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1975 Thorn 9000 & 1979 Thorn 9905 - SYCLOPS

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crustytv
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yes it is there Rob, L120 ( Luma delay) is just visible bottom centre, you can see T90 written on the top. Remember this is a 9905 chassis so best to work from the data Stan has loaded for it and the 9900 series in the library, rather than the 9000, theres a lot of layout changes.

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Posted : 25/08/2014 8:05 pm
Jayceebee
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With all fault finding start with the supplies. You obvoious have HT, check that it is somewhere near the correct value and it is adjustable with the pot on the SYCLOPS control board. To me it looks as though the RGB transistors are hard on or more likely the HT to the RGB transistors is missing. Also check the 25v rail to the decoder is also present.

See if the A1 voltage can be adjusted, the resistor 1.8 or 2.2meg off the bottom end of the pot could also give a bright raster with flyback lines.

This one should be easy but looking at the pics the green gun of the CRT may be down on emission.

John

John.

 
Posted : 25/08/2014 10:25 pm
crustytv
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Update

I can't print at the moment and I'm trying to get my head around this split schematic in R&TS, its making my eyes and head hurt :cch

Anyway, I found the reason for the lit up CRT, F3 the 500mA fuse was blown. I only have large 500mA no 20mm, so fitted the next nearest I had that would fit, a 630mA.

So with the new F3 installed I checked the various voltages.

TP11 should have 87.9V I have 90V
TP12 should have 244V I have 244V
TP13 should have 30.8V, I have 32.4V
TP17 should be 24.4V, I have 25.3V

HT to Chroma board PL4/2 should be 240V I have 246V and the 24V rail at PL4/5 is 23.2V finally A1 is 850V

Pretty much all where they should be

I now have 245V on R,G,B and 26V on the grids. The set now has a blank screen (no raster) checked brightness and contrast, no effect. Removed the fuse to confirm and the screen lit up again, so back in with the fuse. This is similar to the 8500 luminance issue.

That's as far as I've got.

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Posted : 26/08/2014 4:15 pm
malcscott
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Check A1 volts.

 
Posted : 26/08/2014 4:17 pm
crustytv
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Sorry Malc I was editing my post to add the Chroma HT feeds and the A1 when you posted, its 850V

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Posted : 26/08/2014 4:21 pm
malcscott
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Check eht volts/crt heaters.

 
Posted : 26/08/2014 4:27 pm
crustytv
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EHT 22kV
CRT heaters 3.5V :ccf :ccb I would have thought even with it that low, I would get something, even if ever so dim on screen.

Off to find and check R732

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Posted : 26/08/2014 4:33 pm
malcscott
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s/c one of the crt cathodes to deck for a couple of secs see what is on the screen.

 
Posted : 26/08/2014 4:38 pm
crustytv
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I shorted each one in turn and each one flashed Red, Green and Blue accordingly.

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Posted : 26/08/2014 4:45 pm
crustytv
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May be a new one for you Chris, but remember that the heater voltage is being generated at 15.625KHz and is very unlikely to read correctly on your DMM's AC volts range. I expect that what you are reading is correct for that meter and the actual heater voltage is correct.

Graham.

You're right Graham, I was not understanding that :ccg Interestingly I tried my AVO 9 and that couldn't read it at all just 0V

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Posted : 26/08/2014 4:55 pm
Cathovisor
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This is why it's worth investing in a good quality True RMS meter... as your sets get (distressingly :qq1 ) newer Chris, things like this will trip up more traditional test meters.

Leave the AVO 9 for your G6 :bba

 
Posted : 26/08/2014 9:28 pm
crustytv
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Not sure but I think I might be on the track of the issue.

Earlier I established the presence of the two HT rails to the Chroma board, they come in via PL4/2 (240V) and PL4/5 (24V) However when the plug is installed into the socket the 24V rail collapses. I've checked the pin continuity for PL4/5 and it checks out ok.

