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Forum 141

1972 Ferguson Colourstar 3713 - Thorn 8500 Chassis

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crustytv
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Further exploring my way through time and early colour televisions, I find myself in 1972. Placed before me another offering from the Thorn stable, the 8500 series chassis. I have a fair bit of data to assist with this one although as we know, as well as the stock faults of yesteryear we will have a whole raft of other issues which most likely will not covered due the extreme amount of time these sets have now been in existence.

I have the Thorn 8000, 8000A & 8500 yellow manual, the 3 part series covered in Television ( 1976 ) and the McCourt book of known stock faults for this chassis as of 1975.

June 1971 saw the production of the first British colour chassis aimed at breaking through the £200 barrier, the Thorn 8000 series chassis. The 8000 had a 17" A44-271X shadow-mask tube and all solid state circuit.

Along with its successors the 8000A, 8500 and 8800, it proved to be very popular with the viewing public and more reliable than many engineers at first predicted.

Here we have the Ferguson 3713 fitted with the 8500 series chassis which was released in July 1972.

Forum 142
This 8500 differs from the 8000 using a 19" tube, the higher scan drive requirements led to a redesigned line output stage, focus circuit and the use of a choke instead of a dropper resistor in series with the live mains input line. This latter modification provides an HT rail of about 190V in place of the 170V used in the 8000 series. Line output is via a 1700V transistor the BDX32.

Researching the known faults on this series I read that C406 the third harmonic tuning capacitor caused Thorn a fair bit of trouble. This cap is critical in value and voltage, having to operate at 16kc/s.

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The value is of this cap should be .002uF/2000pF the voltage rating at this frequency being 2000V. Further reading revealed Thorn released special capacitors for the job at the time, the value needed to be right as a reduction in value results in excessively high EHT and a duff BDX32.

For modern replacements I ordered 2 x 1000PF, 2KV, 5%, AXIAL Vishay Roederstein MKP1845210204, these will be paralleled up to replace the old one. As Rich Marcon_MPT4 noted,

Having two in parallel will work to your advantage as current through each will be lower

The set itself is in very good condition and came with its original stand. The cabinet polished up nicely and the chassis being full of cobwebs and dust. Only minor rust due to storage conditions. One issue, there are some gouges on the CRT faceplate, too deep to be polished out, shame but this can be lived with.

Before applying power a close inspection of all the boards and components were made, looking for anything untoward, certainly found a dodgy bodge.

Access to the chassis and internals has been well thought out. Two base screws allow the chassis to be extended on its plastic rails, it can also be lifted off the rails further by removing the earth strap from the tuner. Once extended the Timebase panel on the left-hand side can be hinged down and the chroma panel on the right swings out. All in all giving the service engineer excellent access to the components within.

The 8500 main HT fuse was uprated from the previous 8000 series to a 3.15A fuse ( F302) The fuse fitted was 4A but that was not the problem. Someone in the distant past had wrapped wire around the fuse holder.

Forum 144

Now whether this was meant to be a temporary fix due to no spare fuse that got forgotten, or a bodge due to a recurring condition remains to be seen. The 4A fuse that was in place is O/C so I will have to order a few 4A fuses. The other 800mA fuse (F801) is OK.

So some investigation is to why this fuse was bodged is in order before proceeding.

Some further shots of the chassis, work suspended until the 4A fuses arrive. I might get the Thorn 9000 on the bench while I waiting, see what that's got lurking away to reveal.

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Posted : 19/07/2014 4:03 pm
Lloyd
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That looks a nice set Chris! Shame about the gouges on the tube face.

I'd love to get my hands on some more early colour sets, but there's just too much modern crap in my workshop to allow it!

Regards,
Lloyd

 
Posted : 19/07/2014 4:36 pm
abctelevision
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It wasn't me "I swear gov" the previous owner who fitted the wire link across the fuse. Never got round to looking into what was wrong.

I got it in a swop not working 30 odd years ago and its been in my shed when we moved here for the past 27 years!

 
Posted : 19/07/2014 5:22 pm
crustytv
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:aaj :thumb

Hi Dave, I knew it wouldn't be you and guessed it was as received via the rental sell off. As I mention above I think it could be either a quick fix so the engineer could get on to his next job or down the pub, then forgotten.

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Posted : 19/07/2014 5:29 pm
Anonymous
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I really liked the 8k series ,most calls were to buzzing chokes and the bar on the tuner. I must of been lucky I only had 2 failures of the tuning caps (maybe changed before we got the sets)
the original tubes were not the longest lived but reguns were cheap in those days.
good luck with it should be easy after the 3000.
Rob T

 
Posted : 19/07/2014 5:35 pm
ntscuser
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I'll be interested to see it finally set up. We had a similar Fergie for a short period as a loan set while our Bush was away being repaired. It had an obvious green colour cast in every scene which made even the Bush look good by comparison.

Classic TV Theme Tunes

 
Posted : 19/07/2014 7:54 pm
crustytv
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Seeing as I didn't have any 4A surge fuses I thought I would stick in a 2A, the set should actually have a 3.15A. Yes I know 2A is hardly worth trying but I thought to give it a go in an attempt to see if it would fire into life. It did, when powered on a hiss from the speaker as it sprang to life then the fuse blew.

