DG7-32 TV

I need a final video amp to drive the grid. I tried a triode (6n1p, as I have buckets of them), but It smears the video all over the place, and destroys the bandwidth.

That'll be a little thing called the Miller Effect.

Use a pentode.

(Can someone post me a link to FAQ to embed these - I've looked and can't get it to work )

Click on the 'youtube' button in the message composition panel, and in between the tags insert the part of the YouTube URL AFTER the 'v='. It actually tells you this if you hover the mouse pointer on the YouTube button.

Can someone post me a link to FAQ to embed these - I've looked and can't get it to work

I wrote a guide in our FAQ section viewtopic.php?f=45&t=5409 and if you hover the mouse of the youtube button in the post tools you will see a note I left as well which states what to do

Can someone post me a link to FAQ to embed these - I've looked and can't get it to work

I wrote a guide in our FAQ section viewtopic.php?f=45&t=5409

Ah... I didnt realise we had two FAQs! I was looking in faq.php

I have two buckets of EF91s. I'll field one of those into service. I'm suprised I've got as much bandwidth as I have really, as the grid is currently driven from 1 6n1p. Its when I add a second it all goes to pot...

Ah... I didnt realise we had two FAQs! I was looking in faq.php

That probably explains why nobody reads the very first section on the forum "Please Read First: FAQ's" section

Last night I created a simple EF91 video amp, and the results were really good. Switching to test card F, Carole appeared in shades of green, with good line lin, and reasonable frame lin. There was some issue with frame lock, but it wasn't bad... then something went pop, and I've got no video again!

Now I've got the keracolor out of the way, this thing keeps staring at me from the back of the bench.

I found out what had gone pop, it was a zener I use to hold the cathode of the CRT steady. I've sacked it and put a decoupled potential divider in from HT to hold the cathode at about 100V. My original spec was to have no semiconductor junctions in the thing. I have relented, as my EHT is rectified by a grain of sand, as I can't float the heaters on my original EZ80, and I compromised it's H/K insulation.

I've now rebuilt both video stages using EF91s. I've got pots of good video available at the grid of the CRT.

I have been plagued by some line sync issues. The vertical sync sep is locked solid, but it's not so easy with the line. Video kept breaking through and causing false syncing. I've added two simple CR filters to the horizontal sync, which has cleaned things up no end.

I've also added a simple audio amp. An ECL83, driving a 9v mains TX backwards as the OPT. Seems to work surprisingly well.

There's some sort of line fold-over in the line section, as evident in this video, and I could use some pointers, but on the whole, I'm really very pleased with it.

Excellent work Andy, the pause for thought and rebuild has certainly paid off

Any chance of a few pics of the underside and perhaps a circuit. There was a discussion a few moths ago about a simple TV build, I think this certainly qualifies.

P.S.

You have a great presentation style and I enjoy your vids so keep up the great work.

Chris

Hi Doz,

Just wanted to second Chris's point about presentation style. Very good, informative and entertaining too, I really enjoyed watching, Thanks for taking the time to do the videos

Marc.

Excellent work, Doz. Neat construction AND it works!!!

Magic!

Would also like to see the circuits.

Cheers

Ian

Thanks for all the positive comments. I'll start on a circuit diagram tonight.

I'm puzzled by your sync problems - there should be no difficulty getting perfect, unadulterated sync from a single valve sync separator. As you are using grid modulation for the CRT the video will be the wrong way up for feeding direct to a conventional sync separator, so perhaps the take-off point you are using isn't hitting it hard enough?

Obviously the video quality will be improved when you add flyback blanking but it looks to me as if the video is either severely compressed or the HF response if very poor (which could be part of the sync problem) - or both. Perhaps it's easier to see 'in the flesh' ...?

You've done wonders with it so far, Andy. With a little tweaking I'm sure you can turn it into a miracle!

Some scope waveforms of the sync/video could be instructive ...

Schematic as promised. Audio not included.

Just spotted there's a cathode bypass cap drawn the wrong way round, rest assured it's -ve to gnd.

*UPDATED* now. Why is it you stare at something for hours to make sure it's ok, and then as soon as you publish it, you find errors.

I've changed the diagram above now... it's correct ...

.... for the moment.

My edit didn't appear to work. Let's try again.

And the fugly construction...

I think it looks beautiful, much prettier than a printed circuit board any day!

It looks nice and shiny.
Are the cases aluminum or ABS? I know they did or still do them in that style in both materials.

The cases are aluminium. They're from scrap marine satellite transceivers I used to service.