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 Post subject: How far do you go to get perfection?
PostPosted: Mar Sat 10, 2018 3:07 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 281
A restoration I'm just finishing had a larger than usual number of faults. One of these leads me to ask the question - should you consider modifying a TV to incorporate improvements that were introduced in later chassis in the series?

The particular issue in this case was a vertical fault that would, for a few seconds in half an hour, cause the height to fluctuate by about 5 or 10%. Just enough to be annoying. The linearity was also crushed a little at the top 1/3 of the raster and the height control was max'ed out.

By scoping the set (after replacing all marginal parts and determining that the circuit was actually working as intended) I eventually discovered that the jumps were associated with mains voltage fluctuations. Not surprising I guess since the height control comes from B+, not the usual B+boost.

This chassis is an Australian HMV F3 from 1957. The F series was a high-end design, intended to perform well in "Fringe" areas with weak signals and, on Band 1 frequencies, power line noise. All the F series chassis are very similar except for the last in the series, the F5, which has a number of small changes. Significantly, many of these changes address the vertical, such as deriving the height from B+boost and increasing the negative feedback around the vertical output stage. So I bit the bullet and made the "upgrade".

No surprises, the height fluctuations are gone, linearity is much improved and as a bonus the raster no longer "breathes" with sudden changes in average picture level (set has full DC coupling). Much better!!

I suppose, back in 1957, most viewers were not so critical about such things, given that 5 or 10% overscan was common and most of the transmitted content came from film where some sprocket shake was expected. But today, one is less inclined to tolerate such defects.

What do you think? How far should you go in the pursuit of perfection? Should you always track down all intermittents because they will eventually fail completely?


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sat 10, 2018 4:10 pm 
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Location: Detroit, MI USA
I think you have to find all the intermittents, as they are annoying and rarely ever go away on their own.

As far as modifications, if a manufacturer updated their circuit to make the set perform better then it may be worthwhile adding those changes to an earlier version of the same design which exhibits the problems that those changes were added to correct. Some really questionable designs got into production over the years.

In reality most of us only have a few favorite sets that we ever watch, and the rest of them even though they have been restored are seldom used. If I'm going to watch a vintage set, then it needs to work well enough not to be annoying while it's in use.

I can think of a few sets, notably the minimalist Muntz series string big screen (for their day) models that barely worked when they were new, and would need to be completely redesigned to work well. You actually had to hand pick certain tubes in those to get them to work the way they did when sold, which wasn't very good. As someone pointed out the other day in a different discussion, designing a circuit that relies on certain characteristics of a specific brand of tube to work properly isn't the way it should be done.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sat 10, 2018 5:45 pm 
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If there were factory improvements in later models that solved issues I definitely try to put them in. You are fortunate they exist, as that's usually not the case.

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I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sat 10, 2018 5:55 pm 
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Location: Mission Viejo, southern California
It is interesting to read about a TV made the year I was born. Do you have any photos of it?

Many of the Philco radios had numerous revisions. It seems like a good idea to make the changes, especially when the reasons for the changes are documented or obvious.

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many of my radios http://s269.photobucket.com/user/FSteph ... t=3&page=1


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sat 10, 2018 10:25 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 346
Location: New Hampshire
Go ahead & make the factory improvements. Often at seminars & tech bulletins you were
told "while you are in there" or " if you still have them on the floor we will pay you to change it".
This isnt a rare antique car you are working with. In 50 yrs NOBODY could tell if its factory, only
that soldering happened. No white gloves here........
As far as ints go its your call. In the states many ints were ignored by customers or they
decided not to spend the time or money to track them down. They are almost the norm
when a set ages.
We found with series string & compactron sets when they get old most of the low level
tubes were "bad". You could change them one at a time & see very slight improvements
with each one. Trouble is it cost to much to almost re-tube a set.
The last thing is tube sockets. MANY were bad when old. It was a PIA to change them often.
Sometimes there were other fixes but they could be so int that you are never sure.

73 Zeno
LFOD !


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sun 11, 2018 3:26 am 
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Location: Woodinville, WA USA
I think there's nothing sacrosanct about a particular production version of a TV. Manufacturers often made production changes along the way -- why shouldn't we follow along? I recently restored a Heathkit FM-3 tuner, and after some research, incorporated a couple of little improvements that appeared in the follow-on model FM-3A:

https://antiqueradio.org/HeathkitFM-3Tuner.htm

These sets were bought, sold, and used as appliances, not priceless art objects. Of course we appreciate them for what they are, and value their history and originality, but I like to restore vintage sets so that they WORK as their designers intended.

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sun 11, 2018 3:42 am 
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Stephen, I just have the chassis, the owner lives 700 miles away! I'll be visiting him probably in a couple of weeks and I'll get some pics of the finished job.

In the meantime, have a look at this link:

https://www.radiomuseum.org/r/hismasters_f3_a2_f3.html

Note this TV has 24 tubes! Look at that list! Most of them are dual functions as well.

