Repairing an old Singer portable BW TV .. general capcitor question

[leonk]

I recently purchased a 1960's portable TV (branded by Singer, but made by Mitsubishi). This TV uses diodes and transistors! It shows full screen, but I can't tune to any channel .. so I assume there's something wrong in the UHF/VHF board.

I've taken the TV apart, and at this point figured I'll start by replacing all the capacitors in it.. but I ran into a problem.

All the capacitor values are non-standard ones! 2,3, 5, etc. uF 15V electrolytic capacitors.

In the past, I've repaired a couple of TV's and Arcade machine monitors .. but for those, I was able to find capacitors.

When someone run into this kind of problem with these old TV's, what do you do? Will you put a 4.6uF capacitor instead of 5uF, or 5.2 uF (assuming these are real values that I can easily find).

I remember the old arcade machine monitor repair guys frown a lot about changing uF values, but have no issues with moving voltage up.. (up to a point I assume.. 50V instead of 15V should work, but I shouldn't try and put in a 600V, right?)

Lastly, There is one capacitor that looks almost like a match box, and is just as big (big yellow box with with 2 leads on either size) and is marked: 7uF (K) 50WV.

My concern is with this one.. 7uF 50WV is not that big.. why is this one so huge? Also, 7uF? What a strange size.. I can't find it ANYWHERE...

Please advise how you would handle this..

Thanks.

[jpdylon]

what were you using for a signal source?

Is there snow visible and audible, or only a raster?

If there is visible and audible snow but you simply can't get a station, check oscillator voltages. If there is nothing but a raster check RF amp voltages and IF stages.

Have you cleaned the tuner yet? often times this symptom can be caused by a filthy tuner.

Please don't start replacing parts until you've found the actual failure. Replacing caps in the tuner and IF stages may have you needing an alignment afterwords. You might also think about obtaining a sams and or schematic to help you troubleshoot.

Good luck.

[leonk]

Thanks for the reply.. here are my answers:

Originally Posted by jpdylon

what were you using for a signal source?

Is there snow visible and audible, or only a raster?

I was trying to catch a station using UHF/VHF (here in Toronto, Canada, analog signal is still around. I wasn't seeing any snow or audible snow.. all there was on the screen was a solid white/gray raster. The strange thing is that I could actually see the line traces on the screen.

Originally Posted by jpdylon

If there is nothing but a raster check RF amp voltages and IF stages.

The schematics are in the manual (really tiny!) I'll try to follow them. What is an IF stage?

Thanks.

[DaveWM]

that last 7uf cap may be NP which would be a large cap even by todays standards

[leonk]

Originally Posted by DaveWM

that last 7uf cap may be NP which would be a large cap even by todays standards

Sorry, but what is 'NP' ?

[DaveWM]

non polar just a regular film cap

[DaveWM]

maybe the tuner is not working. If you have a schematic and can find the input of the video amp, then I would try injecting a composite signal there.

you can just take the video out put jack from a dvd and use that (not the RF but the video lead). To be safe I would put a meter on the test point to make sure there is no high potential (I messed up and "thought" I had the input of the video amp, when in fact I had a B+, fried my DVD). this will tell you if the problem is before or after the IF/detector stage.

Not knowing the chassis you should make sure its not hot (one side connected to the mains) before attempting working on it to avoid any nasty shocks. back in the 60's there was a lot of ways of hurting yourself (not as many product liabliity lawyers I guess).

I use an isolation transformer on any set that does not have a power transformer, and even then you have to be careful. A motorola I worked on had a big transformer for the filaments but none for the B+, just a voltage doubler and a floating ground for the B-

[wa2ise]

Originally Posted by jpdylon

what were you using for a signal source?

Is there snow visible and audible, or only a raster?

Reminds me when I was a kid, the Admiral Bakelite B&W set we had in the bedroom developed a problem. My father asks "Did it have a raster?" and not understanding correctly what raster" meant (thought it meant a screen of random snow) I said "no". Turns out the set did have a blank raster, and my father complained what an idiot son he had... Well, he forgot that a ten year old kid might not know what "raster" meant...

[Patnukem]

not to hijack your thread but I was working on one of these but I did not have a schematic would there be anyway you could post it?

It would help others to give you a better idea what to check as well.

Ill try to grab my tv next time I am at my storage place then if you need any reference picture or something I can check for you.

[leonk]

Sure thing! I'll get it scanned and post it here.

I have the item, with all accessories, manuals, wires, carry bag, and even original sales receipt!! I don't think there's anything missing (maybe the cardboard box it was sold in??)

[Tubejunke]

Originally Posted by leonk

I remember the old arcade machine monitor repair guys frown a lot about changing uF values, but have no issues with moving voltage up.. (up to a point I assume.. 50V instead of 15V should work, but I shouldn't try and put in a 600V, right?)

There is no "up to a point" concerning maximum rated working voltages and capacitors, as long as the one you are planning on using is rated equal to or higher. With capacitance it's not quite as simple; it depends on the circuit in many cases. Some circuits have a way wider tolerance than others. With that being said, you can go with a somewhat larger capacitance value in a lot of circuits and get a good result; particularly in a filtering circuit.

The rule of thumb here is basically NEVER go with LOWER values.

[leonk]

I have a high res scan of the schematics.. but the file is 3Mb big.. any suggestions on how to share it? what to upload?

[Patnukem]

http://imageshack.us/ would work make sure the re size option is not selected.

[leonk]

Thanks..

here are the schematics. I've replaced 99% of all the capacitors in this TV, and the behavior is still the same. Got snow sound, full raster image, but no snow on the actual TV... just a solid gray image that I can shrink and grow horizontally/vertically, and change horizontal/vertical hold.

It's a pretty easy TV to work with.. It's all 12V or less DC in it (AC to 12V 1A DC transformer attaches to the back). The only thing I'm confused about is the 2 1000uF capacitors (the big ones). They're hooked up very weird (can anyone confirm?) It looks like that the +ve terminals of the 2 caps tie together, then tie to ground, and then have a wire going off somewhere. Where, 1 -ve terminal of the caps ties to ground, and on the 2nd -ve terminals, you got 2 wires, and a diode.

In the schematics, I can only see 1 1000uF capacitor.. I'd like to check my hookup with anyone else that has this tv.

Thanks.

http://img571.imageshack.us/img571/3751/singer.jpg

[Patnukem]

I can check for you when I get mine but it might be several weeks have to wait for a good time with work and stuff (my storage unit is in omaha and I am in KC right now)