Re: Type 503 Oscilloscope Issues

Dave Casey

I would recommend performing the tests Mike has recommended before you
bother with the things I suggested. This seems more plausible for why there
would be arcing in V692 - that light show in there is definitely not normal.

Dave Casey

On Thu, Apr 13, 2017 at 8:15 PM, Mike @mdinolfo [TekScopes] <
TekScopes@...> wrote:


Thanks for reporting your measurements. I am impressed- you did a good
job of thinking through the measurement process and accurately
documenting what you found, and what you did.

The ~560-570 volts that you measured on the (nominal) 500 volt bus with
V620 and V623 removed looks good to me- it is the same as what I
measured with my RM504 with these tubes removed. With these tubes
removed, the bus voltage goes considerably higher than the nominal
loaded bus voltage.

I suggest that you perform the following test next:

This test does unfortunately involve a need to unsolder a few
connections; specifically, the CRT filament winding power leads at T601.
I recommend that you disconnect these power leads at this location,
rather than other possible locations in the scope (and in particular,
avoid messing around with any of the soldered connections at ceramic
terminal strips). The terminals are numbers 11 and 12 at T601,
according to the 503 manual which I have. Make sure to properly
insulate the wires that are disconnected from T601 as they will be
energized at -3000 V for the following test. I suggest that you apply
electrical tape to each of the two wires individually, and locate them
away from the chassis. Then power up the scope, with all tubes in
place, and measure the -100 V bus level.

Note that with this winding at T601 disconnected, there will be no power
to the CRT filament, and therefore there will be no trace that will
display on the face of the CRT.

If the measured voltage at the -100 V bus continues to fluctuate wildly,
then the problem is probably NOT a frame-to-winding short of the T601
filament winding. But, I suspect that the -100 V bus voltage will now
be steady.

If the -100 V bus level is steady (and close to -100 volts), then your
problem is probably a winding-to-frame short of the CRT filament winding
in T601. And at this point, measure the other bus voltages (+250, +85,
+100, +12.6). [If you have capabilities to measure the -3000 volt bus
(safely!), you can do so, but if all of the other voltages are OK then I
expect that the -3000 volt bus level will be OK too.] The -100 volt bus
is a reference bus for the other bus levels; the variations in the -100
volt bus level that you were measuring earlier practically guarantees
that you will have variations in the other bus voltage levels.

Note that my suggestions are based only on my experiences with a sample
size of one (since I have experience only with a single RM504, which is
similar to but not identical to your scope). But let us know what you

If you do get a steady -100 volt bus measurement with the CRT winding
disconnected, let us know. If this occurs, I will post some photos
showing how I installed a supplemental 6.3 VAC filament transformer
(purchased from digikey) to fix this problem with my own RM504.

Good luck! And remember- please be safe in your measurements and tests!
Anything above about 50 or 100 volts can give be very nasty- and
potentially FATAL (no pun intended).

Mike Dinolfo N4MWP

On 04/13/2017 07:31 PM, enchanter464@... [TekScopes] wrote:

Ok. So I have a lot of measurements to report:

1. For the 500 V bus test, with V620 and V634 removed, I first checked
the two tubes. V620 (6DQ6B) gave a heater resistance (pins 2 and 7) of
0.9 Ohm (very low), while V634 (6DJ8) gave a heater resistance of 2.1
Ohm, same as the other 6DJ8 tubes. Also of note, on V620, there was
visible scorching/blackness at the base of the metal components.

For the voltage testing with the new fuse in place, the 500 V bus
registered at ~560 V on the analog meter, and +566-567 V on my digital
multimeter, which was constant over 10 minutes of being turned on (with
the two tubes removed). There was no noise, burning smell, or any
activity from the tubes. Stability-wise, the voltage only increased by
+1V over the 10 minute period, and no fuse blown.

2. With the tubes back in, using the analog volt meter:

a.) At the -100 V bus, it only gave a voltage from -25 V to -50 V
(fluctuating back and forth around -55 V, and steadily decreasing until
around -25 V). There was a buzzing sound and flickering again from V692,
and smell of burning from V620, which got very hot around the tube, and
smell/heat lingered even while the scope discharged unplugged.

b.) At the +500 V, the voltage ramped up to 560 V, and then decreased to
500 V once V692 kick on (and began to buzz), but was steady.

c.) At the +12.6 V bus, voltage fluctuated around 5.2 V, going as low as
4.0 V, but shooting up to around 8.6 V when V692 made sputtering sounds.

d.) At the +100 V bus, voltage was at 55 V, fluctuating between 50 V and
58 V, with spikes at 63 V.

e.) At the +250 V bus, voltage fluctuated around 150 V, going as low as
125 V (when V692 make sputtering sounds), and as high as 160 V.

f.) Lastly, the +85 V bus fluctuated around 90 V, dipping down to around
70 V, and spiking up to 110 V.

For all of the above readings, since V620 was showing signs of burning,
I only had the scope on for less than a minute (once V692 warmed up),
discharging off between each test. In addition, I did not check the
-3000 V bus since I do not have appropriate leads to test such high

At this point, is this still likely a transformer issue, and if so, what
do you suggest on how to further assess the scope?

- Evan




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