TEK 475 - hum on calibrator

[Dave_s]

I'm repairing my TEK 475 and it now has a trace and can display the
calibrator after repairing reference voltage into +15 regulator opamp.

When displaying the calibrator, I see some 60 Hz hum on the square wave
at slow sweep rate. Never saw that before. Also CALIBRATOR sync is not
rock solid but was previously. Possibly 60 HZ is disturbing SYNC???

Disconnected both probes and placed vertical input mode to GND.
Slowed sweep to see see 60Hz easily.

Looked at each input channel display seperately. I see 1/4 divison of 60
Hz hum on both channels. Hum amplitude always APPROX same regardless of
attenuator setting.

Measured all LV PS voltages and ALL are very close to nominal and cannot
measure ripple... too small. Unreg +50 is 0.4vpp and Unreg +15 is 0.5v
pp.

Is there a hum rejection adjustment?
Not sure which stage this 60Hz might be originating.

Thanks for any help. Dave_s

 

[Just Another Theremin Fan]

Also CALIBRATOR sync is not

rock solid but was previously. Possibly 60 HZ is disturbing SYNC???

Is this the rather elderly 200MHz DB jobbie? I ask as my one (wherever
it is) Has always given out 900Hz from the calibrator rather than 1kHz.

On the rare occasions that I use it, I sometimes find that the front
rotary switches give all sorts of oddball problems which a clean
usually resolves.

 

[Jim Yanik]

Is this the rather elderly 200MHz DB jobbie? I ask as my one (wherever
it is) Has always given out 900Hz from the calibrator rather than 1kHz.

Becasue it's only an AMPLITUDE calibration signal *for probe compensation*.
The only TEK scope that had a accurate frequency calibrator in that era was
the 7704A with a special option(rare).It was Xtal controlled.The digital
scopes have more accurate cal signals,WRT timing.

On the rare occasions that I use it, I sometimes find that the front
rotary switches give all sorts of oddball problems which a clean
usually resolves.

Because the HF(and LF) cam switch contacts have a very low wiping force and
are vulnerable to airborne contaminants,plastic outgassing mainly.

 

[tekman]

I'm repairing my TEK 475 and it now has a trace and can display the
calibrator after repairing reference voltage into +15 regulator opamp.

When displaying the calibrator, I see some 60 Hz hum on the square wave
at slow sweep rate. Never saw that before. Also CALIBRATOR sync is not
rock solid but was previously. Possibly 60 HZ is disturbing SYNC???

Disconnected both probes and placed vertical input mode to GND.
Slowed sweep to see see 60Hz easily.

Looked at each input channel display seperately. I see 1/4 divison of 60
Hz hum on both channels. Hum amplitude always APPROX same regardless of
attenuator setting.

Measured all LV PS voltages and ALL are very close to nominal and cannot
measure ripple... too small. Unreg +50 is 0.4vpp and Unreg +15 is 0.5v
pp.

Is there a hum rejection adjustment?
Not sure which stage this 60Hz might be originating.

Thanks for any help. Dave_s

hi Dave,


I don't expect a LV pwr supply problem, the ripple values look very
good to me.

Does the hum disappear when you put on the housing to the scope? At my
Tek465/475, there is always a certain amount of hum as soon as I
dismantle the housing (open the scope). Even when I put the input
switch to GND position. It seems that the Tek465/475 definetely NEED
the cabinet closed for full performance.

Let us know how things turn out, please.

BTW: the service manual is in your PM now.

hth,
Andreas

 

[DaveC]

BTW: the service manual is in your PM now.

"PM"? Same as "MB" (mailbox)?
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.

DaveC
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[tekman]

"PM"? Same as "MB" (mailbox)?

PM means "private mail". MB might also be "MegaByte" ;-))


UUuups Dave, be careful: This topic has been covered in the google ,
espcially sci.electronics.* MANY times. either you used the wrong
"search terms" or..

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So we reply )

scnr,
hth,
Andreas

 

[DaveC]

This topic has been covered in the google, espcially sci.electronics.*

http://tinyurl.com/5d59f

I got tired of paging through...
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.

DaveC
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