Re: Type 503 Oscilloscope Issues

Dave Casey

Sounds good. There may actually be a problem with D672, and you can do some
easy checks to further verify that. The +12.6V supply is used for the
heaters of the input amplifiers (the first amplification stage your input
signal goes through). While the other tubes are heated with AC from a
transformer winding, these are heated with DC for the sake of reduced
noise. Your scope has two possible configurations:
1. The earlier style where the input tubes are V334 and V434, (two 6DJ8s).
2. The later style where the input tubes are V334, V434, V344, and V444
(four 8393s). The 8393 is a late-to-the-party kind of tube called a
Nuvistor, and it's in a small metal can instead of a big glass bulb.

In either case, the +12.6V supply should still be feeding the heaters of
these tubes.

You might re-check D672 with all two (or four, depending on the above)
tubes removed from their sockets and see if it looks more like the others
than it did in your previous round of measurements.

With those tubes removed, you have also broken the return current path for
the +12.6V supply. This means you can also test the D672 using the ohmmeter
function of your multimeter without having to unsolder it. To do this, with
those particular tubes we're talking about removed, measure the resistance
across D672, then reverse your test leads and repeat the measurement. You
should get two very different measurements. When the positive lead of your
meter is on the "pointy" side of the diode's arrow, you should see an open
circuit (greater than 20M ohm). If you do not, then either the diode is not
a diode anymore, or one of the filtering capacitors after the diode (C652c
and C654c) is bad. Because of the capacitors, you may initially see a very
low resistance measurement that steadily increases until it exceeds the
range of your meter. This is normal and is just your meter charging up
those capacitors.

There is only one other place I see the 12.6V rail being used, and that is
to bias the heaters of the other tubes (also a noise reduction technique).
This is on the power supply schematic next to T601 pins 10 and 11. R605 is
going to do a pretty good job of not letting a fault in that part of the
scope overload the 12.6V supply, and you can get a fairly decent
measurement of R605 in circuit (if anything, it has probably drifted higher
than 47k, which shouldn't hurt anything).

With the test leads reversed (positive lead of the meter on the T620
transformer side of the diode), you should see some finite resistance, even
as high as tens of kilo-ohms (depends on the diode and the meter).

Obviously all this is still to be done with the scope unplugged. (Yes, roll
your eyes, but I'm going to remind you every time. And you should hold the
loose plug in your hand every time before you reach into that chassis; it's
a good habit that can save you from making a careless mistake.) It is also
a good idea to keep track of which tube came from which socket and put them
back like you found them. Mixing them up will affect the calibration of the
instrument (or what's left of it after all these years sitting). If you mix
up the tubes you may or may not be able to tell the difference when the
scope is working again, but it shouldn't make or break operation of the
scope (unless one of them is bad).

While they're removed, it's a good time to check each of those two (or
four) tubes for a shorted filament (heater). A shorted filament would cause
an increased load on the 12.6V supply, possibly causing all the other power
supply problems you're seeing, including the blown fuse.

Regarding GFCI - you can rather inexpensively get an add on GFCI that plugs
into the wall and provides a protected outlet. They are sold at home
improvement/hardware stores. At this point, the power switch wiring is no
longer much of a suspect since that wiring didn't actually burn, but if it
were a problem, a GFCI would trip right away and let you know.

Dave Casey

On Thu, Apr 6, 2017 at 2:58 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

1. For the diodes, those values are from my multimeter in diode testing
mode, but it does not display any units associated with the reading (so I
am assuming it is in volts).

4. C614 gives a resistance of 3.2 ohm, so it would seem that it checks

I also checked the burnt black stuff on the wires, which did peal off with
some isopropanol. It appears to be just melted plastic, and it is not from
the yellow wire (since that was clean underneath). As such, the burning
smell may have just been old plastic and dust from various loose components
that were not cleaned off initially (for example, the rubber casing around
the CRT connector has basically dried and disintegrated at this point).

As for the -3000 V supply to pin 5 on T601, it gave a resistance of ~4.40
M-Ohm (my multimeter reads up to 20 M-Ohm, so it should be accurate).

With the video, the only thing with V659 was that it was flickering on and
off (which is accurate in the video no matter the angle), although it is
difficult to make out the behavior of V620. Again, the issue that I
observed with that was just the blue electron streams around the sides of
the wall, and then the redness at the center of the metal plate in the

I won't be doing any voltage testing until the new fuses come in, and they
are the correct type (1.25 A, Slo-Blo), so nothing to worry about with the
dangers of higher rated fuses. I do not have any AFCI or GFCI outlets
available unfortunately, so I will just be very careful in the live testing
once I get to that point. However, I will be sure to check what I plan on
doing here before proceeding, since I definitely do not want to fry
anything (or myself for that matter).

- Evan

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