Author Topic: Crotech 3031 20Mhz CRT scope circa 1985 no vertical line on square wave  (Read 402 times)

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Offline alanjosephbrown

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Hi Folks,

Just having a look a second hand scope (Crotech 3031 20MHz single channel) i had bought yesterday. As per manual i set up for self test/cal, which is a 200mV/1KHz square wave. Problem is that on screen only the horizontal line of the waveform is visible. On the cal post i measure only about 100mV using a calibrated bench DMM and a frequency of 946Hz. On checking the manual it stated that the cal post output could be adjusted by trimming, the stated value of the variable resistor was 100 ohm and the measrred resistance (in circuit) was 1.3ohm up to 84.3ohm. At the lowest resistance setting cal output was at 103.51 mV and then 107.79 mV at the top end. And this is as far i have went. I plan to go through all the components and bench test to see if i can find the culprit. Visual inspection of the inside throws up no clues - no signs of overheating.
Just so you know what you are dealing with, i am a practising electrician with only experience in heavy industry and this was supposed to be my first foray in the electronic black arts however i need a working scope first!! I picked this up for next to nothing so the temptation is to scrap it but i would like to see it working. It was supplied with a great mint condition manual with all the circuit diagrams and parts but unfortunatley the company has liquidated quite some time ago.

What i am hoping is that someone out there has maybe experienced this before and this issue is blatently obvious?

Appologies in advance for incorrect terminologies used.

Many thanks

« Last Edit: January 20, 2018, 02:24:47 am by alanjosephbrown »

Online tautech

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Welcome to the forum.

Probe Cal outputs for scopes come in many flavors of voltages but frequency is in older units can be all over the place and some units allow for it to be adjusted as close as possible to 1KHz but it's never precise and best just thought of as a sanity check.
Probe Cal voltage is p-p and again not necessarily precise as the fast rising edge and flat peaks are what matters for probe compensation adjustments.
The fast rising edge is often not visible but that depends on the phosphor type used in the CRT and it's degree of persistence.
Unlike a DSO where the raster is reconstructed from data points, it's normal the CRO vertical edge is hard to see especially at normal viewing intensity (brightness) levels and/or at slow timebase settings.

Let a normal scope probe rather than a DMM tell you what's going on here as the circuitry for the Probe Cal is normally supplied from LV and in no way could be anything like 100 V.  :scared:
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Offline alanjosephbrown

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Hey tautech thanks for the quick response, my bad i meant 100 milli volts, thanks for the info much obliged!!

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