New Find! motorola VT71m-a
 

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.

Re: New Find! motorola VT71m-a
 

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.

ill just order one

the ballast tube is bad, how do I rebuild?

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

http://antiqueradio.org/motvt73.htm

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

go figure.

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?

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

Ross

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.

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.

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.

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.

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.

one of the main filter caps checks for leakage.

All of the other caps test fine. no shorts.

Better check for shorted electrolytic B+ filter caps. That might have blown up the 200 ohm resistance and the seleniums. I'd just install at some convenient and ventulated spot a new 220 ohm resistor and silicon diodes in place. The extra 20 ohms resistance is to drop the higher voltage the silicon diodes will deliver. But check for shorted caps first! Also all the wax caps will be bad and need to be replaced.

I know the caps need replacing, but I wouldnt necessarily say all the caps are bad.


I have an old 1950's silvertone, completely unrestored, and gives a beutifull picture. with the exception of one electrolytic.

I agree with mbates14. I have seen and own too many electronic devices with all or mostly original caps that work fine. There are probably a few variables besides age that determine caps life span and reliability. The popular idea of ripping all the caps out for new ones is ok if you have the money and time to rebuild television sets or if you wanted to put an old set to every day use. You can use a set with the old stuff but that way you will have to pull the chassis for repairs as they become necessary. There is something to be said for having a set that can be used for years without repair. Guess it all boils down to time and $$, or lack there of.....

the ballast resistor tube tests fine, except for the 200ohm resistor.

that sucker is reading in and along 1.8megohms. hmmmmmm..

need a new B+ supply resistor. all of the other resistors are all way well in tolerance. so thats why all the tubes are lighting.

anyway, the filtercaps have to be bad, because the seleniums were open, and that supply resistor is nearly open.

Guess what guys! my 7jp4 is toast! F**K

well, I recapped the whole set, replaced the seleniums, and the bad ballast resistor.

The set will turn on, filiments will glow. The sound is there, and the HV is there. I can see a slight blue glow, and the filiment lights on the rectifier tube. so my HV is there.

the filiment on the picture tube is on, and i can also see a slight blue glow on the neck of the tube. A spark jumped off of the tube to ground once, but i had to move the tube more away.

there is nothing on the CRT. no raster, nothing. My drive signals are ok on the tube acording the schematics. my volts are within tolerance. by like 1 or 2 volts.

The only thing i didnt do was change the 3 .005 6000vdc caps.

I did replace the .001 6000vdc caps. I had those. I cannot get the .005s No CC.

but, if those were bad, the HV wouldnt be there right? well my HV is there. my volts are on the tube. and the tube is glowing, and there is a slight blue ring around the clear glass on back of CRT.

but, nothing on phosphor. no raster, no pic.

Don't give up on the 7JP4 yet. Replace the .005s. That will probably get you a raster. You can buy those caps at Allied.

Steve

well, what are the purpose of the .005?

every online store I know takes credit card. I dont have a credit card.

I thought if the .005s were bad, there wouldnt be any HV. well, there is. becuase the filiment is glowing in the HV rect.

also to note, im sure it doenst matter, but I ordered the cap kit from justradios, and they didnt have any .005 6000vdc.

They had the .001 at 6000vdc, those are the blue ceramic. I put those in.

uh oh. it might not be the tube, but rather:


after inspecting the HV section, I noticed a (old looking, weird) brown plastic cap, that has 500mmf and rated 20KVDC.

What the?? and ideas?

thats just one.

There is also a large aluminum can cap right near the 25l6 H out.

YES!!, well kinda.

I replaced those 2 oddballs, with the correct ones that were in the schematic (those were actually the .001 6000vdc, with the wrong type installed from in the past).

now i got a raster, but its just a very intense dot in the center of the screen, so I killed hte power almost immediatly.

Those .005@6000V are in the sweep circut, and are known for going bad, even early in the life if these sets, they were usually the cause for their failure, and instead of getting the set fixed, most people wanted a bigger TV now available, so they were put into the attic, with almost no life on the other parts or CRT.
When those caps go leaky, there will be no deflection, horizonal or vertical, so you get the dot in the center of the screen. Since this is an electrostatic deflection set (no yolk), these caps are needed, though I'm sure somebody out there can explain why much better than I could. Sometimes, the caps will cause beam to be deflected off the screen, giving the impression that the CRT is bad.
I would also try to avoid the blue ceramic caps, espically in the deflection circuts, since they are known for causing jittering pictures that you can't get to stand still.

ok.


