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Old 5th Jul 2020, 11:19 pm   #52
1100 man
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Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
Posts: 1,377
Default Re: Marconi 1400 birthday present

Hi Andy,
Still looks like you need just a shade more width! I normally find to get a fairly round circle on the test card, I need to pull the width out so the castellations are pretty much just off the screen. So I would probably want a good 1/2 inch extra each side! I assume the width you have is with the control at maximum?

The width stabilisation circuit seems to be much the same as both my KB VC4 & Sobell 1000. The VDR in both these sets has a cream body with a blue & black splodge. Is that the same as the 1400?

I did find the data on these VDR's- I'll post a link if I can locate it again.

I'm sure both the relevant caps on the above sets (equivalent to C113) are red tubular things about an inch long. One lead out wire comes from the middle which makes them quite distinctive. I've never seen any other caps like them, but they often appear in line stages so I assume they would be 'pulse capacitors'. I'm sure they are rated at 12KV.

When you say low contrast on 405, is the picture OK but just lacking in contrast or smeary and poorly defined?

The contrast controls vary the fixed bias on the AGC line which in turn varies the gain of the 1st IF amplifier, V3 (and the tuners). It's very high impedance so is very sensitive to leaky caps. C13,C15,C16 could all cause problems.

Most of it is common to both 405 & 625 however. Does the local/ distant control have any effect? Might also be worth trying another EF183 as this could be low gain. It might manage just enough signal for 625, but run out of steam on 405.

The output of the 625 tuner is amplified by the 405 tuner, so V3 has to amplify less on 625 than it does on 405 to give the same overall picture contrast.

I'll see if I can find the VDR data.

Found it! https://www.vintage-radio.net/forum/...80&postcount=8. The second one down in the table E298ZZ/06 has the black & blue markings.

All the best
Nick

Last edited by 1100 man; 5th Jul 2020 at 11:29 pm. Reason: more text added
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