Re: Type 503 Oscilloscope Issues

ykochcal
 

Well it looks like good progress.

Looking at the schematic with resistor readings things generally look good.
I assume you mean R621 is 94.6 K ohm (missing the K on your sheet)
And I did not know what R624 was pointing to or referenced.

But given that The readings all seem to me to be about what one would expect
with resistors that have parallel resistor networks as shown.

R692 being the exception that looks too high, did you short your probes and
take a reading before so you can subtract the lead resistance? In any case I
It may work with that value, and you have other issues that are not HV
related.

As for the voltages I would say that the regulation is based on the -100V,
but in this system all the voltages are tied to the transformer winding
ratio (except the -85 which is at the gas tube discharge value)

All your voltages are high (in absolute value) except the -100V

So it would seem that you oscillator is running full throttle trying to get
the -100V where it should be and in that process is pushing all the other
supplies too high.

What is V659's resistance from pin 1,5 to ground? I would think it should be
in the 50K range, possible the tube is shorted?

If the 85V is low, you need to check to be sure it only goes to R640.
And look closely that it's not got a visual short to something.

And then take some voltage readings at all nodes at the each ends of R640,
R641, R642 and R644 mark up a schematic and report

That might help pinpoint the issue.

You have turned R641 how does it feel?

John

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Wednesday, May 03, 2017 10:24 PM
To: TekScopes@...
Subject: [TekScopes] Re: Type 503 Oscilloscope Issues

Hello all,


So I just realized that I had two of the wires flipped from the diagram
that Dave had sent. I wired in sequential order, and did not realize that my
wiring to Pins 2 and 3 should be flipped. With that corrected, the trace
does appear, and the scope seems to be running normally (or at least as it
was before). However, running some sine waves through the scope, there
appears to be some distortion of the curves, with a warping of the image
towards the center of the display. This was present the last time the trace
had worked as well. The scope also seems to be running hot (although it may
have always done that).


I have pictures of the resistor readings and the wiring of the new 6.3 VAC
transformer here: https://goo.gl/photos/4vv9VNR68bkhTUdG9
https://goo.gl/photos/4vv9VNR68bkhTUdG9. Please disregard the tape wrapping
of the secondary wire going to the original Pin 6 wire. I only made it white
and orange to remind me that the wire is being elevated to -3000 V (same
color as the scope's HV wires).


From John, I had overlaid the schematic you sent with the resistor readings
from before. I do have the earlier model of the scope, as from previously
noting that I only have two tubes on the 12.6 V circuit (V334 and V434, both
6DJ8's). My serial number is 004418 if that helps (never did post it
before). From the Tektronix Service Scope issues I have from 1962, I know
that this model does not have some of the fixes that those >7000 had gotten
from the factory, but they were nothing that should prevent it from powering
correctly.

As for R659, it gave the same reading as before, at ~77.6 kOhm. The only
resistor that seems to have taken a beating from the arc was R626 (slight
black scorch mark on the top), but it still reads at 40.1 kOhm.


Since the transformer is now wired in correctly, and the resistors do not
seem to be damaged from yesterday's event, I re-did the voltage measurements
(being sure not to short anything this time).


I tried to adjust the -100 V bus via R641, but the largest value I could
get the voltage to was still just -85 V. The voltage using the original R641
setting gave -80 V when the transformer was correctly wired, so it does
influence the voltage.


With the -100 V bus adjusted, I measured the other buses (although I know
they will be skewed because the -100 V is still not correct).
+250V measured at 305 V
+100V measured at 110 V
+12.6V measured at ~18 V
+500V measured at 470 V
+85 gave the same issue as before (~8.5 V, and no V692 turning on).


As Dave inquired, I am not actually using the analog voltmeter for these
readings. I am currently using my only "good" DMM (the one I used to
calibrate the analog meter), due to misplacing one of the analog meter's
probes. Since it uses non-jacketed banana leads, I am not able to use the
probes from any of my other meters, so I just have to base everything on the
DMM for right now.


- Evan











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Posted by: enchanter464@...
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