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Running a TV62 at 625 line? Anyone done anything like this?

 
Alastair
(@alastair)
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I havent a Aurora or any other stds converter, was thinking/musing the idea of running the set at 625 line and running video direct into the video-amp etc, same with audio.

I wasnt thinking of altering the IF strip--way too much work and wouldnt be practical or reversable....

Any ideas/pointers things to alter, what caps etc for the lopt tuning etc....?

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Topic starter Posted : 08/04/2014 10:51 pm
Lloyd
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I did this a good few years back, before I got my Aurora, I didn't change anything, just used my signal generator and a video ipod which was outputting 625 line video. Tuned it in and cranked the hold control up! The width was a bit low, but it did give a picture of sorts, not very good though. I did have a photo of it, but can't find it right now :ccb

Regards,
Lloyd.

*Edit*

Found it! I can't believe it was 2006 when I was doing that daft experiment..

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Posted : 09/04/2014 12:24 am
Anonymous
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If it was me I'd save for an aurora or use an old PC with VGA timings changed for 405 and modulator from VGA green + H + V + sound card using 2 x SA612.

You'd need to change a few capacitors at the least to do it properly and a new LOPT etc are not 2 a penny.

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Posted : 09/04/2014 10:22 am
Alastair
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Micheal, I'm curious as to why you think the LOPT would be under any more electrical stress running at 15KHz rather than 10KHz, providing the tuning is altered accordingly....

Since the first post, Ive tried the set at around 15KHz line frequency (by my calibrated lug-hole), where the width came in around 1/4"

Ive then linked out one of the two series tuning-caps (both 40pf, so 20pf to the set) and width is restored--in fact, a little too much.
Moving the width setting link--which acts on another tuning-cap and a set of linkable resistors) has restored correct width approximately.

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Topic starter Posted : 09/04/2014 11:25 am
Anonymous
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Ah but the tuning might not be correct initially ... :)

I prefer to experiment on cheap new things due to the high shipping cost of old things.

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Posted : 09/04/2014 4:03 pm
Till Eulenspiegel
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Reference to the circuit diagram of the line output stage will show that the TV62 has an unusual line output transformer. The transformer itself is a very simple affair consisting of a primary winding, the EHT overwind and the EHT rectifier valve heater winding. That's all. The scanning coils are of a special high impedance type and are connected directly between the boost diode cathode and the 400 volt boosted HT line. To be honest I don't think there is any possibility of modifying this line output stage to operate satisfactorily on 625 lines. In the model TV22 Bush pioneered the concept of ramping up the sawtooth waveform in the line output transformer. The line output valve operates as a switch rather than a class A amplifier and I'd imagine it's the same for the TV53/62/3. Curiously some set's will wind up to 625 and others don't.

In the sixties I did try to modify a TV62 for 625. The project was abandoned. After all in those times sets like the TV62 were considered as nothing more than out of date junk.

Till Eulenspiegel.

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Posted : 09/04/2014 4:38 pm
Anonymous
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However it's an engaging and interesting "thought" experiment.

I just bought a new €12.50 LW/MW/VHF battery / mains "kitchen" Radio out of Tesco. I will try fitting it to a totally scrap Philips Radio (only the speaker and shell of 1950s Bakelite cabinet, no original cloth, chassis, knobs, scale front or back or radio back). If the chassis ever turns up I can remove the transistor set as I won't be modifying the case).

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Posted : 09/04/2014 5:02 pm
Anonymous
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The Bush TV53/62 series was considered one of the best sets for TVDX in the 1960's and early 70's.

From memory it worked quite well wound up to 625 lines-There was an article by Charles Rafarel in 1964/65
on modding it and fitting positive/negative video switching..

I'll see if I can find the article.

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Posted : 09/04/2014 8:54 pm
Anonymous
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I'm still trying to locate the article in Practical TV....Some people had the Bush TV 53/62's
running for years on 625 lines as DX sets (including me) so LOPT failure certainly wasn't an issue with these models.

Cheers Hugh

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Posted : 09/04/2014 10:58 pm
Alastair
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MrHoover--

Thanks for the words of encouragement, and confirming the LOPT is up to the job, My simple tests this morning also confirm--at least to me the same....

Well--The story so far.

Out of interest, I THINK this is a later set, it only has one tag-strip on the timebase section--there was something else that suggested this, but I forget now what that was...

There were a lot of waxies and a few out of tolerance resistors that needed replacing. This has all been done.

The set is running at what my ear reckons is around 625/15KHz, and has been for an hour or two.

It has full even width, with no apparent issues of line-linearity that normally show up as vertical bars of shade and light, just an even raster fully scanned now the frame-stage is running right.

This morning, before rebuilding the timebases, I ran the set for a good 3/4 hour then checked the temp of the LOPT.
Side facing away from the line-valves was practically cool, side facing the valves was fairly warm due to heat radiation. (I may do something about that, a heat-shield of tin or summit....

EHT is at 13.2KV, Trader reckon 13.5KV is the right voltage, a tad low, but thats prob due to my poor mains-supply here which varies by as much as 15V during the day.

So--all things being equal, I now need to sort a video feed and audio I/P to it. That should be interesting! We see then if I need to mess with the flywheel-line-sync etc.

I know that many consider this sort of work--almost heresy but in my current financial position having just lost my job, an Aurora is out of the question, and the other suggested mods would need me to buy/design/make items to do the job.

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Topic starter Posted : 10/04/2014 12:37 am
Anonymous
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Glad to hear that it's basically running on 625..

