Re: Type 503 Oscilloscope Issues

Dave Casey

Yeah, I suggested starting with V334 and V434 because you have to start
somewhere. Also, when you pull either of these tubes, you kill the heater
to the other, so you can't run one without the other without putting a
resistor in place of the missing tube's heater (or using jumper leads to
connect only the heater of the pulled tube to its socket).

If those have no effect, I'd plug them back in and try V374 and V384 (since
we also suspect something is wrong with the horizontal amplifier, it
wouldn't be surprising to find the extra power dissipation there).

Also, it's a moot point now, but I believe the design voltage targeted for
the grid of V634 is -2V. It looks like Tek's design parameters for that
circuit came straight out of the RCA tube manual.

Dave Casey

On Sat, May 6, 2017 at 1:35 AM, 'John Snyder' Kochcal@...
[TekScopes] <TekScopes@...> wrote:

I would agree with Dave's analysis.

The voltages now make more since, you can see the voltages on the connected
sides of each pair of resistors are about the same.

The voltage divider R640 R641 R642 with the +85V bus being balanced against
the -100V bus to get a number around 0 on the grid of V634B.

On the grid a more positive voltage is more power to the Oscillator V620
a more negative voltage will turn down the power. It looks like you are
cranked as much as it can go with the Oscillator V620, trying to get the
-100 up. Which it can't most likly because a over load. So everything else
is running over voltage/power now.

V334 is the Horizontal amp input tube and
V434 is the vertical amp input tube.

Keep the tubes so they don't get mixed up and can be put back in the same
socket. If your lucky you will find one or two parts that can be replaced
and with a little care the calibration will remain in tact. I just put my
tubes in a plastic snack bags marked with the tube number so I can get back
in after washing.

If those tubes do not release the -100V there are more tubes that can be
looked at.

Another thing you can do is note the colors on the -100V wire and follow
that around visually. Seeing if you can see a connected resistor that shows
heat stress.

Or you will note on the schematic that the -100 goes to resistors most of
the time. If you measure voltage across them you can get an idea of the
current flowing in each circuit (assuming the resistor is ok) and that can
help locate an overload.


-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, May 05, 2017 9:27 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: Type 503 Oscilloscope Issues

So the +85 is a little high which suggests that V659 might be out of spec
(the later 5651A is supposed to regulate to within 3V), but that shouldn't
prevent the supplies from working properly.

The fact that all your supplies are high except for the -100 suggests to me
that the -100V supply is overloaded. Because it's being pulled down (less
negative) by the load, the power supply circuit is attempting to
compensate. This compensation is not enough to overcome the load on the
-100V supply, so that still measures low. However, the compensation is
driving the other supplies high. Because the power supply uses the -100V
rail as it's feedback for regulation, it doesn't know nor care that the
other rails are high, just that the -100V isn't negative enough.

You mentioned that your scope runs hot. Anything with tubes is going to get
a little warm, but if it's warmer than it's supposed to be, it could be due
to burning more power than it should be.

A good test for my theory will be to find a way to remove the load on the
-100V supply and see if you can then adjust all the rails into
specification with R641. Obviously, when disconnecting the loads, we still
want to make sure that R642 is connected so that the regulator can be
expected to function.

Parts of the circuit that use the -100V supply:
Vertical Amp
Horizontal Amp (where your "warping" is probably happening)
Sweep Timing
Sweep Generator
Sweep Trigger

In other words, most of the scope. That supply goes to a lot of places.

Try powering up the scope with V334 and V434 removed from their sockets and
see what the -100V supply does.

Dave Casey

On Fri, May 5, 2017 at 9:06 PM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

So I re-did the voltages with a new DMM, and I found out that the issue
previously was with the leads. The clips from the older meter were not
shielded/jacketed and caused some shorting between wires that were nearby
to the area it was attached (for the 85V bus). With the new meter and
leads, the voltage measurements for the buses and resistor points are

Bus voltages:
-100V bus = -86.1 V
+250V bus = 308 V
+100V bus = 122.6 V initially, but decreased to around 115 V

+12.6V bus = 14.6 V
+500V bus = 492 V

+85V bus = 90.2 V

Resistor voltages (In order: 1. left of resistor to ground, 2. right of
resistor to ground, and 3. voltage with leads across resistor)
R640 -> 10.6 V, 90.7 V, 74.2 V

R641 -> 10.2 V, 1.7 V, 9.2 V
R642 -> -87.1 V, 1.7 V, 78.6 V
R644 -> -0.3 V, 1.3 V, 1.7 V
R659 -> 308 V, 90.8 V, 214 V

I still have not found the missing lead for the old analog volt meter, so
I ended up ordering a replacement set. Once that comes in, I can re-check
the values again with that meter.

Also, I know some have mentioned measuring the current across the
resistors, but is there a way to do this without desoldering them (to
the circuit and insert the meter)? I don't mind desoldering them, but I
know there were concerns about doing so and the damage it could cause.

- Evan

Posted by: Dave Casey <polara413@...>


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