1975 Thorn 9000 & 1979 Thorn 9905 - SYCLOPS

Well thats embarrassing, a couple of days chasing a non existent fault on the chroma board, that will teach me to take current readings rather than just relying on voltage. What a plonker!! as Dr Smith would say, " Oh The Shame, The Terrible Shame"

R735 3.9R is the culprit, gone high at 4.2M OUCH! Sods law I don't have any 3.9R resistors, closest I have is large 2W 4.1R or a ¼W 3.3R. Not sure what the wattage should be I think its ½W I could do 2x1.8R in series that would get me within tolerance at 3.6R

I'm off to bury my head in the front garden, sticking a sign on my arse saying " Free kick invited"

this is what I was trying to explain we know the 24 volt line was low if you had measured the current thru f4 and it was low it would mean the psu was at fault conversely if the current thru f4 was high it would mean a fault in the signal panel .it was common for those low value resisters to go O/C we refered to them (maybe incorrectly) as fusible resistors.
Rob T.

A good reminder and reality check that I've still got a long way to go to stop making such fundamental errors, sometimes I just don't know where my head goes. I get fixated on a train of thought to the detriment of logic. Like I said Rob, I'm an idiot, no excuses. If I could erase the last couple of days and the evidence of my error I would. Well I wouldn't actually, as it would be wrong and at least my screw up might serve as a reminder for others.

If the resistor has gone high then the 24V to the panel will drop but it won't affect the 24V from the fuse.

...as I said!

Don't worry Chris. It's called a 'learning curve' and we've all done it. Just for test, use a couple of 1.8 in series or even a single 4.7 will probably do. Lets see some pictures next!

I'm an idiot, no excuses.

I disagree and you are learning by your mistakes just like we all did. Remember once again that you haven't got an engineer sitting next to you or in the same workshop to lend a guiding hand like most of us did.

Well done Chris, your getting there!

Don't beat yourself up Chris, you have learned another lesson in proving what is really going on under fault conditions. As to becoming fixated on a train of thought even experienced people(including me) do. I have walked away from a fault finding task many a time for a ten minute break.

Al

I hate doing this but needs must, I put in series a 2.7R & 1.2R giving me 3.9R looks awful but will suffice until the 3.9R I've ordered arrive.

This has resolved the 24V rail and I have sound working too.

The first photo shows what I'm presented with, the cross-hatch shows red and green not converged. Its hard for the photo to show the green but its much clearer in reality. I noticed tapping the cabinet makes the picture start to collapse/bounce back all within a fraction of a second. After about a minute the picture turns blue-ish/grey-ish ( video drives fluctuating?) I noticed yesterday they had dropped from the 244V we had previously.

I think I need to look at this next.

It looks as if the green gun is de-focused. Try each individual gun on the crosshatch and check focus,

OK I unplugged the RGB from the video board and then tried just them one at a time with the cross-hatch.

Green on its own

Red

Finally Blue on its own

Voltages are Red=192V, Green=167V and Blue=179V Looks like the CRT is quite good emission, nice and bright on all three.

Focus is OK

I was going to say check TP717 with it all plugged in. If F4 remains intact the power supply would be the problem. and it was indeed the currant limit resistor.
It looks like the green black level is floating now and that should be an easy thing to fix.

Maybe give the green gun a little tickle with the CRT [strk]wreck[/strk] reactivator!

The resultant colour testcard is pretty good thus far, I've not done any setup at all. The colours are not quite right but equally I've not greyscaled or set any of the biasing, then theres the green gun emission. The main issue I seem to have is when the set is first turned on the whole picture is red, it then starts to flicker after a few mins and the blue seems to then kick in. Then the picture is fine, as can be seem below. I need to look at the components around the RGB circuits.

The CRT certainly seems in better nick than the 8500 I recently had on the bench.

Edit: I cannot tickle it as my Leader although having the right adapter, does not appear to be able to set up G2. I will have to wire up an adapter myself, not sure I would tickle it if I could, maybe if it degenerated further.

 

 

I would check the three black levels for the guns. There is usually plenty of high value resistors there to go intermittent and float including the pots themselves.

I can call with the Dynascan, it has the base for 570/580/590x tubes.

No rush Malc, in fact leave it for a while, I will let you know. I've plenty to get on with and I will need to build my own adapter anyway for the Leader.

I also need to read up on the data Stan provided and familiarise myself with the set, for the life of me I cannot see how you would do grey scale, i'm sure its in one of the manuals somewhere.

My next job is to figure where the fault is with the initial power on, blue missing then kicks in after a few seconds and flashes.

Hello Chris
If the CRT appears to be low emission, check the value of R718 on the power supply is the correct value. It is connected in series with the CRT heater and can increase in value giving the effect of low emission. The latest correct value is 2R2 but a 1R5 or 1R8 resistor could be fitted. Do not rejuvenate the tube before you have checked the value of this resistor.
Regards Stan.

Hi Stan,

Wise words.

In fact something has been nagging at me not to go anywhere near rejuvenation, not for the technical reasons you have outlined, but just something in my head saying no.

This is why I wanted to build and adapter so the Leader, so I can test to find the true state, not to bop.

I've had a little dabble with the drive and bias and things are looking a whole lot better than they were. I also still need to read up on checking the grey-scale and setup. I think the longer its on the better it seems to be getting too. I've not come across this style of set before and from what I gather leave all those rings well alone. They are marked at the time it left the factory with a paint line and it looks to have never been moved, so I've no intention of touching them.

I will check out the resistor you mention and see what its value is. I still need to find this dodgy part as slight vibration or movement make the picture wobble and come back, loose component, dry joint.

Then there's all the checks to do, when I was chasing the non existent fault on the video board there were a fair few leaky caps, I guess I need to continue checking and replacing where necessary to improve things further.

Hello Chris
Just in case you have not found the grey scale tracking procedure, it’s on the last page of the manual, just before the circuit diagram, in the Library. The procedure uses an AVO model 8 20,000ohms/volt meter.
The set had been moved around a few times in it’s life so it may be wise to check and clean all the plugs and sockets and re-solder anything that gets hot and is heavy and is mounted on a PCB. These items can give misleading symptoms due to poor soldering and cracked joints or print.
Also see the fault finding guide for the chassis, also now in the Library.
Regards Stan.

Doesn't look too bad Chris and of course may pick up with use.

R718 was as far as I am aware always 2.2ohm from the factory but the field engineers were allowed to drop it to 1.8 then 1.5 to extend the useful life of the CRT. A lot of the 570X and the 580X which was fitted to the subsequent TX9 chassis seemed to go soft relatively soon.

John

The Dynascan is about the best i have found at boosting colour tubes. Not so good at mono.