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1974 22" Ultra 6714: Thorn 3500 chassis

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crustytv
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The repair of this set started here

To continue the story of this set here. There is a pause before I continue to track the line speed problem.

The tuner in this set has totally failed, it cannot latch on any channel and therefore I have to replace before I go any further. I have one spare in stock but the wiring is missing so I'm transferring it over from the old. I expect to be back at the set here a little later.

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Topic starter Posted : 10/09/2016 4:03 pm
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crustytv
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Stan has scanned and passed me this document. Its from the Newnes Electronics Toolkit and describes the markings, tolerances, types and uses of the various makes of capacitors. The book was printed in 1993 and the data may be out of date for modern types but is still relevant for the type of capacitor we are likely to come across in our sets.

It may also settle some of the discussions regarding  the different makes and types of capacitor.
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Topic starter Posted : 10/09/2016 6:04 pm
Cathovisor
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Excellent work from Stan, as always! thumb_gif

I note the well-used and trusted MKT1813 polyester dielectric caps are listed there, under their time as ERO.

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Posted : 10/09/2016 7:08 pm
crustytv
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Indeed Mike, Stan's library is an impressive one as is his very well equipped workshop, not been up there for a while but he's always tinkering.

Back to this Ultra, as I mentioned above the tuner has totally failed with not one selector latching so I had to address that before moving on. I found a spare tuner in stock but without any wiring and the collars had suffered the usual Thorn problem of perished plastic in the lever end stops. I rewired the tuner and fixed the caps following the excellent method Mikey405 found. You can see the new collars in picture #3

Now all I have to do is remove the ultra tuner and fit this then see where I am with regards to tuning and line speed.

tuner1.jpgtuner2.jpgtuner3.jpg

P.S.

I think you'll find uploading files here much easier. Firstly you can select multiple files in one hit, it loads them into the box then you hit upload. Secondly you can the arrange the order the will be displayed via drag and drop in the attachment box. Finally the file size restrictions are much more generous 500KB. 

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Topic starter Posted : 10/09/2016 7:45 pm
Cathovisor
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All good news. Those collars I best know as screw cups for bay-mounted equipment - I've supplied a few to some other members in the pastwinkfor just this purpose. Which reminds me - must grab a few more!

Look forward to seeing some results with that 3500 - now, where have I misplaced the manual so I can sing along? I know it's in the front room somewhere...duno_gif

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Posted : 10/09/2016 7:59 pm
crustytv
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Cathovisor said
Those collars I best know as screw cups for bay-mounted equipment - I've supplied a few to some other members in the pastwinkfor just this purpose. Which reminds me - must grab a few more!

Ooo if you do get some and have a few to spare I would certainly like to buy some. As you know I've got a number of Thorn sets and and an inordinate number requiring this exact collar repair. I just used my last few, which I believe I got from e-bay but it may well have been your good-self.

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Topic starter Posted : 10/09/2016 8:14 pm
sideband
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Glad you are carrying on with the restoration here Chris. Probably much more peacfull! 

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Posted : 10/09/2016 8:24 pm
crustytv
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Hi Rich,

It certainly will thumb_gif

Well after a rather pleasant evening meal and a couple of glasses to smooth the old bones, I got back to the Ultra. The tuner was well and truly stuck to the cabinet and the retaining metal/plastic screws had to be gently coerced into giving up their grip. Quite why any manufacturer thinks plastic and metal make a good paring is beyond me. Any way the way started to break but no before I managed to get them loose.

With the tuner now removed I found all the collars had failed, this has to be the worst one I've encountered. Normally I'm left with at least one that locks.

Next I'll install the newly repaired tuner.

duff.jpg

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Topic starter Posted : 10/09/2016 10:22 pm
Marc
 Marc
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Hi Chris,

That tuner of yours looks in exactly the same state as the one in my Thorn 8500 did but thanks to the Catho's collar replacement service mine is now as good as new. thumb_gif

Marc.

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Posted : 10/09/2016 10:43 pm
crustytv
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 New tuner installed and now I have managed to tune to where the test card is. Back to where I was yesterday. I can adjust the line speed and just make it out in the raster but I can also hear ( faintly for me) the 625 line whistle change. Now down to some fault finding on the line speed issue. First some voltage checks and then some scoping to see what's going on.