In the 1950's if you lived in a poor reception area (or just wanted "the best") you bought your TV based on the number of tubes it had!
This TV has push-pull audio, a 10" dual cone speaker, 4 stages of vision IF, two stages of sound IF, noise gated sync, turret tuner and Synchrolock hor. AFC. The cabinet is a piece of furniture, made by a piano manufacturer. Picture quality is as good as it is possible to get, this TV can resolve a 5MHz resolution grate, via the tuner no less! And the video is DC coupled.

This thing is so good, it would be a shame to not have it working at its best.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sun 11, 2018 4:56 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 281
In case it's of interest, here is an F5 in a less elaborate cabinet:
Attachment:
F5_front.jpg
F5_front.jpg [ 39.4 KiB | Viewed 1010 times ]


Here's the back view
Attachment:
F5_back.jpg
F5_back.jpg [ 50.07 KiB | Viewed 1010 times ]


and the circuit of the F3

Attachment:
HMV_F3_Sch.pdf [140.4 KiB]
Downloaded 21 times


It's a struggle to get a full TV schematic to fit in the allowable file size and remain readable!

The observant amongst you will recognise the RCA Synchrolock circuit, used in this design with newer tubes many years after RCA had given it up as too complex and expensive! It was a game changer for HMV, it was one of the design features that allowed this TV to work in places where many other TVs wouldn't. They were still using a development of this circuit at the end of the tube TV era. Same goes for the vertical.

Note also the series-connected supply to the IF tubes, used to get the B+ consumption down to fit in the ratings of a pair of 6N3s. This feature was gone by the F5 and a pair of 5AS4s (5U4s to you) were substituted.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Mon 12, 2018 10:59 am 
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Location: Long Island, N.Y.
I've been collecting vintage TV's for decades with all of them being elec. restored. The early post-war models are typically finicky and you learn to be forgiving with them. If you expect them to work like modern TV's you'll be disappointed. Some are solid performers, but most, by far, have there quirks. I have my repairman do any tricks he can to make the set a more reliable player, but some models, like Philco's for instance, are more troublesome.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Tue 13, 2018 7:56 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 281
Yes, well I thought I'd nailed this one, finally. Ran it for a few days - NO! Vertical jumps and shrinks again!

It seems the extra negative feedback was covering up a shorted turn in the vertical output transformer, It just got worse. Primary DC resistance was 380 ohms before, now it's 346 ohms. The evidence is in.....

This transformer I suspect I'm going to have to rewind. Unusual split secondary means I can't use just any tranny. Sigh.....


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Tue 13, 2018 10:23 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1019
Location: Champaign IL 61822
FStephenMasek wrote:
It is interesting to read about a TV made the year I was born. .


I was born in 1944.

I have a working TV built 7 years before that, in 1937. Marconi 702. Its strange!
E.G. a TRF TV! NO!!! I just realized its an FTRF TV. Its Fixed Tuned,
7 stages of RF amp, then the detector, then the CRT ... no video amp.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Wed 14, 2018 2:22 am 
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Location: Durham, NC
Those pre-war TV circuits sure do look strange to our eyes, since they were still trying to figure out what worked. I know the ETF has some of those 1930s electronic TVs in working condition, but I've never seen them operating. It'd be interesting to see how they'd do with weak signals, multipath conditions, ignition noise, etc.

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A collector of TV signal boosters and UHF converters -- God help me!
tv-boxes.com


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Wed 14, 2018 4:35 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 281
I suffered a 24 hour plane flight and a drive more than half-way across the USA to visit the ETF and see these TVs!

Well worth it! You should go!! You probably only live in the next state anyway....

Re pre-war TVs - it was commonly believed in the 30s that there was only going to be enough bandwidth available for one TV channel, hence the fixed tuned TRF receiver. 40MHz in those days was an outrageously high frequency. And it was so difficult to get meaningful gain at those frequencies using available tubes, hence the 6 or 7 gain stages commonly used..

And the Alexandra Palace transmitter was double sideband AM, nothing else happened up at these frequencies back then. Until the Germans started generating navigation beams.

The "battle of the beams" during WW2 is a really fascinating story.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Mar Sun 18, 2018 5:34 am 
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OK, I've managed to get a suitable NOS replacement vertical output transformer for the F3.

It's actually for a Pye T20 and only has a single secondary winding. Apart from the fact it has a screen tap (unusual) it's very close to the same.

So I stitched it in using the F1 circuit for the feedback and I got this, as per quick phone shot, from my 110 degree test CRT:

Attachment:
F3_SS.JPG
F3_SS.JPG [ 29.05 KiB | Viewed 717 times ]

Happy!!

Because the F1 design doesn't have the split secondary the interlace is very slightly degraded over what the F3 can do but, as they say, a blind man would be pleased to see it! And it's got enough grunt to drive the test CRT to full scan so the 90 degree tube should be no problem. The Height control works very nicely, with no effect on the linearity throughout its range.


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 Post subject: Re: How far do you go to get perfection?
PostPosted: Apr Tue 10, 2018 7:41 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 281
Attachment:
HMV_F3.jpg
HMV_F3.jpg [ 67.08 KiB | Viewed 465 times ]


Owner sent me this pic of his F3.

That cabinet has never been refinished, that is the original gloss.

TV was 7 years old when it first showed this picture. Festival Hall, Melbourne.


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