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1974 22" Ultra 6714: Thorn 3500 chassis

Pre War 240/405, Post War 405 Line, B&W Dual Standard, Colour Dual Standard, B&W S/S 625, Colour S/S 625 line, Hybrids. Standards converters & modulators, video recorders.
 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by sideband » Tue Sep 06, 2016 6:43 pm

That green capacitor up near the transformer looks like the type we now change on sight. Similar to the ones used in the G8 that have a nasty habit of going O/C and sending the EHT sky-high.....

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by jayceebee » Tue Sep 06, 2016 6:56 pm

This is looking very good Chris, you've really got the hang of these now. I just hope that CRT holds up.

John.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Tue Sep 06, 2016 7:19 pm

sideband wrote:That green capacitor up near the transformer looks like the type we now change on sight. Similar to the ones used in the G8 that have a nasty habit of going O/C and sending the EHT sky-high.....

Hi Rich,

Yes C518.

The first generation Line Timebase boards had two caps in series in that position, a pair of .056uF 1000V jobbies. Later boards and I believe a mod was to link out C517 and fit a single .028uF 800V cap, not sure about the reason behind it might just have been a stock choice. Some of the boards I've come across have the two caps, some just the singular. Of the singular type I've comes across three types fitted, the green you see on this module, there's a yellow one same size as the green and then a short, fat stubby lozenge type.

I've had to leave the green in place for now, typically I have what I thought a fairly good stocked workshop, but isn't it always the case when you need an odd ball like the 0.28uF, nothing!! Looking online I've equally not found them. However I may revert to the two .056uF 1000V in series as Vishay do these.

http://www.ebay.co.uk/itm/VISHAY-BC-COM ... xyj4hTIjij

For now I will just have to keep a watchful eye and ear.

jayceebee wrote:I just hope that CRT holds up.

John.

Me too

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Tue Sep 06, 2016 11:41 pm

I managed to track down a suitable quality ( panasonic) replacement flyback tuning cap ( C518) for the Thorn Line timebase and thought I would post it here should others find they need to get a stock as I did.

The original single cap was 0.028uF @ 800V the replacement spec is as follows:

0.027µF
Tolerance ±3%
Voltage Rating - DC 800V
Dielectric Material Polypropylene (PP), Metallized

More than adequate for the job and at a very good unit price, I just ordered a batch for stock. This works out cheaper than reverting to the 2 x 0.056uF 1000V Vishay caps I mentioned earlier.

The source: http://www.digikey.co.uk/product-detail ... ND/2179297

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by jayceebee » Wed Sep 07, 2016 8:31 am

I don't think the cap you have selected is suitable for that position, it would need to be rated at around 2kv DC. I suspect if you look closely at the original green one its rating is probably 800v AC, remember we have a huge pulse waveform here.

John.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by Cathovisor » Wed Sep 07, 2016 9:21 am

jayceebee wrote:I don't think the cap you have selected is suitable for that position, it would need to be rated at around 2kv DC. I suspect if you look closely at the original green one its rating is probably 800v AC, remember we have a huge pulse waveform here.

John.

I think Crusty's picked exactly the right kind of cap for the job. Polypropylene, so right kind of dielectric for pulses. It's rare that caps are specified in terms of their AC rating unless they're intended for use in AC positions.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Wed Sep 07, 2016 9:37 am

Exactly what I thought Mike and its designed for High frequency, switching and high current.

capspec.jpg

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by Cathovisor » Wed Sep 07, 2016 9:42 am

Out of curiosity, what does the cap itself and the parts list say, and your spare modules?

Polypropylene dielectric caps are designed with high voltages and especially, fast rise times in mind. They often have quite impressive pulse current handling capabilities too - in the range of amps in some cases.

Just wish I could remember where I last had my Thorn 3K5 manual...

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Wed Sep 07, 2016 10:01 am

All it states is 800V (special) never found out what special meant.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by Cathovisor » Wed Sep 07, 2016 10:23 am

CrustyTV wrote:All it states is 800V (special) never found out what special meant.

But not AC or DC. It probably meant what we are discussing here - polypropylene, rather than polyester dielectric. A polyester-dielectric cap in that position would fail pretty quickly.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Wed Sep 07, 2016 10:33 am

Cathovisor wrote:But not AC or DC.

That's correct

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by Cathovisor » Wed Sep 07, 2016 10:36 am

Right.

I say "buy them". They sound like an ideal device for the job on these sets; that stray nanofarad isn't going to cause any problems :aac

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Wed Sep 07, 2016 10:38 am

Cathovisor wrote:I say "buy them". They sound like an ideal device for the job on these sets

Glad you said that as I ordered 25 last night inno: Why so many? Well the rate I pick up 3K sets, best be prepared :qq1

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by Cathovisor » Wed Sep 07, 2016 10:41 am

NOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!!!!!!!!!! :aae

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by malcscott » Wed Sep 07, 2016 10:49 am

Is the CRT a Mazda?