 

back.jpg

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Topic starter Posted : 10/09/2016 11:02 pm
crustytv
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Well I had a play this evening removing the PSU and Line timebase to the test rig. The intention to get to the bottom of this line lock problem. So first task is to scope the base of VT503 the line oscillator and see what we have waveform wise and how it adjusts.

The wave form is text book, however all becomes clear when adjusting R504 (hoz hold). At the top end of adjustment we have 17.77kHz and at the bottom end we have 16.55kHZ. No chance of getting 15.25kHz needed for lock which should be roughly at the centre of the preset travel.

ltbfreq1.jpgltbfreq2.jpg

That's the easy bit, now to find out why!

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Topic starter Posted : 11/09/2016 9:16 pm
crustytv
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Further investigation also concurs the oscillator drive is not right.

oscdrive.jpgoscwv.jpg

 

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Topic starter Posted : 11/09/2016 10:21 pm
sideband
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I don't know the circuit Chris but is there a way to 'free run' the oscillator? Is there a test point you can short that removes the sync allowing the oscillator to free run? Sometimes a fault in the flywheel sync can cause the oscillator to run incorrectly. You can chase your tale looking for a fault like that.

So if you can remove the sync signal and then the oscillator can be adjusted correctly, its a sync fault rather than oscillator.

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Posted : 11/09/2016 10:47 pm
crustytv
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Already ahead of you on that one Rich, I tried the flywheel short to chassis I was unable to get a float lock.

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Topic starter Posted : 11/09/2016 10:53 pm
sideband
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Oh well it was worth a try....easy to forget some procedures.thumb_gif

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Posted : 11/09/2016 10:56 pm
crustytv
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sideband said
I don't know the circuit.

I will look at this tomorrow without tired eyes and start fresh. I know how folk like to play follow along so here is the circuit.

cct1.jpg

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Topic starter Posted : 11/09/2016 11:00 pm
Jayceebee
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Good to see you all over here. I wonder if the phantom twiddler has been at work. L501 sets the free run frequency but it may be being pulled off the the discriminator stage or a fault around VT501.

Try shorting out the sync in the normal way, set R504 to mid point try adjusting L501 for a floating picture. Remove the short and see what happens, if you obtain lock great, but if you obtain false lock or no lock put the core back where it was and investigate further.

Just a note about the replacement C518, I'm amazed how small it is with that spec. It rather reminds me of the XC condenser scene in the film This Island Earth, I wonder if anyone recalls it?

John.

Edit. You can see the scene here about 8 mins in. 

You haven't got an Interrossitter project on the go have you Chris? tele07_gif

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Posted : 11/09/2016 11:48 pm
crustytv
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Hi John,

I recall that film one of my favourites, in fact I'm a bit of a closet sci-fi buff and have a large collection of videos of this genre. People have often remarked on my having a similar forehead to Exeter, however as is quite obvious I do not hold his intellect. waa_gif

Back to the timebase problem, I will try your suggestion tomorrow.

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Topic starter Posted : 12/09/2016 12:30 am
crustytv
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Before doing anything with L501 ( osc coil) I thought I would check voltages on VT501 ( reactance) As far as I can tell the expected voltages for VT501 are as follows:-

  • Base 4.5V
  • Emitter 5V
  • Collector 18V

My readings are the following:-

  • Base 4.4V
  • Emitter 4.7V
  • Collector 16.3V

I would say that VT501 is working as expected and the 1.7V variance on the collector, perhaps down due to running on the rig and not under normal set load.

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Topic starter Posted : 12/09/2016 11:21 am
crustytv
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I suspect there answer to this is no but I'm curious.

Is it feasible to do this via the rig? By that I mean can I adjust L501 whilst monitoring the collector waveform VT503 Drive)and peaking it to remove the dip in the waveform as seen above.

The correct procedure I will follow and one that requires the LTB to be in the set, states you need to advance the brightness control fully. Then short the sync to chassis at the flywheel TP. Then use an AVO on the 10V range connecting to the slider of R504. Adjust R505 for a reading of 6.2V. With the sync still shorted adjust L501 for a stationary picture, remove short and It should be locked.

Not had a chance to look at this much today but hoping for a late night visit to the bench or maybe first thing tomorrow.

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Topic starter Posted : 12/09/2016 10:01 pm
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