Re: Type 503 Oscilloscope Issues

Dave Casey

Evan -

1. When you test a diode, you're trying to verify that it only passes
current in one direction, which means you have to test each diode twice.
Diodes can fail open, where they pass no current, or shorted, where they
pass current in both directions. This is often difficult to discern while
the diodes are in circuit, as there are other paths for the test current to
travel. The measurements you have taken don't tell you much since you've
made them in circuit. It is no surprise that the 12.6V diode, D672 measures
differently as there is a very direct loop through the filaments of V334
and V434 to ground and back through T620. Your readings are also ambiguous
without units attached. Are you just putting the meter in diode test mode
and telling us the number it shows? Is that kOhms? We don't know.

2. R692 is going to measure much lower than 4.3 ohms in circuit because
there is a parallel path for the current through T620 and V692 (that
horizontal line on the schematic right below R692).

3. R659 can be located by tracing back from V659 and forward from L654. It
gets trickier thanks to the wiring harness; I'll admit I haven't found mine

4. The burned wires in your picture are the primary (mains) side of T601.
The more burned of the two is the Line/Hot from the fuse to an extra tab on
T601 where the wiring changes to the shielded wire running to the power
switch. You might check to verify that C614 (the ceramic disc capacitor
hanging off the bottom of T601) has not shorted. You should see a
resistance of about 3 ohms across C614 while in circuit (scope unplugged,
power switch off, fuse or no fuse). If C614 has failed, it is best replaced
with a modern "X" rated, UL listed capacitor of the same value. If it
hasn't, you have other problems either in or beyond T601. (Has anyone here
who's familiar with the elevated filament short seen it cross to the
primary winding?)

You should repair the burnt wiring before attempting to power the scope
again. The wiring to the power switch (the red wire connected to the burned
wire) and the wiring back from the power switch (the smaller white wire
that runs through the harness with the red wire and is connected to T601
3/4) should be checked for damage also. If the fuse keeps blowing, you
should NOT use a higher rated fuse in an attempt to get more time to test
things - you will only cause more damage. The power switch wiring might be
best tested by using a megger since it is shielded along its path through
the scope.

Have any of your attempts to power the scope been through a AFCI or GFCI
outlet? Did the protection ever trip?

Please be careful and talk through your next steps with us before you do
any more powered testing (you can safely measure the resistance across C614
anytime you like as long as the scope is unplugged). You might also tell us
what resistance you measure between the -3000V supply and the chassis (T601
terminal 5 is a good spot), which should be safe to measure so long as the
scope has been off for awhile (it *should* be safe within a second or two);
resistance should be a few megohms, possibly above the range of your meter
in which case the exact value is not critical. The value is determined by
the resistors along the left hand side of the CRT schematic (R840, 841,
842, 844, 845, 847, 849, 851, 852). If the measurement is lower than a few
megohms, then that filament winding in T601 that we've been worrying about
might be to blame. This is another place where a megger might tell you
something useful.

Don't be too proud to ask questions about anything I've said that you don't
understand; if I'm wrong about anything, the other list members will be
quick to call me on it. We don't want you to learn any of this the hard
way, because you may not survive the lesson.

Dave Casey

On Thu, Apr 6, 2017 at 12:02 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

Hello all,

I feel bad that it has been several days since my last posting.
Unfortunately, my work is at a standstill right now. Over the past weekend
I had focused most of my work on restoring a vintage volt meter (Triplett
630-A), which is now working (and supports up to 6000 V, although I lack
the HV leads, so no -3000 V testing yet). As such, I only now have gotten
time to focus on the scope.

However, when I connected the leads to the ground (chassis) and -100 V
test point, plugged in the scope, and after the brief burning smell,
nothing happened (no lights for the scope, no CRT turning on, no tubes
activating). I discovered that the fuse had blown during my attempt to get
the voltages, but after no luck in going through my fuse stock, I ended up
having to order a set of new ones to come in, so that won't be until next
week earliest.

For the scope, I was still able to check the non-voltage components on the
power circuit. The diodes still check out, with D652 (and others) giving
readings of ~0.65-0.65 on my multimeter. However, only diode D672 gave a
reading of 0.041 on two different multimeters, so I suspect this one is
bad. This one is connected to the +12.6 V circuit.

As for resistors, some also seem to have issues, with R692 (going to V692)
being at 1.4-1.5 ohm, but it is supposed to be 4.3 ohm. I admit I am having
a bit of difficulty in locating R659, so I was unable to confirm if that
one is okay.

Once the fuses come in, I'll let you know how the voltages read out.
However, as for the burning, think it may be coming from the wires leading
to the T601 (see the photo:

- Evan

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