The 24V rail should drop via R174 providing the Luma delay line L102, with 16.2V. As the 24V rail has collapsed its not getting this. I just need to find whats eating the 24V could be C171, C196, C168. Oooo we hear the voice.... "Beware The Bl..........." hmmmmm

Edit:
Don't worry Mike, I think I traveled too far forward in time myself, after this I'm going back to late 60's and early 70's colour TV's and staying there. I'm going to shove a dirty great spanner in the cogs of the TickTock Time machine limiting travel beyond 75. On the bright side most of my other sets are classics of this period. I've still got to get two CTV25's to get on the Bench ( well not on the bench, more the floor) then the HMV 2700 ( BRC 2000), plenty of hybrids, another 3000, A823, Bradford etc. Also still got a couple of 23" 11U dual standards to sort.

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Posted : 26/08/2014 9:36 pm
crustytv
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This is why it's worth investing in a good quality True RMS meter

I already have a True RMS bench meter but never use it that much. I was once chastised, " why on earth do you need precision when working on valve TV's". The point they missed was I didn't, I just liked it and enjoyed using it., so away it went to the top shelf never to be used. Dug it out this morning, dusted it off and took the CRT heater readings. So in answer to Malc's original query, they are at 6.6V well to be precise, they are 6.668V :qq1 (see below)

Anyway today is chase the culprit that is dragging down the 24V rail to the Chroma board.

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Posted : 27/08/2014 8:43 am
Refugee
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I have had that problem with the heater on the VF display on a VCR. It was driven directly from the SMPSU.
The DMM read overload on the 20V range and about half the correct voltage on 100V.
An AVO read low to about the same level as the 100V range on the DMM.

 
Posted : 27/08/2014 8:53 am
malcscott
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Try feeding 24v via an external psu and watch for overheating component.

 
Posted : 27/08/2014 9:04 am
sideband
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I already have a True RMS bench meter but never use it that much.

You will also need it to measure the heater voltages in sets with a diode dropper.

It's a fact you don't need a true RMS meter for most service work but if you already have one, why not use it?

 
Posted : 27/08/2014 9:42 am
crustytv
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With the set powered off I fed 24V to pin 5 from my bench supply, it immediately drops to 14V, no sign of any hot or distressed components.

Thought I had found it as C171 6.8uF & C203 1uF, C126 6.8uF, C142 3.3uF and C173 6.8uF Tants had high leakage but after replacing them, still had the same scenario.

I then did the same test on the live set, feeding the 24V which is pulled down to 14V, its enough to give me a nice snowy raster. So somewhere on the Chroma board lies the culprit.

I'm going to check a few of the other electrolytics/tants that go to ground that might be pulling the 24V rail down, there are many and this could take some time.

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Posted : 27/08/2014 2:39 pm
freya
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I was wondering if you could try isolating the 24v line shown next to the 100Uf you outlined in green.

 
Posted : 27/08/2014 7:23 pm
crustytv
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Hi Stephen,

I've been wondering if I could isolate it most of the afternoon as it would make it easier to narrow the culprit down. At the moment I'm lifting tants one at a time and with just the bench supply powering the 24V rail hoping to see it rise from 14V. I know I could just blitz the board of the blue meanies but I don't want to do that.

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Posted : 27/08/2014 7:30 pm
crustytv
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Still hunting this elusive volt eater.

Under normal mains power the 24V rail is collapsed to a few millivolts.

With 24V fed to the board from the bench, the rail sits at about 14V and the snowy raster is presented.

I've checked a few more tants and they are OK, as Stephen mentioned and I had been wondering, is it possible to isolate this otherwise this is going to be a laborious task of individual checks.

With the set off mains and the 24V rail powered to pin 5 of PL4 by the bench ( dropped to 14V due to the fault), I unplugged the various feeds into the board watching the bench PSU meter hoping to see it shoot back up to 24V.

First PL4 was removed, followed by PL6 and PL3, nothing! Then the final plug PL34, at this point the meet jumped 2V Bugger! not the 10V I was hoping for.

So does this suggest I have faults in two places? one on the tuner which accounts for 2V of the loss and the other 8V somewhere still on the chroma, or is the removal of PL34 just load easing. If it is load then why didn't the other plug removals have the same effect. When the set is powered from the mains with pl34 out, I still only have millivolts in the 24V rail, so the tuner board does not look significant, . :aab

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Posted : 27/08/2014 10:46 pm
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