Prewar Marconi 707 - Videokarma.org TV - Video

yagosaga · #1 05-10-2008, 03:42 AM

Hi folks,

two weeks ago I get my first prewar tv set. It is a Marconiphone 707. The set was repaired in the 1990s. The bad Emiscope crt with broken filaments was replaced by a regunned 9" crt. Nearly all capacitors were replaced by new ones. The line output tube, KT63, was replaced by an EL36.

When I got it, there was no sound and only a small bright spot on the screen. The rectifier tube, U52, had broken filaments. I replaced it with a 5U4. Now the set displayed a bright raster (see first photo). AM sound, but no TV sound. I had to readjust the oscillator. After that, I got sound and video.

Due to the non-fitting crt which usually operates with more than 10 KV, focus is bad and width is not sufficient. The line oscillator was running with the half of the line frequency with double pictures on the screen (see the second and the third photo).

Meanwhile I have set it back to full line frequency, but with the result of a much smaller picture (see last photo). My next steps will be to replace the EL36 by the original KT63 and to replace the 9" crt by a 7" crt.

Kind regards,
Eckhard

Old1625 · #2 05-10-2008, 08:09 AM

Very nice! Thanks for sharing!

John Folsom · #3 05-10-2008, 10:48 AM

Way to go, Eckhard! Do you still have the original 7" CRT? As you may already know, there is a French company which is likely going to attempt some pre-war CRT rebuilds. Talk to Jerome Halphen, he knows the details.

Sandy G · #4 05-10-2008, 10:57 AM

Wow...That is just TOO KEWL !! Way to go, Eckhard !! You Da Man ! (grin)

yagosaga · #5 05-10-2008, 11:54 AM

John:

Quote:

Originally Posted by John Folsom

Way to go, Eckhard! Do you still have the original 7" CRT? As you may already know, there is a French company which is likely going to attempt some pre-war CRT rebuilds. Talk to Jerome Halphen, he knows the details.

I don't have the original Emiscope, but I hope that I can get it in the next weeks. I have also read about the crt rebuilding process in Paul Stenning's forum. It is an interesting thing. But before I deliver it to the French company (if I get the Emiscope), I want to wait for the reports about other crt projects.

At first, I want to replace the present crt-construction in the set with a 7" crt, and I want to solve the focus and width problems.

- Eckhard

yagosaga · #6 05-21-2008, 09:32 AM

Meanwhile I have got a replacement crt from Darius (oldeurope). It is a radar crt, a 7BP7. The crt fits exactly in the Marconi 707. I had to remove a small amount of the cardboard from the center of the yoke, then I could slip the yokes over the neck of the 7BP7.

One remaining fault is in the line output transformer. I do not get sufficient width and linearity. I have to look for a replacement LOT or have to rewind it.

- Eckhard

David Roper · #7 05-21-2008, 12:43 PM

I doubt that set will run right or even acceptably on anything other than a 405 line signal from a standards converter.

yagosaga · #8 05-21-2008, 03:36 PM

Quote:

Originally Posted by David Roper

I doubt that set will run right or even acceptably on anything other than a 405 line signal from a standards converter.

This set is running all the time with a 405 line signal from a standard converter.

David Roper · #9 05-21-2008, 10:50 PM

Aha, I made one of those fatal assumptions.

Only because it appeared to have the same foldover and size issues as Hugo Holden's HMV 904 set did when being fed a 625-line signal:

But not really the same because in this pic the tube is upside down, so the foldover is actually on the right.

yagosaga · #10 05-22-2008, 03:40 PM

Hi David,

the foldover seems to be a general problem of these sets.

Tonight I have removed the lline output transformer and tested various other transformers. The best result I got with a flyback transformer from a hybrid color tv set. The width was much wider than the screen width. And I had some arcing in the output tube. So, I replaced the KT63 with an EL36. I added 800 Ohms in the B+ voltage, and I set picture width at minimum. The feedback cap C85 with 230 pF was replaced by 120 pF. With that I got a reasonable result.

This is not the final solution. But this shows the way to go.

Note, that no centering adjustments were done.

Regards,
Eckhard

yagosaga · #11 06-20-2008, 05:07 PM

Quote:

Originally Posted by yagosaga

One remaining fault is in the line output transformer. I do not get sufficient width and linearity. I have to look for a replacement LOPT or have to rewind it.

Hi,

the lopt was actually bad. Ed Dinning in UK has rewound the lopt. Today I got the rewound lopt, and tonight I have installed the rewound lopt. The tv set has full line width without any further complications. Ed did a great job. I am so happy that the lopt is working properly now.

Some work on the focus coil is on my to-do list.

Kind regards,
Eckhard

yagosaga · #12 07-21-2008, 09:41 AM

Brian · #13 07-21-2008, 10:13 AM

I grew up with hard wiring b/f pcb construction but, after seeing that chassis I know a couple of things: 1) I'd hate to be an assembler, 2) I'd hete it even more to dig into it and do a lit of repairs and 3) I know think there is a reason that PCBs may be an improvement. Really nice to see a pre-war set operating again.

yagosaga · #14 07-21-2008, 02:30 PM

Hi Brian,

PCBs are easier to handle. But this hard wired chassis has its own beauty too. Especially I hate the repairing when I had to replace a cap in the farthest dark corner of the chassis. But nothing is impossible.

Here is a look at a nearly unmolested chassis:

http://bs.cyty.com/menschen/e-etzold...herChassis.jpg

Only the EHT caps, a few other caps and a potentiometer was replaced. But this photo is good for a comparison.

- Eckhard

yagosaga · #15 07-28-2008, 03:04 PM

Hi,

Today I have got the original Emiscope 3/2 for the Marconi 707. Some photos are here:

http://bs.cyty.com/menschen/e-etzold...scope3-2_1.jpg
http://bs.cyty.com/menschen/e-etzold...scope3-2_2.jpg
http://bs.cyty.com/menschen/e-etzold...scope3-2_3.jpg
http://bs.cyty.com/menschen/e-etzold...scope3-2_4.jpg
http://bs.cyty.com/menschen/e-etzold...scope3-2_5.jpg

The CRT looks like an Erlenmeyer flask. The EHT connection is in the socket. The picture tube has white getters, it is gassy. But I could not find broken glass. I am glad, that I have got the original CRT also, even it is not working.

Kind regards,
Eckhard