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Old 4th Oct 2017, 10:38 am   #21
Nuvistor
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Default Re: My first antique TV-Olympic Model 755-need help.

The arrangement of using the sound output valve to supply a lower voltage from its cathode to the signal circuits HT was very common.
The source of the 140v line is the 6V6 cathode.

Edit. Hi Lawrence, thatís my usual problem, donít think you have a monopoly on it.
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Old 4th Oct 2017, 10:48 am   #22
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Default Re: My first antique TV-Olympic Model 755-need help.

I'd made the same mistake . The output valve cathode supplies 140V rather than being fed with 140V. Upside down American circuits (sorry I mean schematics) always confuse me.

It might be worth the OP checking those voltages.
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Old 4th Oct 2017, 11:32 am   #23
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Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
Originally Posted by ct92404 View Post

The most confusing thing is that I don't see how it would even be possible to connect C16 the way it's shown in the diagram. If anyone has a chance to look at the schematic at the link I posted before, please look at page 110.
Hi,

Don't be alarmed, what you are looking at is a very common configuration used in American television design. Admiral were using it as early as 1948.

A bright spark engineer figured out that the audio output stage, in its entirety, could be used as a DC voltage dropping device (ballast), to lower the voltage and provide power to other circuits in the set. This saved wasting power(heat) in a dropping resistor when a valve is already there doing it.

In Admiral sets, they used the voltage to power the entire IF stages. In your set they use it (the 140v) to power the video driver amplifier and looks like the tuner & ist IF amplifier. In any case current from the cathode of the audio output valve creates the 140V rail, that is where that 140V power rail comes from. But of course it needs the filter electrolytic caps (which are wired correctly) to remove any signals at audio frequencies. Clever isn't it ?

Just some words of advice when attempting to run a set of this vintage for the first time:

Never power a set this old up with any of the original electrolytic capacitors in it. If you do you will be chasing exploding capacitors and faults until the cows come home.

Document the wiring as best you can with diagrams and photographs and compare this with the schematics, sometimes you will find modifications.

Replace all of the electrolytic caps in the set before you do anything else.There is no such thing as a reliable 60+ year old electrolytic capacitor, despite what others may tell you.

Then check all the other capacitors as best you can, especially wax paper caps feeding valve grid circuits, replace if they have any detectable leakage at all. Replace any burnt looking or obviously out of spec resistors, only then power it up.

You will then have a good probability of no disasters/setbacks and a minimal number of faults to find after that basic work is done. Rather than being presented with a cocktail of difficulties due to multiple sub-circuit faults.

It is a great set you have there, well done for noticing it and wanting to repair it.

Hugo.

Last edited by Argus25; 4th Oct 2017 at 11:46 am. Reason: Add info.
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Old 5th Oct 2017, 10:47 am   #24
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Default Re: My first antique TV-Olympic Model 755-need help.

Thank you everyone for your replies and all your help so far! I love coming to this site. You guys are always so friendly and helpful!

I found out I made a BIG mistake! What I originally thought was the audio transformer is actually the "vertical output transformer." And what I thought was C16 is actually a completely different capacitor (C47)! So it turns out, I never did anything with C16 and it's still hooked up. I double checked the other capacitors I did replace - C50, C51, and C15 and I did connect them right and they seem to be fine. I did a quick test and powered the tv up for a little while. The screen is lit up again now, and no weird smells or boiling capacitors this time!

But now I'm confused again. I can see where the positive side of C47 is supposed to connect to the red wire of the vertical output transformer (page 109 on the schematic). But I don't understand where the negative side of the capacitor is supposed to connect. When I disconnected the wires on the capacitor "can" earlier, C47 was connected to one of the terminals. The can didn't have any labels, so that's why I had such a hard time figuring out the connections. Now I can't understand how the NEGATIVE side of C47 could connect to any of the terminals on that capacitor can, since they all would have been positive. (The can case is the common negative). But somehow it was!

On the schematic, the negative side of C47 goes back from page 109 to 108 and is shown to connect to the focus coil. But it's also shown to connect to line 4 (upper left corner of page 108). Line 4 goes back to page 107, and is a POSITIVE line. So how can the negative side of C47 connect to that?

Am I reading the schematic right? I bought a new capacitor for C16, and I don't want to hook it up wrong again and blow up another one!

(By the way, that was actually my first time ever burning up a capacitor! First time for everything, I guess!)

