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 Post subject: Olympic Model 755 tv - need help
PostPosted: Sep Sat 30, 2017 7:45 am 
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Joined: Jan Tue 03, 2017 6:43 am
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Location: California
I just bought my first antique tv :mrgreen:

It's an Olympic Model 755, I think from 1952. The wooden cabinet is in pretty good condition, and everything seems to be intact. The chassis was filthy with dust, and I pretty much finished cleaning it. But the tv isn't working. I bought it from someone selling it online, and he powered it up to try to test it earlier. It does power up - the vacuum tubes glow and the cathode ray tube lights up. There is a line forming on the screen that kind of flickers, so at least I know the high voltage supply is working and the CRT at least seems to be working. But there is a loud hum, so I know there must be some bad filter capacitors at the very least.

I do have some experience fixing old vacuum tube radios, but I really don't know anything about antique tvs. I figure my first step should be to try replacing electrolytic filter capacitors. But the problem I'm having right now is I can't find much information about this tv. I can't find a schematic - not even RadioMuseum.org seems to have it. Does anyone know where I can get a diagram? I can't really go any further without that.

Also, what should the screen look like if the CRT is working, but without the tv picking up a signal? On a modern tv, you just get "snow" on the screen, but what would "white noise" look like on an antique tv like this? Ultimately, I'm thinking about hooking up a converter so I can connect a modern coaxial cable to the antenna connection. But in the mean time, what would I look for to know that the tv is at least working, before I feed it a signal? I'm really excited to bring this antique tv back to life, but this is all new to me! I'd appreciate any help.

- Chris


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Sun 01, 2017 3:49 am 
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Location: Durham, NC
Congrats on joining the group of crazies that love old TV sets! There are lots of folks here with more hands-on experience than me who will no doubt chime in on the best way to proceed, but it looks like your set is a 1952 model, so the SAMS photofact should be available from one of several sources if no one has a free copy available for you. My Rider's files are all in-transit right now, as I move from Massachusetts to North Carolina, or I'd be looking it up for you.

"Snow" is the same on old monochrome TVs as new color sets; random dots, constantly moving and changing. It's not a particularly helpful diagnostic tool,so getting a standard definition signal source like a converter or a VCR or DVD player is a very good idea. Even an old video game would provide a known pattern to check things out with.

Keep us posted on your progress!

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


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Mon 02, 2017 3:45 pm 
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I'll see if I can find the riders on it tonight.

On a set that old you don't want to run it much if at all on the original capacitors since irreplaceable things can get damaged by bad caps. If you just have a varying horizontal line that means your vertical section is not running (bad paper and or lytic caps would be the most likely cause)....Caution :!: if the line formed is bright it can BURN the phosphor off the inside face of the CRT along that line ruining the CRT. A good CRT usually runs north of a C-note if one can even be found.

The adapter that converts 300 ohm twin lead antenna to 75 ohm F-type connectors modern TV antennas use is called a balun....While a balun is the best thing to use to connect a converter box (or other modern signal source) to the set, a balun is not strictly necessary. In a pinch you could hook a thin wire (solid core telephone wire works decent) to the ant terminal on the TV and poke the other end into the center hole in the female F-tpye connector on the converter box...The signal may not be great with this method, but it will usually work when basic verification of operation with signal is needed.


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Fri 06, 2017 9:53 pm 
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One common problem with early TV sets is an open vertical oscillator or output transformer; that would be the first thing to check. Any 1940s or 1950s set is going to need recapping, as they are full of paper capacitors, just like the radios and amps of that era. The electrolytic caps are going to need replacement as well.

Be careful around the CRT when you are working on the set; the neck is fragile and easily broken.

There are a lot of us here with television experience, if you need help.

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Tim KA3JRT


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Fri 13, 2017 3:06 am 
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Location: California
Hi,
I'm really sorry it took so long for me to reply. I did find a manual. The diagrams for my tv are on pages 107-110:

http://www.americanradiohistory.com/Arc ... 952-TV.pdf

I've been busy replacing capacitors, and I was talking with some people on a forum on another website. The first capacitors I replaced were the electrolytic filter capacitors C50 and C51, (and also C16, since they were all in the same "can"). That got rid of the loud hum. I replaced some other electrolytic capacitors, and a bunch of wax ones...holy cow, there are a LOT of capacitors in this thing! The CRT came back to life, and is fully lit up instead of just the few horizontal lines I was getting. It responds well to the brightness control, going dim or brighter smoothly as I turn the knob. The horizontal, vertical, and height controls also seem to be working.

