EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: SLR on July 26, 2020, 04:33:13 pm
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All-
I recently acquired an EICO 430 and have been have some “interesting” results which I hope you might help me fix.
In the Horz/Ext mode using a simple curve tracer, I am getting a strange “golf club” end when testing diodes. (see photo attached). This only occurs only with diodes—the XY images using the same curve tracer testing capacitors and resistors look as expected. I thought it might be the tracer or my probes, but I tested the same diodes and curve tracer on other oscilloscopes and there was no “club”.
My (very modest) electrical expertise suggests something (a capacitor?) is not right in the vertical circuit in the H/E mode, but I’m just guessing.
Do you have any idea of the part or area I should look at and test?
Much thanks for any help you can provide.
SLR
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Looks like some kind of hysteresis or phase shift. By hysteresis I mean, perhaps diode charge storage that causes the curve not to retrace itself. The EICO is a low end unit and I wouldn't trust it for much more than a general idea of what's going on.
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You have replaced ALL electrolytic capacitors in this almost 60 year old scope, right?
Only three in the front end, and four as C14 in the power supply. They could cause a delayed trace shift or hum in the power supply. Only a few $ to change them out:
C4 10uF 50V
C5 10uF 150V
C23 20uF 50V
C14a 40uF 450V
C14b 20uF 450V (might be 3 of 40uF on bom?)
C14c 20uF 450V
C14d 20uF 450V
edit: fixed scrambled image link
Eico 430 1953 service manual here: https://elektrotanya.com/eico_430_3inch_25mv_500khz_10-valves_oscilloscope_1953_sm.pdf/download.html (https://elektrotanya.com/eico_430_3inch_25mv_500khz_10-valves_oscilloscope_1953_sm.pdf/download.html)
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It looks like the bandwidth of your vertical channel is
too narrow. If the curve tracer uses a sawtooth sweep
instead of sine or triangle this problem occurs because
the 'reset' part of the waveform is too fast for the scope
and it shows up as a hook or as phase shift. Somewhere
you are seeing the result of a delayed R/C phase shift.
Are your probes 'compensated' to your scope using a
square wave input? I don't think the scope is bad, but
internally there may be compensation trimmer capacitors
to make square waves look square and you will find that
your scope is out of adjustment showing either overshoot
or undershoot on square waves which will also cause
that weird phase delay hook.
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Many thanks for all the helpful comments!
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Let us know if you find the problem :)