Here's the link to the schematic again, if anyone needs it:

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

Thanks again for everyone taking the time to help me with this. I really appreciate it.

- Chris
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Old 5th Oct 2017, 10:58 am   #25
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Default Re: My first antique TV-Olympic Model 755-need help.

It's just another case of a capacitor having different positive voltages at each end. One end will be more positive and this should be the positive end.

If in doubt just connect your meter across the capacitor's ends, observing polarity. So long as the meter doesn't read backwards, or show a '-' symbol, the cap is the right way round.
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Old 5th Oct 2017, 4:17 pm   #26
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Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
Originally Posted by Nuvistor View Post
Hi Nick,
There were transformerless designs but not AC/DC, Sylvania used them and some portable sets.
Various references in the magazine Radio-Electronics from the early 1960ís.
This page from http://www.americanradiohistory.com/...cs-1964-02.pdf
Apologies the screenshot is a bit blurred but the download from American radio is clear.
Almost every US manufacturer used a voltage doubler in some of their set models, usually in the lower end products or portables.
The smaller screen portables could be designed to run on 140v of HT, where the current demands weren't as high.
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Old 5th Oct 2017, 4:32 pm   #27
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Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
Originally Posted by 1100 man View Post
Were American sets of that period designed for AC/DC mains and therefore 'live chassis' or did they have mains transformers? Thinking about it, if they were designed to operate from 110v, it would not be possible to get enough HT (usually 200V or so) with UK type design. I imagine, therefore, they were for AC only and will have a transformer. I would be interested to know!
Outside of New York, I don't know if DC mains was still in use in the 1950's in certain areas.
Almost all the larger, older cities in the US had DC districts where larger buildings with elevators (lifts) were common. Many areas, it was being used until the late '60s. Around that time, the electrical suppliers ceased supplying DC power. The building owner had to furnish an alternate source of DC, either a rectifier or M-G set.
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Old 6th Oct 2017, 2:21 am   #28
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Default Re: My first antique TV-Olympic Model 755-need help.

I made good progress today.

I went ahead and connected C47 as it's shown in the diagram, with the negative side going to line 4. I also replaced C16 (now that I found the right one!). So now, I've replaced all the capacitors I planned for..C50, C51, C15, and now C16.

I powered up the tv and had it on for several minutes. There was no weird smells, no smoke, and no other obvious problems. I must have hooked up C47 right this time! It didn't seem to have any problems and wasn't hot. Considering all the confusion I had with the wiring, getting the capacitors hooked up right and getting the tv to power up with nothing burning is major progress for me!

The screen is lighting up, and the horizontal and vertical controls seem to be responding better. But I'm still not getting a signal. I tried to hook up a vcr to the tv and it didn't play. There is still humming, however, I am also starting to faintly hear the "hiss" of white noise on the speaker, which I definitely was not getting before.

There is another can with electrolytic capacitors, and my next step will probably be to replace them. I won't have any more money for a few days, so I'm going to have to take a break for now. But I think I'm definitely seeing some improvement with the tv, little by little.

- Chris
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Old 6th Oct 2017, 10:18 am   #29
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Default Re: My first antique TV-Olympic Model 755-need help.

Two posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...818#post980818
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Old 7th Oct 2017, 12:42 am   #30
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Default Re: My first antique TV-Olympic Model 755-need help.

The 140 volt line is generated by the audio output tube. It feeds to the IF and RF.
The cathode current of the audio powers the RF-IF.

C16 would explode if the 140 volt line is shorted to ground. You could use
a 450 or 500 volt cap from the audio screen to ground instead
of to the cathode. That way it would not explode ... the audio tube would overheat.
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Old 8th Oct 2017, 3:58 am   #31
ct92404
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Default Re: My first antique TV-Olympic Model 755-need help.

I've made some incremental improvements, but I'm stumped again.

The remaining electrolytic capacitors were cheaper than I thought they'd be, so I went ahead and bought new ones. I'm pretty sure I've replaced ALL the electrolytic capacitors on the tv now.

I can see improvement with the screen controls, especially the brightness. It's much smoother now and the entire screen responds well and goes bright or dim as I turn the knob. But I'm still NOT getting an actual image. I hooked up a vcr to the tv. I set the tuner on the tv to channel 4, and I'm getting sound from the tape. (The humming is gone completely now). But the sound is kind of muffled, and I'm not getting any video from the vcr. I'm just getting black and white lines that are kind of flickering.

I have no idea what's going on now.

