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Old 21st Oct 2017, 11:11 am   #61
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Default Re: My first antique TV-Olympic Model 755-need help.

I would suggest that you first check R41, R42, R43 and R44, as it's a nice easy test to do.
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Old 21st Oct 2017, 11:13 am   #62
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Default Re: My first antique TV-Olympic Model 755-need help.

Ditto.

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Old 21st Oct 2017, 11:16 am   #63
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Default Re: My first antique TV-Olympic Model 755-need help.

Then you could try swopping the 6SN7 with another of those in the set to see whether the symptoms change.
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Old 21st Oct 2017, 1:44 pm   #64
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Default Re: My first antique TV-Olympic Model 755-need help.

Just a thought, the voltages given in the schematic are given with no signal input.

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Old 21st Oct 2017, 2:01 pm   #65
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Default Re: My first antique TV-Olympic Model 755-need help.

Hi,
Has the circuit diagram been posted in this thread? If so, could someone point me in the direction of it as I would like to have a look at it!!
All the best
Nick
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Old 21st Oct 2017, 2:06 pm   #66
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Default Re: My first antique TV-Olympic Model 755-need help.

http://www.americanradiohistory.com/...an-1952-TV.pdf

Starting at P 107.

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Old 21st Oct 2017, 11:09 pm   #67
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Default Re: My first antique TV-Olympic Model 755-need help.

Lawrence, I didn't know that. I'll check the voltages again with the dvd player disconnected. Still, I think there is something definitely wrong there. Other than the problem areas I mentioned, the voltages were about right and fairly stable.

I've always been told that when resistors fail, they usually have higher resistance and they don't go lower or short out (or at least it's extremely rare). But I'm wondering if one of the resistors went bad and could cause a short giving a direct pathway for the 275 plate voltage to other parts of the tube? I don't have the diagram with me at the moment, but I remember that a 275 v pin is connected to another pin THROUGH a resistor.

- Chris
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Old 21st Oct 2017, 11:22 pm   #68
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Default Re: My first antique TV-Olympic Model 755-need help.

Quick rule of thumb for ordinary carbon resistors, higher values 100K and above go high, low value 10k and below tend to go low if the low value resistors are overrun.
Of course there will always be one that drifts the opposite way and there is leeway in those values.
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Old 21st Oct 2017, 11:23 pm   #69
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Default Re: My first antique TV-Olympic Model 755-need help.

See posts #61 and #62.
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Old 22nd Oct 2017, 12:11 am   #70
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Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
Originally Posted by ct92404 View Post
go lower or short out (or at least it's extremely rare). But I'm wondering if one of the resistors went bad and could cause a short giving a direct pathway for the 275 plate voltage to other parts of the tube? I don't have the diagram with me at the moment, but I remember that a 275 v pin is connected to another pin THROUGH a resistor.
Chris,

A lot can be worked out from the plate voltage on a valve(tube) signal amplifier stage.(I'll use the word tube since its an American set)

The first thing to do is look at the the plate load. If it is a transformer (RF or audio type) the DC resistance is relatively low, especially RF coils. So in this case, over a very large range of the tube's plate currents, the tube's plate voltage will appear with the meter to be about the same as the HT voltage or the voltage supplying the other terminal of the load.

In these cases the only way to get a handle on the tube's plate current, is to look at the voltage across the tube's cathode resistor and divide the resistance value into that to find the current. For example you could have a signal stage in the radio frequency section where the tube was biased off, or lost its emission and had little or no plate current, the plate voltage would not be very informative and the cathode voltage would be a better test.

When resistors are used as plate loads, for audio amplifier circuits, normally the voltage you see on the plate is very roughly 50% to 60% of the HT supply. This is so the voltage there can swing around without clipping to the HT voltage or bottom out when the tube is fully conducting.

In RF circuits or IF amplifier circuits, even though the voltage drops across the RF & IF coils themselves are very low, there are often series resistors in the HT supply of a few k ohms or more and filtered to ground with bypass capacitors. The voltage drops across these resistors gives an idea of the plate current to the tubes they supply. So that is a way of getting a handle on the plate currents.

In tube TV circuits where are sync separators, or circuits that handle pulses, usually the plate loads are resistors, the DC voltage on the tube plates could be quite high or even in some cases low in the absence of signal, depending on the design of the stages. In this case the DC levels on the plates with a meter can be less informative. Also using a digital meter, set on DC, might show all sorts of fluctuations say when being fed with a DC voltage with a large AC signal amplitude pulse on it, like for example separated horizontal sync pulses or separated vertical sync pulses.