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Wed Sep 07, 2016 10:50 am

yes

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Wed Sep 07, 2016 10:51 am

Cathovisor wrote:NOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!!!!!!!!!! :aae

All because I know how much you love a good 3k story, I will torture thee :aah

Call me mad if you will but If I could travel back in time I would set up a CTV panel repair/replacement shop for the independents that couldn't be bothered. "Send me your old and I will sell you a refurbished with discount" I would happily work away on 3k panels day in day out.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by slidertogrid » Wed Sep 07, 2016 11:31 am

This is exactly what a friend of mine did back in the mid 1970's, he had a 19" set he used as a jig. A few other small dealers would send modules in for repair. It's strange how non BRC dealers would run scared from these sets, the main excuse I heard was that they used "special" transistors which they didn't have in stock... :aaq
Another set saved Chris! I can't wait for the next instalment..
:thumb
Rich.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by malcscott » Wed Sep 07, 2016 11:40 am

I must have spent many hundreds of hours working on the 3000 test rig when i worked for Rediffusion. The psu and ltb panels were stacked like army blankets awaiting repair. :ccf :ccf Malc.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Wed Sep 07, 2016 11:43 am

slidertogrid wrote:Another set saved Chris! I can't wait for the next instalment..
:thumb
Rich.

Lets hope so Rich, and as for the next instalment..... Before I reinstall the PSU and LTB modules and power up there's one last thing to check. From bitter experience its a must otherwise the PSU 30V rail gets stuffed and in extreme cases takes out a whole load of components in that rail. The thing to check is on the I/F panel and is C132 (400uF 40V), a dead short here causes all that damage. I only found out the hard way on a past set. :ccg

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Thu Sep 08, 2016 1:09 pm

Update

C132 on the I/F panel tested OK so it was time to reinstall the PSU, LTB modules and go for the first full set power up. Powered on, sound came up , EHT rustle and a rather sickly Green raster appearer which slowly turned Red.

The tuner is suffering the plastic collar rot so only BBC1 is locking ( thankfully) I will have to repair all those later. Fed in Test Card F and tuned until signal found however unable to obtain horiz lock, vert lock is ok. During all this checking there was a loud crack, made me jump sounded like a tant going off.

This for me is the best bit, so pleased that I wasn't presented with a test card and colour working etc. Plenty of fault finding fun to be getting on with starting with the line speed which might be C506/C502 we shall see, then on to the video board for the excessive Red which might be VT208, VT209, C215, R250 . At least we're out of the blocks up and running.

More to follow........

if.jpg
The last preventative check was here was C132. It had been replaced long ago but I still tested it.

flight.jpg
Test Card F is in there..... Honest!
Last edited by CTV on Thu Sep 08, 2016 1:10 pm, edited 1 time in total.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Thu Sep 08, 2016 8:49 pm

Little bench time this evening so the Line Timebase module was removed from the set and back to the bench. C502 & C506 were innocent. As a good measure I also removed and tested R524 ( 250R) horiz pre-set, it was OK. Removed the Beam limiter board to reveal the underside of the TMB panel this revealed two faults. The source of the loud crack was in fact R530 ( 5.6K) exploding, all that was left is a little stubby end. R530 receives the chopper pulse and is connected to the collector of VT502 the line Oscillator. dmm The loose wire is the from the junction of W904/R907 ( beam limiter) connecting to the line output stage earth.

I will get these two faults fixed then to be sure VT502 is OK I will run the LTB up on the rig and scope the oscillator.

fault-1.jpg
Yellow shaded boxes, top shows remains of R530. Bottom the wire broken and thereby not connecting to the LTB earth.

r530.jpg

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by jayceebee » Thu Sep 08, 2016 10:50 pm

The resistor remnant shown is actually R528 18 ohm which is in parallel with L502. It suffers if L502 is dry jointed or o/c as all the line scan current will flow through it and the print does at one end does look a bit iffy. C506 and C511 were the most common causes of line osc drift, does adjusting R504 have any effect on the line frequency?

John.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Fri Sep 09, 2016 12:00 am

jayceebee wrote:The resistor remnant shown is actually R528 18 ohm
John.

Quite right.

I remembered the resistor under there was sheathed but no recollection of its id or value, a hasty glance at the board layout diagram showed R530 as sheathed, mistake! Double checking the physical on a spare LTB, as you say its R528 (18R). I fitted a new 1% 1W more than adequate for the original ¼w specified.

Regarding the track looking dodgy, it does but there is continuity. It looks to have had a pad lift with a resultant crease in the track, which is probably what you see. At some point in the past a previous repairer has daubed a load of solder. Despite having continuity I decided to run a jumper to save having to revisit in the future should the track fail.

C502+C506 were fine and as a precaution I had already replaced C511 as it was on of those little orange jobbies that don't fair too well. As mentioned earlier R504 was tested and found to be OK and from what I could see on screen did have an effect on line frequency.

Finally I ran a new junction wire from the beam limiter to the line output stage earth. That's for now, tomorrow I will recheck as mentioned above before trying again in the set.

tmbprts.jpg

tmbrep1.jpg
New R528 fitted and sheathed. L502 Track was OK but I ran a jumper for future proofing. Also Ran a new junction wire from the beam limiter to the LTB Earth.

 
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Re: 1974 22" Ultra 6714: Thorn 3500 chassis

Post by CTV » Fri Sep 09, 2016 4:14 pm

As fortune would have it the replacement flyback tuning caps just turned up via UPS. Snipped out C518 which looked OK if not a little discoloured but I'm glad this green meanie is now out, one less thing to worry about when testing the set for any length of time. A little later on I hope to get this back into the set and report back the results.

C518.jpg
New Polypropylene .027uF 800V tuning cap. Green meanie to the right.

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