TEK 475 - hum on calibrator

Dave_s 21 years ago

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

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Just Another Theremin Fan 21 years ago

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.

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Jim Yanik 21 years ago

"Just Another Theremin Fan" wrote in news: snipped-for-privacy@f14g2000cwb.googlegroups.com:

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.

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

Jim Yanik jyanik at kua.net

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tekman 21 years ago

opamp.

wave

not

cannot

0.5v

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

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