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Old 19th Feb 2021, 6:17 pm   #81
Travellingwave
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Default Re: My first telly TV22

I have replaced it - I think , will double check

I dont think my DMM is loading the circuit much - I put a 150 K resistor in series with the DMM lead and the reading was the same , suggesting no current flow.

When I adjust the brightness the picture size changes - gets bigger when brighter - which I have read may the the tube gone soft?
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Old 19th Feb 2021, 6:38 pm   #82
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Default Re: My first telly TV22

It can also mean a tired EY51.
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Old 19th Feb 2021, 6:44 pm   #83
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Default Re: My first telly TV22

Quote:
Originally Posted by Travellingwave View Post
I have replaced it - I think , will double check

I dont think my DMM is loading the circuit much - I put a 150 K resistor in series with the DMM lead and the reading was the same , suggesting no current flow.

When I adjust the brightness the picture size changes - gets bigger when brighter - which I have read may the the tube gone soft?
Does the filament of the EY51 glow less as you advance the brightness?
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Old 19th Feb 2021, 8:04 pm   #84
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Default Re: My first telly TV22

Hi
Thanks for all the suggestions.

When I measure the A1 volts the rightness does drop a little bit

So in the dark I can see that the EY81 filament stays the same as I change the brightness

I can also see in the dark that there is a corona discharge around the anode cap of the CRT , which gets stronger as I reduce the brightness - so my take is that as the beam current increases the EHT is dropping , does this suggest a tired EY?

Any way wine is open now so time to stop messing with high voltage
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Old 19th Feb 2021, 8:05 pm   #85
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Need to sort out contrast pot - but I doubt its to do with this problem as its so far back in the RF deck right?
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Old 20th Feb 2021, 10:52 am   #86
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So Im thinking to rig up a temporary A1 supply of about 300 Vdc , I think my capacitor reformer will do, so I can see if its likely to be the metrosil.

I also have a couple of high voltage diodes from a microwave oven, would one of these serve as a temporary replacement for the EY as I dont have a spare one

This way I could eliminate two of the possible problems.
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Old 20th Feb 2021, 11:15 am   #87
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Default Re: My first telly TV22

Hi Travellingwave,
Microwave oven diodes may not have a hgh enough voltage rating to replace the EY51.
If you put the two diodes in series you will get double the voltage rating, but it still mich not be high enough.

I don't think I have a spare EY51 here, my valve store is at my parents place.
I do have some much more suitable diodes though and some ECL80s. I will see what I can pop in the post when I send the tag board. I can also send you the 1N4007 diodes for my A1 mod. Again, spare metrosils are at my parents place, which I can't visit with current Covid restrictions.

I would be interested to know how you get on with your temporary A1 supply.

On the subject of contrast, have you tried replacing the 50uF elctrolytic in the RF unit? If not then do this next and see what effect it has on the contrast control.

Cheers
Andy
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Old 20th Feb 2021, 12:45 pm   #88
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Default Re: My first telly TV22

Thanks Andy , I’ve got a load of IN4007s so may try ten in series that should give me 10kv in theory. Would also allow me to try your mod if the metrosil turns out to be faulty.

I did replace the electrolytic you mentioned , the contrast works but all the action is at one end , might be a resistor gone high. Anyway will look at that later

Is there any risk in having the A1 supply on the tube before the Lopt fires up?

Interesting warm up on this set . I get full HT pretty quick , then quite a long wait before a bit of life from the speaker , the. The audio kicks in and finally the EHT
I guess this is pretty normal

Cheers
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Old 20th Feb 2021, 12:47 pm   #89
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Thanks for the offer of spares by the way very kind of you.
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Old 20th Feb 2021, 1:54 pm   #90
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Default Re: My first telly TV22

It's easy to get hung up on A1 voltage. As most are aware, placing a conventional DVM (10Meg impedance normally?) will instantly load the circuit at this point and may only give a reading around 200v, or 196v in my case.

An infinite impedance meter will give the correct figure of around 300v, however my DVM tries to read the instantaneous voltage, but after much less than a second the loading has made the voltage decay away.

I found a little trick that gets a pretty good ballpark measurement (even on my cheapest Chinese ‘Aneng’ DVM).

My procedure is to set the DVM to manual range, and start recording the DVM display on my phone, touch the probe onto A1, remove probe, stop recording. Now go to your phones edit mode and analyse frame by frame - you should be able to see the voltage rise extremely quickly in the first few frames, before admitting defeat and rolling back down to 200v or so. In my case I had readings of 278v, 280v and the best one captured was 297v (see pic), so I’m quite happy that It’s near enough.
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Old 20th Feb 2021, 3:12 pm   #91
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Default Re: My first telly TV22

So I’ve tried a number of things as follows

1. Rigged up separate A1 supply via a 150 k resistor , giving me about 310 v steady at A1

This resulted in the picture being dimmer but more in focus . It also messed up the vertical linearity giving me an egg shaped test card , I was able to adjust this back to round
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Old 20th Feb 2021, 3:15 pm   #92
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I think this was the right picture it looks quite good in hindsight
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Old 20th Feb 2021, 3:15 pm   #93
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Picture
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Old 20th Feb 2021, 3:19 pm   #94
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Default Re: My first telly TV22

I tried a microwave oven diode (12kv PIV rating according to data sheet)
This resulted in a much dimmer picture - my impression lower EHT but I can’t measure it so might be wrong. I connected it in parallel with the EY51 my theory being that if the EY is low on emission this would help it along?
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Old 20th Feb 2021, 3:22 pm   #95
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I strung 8 IN1007 in series and got similar result.
Removed so back to EY on its own
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Old 20th Feb 2021, 3:22 pm   #96
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Default Re: My first telly TV22

Quote:
Originally Posted by Travellingwave View Post
I tried a microwave oven diode (12kv PIV rating according to data sheet)
This resulted in a much dimmer picture - my impression lower EHT but I can’t measure it so might be wrong. I connected it in parallel with the EY51 my theory being that if the EY is low on emission this would help it along?
Have you readjusted the ion trap magnet under these new conditions?
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Old 20th Feb 2021, 3:26 pm   #97
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The last thing I did was replace R24 on the trader sheet , this is the resistor between CRT cathode and ground , it had gone high to about 320k , should be 270k , actually it measured about 55k in circuit so must be a parallel path vis the supply rail.

Replacing this seems to have made the picture even fuzzier - like there is now noise in the signal , also more what sounds like vision signal breaking through onto audio

I might put origional resistor back in , though it does not make much sense to me
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Old 20th Feb 2021, 3:27 pm   #98
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Hi Helder to be honest I don’t think I did
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Old 20th Feb 2021, 4:26 pm   #99
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Hi Petedox
Thanks for the tip . After a bit of fiddling about I realised my DMM has a fast peak hold function

Result 305 volts on A1
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Old 20th Feb 2021, 4:28 pm   #100
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The noise after changing the resistor was actually a bad aerial connection , cleaned it all up

Actually not looking too bad , maybe that high resistor was having a bit of an impact
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