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There's going to be some normal AC voltage detected with a Digital
Volt meter on the input to the blanking circuit. That's because the
0.5 signal is momentarily forced to -8 V at the end of each horizontal
sweep to blank the trace while moving back to the left-hand side of
the screen.

The AC is not sinusodial, but pulsed in nature. The expected signal
measured at test point 87 is shown here:



It is believed the vertical scales published in graph 87 and 88 are
not correct.

Another potential +5 volt noise blanking source is applied to the base
of Q1466 through voltage divider R1466 and R1467. A changing +5 volts
here would definitely vary the blanking threshold voltage at 120 HZ!
This adds additional weight to support the current +5 volt noise
blanking theory.

It all points back to capacitor C1512 or C1513. Can anyone recommend
a supplier for these caps. The parts list calls for Sprague
capacitors, now called Vishay / Sprague. Mouser and DigiKey carry
Sprague capacitors, but can't find the exact replacement parts in
their on-line catalogues.

Ron




--- In TekScopes@..., "jordankersten" <jordankersten@>
wrote:

I checked the blanking input and it reads .5VDC...right where it

should be. However,

.090VAC was also present. Does this seem like I've got some ripple

in my blanking input?

Would that bad/leaky 110V supply cause this?

--- In TekScopes@..., "rlc9878" <rchildre@> wrote:

Notice the Regulated +55 Volts signal through the Zener VR1515 at the
bottom of the 110 Volt regulator circuit? It is believed the root
cause is 120 HZ noise on the +55 Regulated supply.




A careful inspection of the +5 volt regulator circuit reveals that
regulated +55V also supplies a voltage divider circuit (R1559 and
R1554) to provide a +5 voltage reference to OP amp U1554A, pin 9. If
the +55 V signal is noisy, the same noise will be reflected in the +5
volt signal with a ratio of approximately 1/10 amplitude of the +55
volt noise.

+5 volt ripple can propagate to the blanking circuit when the voltage
supplied by the intensity adjustment pot is not high enough to block a
noisy +5 volts signal via diode CR1463. The blanking input is
normally around 0.4 to 0.5 volts, so a little bit of ripple is all
that's required to cause a blanking issue.



The most likely caps needing replacement are C1512, C1513. The
schematic indicates +78 volt reading to ground between caps C1512 and
C1513. These affect the +110 V, +55 v and ultimately +5 volt regulated
supplies.

Noise on the +5 volt supply can affect blanking when the intensity
adjustment voltage is less than +5 volts and voltage limiting diode
CR1463 is allowed to conduct.

On my scope, the blanking issue goes away when the intensity is
increased high enough, adding additional weight to this theory.

Ron



"One experiment is worth a thousand calculations."







--- In TekScopes@..., "jordankersten" <jordankersten@>
wrote:

Ok, all voltages (regulated and unregulated) are fine except the

110V supply. I get 120V

unregulated, which I think is fine. I only get 100V regulated.

Also, the 110V unregulated

reads 25VAC and the 110V regulated reads 11VAC. All other supply

voltages (regulated

and unregulated) read 0 VAC. I would suspect the 550uf power supply

cap, but would that

explain why my regulated 110 voltage is 10V low?

--- In TekScopes@..., "jordankersten" <jordankersten@>

wrote:

Wow, how did I miss this: the 110 test point reads 100V DC and 11

volts AC. Something

is

up there.

--- In TekScopes@..., larrys@ wrote:

How's the +110?
-ls-

glydeck@ wrote:

Jordan,
I don't have a 465B manual with me, but 40 mv rms does sound a

little on the

high side. I believe that would be about 113 volts pp, [(RMS

x 1.414) x

2=P-P not exactly a sine wave but close enough]. Having a

scope to trouble

shoot the scope is always helpful, but in a case like this

I've used clip leads

to parallel an equivalent or larger capacitor across the

suspect power supply

filter caps in the circuit, and then observe the results on

screen. (Make

doubly sure the working voltage of the cap is high enough.)

This test doesn't

always work because the filter cap could be failing toward

short and not

failing toward open thereby negating the added capacitance.

Ron,
Nice picture, labels and all. I can even count on the

graticule 8 small

divisions between the start of each darkened portion. That

translates to an 8

ms period, or a frequency of 125 Hz. This would be well

within the margin of

error to be 120 Hz ripple from the full wave bridge in one of

the supplies.

I agree you have a similar problem with AC mains.

George



In a message dated 9/29/2008 8:17:21 P.M. Pacific Daylight

Time,

jordankersten@ writes:

Just turned off the auto-ranging and took readings again.

They all read

about .003VAC
except the +55 rail. It reads .040VAC. Is that enough to cause

my problem?

I also have the same issue with my Tek 465 scope. A picture

was added

to your photo album:

_.<WBR

()

These are my settings:

1) Set Horizontal sweep to 5 milliseconds per division.
2) Set Vertical to Channel 1.
3) Set Channel 1 to Ground.
4) Center trace vertically.
5) Set Trigger Mode to NORM
6) Set Trigger Source to Line to synchronize to 60 HZ.
7) Reduce the intensity (counter-clockwise) to a bare

minimum to see the

trace.

I'm thinking +15V power supply is the culprit, but not sure.
Unfortunately, I don't have another scope to test this one.

Using the

scope to test itself is no good because the blanking circuit

blanks

it's own trace.

But, it's definitely caused by 60 HZ noise, somewhere...
Ron



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