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Old 25th Apr 2021, 9:25 pm   #1
GadgetM
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Default Ferguson Moviestar TX9 varying brightness

Hi everyone,

for some unknown reason I bought an old Fergie TX9 14" last week
It does work and has a really good picture but a strange characteristic:
When switched on the A1 (or is it G2) seems high with a very high and slightly blue-ish black level, it then gradualy gets darker and less blue. Thr trouble is it gets too dark (and quite green), then by the 20 minute mark it's settled down and brightness and greyscale back up to a good level.

Tube seems strong and sharp focus so I don;t think theres an issue with the tube, but I could be wrong.

I've checked the 115v, and cleaned pots etc but still doing it.

Anyone seen this before? any tips?

cheers
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Old 26th Apr 2021, 1:35 pm   #2
slidertogrid
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Default Re: Ferguson Moviestar TX9 varying brightness

I haven't got a circuit to hand but I would look for a dried up Electrolytic capacitor on the A1 supply. The K30 / KT3 chassis had a 4.7 uf that used to dry up and cause this type of effect. I don't know if the TX9 has a similar cap I can't remember having to change one but the sets are a lot older now...
I know you said you think the tube is ok but if you want to double check there is a series resistor in the CRT heater supply (usually a blue thick film type) you could try temporally shorting it out to see if that changes the effect during warm up.
Just out of interest is your chassis the early one with the big choke or the later one with the modified power supply circuit?

Rich.
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Old 26th Apr 2021, 2:40 pm   #3
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Default Re: Ferguson Moviestar TX9 varying brightness

Wasn’t common back in the day on the TX9 but Rich is on the right lines with the 4.7 uF electrolytic but it decoupled the HT to the video output stages not the A1 supply, it’s C182 on the TX9.

You would be very unlucky indeed if it was the CRT, only the 20” models using the Mullard A51-580X were prone to going low emission.
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Old 28th Apr 2021, 8:34 am   #4
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Default Re: Ferguson Moviestar TX9 varying brightness

I too would be surprised if the CRT is faulty. Remember on these the A1 supply is separate to the LOPT and I'd check the control and feed components. Hanging a meter on the A1 then the RGB feed will soon determine where the fault is.
Can't be sure, but wasn't there a preset brightness control in these? Those black and white pots can give trouble.
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Old 29th Apr 2021, 9:22 am   #5
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Default Re: Ferguson Moviestar TX9 varying brightness

Hi everyone, really appreciate your advice.
Thanks Rich, jayceebee and Glyn, there is indeed a 4.7uf cap in the A1 section, which I'm placing on order along with a few others for good measure. I'll check that series resistor in the heater circuit too. I've got rather a lot on this week but as soon as I get to work on this again I'll post the results here.
cheers
Meurig
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Old 29th Apr 2021, 5:07 pm   #6
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Default Re: Ferguson Moviestar TX9 varying brightness

Just to be clear the 4.7uF mentioned on both the TX9 and K30/KT3 is in the HT supply to the RGB output transistors, not the A1 circuit.

John.

Last edited by jayceebee; 29th Apr 2021 at 5:15 pm.
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Old 4th May 2021, 6:46 pm   #7
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Default Re: Ferguson Moviestar TX9 varying brightness

Hi everyone,
I changed the 4.7 cap today, sadly it didn't fix the issue, even though the cap I took out had very high esr and was almost double it's rated value.
The picture still comes on really bright, then over ten minutes or so goes very dark, before gradually returning to a sensible level by about 20 minutes. The trouble is I wouldn;t know what readings to expect if I took voltage measurements. Any other ideas guys?
Thanks
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Old 4th May 2021, 7:50 pm   #8
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Default Re: Ferguson Moviestar TX9 varying brightness

I've measured the A1 as it arrives at the tube base comes on at 493 then gradually rises to 496.1

Last edited by GadgetM; 4th May 2021 at 8:10 pm. Reason: typo
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Old 4th May 2021, 8:19 pm   #9
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Default Re: Ferguson Moviestar TX9 varying brightness

The A1 supply change is negligible and I wouldn't really worry a swing of 1.4V. It might be a good idea to remove the CRT base and inspect the pins for corrosion though. Did you try cleaning the preset brightness which was mentioned by Welsh Anorak, it's R94.

Do you know which version of the chassis is fitted? This can be determined by the large IC type and if it has a large choke and capacitor in the PSU.

John.
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Old 4th May 2021, 8:31 pm   #10
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Default Re: Ferguson Moviestar TX9 varying brightness

I think it would be also worth measuring the voltage to the drives on the CRT. I wonder if the problem is in the video drive circuit?

Check the power supply to the drive transistors on the CRT base and see if they are overdriving the tube during the fault.
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Old 4th May 2021, 8:38 pm   #11
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Default Re: Ferguson Moviestar TX9 varying brightness

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Hi John, Yes cleaned pots /presets, do the pics answer your question on chassis?. Not taken the tube base off yet I'll have a look at that tomorrow.
cheers
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Old 4th May 2021, 9:06 pm   #12
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Default Re: Ferguson Moviestar TX9 varying brightness

That's the very first version, PC1001.
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Old 5th May 2021, 8:35 am   #13
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Default Re: Ferguson Moviestar TX9 varying brightness

You'll see the prestes are covered with a sort of whitish dust. This is common and does lead to them being intermittent. Cleaning or, ideally, replacement will go a long way to curing any current or future intermittent faults.
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Old 5th May 2021, 9:34 am   #14
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Default Re: Ferguson Moviestar TX9 varying brightness

Thanks Glyn, Yes those presets were all covered in th white dust, almost like salt crystals.
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