the next step is to take the chassis out so the VHF tuner can be investigated, particularly to look at the UHF biscuit mentioned by Dazzlevision.
Hello,

I have found the fixed contact block moulding to have warped, resulting in some of the UHF biscuit studs not actually making contact with the spring contacts. So well worth checking and cleaning.

Regards,

Dazzlevision
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Old 2nd Mar 2015, 6:58 pm   #35
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Default Re: Thorn 850 chassis - tuning problem

Check to see if you have HT on the UHF tuner and the valves are the correct way round, PC88 towards the aerial input. If you have a signal generator, feed the output at IF via a .01uf to the output terminal on the UHF tuner. This will give you an idea if the tuner or the switching is faulty. Check to see if the tuner drive is actually activating the vanes inside the tuner. It may have broken or it's fixing become loose. It is OK to remove it's cover but don't prod around inside it. John.
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Old 2nd Mar 2015, 8:15 pm   #36
dazzlevision
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Default Re: Thorn 850 chassis - tuning problem

Quote:
Originally Posted by Heatercathodeshort View Post
It is OK to remove it's cover but don't prod around inside it. John.
If you do open the UHF tuner up, look to see if there are any dry joints or wires come adrift and any burnt/overheating resistors (including the one on the outside - probably - soldered between two feed-through capacitors - HT decoupling).

It also wouldn't hurt to put a tiny drop of contact cleaner/lubricant on the rotor earthing spring contacts and then swing the tuning back and forth across the whole range of the tuner a few times, to overcome any possible tarnishing.

I've found in the past, that a quick way to test if there is a path for the UHF tuner's IF output through the VHF tuner and into the main vision IF strip, is to touch a long screwdriver blade onto the UHF tuner's IF output coax centre conductor (be careful, it may have HT on it). It acts as an aerial and should produce some sort of (changed/additional) patterning on the screen and some audio output - apart from a "click" when you first make contact.

Dazzlevision

Last edited by dazzlevision; 2nd Mar 2015 at 8:21 pm. Reason: Added text.
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Old 3rd Mar 2015, 12:08 am   #37
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Default Re: Thorn 850 chassis - tuning problem

Ive just seen this and cant believe it, I restored this set a few years ago, it was in a terrible mess cosmetically and needed lots of electronic restoration too including a replacement FOPT.
Where did you get the set Steve?

Its late and I don't have time to ramble on any more about it tonight but will continue tomorrow, I have dug out a few pics that I had taken during the restoration and posted for you to see. (I have more) Tas, you should remember this one as you gave a hand with the restoration on the day we came back from Mikes parents in Devon.
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Old 3rd Mar 2015, 12:11 am   #38
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Default Re: Thorn 850 chassis - tuning problem

I meant to add that I had no joy with the UHF tuner either, the valves in it are NOS so keep hold of them, I did lash up the tuner from my Fergy 3629 Personal on the bench and plug it in and it worked fine so it is definitely a problem with the tuner and not the rest of the set.

Cheers
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Old 3rd Mar 2015, 12:38 pm   #39
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Default Re: Thorn 850 chassis - tuning problem

A few more,

Pic 1 shows the timebase chassis recapped but not the IF.

Pic 2 shows the set running with the faulty FOPT, it was arcing internally, luckily I had the correct transformer for this set, a C core type in the spares box

Pic 3 shows the new transformer connected with primary and secondary around the wrong way hence flyback lines.
Pic 4 shows results with the connections corrected but still looking poor due to the original capacitors still in the IF deck.

Cheers
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Old 3rd Mar 2015, 12:40 pm   #40
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Default Re: Thorn 850 chassis - tuning problem

Cheers
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