The flywheel sync circuit works fine on 625 with no mods...If the diodes are original it's a very good idea
to change them...From my fading memory we used BA148's as a replacement at the time with a series resistor,around 4.7K or even a bit higher.

Yes,getting video into it could be an "interesting" exercise...

As long as you can put everything back to 405 at a later date I don't consider it heresy.

Now if I could only locate that article on conversion...It even mentions running on 819 but for that recommends an extra turn on the EHT rectifier heater winding on the lopt.

Cheers

Hugh

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Posted : 10/04/2014 8:44 am
Alastair
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Absolutely Trevor--Will do if I can get video into it today sometime, and it has reasonable performance--Or not which ever is the case!....

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Topic starter Posted : 10/04/2014 9:07 am
Anonymous
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I've found a copy of the article on converting the TV62..From the June 1964 issue of Practical TV.

It's coming from a DXTV angle but the 625 timebase conversion is valid.

It's too big to upload here in reasonable quality so if anyone wants a copy please PM me.

Hugh

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Posted : 10/04/2014 9:36 am
Alastair
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Ah--Thats brill. Interesting the video-amp stage needs its bias altering, I thought as much by comparing the TV62 to the later TV125 that uses a very similar video stage and the same PCF80 valve...

All great stuff to think about! Cheers! :thumb

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Topic starter Posted : 10/04/2014 10:05 am
Anonymous
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Interesting the video-amp stage needs its bias altering, I thought as much by comparing the TV62 to the later TV125 that uses a very similar video stage and the same PCF80 valve...

Look forward to seeing some good pictures!!

Considering the TV125 was quite a bit later than the TV62 the design hadn't moved forward very much in the
intervening years...It's almost like a dual standard TV62 with a bigger CRT & a few later valves here & there.

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Posted : 10/04/2014 11:35 am
Till Eulenspiegel
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From the 1953/4 radio and Television servicing book. An expaination how the directly driven scan coil line output stage works and the line TB circuit of the Bush TV24C which was one of the first Bush TVs to employ this arrangement.

Till Eulenspiegel.

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Posted : 10/04/2014 11:37 am
Alastair
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Well--after much thinking and tinkering, it does actually work!

--As they say in Forum Parlance, things claimed--with no pictures are meaningless--So as currently I haven't taken any--I'll not claim anything!

One thing Ive found is a fault though--probably like it on 405 too. There's a very slight line instability--like the source was a video recording, a very small shake to the picture--but isnt constant, not line-tearing or anything extreme, and most of the time not even noticable....

Line and frame sync are rock solid.

There's two diode-like devices in the line-flywheel circuit, Metal-rectifiers by Westinghouse--I'm suspecting one or both of these is messing round,--I think someone mentioned them in another thread....

Also, I noticed that the picture will vary just slightly in size both horizontally and vertically--in a completely random manner, scene changes and brightness do not seem to affect this fault. This however, could well be my mains-supply here which is pretty cruddy.

I think my CRT has the heater issue too. There's only 4.6V across it and other valves in the chain have between 6.4 and 6.8V across them There's no leaky caps in the chain that I can find. The picture is a little silvery and a bit negative until its been on for about 10 minutes, then its fine. Brightness when warmed up is pretty good. Cathode although evenly heated is pretty dull heat colour.

What Ive actually done to the video-amp and the cathode-follower stages is this--
I've made it a little more like the Bush TV125 set, the pentode-section now has a screen feed resistor of 5.6K decoupled to deck by a 50pF cap.
The original 33K (R35) from the +B to cathode-resistor R36 has been removed (but still in chassis) to remove the supply/potential-divider to the Cathode of the pentode to reduce the bias.

The 10K anode-load has been changed from 10K to 15K to give a little more video gain, as the normal line-level from the usual sources is too low for the set. Still pretty under-contrasted, but didnt spoil my enjoyment of a full movie. The cathode-follower had a new cathode-resistor of the correct value.
I definitely need to make a video pre-amp to help out with this lack of contrast.

The line-level video is supplied direct into the G1 of the pentode-section, via a 100 ohm grid-stopper and the grid resistor of that valve increased from 3.9K to 33K The original source of signal was via an inductor that's been disconnected.
All the above sounds like a lot--but fully reversable in about 15 minutes....

And--Yes.....I will get some pictures done before going much further!--Even one of a Test-Card, but it'll have to be via my PC and an HDMI to AV box as the thing trips the RCD if plugged into anything else attached to the mains!

Guess this Bush TV62 is the Only One On Earth with an HDMI socket! (Well--sorta) :thumb

--Really WILL have to get an Isolation-Transformer.....One day! :aaj

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Topic starter Posted : 11/04/2014 12:21 am
Anonymous
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Sounds like it's coming on well.

Yes,get rid of those westinghouse metal rectifiers in the flywheel sync circuit,almost certainly causing the twitching effect...We used to use BA148 diodes,each with a series resistor of around 4.7K but it's not too critical.

Look forward to seeing some pics...Thought you might need a video preamp to lift the video input level a bit.

Cheers

Hugh

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Posted : 11/04/2014 8:11 am
Alastair
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That's one thing I don't have a lot of--Diodes for some reason!

Ive found some 1A FR diodes at 600V on maplin, so those should do it--Even got 'em in stock at the local shop!....

Work in progress as they say, looking at designing a small gain stage for the video too, Maybe if I get time, I'll make it a Valve amplifier, I have loads of suitable types around....

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Topic starter Posted : 11/04/2014 1:01 pm