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Old 30th Aug 2016, 4:40 pm   #1
acottrell
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Default Help with my Second Rediffusion MK1

Hi Guys
Started my second Mk1. I have had to replace the HT cap looking really rough.
On switch on the EHT comes up as you can feel the static on the screen. The valves light up normal no glowing but I have no raster. If I turn the set off I get a large spot on the screen and if I turn it back on straight way you can see raster for a split second. I have a healthy A1 voltage to the tube base. I have tried a known good Decoder and RGB panel but still the same so can only think my problem is in the EHT tower. Can any one give me some advice. How does the beam limiter work and how can I check this out. I have a feeling I might need to look round this area?
Thanks
Ade
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Old 30th Aug 2016, 6:20 pm   #2
MALC SCOTT
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Default Re: Help with my Second Rediffusion MK1

Check eht/focus volts. Try s/c one of the RGB transistors collector heat sink to chassis for a second and watch screen, Malc.
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Old 30th Aug 2016, 6:25 pm   #3
acottrell
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Default Re: Help with my Second Rediffusion MK1

Will have another look tomorrow. What volts am I looking for on the focus unit?
Thanks
Ade
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Old 30th Aug 2016, 7:25 pm   #4
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Default Re: Help with my Second Rediffusion MK1

Hi Ade.

If you've already replaced the RGB and Decoder boards, and after you've checked what Malc suggested above, my first thought would be to check the voltages and waveforms round TR408 / RV405 / D405 etc. on the timebase panel. This controls the blanking and a fault round here could well cause the problem you're seeing. It would also pay to make sure the luminance is reaching the decoder / RGB panel too.

Thanks Ade.

Kind regards.

From Mike.
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Old 31st Aug 2016, 7:18 am   #5
acottrell
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Default Re: Help with my Second Rediffusion MK1

Thanks Guys
Will have another look later
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Old 31st Aug 2016, 5:09 pm   #6
acottrell
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Default Re: Help with my Second Rediffusion MK1

Hi Guys
Tried shorting the collector of one of the output transistors to chassis and nothing happened on the screen. What I did get this time is a purple flashing PL509. I have replaced it and up came the picture. The EHT must have been low just enough the feel the static on the screen.
Now I have a b&W picture and lack of height to sort out. The Second decoder colour comes up for 5 mins and then disappears not going to look at this until I have sorted my lack of height. Will post some pics once sorted
Thanks for your help
Ade
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Old 31st Aug 2016, 7:57 pm   #7
Tazman1966
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Default Re: Help with my Second Rediffusion MK1

A-ha! I had similar symptoms with one of mine and discovered that the LT derived rails were low. It was caused by the reservoir electrolytic in the power supply (C602) having dried up. Once replaced, I was rewarded with full frame scan and reliable decoder operation. If this is the case, you'll also find the sound louder and the brightness better too!
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Old 2nd Sep 2016, 9:33 am   #8
acottrell
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Default Re: Help with my Second Rediffusion MK1

Thanks Tas
Probably not the done thing but tried the time base panel from my working MK1 in it and I had full frame so do have a problem on the panel. Once I get more time should hopefully find this an easy fault, Decoder the same. One of my faulty decoders if I disable the colour killer it comes up. Will get the frame right first
Ade
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Old 2nd Sep 2016, 3:26 pm   #9
acottrell
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Default Re: Help with my Second Rediffusion MK1

Hi Guys
Just out of interest and for future if you have no raster and short out the collector to chassis on the colour amps the raster then comes up apart from knowing the EHT is ok what else does it tell you?
Thanks
Ade
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Old 2nd Sep 2016, 3:28 pm   #10
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Default Re: Help with my Second Rediffusion MK1

it tells you that the frame/line output is working and there is emission from the crt.
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Old 2nd Sep 2016, 3:42 pm   #11
acottrell
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Default Re: Help with my Second Rediffusion MK1

Thanks Malc
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Old 2nd Sep 2016, 11:40 pm   #12
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Default Re: Help with my Second Rediffusion MK1

Quote:
Originally Posted by acottrell View Post
Probably not the done thing but tried the time base panel from my working MK1 in it and I had full frame so do have a problem on the panel.
If you have the luxury of a working panel I'd say it is the done thing. Maybe now, as a hobby, you can take your time and investigate, but when these were current time was of the essence. The Field Engineer would have done just that, and left the fault finding to the workshop. The Workshop Engineer would have the experience (and test jig) to rapidly dig deeper. Your turn now!
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Old 5th Sep 2016, 6:33 am   #13
acottrell
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Default Re: Help with my Second Rediffusion MK1

Thanks Bill
I have replaced two capacitors 10uf C423 and C422. Changed them both but C423 was leaky this has now restored my height.
just the colour now. Disable the colour killer and the colour is restored on one of my panels so going to look round the colour killer area.
Thanks
Ade
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Old 9th Oct 2016, 6:37 pm   #14
acottrell
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Default Re: Help with my Second Rediffusion MK1

Hi Guys
Just thought I would give you an update set now working replaced the crystal and a transistor in the colour killer stage (took a while to find however brushed up my scope skills) now working fine. CRT a little flat but hey it is over 40 years old
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Old 9th Oct 2016, 6:48 pm   #15
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Default Re: Help with my Second Rediffusion MK1

Excellent result!
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Old 9th Oct 2016, 7:20 pm   #16
acottrell
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Default Re: Help with my Second Rediffusion MK1

Thanks Tas
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Old 9th Oct 2016, 7:52 pm   #17
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Default Re: Help with my Second Rediffusion MK1

Lovely to see a nice old set restored to vision !
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