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516 Triggering not working


Mark Vincent
 

I have been working on the circuit for a week after the 27mmfd to the Sweep
Gating Multivibrator and Hold-Off Circuit with no success. All I get now is
a spot, no horiz. line. The part before the 27mmfd condenser, C44, works
and was adjusted so signals will trigger on the plate side. Lifting the
"hot" side or a signal into the Ext. Trig. post will not start the
sweeping. There is zero output unless R114, R184 and R185 are unsoldered.
Using the cal. voltage output of another scope up to 100V made no
difference. Output was zero with the resistors soldered in. This is with
the scope on or off. Touching any of these resistors to its respective
point with them unsoldered will make the signal go to zero. The bias of
V135A and V185A is deep enough to cut these sections off. The valves are in
very good to like new condition and the resistors are 1% types. The power
supply voltages are right. The unit has been overhauled. This problem
suddenly came up. The diode across the 4700 ohm resistor is good and
another one did not make any difference. The hold-off caps are Orange
Drops. The timing cap is the original type. I did a resistance A-B with my
working 317. The resistances were the same. Reference was to the chassis.
Putting a signal in a vertical or horizontal input causes the spot to be
two spots with a thin dim trace in between them. I cannot figure out what
happened to cause the loss of triggering. I know this circuit is used in
other 300 and 500 series. Thanks.

Mark


John Williams
 

Hi Mark. Sounds familiar. I have been fooled many times by the famous Tektronix sweep circuit. So if I sound like I am a bit dense please forgive. First, are you sure the sweep circuit is what is dead? If you check with your scope on either side of R330 / C330 you would normally see the sawtooth there if the sweep is running. This would be dead here if the sweep is dead. There should be sawtooth here if the sweep is running. If dead, you can check to see what state the sweep generator is stuck in. It can be run-up or not run-up. The procedure for checking this is on page 5-8 / 5-9 of your instruction manual, troubleshooting section. Once you have determined this the text should help guide you through further testing. Good hunting.


John Williams
 

Sorry Mark I forgot to ask if you have checked all of the power supply voltages. The sweep is especially sensitive to these voltages being incorrect.


Mark Vincent
 

John,

Looking at page 5-8 indicates mine is with the high voltage at the plate of the Miller tube V161A. Grounding the grid did make the difference in the paragraph; stuck in run up position. Zero sweep signal at R330. The only way a horizontal line can be generated is by putting the scope in the ext. horiz. mode and feeding a signal to the ext. horiz. post. Any signal stops on the right side of C44 if the scope is on or off. Varying the pots in the triggering circuit and varying the B+ did nothing. I know the triggering can be touchy. The B+ voltages are spot on.

Mark


John Williams
 

Mark the first thing I always suspect is causing the problem is a tube. Rightly or wrongly. But is the easiest check to make. I turn out the lights (keep flashlight in hand) and look for a tube that is dark. Take each tube out of its socket and spray contact cleaner in the socket. I even clean the tube pins gently with a piece of sandpaper. I spent a week on a 541A that had the same symptoms as your scope. I finally resorted to wiggling each tube in the trigger circuit and I’ll be darned the thing stated to work. I also try cleaning the pots and switches. I know you no doubt have thought of a lot of this. It just takes time and patience. More of the old scopes have been scrapped for sweep problems than anything else, so please don’t give up.


Tom Lee
 

Hi Mark,

Of the things that tend to be overlooked when debugging VT circuits, the level-shifting neon bulb surely ranks high (here, that's B167). If the bulb no longer fires, the loop breaks and the sweep stops sweeping. It's easy to check, so be sure to include that among the things to look at, if you haven't already. And while you're in that neighborhood, check R167. If it's a carbon comp (and I think it is), it could have drifted upward in value to the point where it's a near open. That would have the same effect as a bad B167.

--Cheers,
Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/14/2020 14:34, Mark Vincent wrote:
I have been working on the circuit for a week after the 27mmfd to the Sweep
Gating Multivibrator and Hold-Off Circuit with no success. All I get now is
a spot, no horiz. line. The part before the 27mmfd condenser, C44, works
and was adjusted so signals will trigger on the plate side. Lifting the
"hot" side or a signal into the Ext. Trig. post will not start the
sweeping. There is zero output unless R114, R184 and R185 are unsoldered.
Using the cal. voltage output of another scope up to 100V made no
difference. Output was zero with the resistors soldered in. This is with
the scope on or off. Touching any of these resistors to its respective
point with them unsoldered will make the signal go to zero. The bias of
V135A and V185A is deep enough to cut these sections off. The valves are in
very good to like new condition and the resistors are 1% types. The power
supply voltages are right. The unit has been overhauled. This problem
suddenly came up. The diode across the 4700 ohm resistor is good and
another one did not make any difference. The hold-off caps are Orange
Drops. The timing cap is the original type. I did a resistance A-B with my
working 317. The resistances were the same. Reference was to the chassis.
Putting a signal in a vertical or horizontal input causes the spot to be
two spots with a thin dim trace in between them. I cannot figure out what
happened to cause the loss of triggering. I know this circuit is used in
other 300 and 500 series. Thanks.

Mark