New Find! motorola VT71m-a

[mbates14]

one of the main filter caps checks for leakage.

All of the other caps test fine. no shorts.

[wa2ise]

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.

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.

[mbates14]

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.

[Tubejunke]

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.....

[mbates14]

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.

[mbates14]

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.

[Steve K]

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

[mbates14]

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.

[mbates14]

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.

[mbates14]

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.

[mbates14]

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.

[maxm]

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.

[mbates14]

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.

[Charlie]

Slowly, but surely, the set is awakening!

[bgadow]

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.