Motorola 21BT130BE-1 troubleshooting: Brightness Debacle!!! - Videokarma.org TV - Video

cluelessgame · #1 03-12-2014, 08:47 PM

*I hope this is the correct forum for this thread*

I am going to try and restore this gem over the break and had a few preliminary questions before I dove in. Being a newer (mid-to-late 60's) set, I've decided for this set to only replace-as-needed...I know that sounds a bit lazy, and maybe later I'll go back and do a full resto, but at least for now I just want to get it stable.

This set powers up fine and reception, picture quality/geometry, contrast and brightness all seem fine - initially. The issue with this set that after about 30 minutes or so, the screen seems to get dimmer and dimmer, and bright objects blank out everything right of it. After about 45 minutes, the whole right half of the screen is black. My guess is that there's something in this set that doesn't like heat, but I don't know where to begin looking. I've attached the PhotoFact for this set if it helps.

Also, when powering down the set, the rasters collapse to a single, sharp, VERY BRIGHT dot in the center of the screen that lingers for several minutes. Is this a normal thing? I'm worried that this could potentially burn a dot onto the phosphor. Your thoughts?

Zenith26kc20 · #2 03-12-2014, 09:34 PM

crazy to say it, check the horizontal blanking diode.

cluelessgame · #3 03-12-2014, 09:52 PM

Quote:

Originally Posted by Zenith26kc20

crazy to say it, check the horizontal blanking diode.

will do, just curious tho, what makes you think that?

Electronic M · #4 03-13-2014, 01:01 AM

How do the tubes in the horizontal test? Some sets will do stuff like that if the HV rect, HO tube, damper, H osc. are not fairly strong (good means nothing in some sets, but the upper end of good usually means strong enough).

If you have a way to check HV, I'd monitor that.

As a rule of thumb replace ALL white tubular ceramic caps, as well as plastic cased types like bumble bees and black beauties. Also if your going to stick with cherry picking bad parts to replace, want it to be somewhat reliable, and fixed as best it can, then I recommend getting a tuning eye type capacitor tester (ESR or other solid state testers won't test at full rated voltage which is important to do) and checking all caps that are not rated in pF (mmF) including the lytics.

Zenith26kc20 · #5 03-13-2014, 08:27 AM

From a quick look at the schematic, that diode goes to the vertical circuit and possibly bleeds the screen voltage off. The bright dot on turn off may indicate the diode is open and the boost cap stays up awhile. I worked on many of those sets years ago but never paid attention to the diode because it never failed. About 50 years may have taken a toll on it. I would love to scope the video when that set is cutting up.
When I worked at my first TV repair job (long(!) ago) we took one of those Motorla sets and lengthened the cord, glued a rabbit ear antenna to it and put it on the roof! It would come on with the bench and we could hear it playing. In the evening we would look to see if it still had a picture. It stayed there for months, still working unless it rained, then we didn't turn it on. One day we came to work, opened the back door and it was gone. The thief did leave a rather nice extension ladder for us. It was replaced by a Silvertone that lasted about a week. The Motorola was nicknamed "the satellite!"

DaveWM · #6 03-13-2014, 08:40 AM

Quote:

Originally Posted by cluelessgame

will do, just curious tho, what makes you think that?

Second that, losing half the screen horz would tend to indicate a problem that is blanking at the horz rate. Most BW sets do not even have horz blanking (well older sets anyway) the just rely on the sync pulse to cut off the CRT. this is where a scope would come in handy as you could examine the signals at the CRT to see where the defect is. I am sure its the pin 3.

would like to add that I would replace all the caps in the boost filter supply, and also check resistors and replace caps in the horz blanking.

As too why the spot, that's a tough one. fix the blanking then try running it for an extended time (like 10 min) see if turning it off then still gives the dot.

I have a set that if I turn it on for just a short time (like right after the raster comes up) then turn off the spot is there, very bright very long. But if I watch for any length of time its not a issue.

I recall reading somewhere that a weak tube can do this as well but not sure about that, I do have a tube (27ZP4) that did this. it was weak takes a few min to come up to decent brightness. The spot bothered me on that one so much I made up a little cap/diode/bleeder resistor in the brightness circuit that would cut off the tube by discharging the cap into the cathode with a pos voltage when the set was turned off (think that was it, its been a while).

DavGoodlin · #7 03-13-2014, 12:27 PM

My "first tv" is about 2 years older with all but 3 of the same tubes.

It always had the bright dot after turning it off, still does. Maybe I need to look at that.

I used to tell my little brother it was the martians spying on us.

bgadow · #8 03-13-2014, 09:11 PM

Quote:

Originally Posted by Zenith26kc20

From a quick look at the schematic, that diode goes to the vertical circuit and possibly bleeds the screen voltage off. The bright dot on turn off may indicate the diode is open and the boost cap stays up awhile. I worked on many of those sets years ago but never paid attention to the diode because it never failed. About 50 years may have taken a toll on it. I would love to scope the video when that set is cutting up.
When I worked at my first TV repair job (long(!) ago) we took one of those Motorla sets and lengthened the cord, glued a rabbit ear antenna to it and put it on the roof! It would come on with the bench and we could hear it playing. In the evening we would look to see if it still had a picture. It stayed there for months, still working unless it rained, then we didn't turn it on. One day we came to work, opened the back door and it was gone. The thief did leave a rather nice extension ladder for us. It was replaced by a Silvertone that lasted about a week. The Motorola was nicknamed "the satellite!"

I've seen an article, I think it was in the "Servicer" that came with Sams Photofacts, about a TV shop somewhere that mounted a CRT on their roof with l-o-n-g extension cables as a gimmick...I seem to recall it also being a Motorola.

Zenith26kc20 · #9 03-14-2014, 08:21 AM

Since we had a Motorola distributor in the city, we sold a lot of Motorola. I always liked that chassis. Mostly bad electrolytics and vertical output tubes. It wasn't as hard to repair as the Admirals that also sold well here. Does yours have the lighted dial numbers? Not all of them did.

DavGoodlin · #10 03-15-2014, 12:23 PM

Does yours have that tall, square plug-in integrator K5 next to the 6GK6 vertical output tube also?

dieseljeep · #11 03-16-2014, 11:10 AM

Quote:

Originally Posted by DavGoodlin

Does yours have that tall, square plug-in integrator K5 next to the 6GK6 vertical output tube also?

The earlier models had that. Motorola made a replacement for it, that was a cluster of caps and resistors.
The next year model, after this was a hybrid. The model built next was all solid state.