I have a vcr hooked up to the tv. (I'm just using the RF out from the vcr and have it hooked up to the tv's antenna screws with a coax cable and matching transformer). But there is something weird going on. I can get sound from the vcr fairly well if I have the tv tuner on channel 4. At one point, I was able to get a faint, fuzzy image if I had the tv on channel 3. (Though the sound on channel 3 would just be buzzing). I don't know why the video and sound seemed to be on different channels like that, but I assumed it was just because of more bad capacitors that I had to replace. So I kept working and replacing suspect capacitors. But now something has happened to the image. I lost the video signal, or at least it's really faint. It seems like the picture and sound are both coming on the same channel now, channel 4. The sound is still coming in, but the video is barely visible and now somehow the sound is interfering with the image. The screen flickers along with the sound, like somehow the sound is feeding into the picture. Does anyone have any idea what could be causing this? :(

I was very careful about getting the right voltage and capacitance values when I replaced the capacitors. I don't think I did anything wrong with that.

I noticed today though that there are still a few "bumblebee" capacitors that I missed. I'm probably going to replace them tomorrow. Could bad capacitors cause a problem like this, with the sound interfering with the image? I'm wondering if now that I might have "fixed" the tv enough that the sound and video are on the same channel, if maybe a new problem is showing up that I otherwise wouldn't have seen? (I actually kind of hope that's the case and I didn't do something wrong!)

I'd definitely appreciate any help with this! I'm in completely new territory! I've never tried to fix a tv before. I was so excited to have an antique tv like this, but it's turning out to be a lot harder to fix than I thought it would be! I'm still hoping I'll get it working eventually.

- Chris


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Fri 13, 2017 6:54 pm 
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Joined: Jan Sat 16, 2016 10:03 pm
Posts: 316
I have never had good luck using a VCR as a video source for these vintage televisions. You may want to try a DVD player with a RF Modulator. However when using a DVD player some discs will have macro-vision software that can interfere with the performance of the display. If you encounter macro-vision just copy the original DVD disc to another blank disc using your computer and the macro-vision will not be copied to the new disc.


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Fri 13, 2017 7:36 pm 
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Joined: Aug Thu 20, 2015 3:09 pm
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Location: Albion, CA, USA
Loss of video can be a loose tube or poor contact. Have you done a thorough cleaning of all contacts in the tube sockets, tuner to IF if it has a plug, CRT socket etc?


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Fri 13, 2017 11:06 pm 
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Location: Woodinville, WA USA
Yes. Cleaning tube pins and sockets is such a basic task that I always do it before other restoration jobs like recapping:

https://antiqueradio.org/FirstStepsInRestoration.htm

Many tuners can benefit from a cleaning of their contacts, too. Again, I normally do this task sooner, not later.

Regards,

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


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Sat 14, 2017 1:36 pm 
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Yes, get rid of all the "bumblebees" and any wax caps. Watch out for old replacement caps, as they are likely to be bad as well. Resistors are another problem in early TV sets, so it would be a good idea to check them.

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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Mon 16, 2017 2:05 am 
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Location: California
I got the picture back. It's still really fuzzy, but at least I have some kind of image again! :)

After reading through some other topics on the forum, I got an idea to try adjusting the horizontal oscillation frequency. The adjustment is on the back of the chassis. I put the horizontal hold around the halfway point and then carefully turned the frequency adjustment. After fiddling with it for a while, the image came back. It kind of makes sense, since a couple of the capacitors I replaced were on the horizontal oscillator coil. With new capacitors, I guess that would have thrown off the resonance of the coil and so that's why I had to adjust it. I'm definitely learning a lot from trying to fix this tv. Before all this, I didn't even know what a "horizontal oscillation coil" was!

I decided to go ahead and use a DVD player instead of the VCR. Actually, the player I'm using is a combined vcr\dvd player, and you just select between the two. With a dvd, I can just leave it on the title screen and use that to test and adjust the tv.

