EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: homerwsmith on April 14, 2014, 03:42:20 am
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Dear Gentle Folk,
Using an older Philips transistor scope, dual beam 10 Mhz, works well for its age.
I have the manuals.
Recently while tuning a Teac XL-2000R, the straight line traces with no signal while normally centered suddenly dropped
to near the bottom of the screen and started to display a 120Hz soft rounded triangle wave. Straight sides, rounded top.
It appears on both YA and YB channels.
The amplifier knob does NOT affect it.
The time base knob, DOES affect it like a normal signal.
The X and Y Position knobs DO affect it, and I can bring it back up to center screen,
but when the scope goes back to behaving again, the straight traces are too high.
The AC - 0 - DC switches on either YA or YB do NOT affect it, its always there.
The X Via YA, shows a 45 degree line as if the signal was really there.
If I add an input signal of any kind, it adds into the existing aberrant signal as expected, beating and all.
The amplifier knob affects the input signal, but not the aberrant traces which are about 2 cm high
regardless of amplifier setting.
I have the scope manual, I know some electronics, but before I go in and blow myself up at 8.5KV can someone give me a clue where I am looking?
1.) Its probably not in the input streams of YA or YB because both are bad at the same
time and they are quite independent.
2.) I can't see how a vertical signal could get into the X time base circuity.
3.) It could be a power supply issue, with perhaps a bad capacitor putting unfiltered
power into both the YA/YB circuits but the amplifier knob only affects input signals, and not this
aberrant one. With the amp up high, the input signals are huge but still add in with the smaller aberrant ones as expected.
The scope usually comes up working fine, then after about 5 minutes starts
this odd behavior.
Thanks in advance,
Homer W. Smith
Lightlink Internet, CEO
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Use an external DC 24V 2A supply instead of mains to feed the scope and see what happen.
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OK, thanks for that. Didn't get to the 24 Volt power supply, but noticed knob on the
back offering different volages, 220 240 110 and 120. It was set to 120, probably by me
the last time I played with it. I set it to 110 and the scope works fine, brighter too. Our voltage
is about 116.
Shall consider this closed for the time being.
Homer Smith