New Find! motorola VT71m-a

[mbates14]

I got this from a teacher for a graduation gift.

well, i need to restore it.

I powered this beuty up on the variac, and not to my surprise, the tubes lit with no audio or picture. needs new caps, and both seleniums.

Charlie, do you have the sams?

also, this thing has the so called "ballast tube" that needs replacing. a couple of those filiment resistors are burned open, and there are others that are intact. (shorted 'lytics i think).

also, anyone that has restored these in the past, is there any advice i need to watch out for?

i restored 2 other TVs, and one radio, but i always ran into a tricky problem.

also, how am I going to rebuild the ballast tube with modernized resistors.

Any ideas? sams would be usefull here.

lol.

[Charlie]

Originally posted by mbates14

Charlie, do you have the sams?

I'm not at home right now. Currently on board ship docked in Philadelphia and won't be home till mid-late June. If you haven't found one by then, I'll look and see if I have it.

[mbates14]

ill just order one

[mbates14]

the ballast tube is bad, how do I rebuild?

[Tim]

Phil Nelson has a great article on his site about rebuilding the ballast.

http://antiqueradio.org/motvt73.htm

[mbates14]

oops, its a ts-4d in a ts-4j cabnet.

go figure.

[mbates14]

I contacted justradios, they are making me a capacitor kit, and going to send the sams.

I have one problem:

this chassis uses (5) .005 6000v capacitors.

they dont carry those, and I cannot find them at allied electronics.


any ideas?

[Stlouisian]

You can use the .0047 mfd 6000 volt capacitors that Allied carries to replace the .005 mfd 6000 volt capacitors.

Ross

[mbates14]

I did an analysis on the chassis, and I believe its the powersupply that is not working.

like phil nelson said on his site, that he replaced the filters and selenium, and up pop the picture.

well, i powered mine on through a variac, the resistor "wires" on the ballast tube glow red for a second or two before the tubes start to warm up. There is alot of heat on the ballast tube. but, on the other mica, the resistors were cold.

So I think, that someone "just plugged it in" sometime in the past, and the shorted lytics blew out the seleniums. so there is NO volts on any tubes period. except for filiments.

There have been a lot of discussions of these sets on the antiqueradios.com TV forum, including many that I have posted.

http://antiqueradios.com/cgi-bin/for...=20&LastLogin=

I'm travelling at the moment and unable to give detailed answers to your questions, but do a search at the link above and if you have further questions, feel free to email me. I have restored 3 recently and pretty much know them inside out (which may not necessarily be a good thing .

As for the power supply problems, I would start by assuming that all rectifiers, filter caps, and ballast sections are bad, and replace everything before applying power again.

These are neat little sets, humble as they are, and my friends love to watch them in action. Good luck restoring it and again, contact me privately if you have questions.

[bgadow]

This is the model I have. It sorta worked when I got it; being stupid, I pulled a tube while it was powered up and watched a couple elements in the ballast burn up. Anyway, I managed to fit new wirewrounds in the ballast can, it was tight and if they were all burnt out my method wouldn't have worked, but it has lasted many hours so I guess I didn't do too bad.

Somebody should buy a large quantity of those 6K caps to resell-Allied isn't the easiest place to deal with, at least not on such a small order.

It took me awhile to find the right schematic as the ones I found were for a different version. Somebody online sent me the correct one.

[mbates14]

there are several several several different chassis.

On the 6000V capacitors, the ASC metalized polyester caps seem to work great. For the .001 cap, the ASC part # is X675-.0010-10-6000 and the Allied # is 225-6015. On the .0047 cap, ASC # is X675-.0047-10-6000 and the Allied # is 225-6075. They are around 2.50 each.

[wa2ise]

also, this thing has the so called "ballast tube" that needs replacing. a couple of those filiment resistors are burned open, and there are others that are intact.

Do all the tubes' heaters light up, or just one of the strings? In any event, if a string's voltage adds up to about 85V, you could use a diode (like an 1N4004) with no filter cap to run that string off the 120V powerline. Remember that it's V^2/R = power, and that a diode would cut that power by half. So (1/2)(120V^2)/R =~ (85V^2)/R Tubes generally perfer that their heaters are on average at more positive voltage than their cathodes, so connect the diode's cathode to the top of the string.

If the string adds up to less than 85V, pad it up with a power resistor of appropiate size. using the heater string's rated current, and the amount of voltage you need to make it add up to 85V, R=V/I. And the power size needed: I^2*R =P This cuts waste heat down.

[mbates14]

Ive gottten a schematic, and I tested all of the ballast resistors.

they are fine, except for one.

the 200ohm one is bad.

the others are fine, and ALL of the filiments light.

there is dead 0 volts. both seleniums are open, and the B+ supply resistor (200ohm) reads about 1 to 2 megohms.