I missed a cap in the corner by the 25l6.

I replaced both .001 with blue ceramic.

the 3 other .005s i havent replaced.

The set now powers on with no vertical sweep. only horizontal.

the .005 caps are in the vertical section, so im guessing those are at fault.

Slowly, but surely, the set is awakening! :)

justradios (or somebody else convenient) really should stock those hv caps. They could just buy a bunch of them from Allied and resell them at a profit. Maybe its just me, but dealing with Allied was a pain in the neck.

Maybe even somebody on the group could buy a bunch of them, then when somebody needs a few they could go to them and get just a small amount, instead of having to go through Allied and their minimum order, which is a real pain if you are only planning to do one set.
You could sell them at a small profit, it would still would be cheaper if you just need a few, and you could save on shipping, probably less than 1.00 for first class, caps don't weigh very much.
Just a thought...:)

you got that right. its 25.00 no matter what.

Actually, I did that the last time I ordered, with the idea of selling some of the surplus.
If I actually got around to recapping all my electrostatic sets I wouldn't have any surplus :p:

However since that's probably not going to happen soon I would be willing to part with 3 of the .0047@6000 for $3 each (what they cost me basically after tax&shipping) and I'll pay the shipping.

I'll throw in a couple .001@10,000v Ceramics that work great for the Horiz circuits.

Eric

actually, 2 of the .005s at 20kv I had are hermetically seald, and I reinstalled those. I now a full raster. its a gray raster with retrace lines.

for some reason, there is no picture, and no audio. (dead audio).

just a full raster. thats it. entire set was recapped. Also, it looks like the vertical linerity is bad.

I have volts going to all my tubes.
my tubes test ok.

also to note, the focus isnt that great, and the contrast control has absolute no effect.

i wonder what the issue is.

I know there is a bad problem somewhere, because when I pull out the 6au6 video amp tube, the raster doesnt change at all, except fade, because the string of tubes are knocked out.


I have a scope, but I have no idea what to look for.

well crap, it had very week sound for a little bit, and I was hooking cable up, I got a station, and then something happened and the sound went back out again.


as always, nothing on the raster.

If anyone would aide me into an area of the circuit thats causing the trouble, let me know.

I tested the audio stage, by injecting a tone into the volume control. it came out loud and clear. Im leaning more and more to the IF stage.

Im not sure exactly how to test the video. I know audio works, by injecting a signal to it, right after the limiter.

anyway, I have a good calibrated oscilliscope, but I have no idea where to probe it, and no idea what waveforms to look for.

Help! Thanks!

BTW, I got the .001s and the .005s replaced. I have full raster. no pic.

UH OH. I ran into a mindboggler, which would cause this trouble.

I was looking at the sams, and it has all the volt measurements from ground point.

Everything looked good, until I ran into the 12AT7 Osc/mixer.

it has a problem. The mixer plate, grid, and cathode volts are fine.

BUT> here comes the problem

the tuner osc plate voltage is normal, the tuner osc grid volts is supposed to be at -3.2vdc, I get about 141vdc.

the cathode is supposed to be 0v (ground somehow). it reads about 130vdc.

Thats my problem I think.

Whats going on here? the resistors in this area test fine.

Im studying the schematic, but Im comming up with nothing.

here comes the funny thing, it doesnt happen till the tubes warm up.

I changed it, and it still does it.

its solid state PS so the voltage immidiately shows at power on.

I pulled the tube, and the volts dissapear off of the cathode pin.

i plug it back in, it begins to climb as the tube warms.

ALAS!! The set has awakened!

I found out that someone in the past has f**ked with it, and the RF Choke to the oscillator for the tuner, has been wired to B+

Well, I took it to ground like it was suppoesed to be, and up popped the picture!! I can now pull in local stations.

Its not fully awakened yet. The sound is still dead, and there is a problem with the vertical.

even though the caps have been replaced, the top half of the vertical is streteched, and the bottom half collapses to a line. You cant see the bottom of the pic. its a line. the top half works.

Any ideas? it looks like bad vertical linerity on steroids.