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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 11th Nov 2018, 2:02 pm   #181
Argus25
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Default Re: HMV 905 Project

I think a single stage R-C filter for the vertical integrator can actually work well sometimes, it requires a suitable vertical sync injection level into the vertical oscillator, and when that is ideal, the interlace is pretty good. I think when the vertical integrator is better (multi-stage), there is a wider range of vertical sync injection levels that give good interlace.
I have not remedied the issue on my 904 yet.
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Old 25th Jan 2019, 9:38 pm   #182
beery
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Default Re: HMV 905 Project

Hi all,
just to let you know that an imminent house move finally forced me to put the HMV 905 back together, perhaps before I was entirely happy with it.

When I had the chassis on the bench to do the IF sweep I had no idea that I was about to move house. So without knowing if it was really at its best, I had to pack up all the test gear and assemble the set with the CRT for the first time since I restored it...

Now, this is not all bad news as it has worked out well, with a bit of extra space in the new place. The other nice thing is seeing the whole thing back together.

The focus had been a bit uneven, so before I assembled the set I put the focus coil on a variac via a 110V step down transformer. Giving it a few minutes of AC and then gently winding it down to zero did the trick and the result is a more uniform focus across the picture.

Performance... After sweeping, the bandwidth is still quite poor in fact it might be worse, but the HF noise has disapeared unless the contrast is turned right up. So the results are quite viewable and the sound is realy crisp. There is an intermittant fault which causes wobbly line sync, it needs to be scoped really to find it.
There is a bit of IF instablilty in radio mode due to not having a metalised MHD4 (hard to find).

Overall I am very pleased with the results. After the move next week, I hope to get the chance to spend some time watching the set.

This is just about the end of a very long journey with this set

Cheers
Andy
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Old 25th Jan 2019, 9:53 pm   #183
FERNSEH
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Default Re: HMV 905 Project

Hi Andy,
I might have a spare metallised MHD4 or the Tungsram equivalent, DDT4

DFWB.
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Old 26th Jan 2019, 9:43 am   #184
Jac
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Default Re: HMV 905 Project

Great result Andy!

You have a much better tube than my 706 has (large ion burn).

Jac
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Old 26th Jan 2019, 12:23 pm   #185
Heatercathodeshort
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Default Re: HMV 905 Project

The Marconi 707 I am working on at the moment has an intermittent grid/cathode short and a large ion burn. The emission is 100%. It does settle down after about 5 minutes.
Tapping the tube neck can cure/cause the fault to occur.

I'm stuck with a difficult choice. Leave it alone or attempt to dislodge the short with a charged electrolytic. I am going to leave it alone. The second picture shows the screen under bright bench lighting. John.
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Old 26th Jan 2019, 5:20 pm   #186
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Default Re: HMV 905 Project

Exactly the fault I encountered on the smaller 3/1 variant on an HMV 904 recently. I also decided to leave it alone, replacing eventually with a 5FP4.

A standard fault on these tubes?

Steve
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Old 26th Jan 2019, 9:03 pm   #187
Heatercathodeshort
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Default Re: HMV 905 Project

I think a standard fault with all EMI side wire tubes. I have a 3/6A that has a hard H/C short and many years ago Gerry informed me it was very common. John.
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