It is possible therefore that some fluctuations you are seeing are measurement artifacts. Analog meters tend to ignore the pulse component and average out the values. It could be though that when you are noticing the fluctuations when the picture sync is lost that this represents the sync pulse signal coming and going due to a fault further upstream and not in the actual stage you are testing.

This is really why, especially in parts of the circuit where pulses are present, you really need to put a scope on it to see what is going on there, the meter has very limited utility in these sub circuits.

Also, make sure to keep your tests well away from the anode of the horizontal output tube and the efficiency diode area, the peak voltages there can be very high and just as, or more destructive, to test instruments as the actual EHT.

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Old 22nd Oct 2017, 3:44 am   #71
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Default Re: My first antique TV-Olympic Model 755-need help.

I think I've actually reached my limit for the time being. I'm really starting to get frustrated and burned out with this tv now. I probably can fix it eventually, but for now I've pretty much reached my limit mentally. Tonight, I started to get V16 confused with another tube and at that point I think it's time to stop! I'm going to double check everything I did, and then I think I'm going to put the tv back together and put it somewhere aside for a while. I hate quitting things, and I've been trying to just keep going with it, but now I'm afraid that I'm going to start making mistakes if I try to keep going.

At least it's better than it was. There was NO image at all when I first got the tv. But it's a LOT more complicated than I thought.

- Chris
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Old 22nd Oct 2017, 5:34 am   #72
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Default Re: My first antique TV-Olympic Model 755-need help.

Chris,

Well TV sets, even of this vintage are a lot more complicated than radios. And you can't be expected to know all the details right off the bat.

The technology that makes them work was worked out by a lot of very clever people in the past like Blumlein in the UK and Zworykin in the USA and for your set, teams of very clever engineers at places like RCA.

The only real way to get to grips with this technology is by reading (the book by Grob will do it). Then to get total control over the set you need an oscilloscope, a TV signal & pattern generator, and preferably too a sweep generator if you need to set up the video IF stages.

On occasions, it is possible to get a vintage TV like this up and running, just by re- capping it, but if that is all that is takes you would be pretty lucky.

Don't think of all this as a negative thing though, anything you learn will make you better & smarter and give you more tools to tackle another problem later.

Many of the folks on this forum here have been repairing both analog monochrome and color TV's for years and it all seems second nature, sort of making it look easy like riding a bike if you know how, but it is a bit of a long road to get there.

Unfortunately as time passes, less and less latter day trained engineers (who have been trained to code PIC micros etc) have any familiarity with this old tech.

Hugo.

Last edited by AC/HL; 22nd Oct 2017 at 2:14 pm. Reason: OT aside edited
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Old 24th Oct 2017, 9:04 am   #73
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Default Re: My first antique TV-Olympic Model 755-need help.

I got it!! I have a clear picture on the screen now!!

I was just about to give up. Last night, I was just double checking all my work to be sure I hadn't connected anything wrong, and then I was going to put the tv back together and put it aside for a while. As I matched up the wiring with the diagram, I started to identify more parts and things were making a little bit more sense. I decided to measure the resistors connected to V14 again (now that I knew for sure which tube it was!) and sure enough, there were a couple that had bad measurements. R41 was WAY off, it's supposed to be 22k, but was only measuring 5.48K. R42 is supposed to be 1 mega ohm, but measured about 1.3 mega ohms. I thought that maybe that could explain the weird voltages I was getting at V14. So I replaced both of those resistors.

The guy at the electronics shop where I've been getting most of the parts is an old timer and is familiar with vintage electronics. He had suggested earlier that I replace R44 at V14, and R65 at V16. Those are high value resistors, over 3 mega ohms, and my meter doesn't go that high. So I just had to take a chance and replace them.

I finished putting in the new resistors...and nothing. There was maybe a slight improvement in the image stability, but nothing major. BUT while I was looking at the diagram showing a top view of the chassis (on page 110), I noticed a couple of adjustment screws I hadn't see before! They're in the corner, I guess they're variable capacitors inside the tuner - C303 and C304. I figured I had nothing to lose at that point, so I decided to try to adjust them and see what happens. As I gently fiddled with the screws and also adjusted another screw in the front of the tuner and turned the fine tuning, the image started to hold more still. The horizontal hold stopped going nuts...and then bingo it suddenly turned clear!

I've attached some pictures I took of the tv. I was playing the same astronomy DVD from before. I was just using my cell phone camera again, so some of the pictures look a little bit blurry or overexposed, but that's just the dumb cell phone. The actual image on the tv looks amazingly sharp now! It's unbelievable how much of a difference there is from before! There might still be a tiny bit of noise, but I think it looks really good, considering how much trouble I was having before!

