1965 Motorola Black and white TV, curbside find

[MRX37]

Better top chassis shot, if it helps:
http://i.imgur.com/zm3r8Pd.jpg

[rca2000]

Looking at the chassis tube layout...I see where you need to check for B+. You can either reach it on TOP of the chassis or UNDER It.

Next to the H-put tube...is a terminal strip. The THIRD terminal post is B+. CHeck this to GROUND...and see what it reads. you can ALSO measure this under the chassis, the third termnal is under that black capacitor.

I see what look like a diode under the chassis. It is under that red cap...above the brown tubualr cap--which is likely one of the doubler caps....ASSUMING...this set uses a doubler. The other doubler cap is likely part of the can cap on the right of the chassis.


IF this is a doubler power supply...you should have at least 275-285 volts B+ If NOT a doubler....only 140-150 volts.

[rca2000]

And as others have said....do NOT measure the "Boot' or Boost voltage...UNLESS you have a meter than can read 1KV or so....it could eb that high. Measure ONLY the b+ right now..I suspect it is LOW....from your symptoms..

[MRX37]

B+ voltage starts off at 310 volts, then drops to ~265 on warm up. Seems like it's in range according to DieselJeep's post.

[rca2000]

That sounds fine. NOW...you need to see if HV is up to snuff... For that you will need a HV probe.... a set like this should be producing at least 15-20KV... NO lower than 15KV..

[MRX37]

That's where I hit a wall... HV probes aren't cheap, and I'm flat broke.

[Electronic M]

Originally Posted by MRX37

That's where I hit a wall... HV probes aren't cheap, and I'm flat broke.

Hit up a repair shop or ebay for a cheap used one.

[MRX37]

Well for now lets assume there's little to no HV. I think the image blooming out before it totally failed is enough evidence to suggest that

Lets also assume that the HV rectifier tube is okay, seeing as those should gradually fail and the last time I ran the set, the image was clear and bright.

And lets assume...hope to god... that the flyback didn't decide to up and quit on me.

What else could be causing this lack of picture issue?

[Electronic M]

The HV rect. could be the problem......It could be that the previous issues were a weak HV rect, and the filament opened since then killing the tube. A good way to tell in absence of a tube tester is to ground the blade of a flat edge screw driver, with the set off lift the top cap on the rect enough that some of the metal is exposed, turn the set on and try to draw an arc/corona off the HV rect top cap. If all is well with the fly etc. you should be able to draw a 1.5" or longer arc from the HV rect. top cap. On some of the rubber tire fly roundys one can draw 1/2" of spark off the fly tire.

[MRX37]

Not enough room in that flyback cage to get a blade under that top cap. Also I wouldn't trust myself to attempt this as I'd probably jump at the arc and the screwdriver might go somewhere it shouldn't.

I think it's worth locating a replacement tube to try, because I'm getting the feeling if it's not that, then it's probably the flyback... in which case might as well junk the set.

Could I visually inspect the rectifier tube and see if I can spot an open filament?

[Olorin67]

if the filament is intact, you will see continuity across those 2 pins on the tube. What tube does the set use?

[Electronic M]

A continuity check at the heater pins of the unplugged tube should tell you a good bit about it's health, now that I think about it.

It is easy for a set with a good fly not to produce HV.....It could be caused by a large number of things such as the H osc. not working to spec., the H output tube being weak or miss-biased(any element too far off could kill HV and or cause redplating), horizontal stage fuse being blown, yoke bad or unplugged, HV rect tube bad, or it's heater winding shorted to ground, bad damper tube, etc., etc, etc.....

Don't give up. Get a schematic for it if you don't have one, and check measurable listed voltages in the horizontal section of that schematic and look for discrepancies.

You should put a tube tester on your list of equipment to get. They won't find all problems in horizontal stage tubes, but they will keep you from putting tubes in your set that are DEFINITELY not going to work, and you can never be sure a NOS tube is good without testing it.

[old_coot88]

Originally Posted by Electronic M

...A good way to tell in absence of a tube tester is to

GROUND THE BLADE

of a flat edge screw driver, with the set off lift the top cap on the rect enough that some of the metal is exposed, turn the set on and try to draw an arc/corona off the HV rect top cap.

O jeezus. Not this old chestnut again. Yikes. There is no need to ground anything. If the fly is working OK, the screwdriver will draw a healthy 'air arc' a half-inch or so long. (Of course the screwdriver needs to have a well insulated handle to hold it by.)

Striking a direct-to-ground arc off the HV secondary is compounded by the fact that the winding is forced to carry B+ current plus the excessively high RF component. Please don't tell noobies to do this.

[MRX37]

The HV rectifier tube is a 3AT2. let me know what pins to test and I'll pull it and see if there's continuity

[Olorin67]

heater is pins 12 and 1, each side of the heater is connected to multiple pins
http://www.mif.pg.gda.pl/homepages/f...23/3/3AT2B.pdf