New Find! motorola VT71m-a

[Steve McVoy]

If you didn't change the scope sensitivity between pictures, then it looks like the amplitude of the second trace is about 1/2 of the first. Check or replace the components I mentioned in my last post. You should have the same amplitude on each grid.

[mbates14]

I didnt do anything to the scope.

I adjusted it according to the first wave, and just removed the wire, and put it on the second one. Didnt touch any controls.

[mbates14]

well, the 100k resistor reads about 114k.

the 220k resistors reads about 264k.

the 100mmf cap is a huge one, and I dunno where to get one like that. its dark in the middle from heat.

[Steve McVoy]

How about the .004? Have you changed it? Any 100 mmf 600 v cap should work for C57. The resistors are close enough.

[mbates14]

the .004 has been changed, and I changed the 220k to be sure, it didnt hardly make a difference, so the only thing left is the heat scorched looking 100mmf. Its a long cap that looks like a resistor.

the cap tests at 170pf on the DMM

MMF means PF I think

where am I going to find a 100pf at 600vdc?

[mbates14]

wait........


you know what, After close inspection to the schematic and the actual point to point wiring, the .1mf between the .05, and the pin 6, or the cathod of the second side. IS MISSING.

its not in the chassis.

I wonder if its supposed to be there, or its just plain missing. That might cause this.

[mbates14]

other words. C-56 isnt in the chassis.

[mbates14]

DOH!

I found it. its between the plate of the OSC tube, and the ground.

sheesh.

[Steve McVoy]

It should be there. Try putting it in. That could solve your problem.

[mbates14]

I said I found it. I found it over at the osc stage.


so, the only thing left is the 100mmf cap.

its an oblong cap that is crusty and almost black in the middle from age and use. its the long cap that sits right under the VO tube socket.

it reads as follows: .170 nf

on the dmm. is it bad? Thats the only thing left. the paperwax caps have been replaced the resistors have been checked and replaced.

[mbates14]

oops forgot picture:

BTW. Disreguard the paper caps. Theyve all been replaced. along with the black ones you see.

[Steve McVoy]

I suspect that the 100 pf is OK, so before you mess with it, try this:

Take out the 1B3 so you have no high voltage. Then you can look at the waveforms directly on the the deflection plates of the CRT. See if they are about the same amplitude. If they are, I'm stumped. Maybe the CRT has something wrong with it.

If they are not the same aplitude, remove the two .005 coupling capacitors from the plates of the 6SL7 and swap them. See if the distortion changes. It is possible that you have a bad capacitor from Allied.

[Eric H]

As I understood it the 6000v caps are NOS but still quite old, Is that correct?

Mike, I will send you a couple of fresh .005@6000v caps to try.

Meanwhile you could do as Steve suggests and switch those caps around and see what happens.

A word of to the wise Mike, if you have to choose between my advice and Steve McVoys go with his

I am just a humble parts changer for the most part while he is the wizard.

Eric

[andy]

...

He's already changed those caps, scroll back up to the post with the photo and read it again.

Hurry up and fix this thing already, this is like watching a soap opera! LOL

Anthony