The only problem is that now I'm not getting sound. There's just a buzzing noise if I turn the volume up. I'm optimistic that maybe something else just needs to be adjusted somewhere. As I'm writing this, I do see a couple of other adjustment screws in that area by the tuner - C301 and C302. Maybe adjusting those will get the sound in tune?

I can't believe it, right when I was about to give up, I finally got the tv working!! Well, mostly, I just have to fix the sound. But I'm thinking that should be easier than fixing the image!

- Chris
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Old 24th Oct 2017, 8:45 pm   #74
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Default Re: My first antique TV-Olympic Model 755-need help.

Hi Chris,
Congratulations!! You must be well chuffed That looks a pretty reasonable picture too!
The IF signal leaving the tuner contains both the sound and vision information along with the vertical and horizontal sync pulses all mixed up together!!
The sound carrier (its not actual audio at this point but FM modulated onto a carrier like an FM radio) is a very narrow band of frequencies within the IF. This is 'picked' out of the IF signal by L5 in the anode circuit of the 1st common IF amp, V8. It's then dealt with totally separately to the vision signal.
So now as a sound IF signal, its amplified first by V3 and then by V4. It's then 'demodulated' by L2/3 and V5 to turn it into actual audio which is passed via the volume control to the audio amplifiers.
By making adjustments in the tuner, it's possible that the sound IF signal is not at the correct frequency to be picked off by L5, hence you just get a buzz.
Certainly in the UK system, tuning to get good sound is way more critical than to get a good picture and I imagine that will be the same for the US system.
You may find that adjusting the fine tuning will pull in the sound or you may have to carefully re- adjust the tuner adjustments.
That is of course assuming that there isn't a fault in the sound stages.
V14 is the sync separator which splits off the sync pulses from the video signal. The horizontal syncs are used to set the point at which each line starts and the vertical syncs the point at which the frame scan starts.
With the poor signal you had coming out of the tuner, these sync pulses would have been all over the place hence why the picture was very wavy and unstable. Also the 'hold' controls are much more tetchy with poor sync. All that would have led to erratic voltages around V14.
Well done on the progress so far,
All the best
Nick
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Old 25th Oct 2017, 5:28 am   #75
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Default Re: My first antique TV-Olympic Model 755-need help.

Thank you, Nick!

I tried adjusting the tuning again and the adjusting screws for the other variable capacitors I saw in the diagram, but I didn't really get anywhere. The picture looks good, but I just can't get the sound to work.

I think at this point, there's not really much else I can do. I'm really going to have to study about how these old tv's work. There are too many things to adjust - I feel like I'm playing with a Rubik's Cube! I don't really want to mess with it anymore.

I have a good image now, and I'm really happy with that. Considering the fact that the tv wasn't really doing anything when I got it (just a loud humming and a few flickering lines), and now I have a clear image, I'm happy with how the tv is working so far!

I decided to put the tv back together and just enjoy it as it is for now. I'll come back to it and try to fix the sound later when I've had a chance to do some more research. Tonight, I hooked it up to my cable box (FIOS, actually) and watched some television on it. There was a series on, called "Terror In The Woods," talking about paranormal experiences people have had...since Halloween is coming up! LOL perfect to watch on a vintage black and white tv in the middle of the night, I guess!

I attached a few pictures of the tv put back together. The image on the tv looks pretty good, the pictures from my cell phone camera just aren't that great.

By the way, Hugo, if you're still reading this thread, I ordered the Grob's television book from Amazon tonight! Maybe after I've read it, I'll be able to work on the sound problem. Thank you for suggesting it.

I do have one other question...how long would it be safe to have an antique tv like this on for? I've listened to my old tube radios for several hours with no problems, but I don't know about a tv. I wouldn't watch it for hours on end or all day of course, but how long would be safe to watch it and not risk damaging anything? I don't want to overheat it or burn out the CRT! But after all this work, I do want to power it up and play with it for a while now and then.

- Chris
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Old 25th Oct 2017, 6:25 am   #76
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Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
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I do have one other question...how long would it be safe to have an antique tv like this on for? I've listened to my old tube radios for several hours with no problems, but I don't know about a tv. I wouldn't watch it for hours on end or all day of course, but how long would be safe to watch it and not risk damaging anything? I don't want to overheat it or burn out the CRT! But after all this work, I do want to power it up and play with it for a while now and then.
Chris,

It's probably fine to leave it on while you are there, but I wouldn't leave it unattended for hours, just in case an issue cropped up, but probably it would be OK.

You did make amazing progress with this TV considering the limited test gear etc, again well done.
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Old 25th Oct 2017, 9:12 am   #77
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Default Re: My first antique TV-Olympic Model 755-need help.