Does anyone have an idea what could be causing this, why I'm not getting video? Could one of the tubes be bad? Or could a bad capacitor cause a big problem like this? I could replace the wax capacitors as was suggested earlier...but there's a LOT of them! When I've fixed antique radios before, I usually didn't bother to change the wax capacitors and the radios would still work fine. I only changed electrolytic capacitors. But could "waxies" cause big problems in a tv?

I also noticed that if I move a knob labeled "picture", the screen kind of flickers. I'm not sure what it's supposed to do. I found it in the schematic, and it looks like it's just a variable resistor.

I'm attaching a couple of pictures of what the screen looks like. If anyone has some ideas of what should be my next step, please let me know! I've made progress, but I'm really stuck now! I appreciate everyone's help with this so far.

- Chris
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Old 8th Oct 2017, 5:12 am   #32
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Default Re: My first antique TV-Olympic Model 755-need help.

Generally, there are three or 4 basic types of capacitors in the set.

There are "Waxies" or wax paper capacitors. These use paper insulation between the foils. These sometimes, in American TV sets, have the appearance of a black plastic body with colored stripes, but more usually, they have the yellow/brown colored waxed outer cardboard sleeve. The paper absorbs water over time and the salts in the paper cause leakage and this also alters the effective value of the capacitor, making it behave like a network of 4 components,

1) the original value
2) a resistor in parallel
3) a larger value capacitor with a resistor in series, paralleled across that.

These changes to the waxie can severely mess up the operating conditions of many valves(tubes) and even change the operating frequencies of the time-bases.

Then there are the Mica capacitors. In many sets these had a molded plastic case, sometimes with colored dots to identify the values.

Then there are "Dogbone capacitors" that are hollow cylindrical type used in RF circuits.

Then also the disc ceramic capacitor is a common part.

Since you have replaced the electrolytic capacitors, as I advised would be a good idea on a previous post, you have rehabilitated much of the circuitry, to the extent that the scanning circuits and CRT support circuits are working basically well.

But there is obviously little signal if any coming out of the video detector. There is a trace of signal perhaps.

A lot of things could cause this problem. However, since the ceramic disc capacitors are probably ok, the Dogbone caps are usually ok and a lot of the time the molded micas are ok (but not always) and you have replaced the electrolytics, that leaves the waxies yet to be dealt with.

The best move which might help you make progress (In the absence of a lot of fault finding experience and test equipment) it to replace every waxie in the set and see what happens next. It only takes a small amount of leakage in these to corrupt the function of many of the sub circuits they are in.

This is one reason why with a very old set, its best to change the electrolytics and the waxies first, then you are left with a smaller number of faults to hunt down after you power it, but often there is still one or two to resolve.

PS: the "picture control" is probably the contrast control , or video signal gain control, and it would be normal for it to make the image flicker on rotating it, especially if it needed a clean.It alters the cathode bias and therefore gain of the video driver stage.
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Old 8th Oct 2017, 11:49 pm   #33
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Default Re: My first antique TV-Olympic Model 755-need help.

Ok, I'll start replacing the wax capacitors tomorrow. I guess I'm going to have a lot of soldering to do.

I just hope I can get some kind of image soon. This is getting frustrating. I know it could be worse...at least the CRT is lighting up and responds to the screen controls, and I am getting sound (although it's muffled). The tv isn't totally dead. If changing the other capacitors doesn't fix the problem though...eek...I'll really be out of ideas then. I'll have to see what happens! Maybe with some luck, I'll have good news to report in a day or two.

BTW, I did try cleaning the controls with an electrical contact cleaning spray. The sound knob especially was really scratchy, and cleaning it with the spray fixed that problem. I also tried spraying the "picture" control, but that didn't really seem to do anything.

- Chris
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Old 9th Oct 2017, 12:59 am   #34
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Default Re: My first antique TV-Olympic Model 755-need help.

Quote:
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I also tried spraying the "picture" control, but that didn't really seem to do anything.
The design of the picture control is such that it has to pass the cathode current of the video driver tube. So with a DC current passing through the control it does tend to magnify imperfections in the control as the track wears and ages and result in the type of noise you see on the screen when its rotated. (Other circuit designs isolate the DC from the control)

Still if you had a decent video signal it will likely be a lot less noticeable. If it remains noisy and objectionable in the long run, you always have the option to replace the control later.
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Old 9th Oct 2017, 2:44 am   #35
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Default Re: My first antique TV-Olympic Model 755-need help.