Maybe the same fault condition exists but the 2A fuse is hardly adequate to be useful. I suspect the same fault condition still exists that resulted in the person wrapping wire around the holder. Anyway I decided to live on the wild side and put back the wire back around the holder, powered on with a hand ready to unplug rapidly.

The set sprang to into life, sounded like the degauss kicked in too. There is a constant hiss from the speaker so the audio output stage is working, the CRT heaters were glowing nicely too, but no raster totally blank. Frequency counter however confirmed the line-TMB is running. Checked the 45V rail at F801, reads 52V and the input at fuse F802 ( wire wrap in this case) reads 243V

Next I checked the 25V stabiliser voltage was present at the emitter of VT702, it was. Next the 180V rail was checked at the junction of R703 and it was 175V. W703 has 190V at the Cathode, 220V at the anode and 190V at the gate.

I checked the A1's G=172V, R=560V and B=563V
Interesting that green is so low compared to Red and Blue

Oh and the brightness circuit seems to be non existent as operating the control has no effect. Will need to check around W110, VT116, VT121, VT118-VT120 this could all be related to this.

Whilst measuring I glanced at the screen, a dim frame collapse had started to become visible. I switched off to go read up some more and have a coffee. Upon my return powered up with the exact same scenario, after about 10-15 mins the dim frame collapse becomes visible. There was a point briefly when I looked around and the collapse was bright, I've not been able to get back to that point.

So to summarise I have a slightly high (7V over-voltage)45V rail at 52V, the 25V rail is correct, the 180V rail is 5V down. I need to figure the constant F802 fuse blowing. I've replaced the Thermal cut-out as a precaution (see below). The brightness circuit seems non operational and the green drive is well down compared the R/B

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Posted : 19/07/2014 9:00 pm
Anonymous
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Frame collapse
VT 409 (BC142) VT410(2N6178) VT411 (2N6180)
Rob T

 
Posted : 19/07/2014 9:19 pm
Anonymous
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the output pair can be replaced with bd 139/bd 140
Rob T

 
Posted : 19/07/2014 9:26 pm
crustytv
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Sod's law, even though I've been building up my transistor stocks, as usual BD139 are not there. I've just ordered a dozen.

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Posted : 19/07/2014 10:35 pm
Refugee
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Check if the fuse powers the degauss coils.
The bodged fuse will need to be a slow slow blow type as the degauss draws 8 amps for the first 200ms.
In my CRT days a lot of sets had a button on the front to activate the degauss after the swivel stand had been rotated. This used to make the moving iron meter on the Variac "wack" the top stop if it was pressed.

What powers the frame output stage? Would it happen to be the suspect 45 volt rail?
The 45V rail aught to look better once the loading from the frame output stage is back again if this is true.

Have a look at this thread for more fuse info.
viewtopic.php?f=15&t=8612

 
Posted : 19/07/2014 11:13 pm
crustytv
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The fuses I've ordered are Glass Anti-surge Slow Blow Fuse 4A.

Here's the power circuit, HT2 powers the frame-tmb and yes the fuse is in line with the degauss

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Posted : 19/07/2014 11:25 pm
Refugee
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Yes that is what I thought it might be.
The other check I would do on the screens I used to fix is to take the degauss dinger (X801) out and hold it by my ear and shake it. If it rattles the job will bounce with a blown fuse again or better just replace it anyway.

 
Posted : 19/07/2014 11:53 pm
Anonymous
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Chris why 4 amp fuses I only ever used 3.15 amp?
here is a funny pic of more fuses

 
Posted : 19/07/2014 11:55 pm
crustytv
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:aaj Excellent

Re fuse:
I ordered 4A and 3.15A, assumed the 4A was a later addition/mod. I can refit 3.15A if that's what it should only have.

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Posted : 20/07/2014 12:03 am
Anonymous
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:aaj Excellent

Re fuse:
I ordered 4A and 3.15A, assumed the 4A was a later addition/mod. I can refit 3.15A if that's what it should only have.

I don't recall any mod to 4 amp others may know differently .
Rob t

 
Posted : 20/07/2014 12:07 am
crustytv
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:thumb OK most likely all an engineer had in his tool box at the time.

Either 3.15A or 4A are academic at present as whatever is causing ( likely the degauss as Ref suggests) the surge is taking anything out in that position.

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Posted : 20/07/2014 12:16 am
Refugee
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The fuse choice is a compromise and Thorn will have called a meeting relating to nuisance fuse blowing.
In the times the set was made how many warranty returns are you prepared to put up with?
The answer is as few as possible and newer production will have been shipped with 4 amp and it will save 90% of the fuse related returns and write off perhaps 1% of other returns with the reduced protection due to burned transformers causing enough damage to write the set off completely.

If a 3.15 amp fuse can be found that can take three times its rating for 250ms of inrush currant and be reliable it should be selected.
A couple of hours in front of a screen with data sheets being displayed comes to mind here.

These days the choice is worry or risk of a fuse blowing while you are showing the set off.

 
Posted : 20/07/2014 1:18 am
ntscuser
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A couple of hours in front of a screen with data sheets being displayed comes to mind here.

Did they have such a thing in 1972, I thought PC spreadsheets and the like were a relatively recent invention?

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Posted : 20/07/2014 2:48 am
Refugee
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I was referring to what we would do now to find a suitable fuse. The 4 amp fuse was the only way back then without having special stock made up.

 
Posted : 20/07/2014 3:42 am
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