Hi Chris,
That looks great now it's back in the cabinet! The CRT looks pretty good and even the subtitles are perfectly readable in your last picture!
I think some in depth reading & research will be really helpful: I don't think you will get much further without more understanding. TV's, as you've found, are amazingly complex- the technology of the time was pushed to it's limits and often beyond! The amount of stuff crammed into a TV signal is amazing- all the picture information, sound, horizontal & vertical sync pulses, equalisation pulses and then later, colour subcarrier, colour burst, teletext and finally stereo sound! (Did the US get stereo sound?)

The biggest single threat for an old TV is the LOPT. The CRT will stand many ,many hours of use (unless you are very unlucky) as will all the other components.
It would be worth measuring the LOPT temperature after the set has been on for 1/2 hr or so. Turn the set OFF and stick your finger on it (it will bite you otherwise ) The ideal is mildly warm but more likely it will be uncomfortable to keep your finger on it. If it burns your finger, its too hot!!
If it does run hot, you can mount a small fan to blow air over it- I've found that makes a big difference. My GEC 2000 LOPT settles at 56 Centigrade which is hotter than I would like but it doesn't seem to mind!
Apart from that, you don't even have a big mains dropper belching heat (as here in the UK) as you have a mains transformer. We have to get rid of 30-40 Watts of heat which makes everything else run hot! so your reliability should be good.
In my view, these TV's were designed to be used and its great to watch vintage programs on them regularly.
Last year there was a vintage TV day at the Museum where quite a few TV's, some pre- war, were run continuously all day. To my knowledge, there were no failures but I bet the rooms were hot by the end!!
So sit back and enjoy!!
Well done
Cheers
Nick
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Old 26th Oct 2017, 1:21 am   #78
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Default Re: My first antique TV-Olympic Model 755-need help.

I think I'm also going to have to find some plastic bladed screwdrivers. I've been looking online a little, and I see such a thing is available. There are a couple adjustment screws for coils under the chassis (like L3) and trying to adjust that while the circuit is live with a regular screwdriver would be WAY too dangerous...one slip and it could short out connections and damage parts or if I somehow lost my grip and touched the metal part it could even give me a nasty shock.

What do you guys usually do? Are non-conductive screwdrivers hard to find? Do they really work or break easy? Working on a live circuit with high voltages like this in a tight space with so many wires and connections makes me a bit nervous. The only thing I did under the chassis with the tv live was just measured voltages with my multimeter, and even that, holding the probes, made a little nervous at first. 400 volts and probably a large amount of current available too. Eeek!

- Chris
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Old 26th Oct 2017, 9:32 am   #79
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Default Re: My first antique TV-Olympic Model 755-need help.

Hi Chris,
What you need is a 'TV trim tool kit or TV alignment kit'. The cores of IF transformers are made from materials which have inductive properties- ferrites or dust/ iron amongst others. These are fragile and break quite easily. They often have hexagonal, square or slotted holes in them for adjustment and need to be adjusted with the correct PLASTIC or non ferrous tool. Using a normal screwdriver will tend to break them. Also a steel screwdriver will affect the tuning of the coil when placed near the core. I don't know what sort of cores your TV has.
Beware of having a 'twiddling' session though. All the adjustments are critical and inter- related and really need to be set up with the correct test gear. It's possible to end up in a complete mess with the set badly out of alignment!

As to making measurements with the set powered, I would make the following points. The old adage of 'keep one hand in your pocket' is a good one. Get used to NEVER having your spare hand resting on the chassis. That way, if you do touch the HT, the shock will be much reduced.
Have the ground lead of the test meter attached with a crocodile clip rather than having to hold it on the chassis. You can then concentrate on where the measuring lead is going.
Work in good light with the chassis well supported and things are much less likely to slip.
Always exercise caution and think about what you are about to do!!
Hope that helps
All the best
Nick
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Old 27th Oct 2017, 1:00 am   #80
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Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
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What do you guys usually do?
If buying on line, search for "radio tv trimmer tool" or "radio tv alignment tool." Plenty available. I found mine at the local independent electronic component/tool store.

One key reason to use the plastic tools is that any metallic tool such as a screwdriver will actually affect the tuning. You need something non-inductive such as plastic.

By the way these tools are not necessarily a panacea. The problem with vintage equipment is that over the decades the ferrite slugs have often become jammed. The plastic trimmer tools are the right way to go because anything stiffer could break the slugs.

If the slugs are jammed, it's another ballgame. Forum members will be able to advise you on that. As I understand it there are sometimes ways to free the slugs so that they will rotate freely again. But sometimes you have to break them deliberately, drill them out, then insert new ones.
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