The picture is better than I thought. But it's still really fuzzy and it's definitely getting interference from somewhere. I try to adjust the vertical and horizon holds and get a somewhat decent (albeit fuzzy) picture, and then the next moment it starts jumping around again. I think I might have found a clue though. There is yet ANOTHER bumblebee capacitor, connected between pin 3 on the 6v6 tube and the chassis. It was hiding in a corner on the chassis and I somehow missed it! What I'm confused about though is that on the schematic (on page 110), the only capacitor that connected on pin 3 is supposed to have its other lead connected to pin 4 and the audio transformer. Why is the capacitor connected to the chassis on my tv? :? It's soldered onto a big lug on the chassis, it definitely looks like something the factory did. Maybe they changed it at some point? If that capacitor was bad, would that cause the sound to distort the image? I'm going to change that capacitor regardless (I thought I got them all!), but I'm just wondering if it could be a culprit causing the interference.

- Chris


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Mon 16, 2017 5:15 am 
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One adjustment you haven't mentioned adjusting is the fine tuning. That is typically the knob just behind the channel selector knob. Try adjusting it.

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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Thu 19, 2017 4:51 am 
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The 6V6 is the sound output and wouldn't likely have any effect on the video. The additional cap on pin 3 should be replaced of course but probably it cuts down on the higher frequency sound, so it effects the tone.
One thing you should be warned about is de-tuning the IF circuits. IF alignment on a TV is a lot more complicated than an AM radio IF alignment. On AM you 'peak' the adjustments but on a TV you need a wide flat IF response. A sweep generator and oscilloscope is usually used, etc. So stay away from mica caps and coils around the IF sections. Stay away from them also in the tuner section for the same reason. Replacing the wax and bumble bees shouldn't be a problem they don't set frequency in these areas.
That being said, the sound is on a FM modulation of a 4.5MHz signal that is on one side of the video signal. You might notice a 4.5MHz trap around V12, pg 109. The trap keeps the sound carrier out of the video. The video is AM modulation, and is about 4+MHz wide.

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Tim
"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Thu 19, 2017 9:53 pm 
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This TV uses a stacked B+ design where the 6V6 acts as a voltage divider with the cathode being at 140V. That 140 volt buss supplies several circuits including the IF so it is essential that the 6V6 be healthy and biased correctly to get a good image.


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Fri 20, 2017 9:07 pm 
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Pin 3 is the plate of the 6V6, so the capacitor is the plate bypass. It would have the same effect if it was connected to pin 4, as it would be coupled to ground through the filter capacitor on the B+ line.

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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Sat 21, 2017 1:23 am 
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Tim Tress wrote:
Pin 3 is the plate of the 6V6, so the capacitor is the plate bypass. It would have the same effect if it was connected to pin 4, as it would be coupled to ground through the filter capacitor on the B+ line.

The only difference is that the capacitor would see less voltage across it if it were connected to pin 4. Probably there was an early design change to move the connection from ground to pin 4. Early enough that the schematic does not mention the design change.

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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Sat 21, 2017 3:45 am 
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Tim Tress wrote:
Pin 3 is the plate of the 6V6, so the capacitor is the plate bypass. It would have the same effect if it was connected to pin 4, as it would be coupled to ground through the filter capacitor on the B+ line.

I beg to differ, it wouldn't have the same effect as pin 4.
Pin 3 of the 6V6, the plate is the sound output. A cap on pin 3 would ground out the sound completely if it was large. The cap isn't large so it only reduces the higher audio frequency.
If the cap were on the other side of the audio output transformer, pin 4, it would be a stage bypass.

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Tim
"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Sat 21, 2017 3:22 pm 
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I should have made it clearer; the capacitor connected BETWEEN pins 3 and 4 would have the same effect as between pin 3 and ground. A capacitor with a higher working voltage would be needed in that case, as the peak voltage would be higher; depending on the plate voltage, it's not uncommon to see that cap rated at 800 volts or more.

Back to the original problem..... Have you checked the tubes in the tuner, IF strip, AGC, and video stages? That "stacked B+" circuit caused a lot of trouble, as it can affect many different stages. Make sure that the coupling capacitor to the grid of the 6V6 isn't leaky, and check all of the biasing resistors in that circuit; do some voltage tests.

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 Post subject: Re: Olympic Model 755 tv - need help
PostPosted: Oct Sat 21, 2017 11:57 pm 
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The problem could easily be resistors that have changed value. I've seen things from 1959 where a majority of the resistors are out of spec. No fourth color band=+- 20%, silver fourth band=+-10%,
gold fourth band=+-5%. You probably couldn't find new 20%, 5% don't cost much more or the same as 10%, so replace with 5%. If you have to unsolder one end to measure a resistor, because something is in parallel that would effect the measurement, you may as well replace it, you're half way there already.

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Tim
"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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