Re: Type 503 Oscilloscope Issues

Dave Casey

Evan -

R605 is high, but it is not likely to cause any significant problems. R606
and R607 are effectively paralleled in circuit because of the low
resistance of the 6.3V winding on T601, so the measured value is
reasonable. (Basic circuit theory here for combining discrete circuit
elements: Requivalent = R1*R2/(R1+R2) = 100*100/200 = 100/2 = 50 ).

With V20 and V634 removed, you can get readings of the 500V bus at D612 or
either side of R626. Be sure to follow the advice others have given before
about making sure the charge on this bus has dissipated before contacting
it. Regardless of whether or not the tubes are still installed, you should
see the resistance to ground steadily climb until reaches about 300k (more
if you're on the far side of R626). If it is less than that something's not
right, probably with C611/C612, D611/D612, or R611/R612.

The +85V bus can be measured at the socket of V659. If you look at the back
of the socket, you will see only three pins are actually connected. Two are
tied together; these are pins 1 and 5 and are the +85V bus. If you look
closely, you will see pin 2 is the only other connected pin, and it is tied
straight to the chassis ground.

A variac lets you do several things. The manual is showing it used to
adjust the input line voltage to exactly 117VAC for the purpose of precise
measurements or calibration. But the variac is also a useful
troubleshooting tool. It allows you to ramp up the voltage slowly, and do
sanity checks along the way. For example, if you have a failing filter
capacitor such as C611 or C612, you could use the variac to supply only 10%
of the nominal supply voltage and expect to see 10% of the expected no-load
output voltage on the +500V bus. At this point, leaky capacitors wouldn't
be stressed as much because they're not near their rated voltage. You can
expect the +500V bus (with V620 and V634 removed) to linearly track the
variac setting. If you observe a non-linearity, then it's possible that the
diodes or capacitors are leaking at higher voltages.

Keep in mind also that the GFCI only protects you on the input side of the
T601 power transformer. It's like wearing a helmet - it makes what you're
doing safer, but you shouldn't expect it to make you completely safe. The
+500V supply will still kill you even with the GFCI.

Dave Casey

On Thu, Apr 13, 2017 at 12:09 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

Hi everyone,

I just got in today the fuses and GFCI adapter (just in case), so I went
ahead with the powered-off tests first.

My scope is one of the earlier models, having V334 and V434 both as 6DJ8
tubes. I confirmed on both tubes that the heating filaments are good (V334
gave a resistance of 2.7-2.8 Ohm, and V434 gave a resistance of 2.8-2.9
Ohm). With the two tubes removed, I retested D672, which gave similar
readings this time to the other diodes on the power circuit (with a diode
testing reading of +0.637 V, or a resistance of greater than 20 M-Ohm).

However, I am unsure on R605, which gave a reading of 111.5 k-Ohm (instead
of 47 k). The resistors ahead of this from T601 leads 10 and 11, R606 and
R607, both gave readings of 51.2 ohm, which is less than their 100 ohm
specification. I am not sure if this is just a reading error since they are
all soldered together, or if they could be to blame for the observed

I would proceed with the power-on testing (having removed V620 and V634),
but I have not been able to locate the +500 VDC bus. My manual only shows
the points to connect for the +250, +100, +12.5, -100, and -3000 V points
for testing, but not the 500 V or 85 V ones. Would someone be able to point
me to the right direction for this test? (It is late right now, so I will
hold off until tomorrow to actually run the tests).

Also, a side question, but in the manual, the test setup shown has the
oscilloscope plugged into a Variac for power. Is this suggested / has a
purpose? (I do have one that I can use)

- Evan

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