slyvania roundie

[DaveWM]

I did fool with the AGC seems to be working (goes from washed out to over contrast, but never breaks sync, but then again the box may not be putting out a strong enough signal to make that happen).

Good new!! after yet another tube swap on the 6GF7, I get a good clean sweep. I can adj the vert hold for an easy to obtain non paired up screen.

It pays to have a LOT of NOS tubes. While I have a good Mu tube tester I dont know how sensitive the grid emisson test is (its a mercury 1000).

I am calling this one fixed, at least that part.

One last little issue, seems like I often have to change the channel back and forth when the set is 1st turned on or even if I just switch signal source.

The screen will black out, sound is still there a bit weaker but there. one flip back and forth and all is good.

[earlyfilm]

Originally Posted by DaveWM

One last little issue, seems like I often have to change the channel back and forth when the set is 1st turned on or even if I just switch signal source.

The screen will black out, sound is still there a bit weaker but there. one flip back and forth and all is good.

Try this theory on for size:

Horiz drops sync and the keying pulse hits the signal in the wrong place and this confuses the AGC.

The noise from channel flip wakes up the AGC.

This could be and probably is related to the problem with line pairing and signal level shifts.

Recheck resistor values, especially the high value ones in the AGC and any caps for leakage, where a signal from a higher voltage source is fed into the AGC (or to sync circuits.)

(Warning: With that said, I am not familiar with this model set and have not seen a schematic.)

James

[DaveWM]

James,

for fun I am going to put a bias box on AGC, try clamping it and see if that does anything.

I will check the values as you described. I "think" the fix of the 3rd IF tube is working now, but I have to try it a few more times to be sure.

The agc was acting 'funny' as I was not able to distort the pic or break sync when set to max CW, the new 3rd IF seems to have fixed that.

Oh an I did not do one of the things I promised myself a while ago, always scope the AGC voltage. Look for horz hash or any thing other than a nice DC voltage.

Meters dont catch this so a scope is a must. So will be scoping the AGC voltage as well.

[DaveWM]

scoped the TP for agc, def some horz noise on it. there is a .1 cap from AGC to ground that I assume is there to filter it. Looks like I will end up pulling the chassis again, I dont recal checking that cap. Could be some stray horz getting into the wiring from the sweep board to the IF board. With out pulling a working set out I dont know how much is acceptable.

[DaveWM]

ok both the good set and the bad set have a horz pulse in the AGC BUT on the bad set there is a very strong vert pulse as well, I dont know why I did not pick up on this before.

I am going to check over the vert bypass cap again.

[DaveWM]

this this is really kicking my rear. The AGC just does not seem to work right, full CCW just dims the pic some and full CW just a bit more contrast. I am used to going from full snow to overloaded pic (dark screen loss of sync).

AND its back to its old tricks of having to turn the channel to get the pic to come in (the AGC voltage skyrockets, over 10v+ flipping the channel returns pic and drops down AGC to near zero.