Well I think that while I'm working on the tv tomorrow and replacing the capacitors, I'm going to also check the picture control and measure the resistance and voltage across it. The knob did seem to feel a bit little loose, and I'm wondering if it could be so worn out that it's making intermittent contact? In the diagram, there is a capacitor going from pin 3 on the 6SN7 tube to ground, and the picture control is a variable resistor connected across the capacitor. (Page 109). If the wiper or whatever is in the picture control is worn out and not making contact, so that the variable resistor sometimes just isn't connected , what effect would that have?

Maybe I'm overthinking it. But I'm new to trying to fix televisions. I'm going to replace the wax capacitors regardless, but now I'm now I'm also wondering if the picture control could also be on the list of suspects.
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Old 9th Oct 2017, 3:41 am   #36
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Default Re: My first antique TV-Olympic Model 755-need help.

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If the wiper or whatever is in the picture control is worn out and not making contact, so that the variable resistor sometimes just isn't connected , what effect would that have?
If the control is wired exactly as in the schematic, then if the wiper goes open, there is a very large change in the cathode current of the video driver stage, it jumps to zero. Ideally, to help mask over imperfections, the central or wiper connection should be connected to the free end of the control, that way when the wiper goes open the value can only get to 5k. The higher the resistance placed in the cathode the lower the gain of the video driver stage. Certainly if that control was open, you would have negligible video signal ending up being sent to the CRT even if it was ok out of the detector. Try either shorting out the control, or connecting its center leg to its free end.

The purpose of the small 47uuF capacitor across it is to maintain the video driver stage's high frequency response, otherwise at low contrast setting (the control at its max value or close) the picture could look soft with a loss of high frequency detail.
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Old 10th Oct 2017, 11:27 am   #37
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Default Re: My first antique TV-Olympic Model 755-need help.

I'm still in the middle of replacing capacitors, but...I have a faint image on the screen now!!

I have a vcr hooked up to the tv, playing a tape with a documentary about hummingbirds. The image is really fuzzy, but I could definitely see a bird flying around. And a guy talking. So, I'm getting there! Instead of the CRT just glowing and showing squiggly lines, it's beginning to look like a tv screen!

I still have a lot more work to do, but the tv is already better than before...it wasn't doing anything when I got it!
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Old 12th Oct 2017, 9:33 am   #38
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Default Re: My first antique TV-Olympic Model 755-need help.

I hope someone is still reading this. I've really run into a strange problem now.

Earlier, I managed to get a fuzzy picture on the tv. The weird thing was that the sound was coming in more clear if I had the tuner on channel 4, but I would only get a discernible image if I had it on channel 3. Still, I was happy to get ANY kind of visible image and I figured the tuning was just off because of bad capacitors.

So I continued to work on replacing caps. I was very careful to leave some of the leads when I cut out old capacitors, so that it was easy to see where they connected and I could connect the new capacitor at the same connection points. I was also careful to make sure I had the right voltage\capacitance values. But now I've had a big setback. It looks like I've lost the video signal, or at least it's really distorted. Somehow, the sound is interfering with the image. The screen flickers coinciding with the sound. I think the sound and image are both coming in on the same channel now, but the image is barely visible and there is a lot of interference.

What in the world could be causing this? It's like somehow the sound is feeding into the image. I think the last capacitor I replaced before I started having this problem is C13, but I don't see what I could have done wrong. There is a small metal separator on the chassis in that area, which I guess could be meant as some kind of shield. But I tried moving the wires connecting C13 around behind that metal part and it didn't have any effect.

This is so frustrating! Just when I was making progress, I really hit a brick wall!

-Chris
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Old 12th Oct 2017, 10:02 am   #39
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Default Re: My first antique TV-Olympic Model 755-need help.

C13 is an audio coupling capacitor, what would be called "that capacitor" in a radio receiver. I cannot see how changing it would cause the symptoms you describe. Page 110 in the service sheet refers.

A sound on vision problem would more likely be located in the IF stages of the set.
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Old 12th Oct 2017, 10:27 am   #40
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Default Re: My first antique TV-Olympic Model 755-need help.

I'm not familiar with the US TV system but so far as I can make out from the schematic it's FM sound, might be worth checking the 140 volt rail with the volume set to minimum just to make sure the sound output valve's operating point is where it should be, poor regulation on that line could modulate the vision/video with the sound content.

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