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Thanks for your suggestions. I did the oscillator part first.
1. reducing the -5500 cathode voltage to -4400 V has no effect. Oscillation still stops at about 2 1/2 minutes
2. +120V and +30V (derived from the secondary of T710) are all good. I lifted R596 to reduce the 120V load but see no effect.

As I mentioned in my reply to Chuck, disconnecting the CRT has a very positive effect and oscillation doesn't stop anymore. I'm going to look at the DC restorer circuit with the CRT disconnected next.

Question though. You mention that CR769 doesn't clamp if the voltage never goes below 10V. I understand. But the voltage at R768 (on the side of C768) definitely goes below 10V. So something on the other side of R768 (which is the nexus of CR769, CR776 and R777) prevents that signal from going below 60V, correct?
Could this be related to the voltage shift at C768? C768 is pure AC centered around 0 when R768 is lifted but biased to 100V when R768 is connected?

JdR

Hi Chuck,
I've followed your discussions regarding the use of steppers for universal windings and have at least
a superficial understanding of the torque "jerk" that's troublesome. It has occurred to me that using a
(modulated) eddy current brake might be useful for smoothing the piecewise motion of the offending
stepper(s).

I also have a hunch that a little time hunting through the USPTO might turn up something.

Just my $0.02 of speculation ... I'm not a motion control guy, nor do I play one on TV..
-Roger

I have a 454 that will not trigger on A or B timebase. I bought knowing of
the triggering problem. I have scoped the signal and it will not go through
T670. Overhauling the unit did not help. The power supply voltages are
right on and the small 1mfd decoupling condensers are now film types. The
voltages at points BE, TP646 and CB are what the manual says by the
adjustments. The 8393s tested very good to new. I pulled out known working
tunnel diodes from another unit that works. That did not work. Swapping the
transistors from the known working one to this one did not work. Slight
adjustments of the pots to the three above points did not work. I tested
the diodes for leakage. They are fine. This looks like something common to
both like the power supply or trigger preamp. Those are working as they
should. The power supply outputs are within mV of the rated voltages. Using
the EXT. trigger input did not work. I checked the connectors on the boards
for corrosion and found none. I did tighten the female ends before
reseating them back on the pins. The unit is clean so dirt should not be a
problem. I know the A SWEEP TRIG'D light is good. I ohmed it out. I cleaned
the transistor sockets and moved the transistors up and down a few times to
make sure they were making contact. I resoldered connections in case one or
more were bad. Nothing I have done will make it trigger. I have worked on
this for a few days and cannot find the problem(s). Does anyone here know
of something I missed or what? I waited to ask here until I did what I
could. At this point, I need help. Thanks.

Mark

@zenith5106 Thanks a lot!

Unfortunately, I do not see the information on how to calibrate R12 :-(
It looks like every manual promises the information will be in another manual. It is highly frustrating.

The P6042 service manual says "...adjustment procedure for R12 supplied with T1. Order Tektronix Part No. 050-0439-00"

The 050-0439-01 instructions you so kindly provided say: "If R11 was installed...adjust R12... to the CLOCKWISE extreme" and "If R13 was installed...adjust R12... to the COUNTERCLOCKWISE extreme", followed by "Refer to Manual Insert for recalibration"

And the manual insert (that you also kindly provided) totally ignores the issue :-( :-(

I can hardly believe that fully clockwise or fully counterclockwise is the final position of the adjustment. If that was the case why use a variable resistor?

Steve,

Some time ago I got hold of a 520 digital scope. To my surprise it passed all self tests and looked clean and pristine inside, so I decided to sell it as working OK, with a lot of pics including of the selftest.

The buyer got the item, said its OK but didn't pass selftest and asked for a partial refund. I knew the unit was very well packed, so I became suspicious. I refused and the seller did not really argue about.

As I did not offer to take back in first place, and didn't use paypal or anything similar, Ebay could not interfere. They're trying to change things in Germany these days, so they will be always in the loop. If this suceeds I might not offer anything else than local pickup in future.

cheers
Martin

---
Put it all together and stuck it in the 3S2. Of course the triggering is still
screwed up in the 3T77A so I will eventually get back to work on that. In the
meantime, the manual does say that a non-sampling timebase can be used at .1
ms/div or slower, so I flipped the internal slide switch on the 3S2 and stuck
a 3B3 in the right-hand slot.

The S-1 head will indeed display the output of my 114 pulse gen (approx 17 ns
risetime) through a 20 db pad. I stuck a tee at the input and ran a cable to
my bench scope to verify the signal and it's correct. But the 561A/3S2/S-1 is
showing a VERY slow rise and fall time, with about 8 or 9 dots at the edges,
which is an indicated time of close to 100 us, not 17 ns! Something ain't
right but I am not sure what it is. The manual says I should be able to use
the 3S2 with full bandwidth of the sampling head, even though I can't display
anything really fast.

I'll try the file one more time: /g/TekScopes/files/Tek2465A_CT_RAM_022521.pdf

Hopefully this works...