New Find! motorola VT71m-a

[maxm]

Originally posted by 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.

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...

[mbates14]

you got that right. its 25.00 no matter what.

[Eric H]

Originally posted by maxm
Maybe even somebody on the group could buy a bunch of them,

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

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

[mbates14]

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.

[mbates14]

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.

[mbates14]

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.

[mbates14]

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.

[mbates14]

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.

[mbates14]

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.

[mbates14]

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.

[Charlie]

Originally posted by mbates14

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

Did you try adjusting the verticle height/linerity controls? I had a set once that I thought had a faulty part in the vert circuits, and after going thru everything, it turned out that the controls were simply turned all the way to one side. I felt pretty stupid after making that discovery, but I bet it happens often.

[mbates14]

it doesnt have a vertical linerity control, at least I cant spot it.

Also, the sync/clipper circuits arnt functioning either.

I can lock the vertical and horizontal to one scene, but if the scene changes, or the brightness of teh picture changes in the scene or if the picture starts to move like in a commercial, it will loose lock. horiz lock is worse than the vert.

I think that clipper circuit failed, because I pulled out the clipper tube, before the string of tubes started to fade out (wired in series). no change in picture. The sync circuit has problems. and the sound is absent.

[mbates14]

I know ive kinda been running on and on and on with this thread, and it is probably misleading.


I am sorta treating it like my blog. But, go ahead and ignore everything above this point:

The set is in now working order, the sync is locked, the sound is preasent, and the picture is on the CRT.

The problem I cannot seem to be able to figure out is the focus is sorta bad, and the linerity is very very bad.

The tuner happened to be on CSPAN lastnight, and dick cheney had a very bad case of the coneheads.

Also, I checked the (2) 4.7megohm resistors between the two .005s at the vertical output plates, they check fine, but I replaced them anyway. The two 1megohm resistors off of the focus pot test within tolerance.

The 2.2meg series resistors on the focus pot test fine also.

I dunno what the deal is.

I checked the (4) 10megohm resistors around the vertical output section, they all test perfect.

I dunno what to do now. Its weird. Unless one of my HV .005s are already bad. I dunno. I can center the picture. but the linerity is horrible, and the focus is somewhat bad.

Help. I may need a new batch of .005s

[Steve K]

On those electrostatic sets I often replace a lot of the resistors over 1 meg especially in the sweep and HV circuits. Your vertical problems could be caused by one of the mmf caps being bad. Also, recheck the caps that you replaced, it is not hard to put in a .001 when it should be a .01 or something like that.

Steve

[mbates14]

I have no idea how to test an MMF cap.

I have a Capacitor checker on my DMM. thats it.