Vintage analog oscilloscope, wrong voltage readings(9.5v shows 13v) - Page 1
Forum section: electronics / repair

author: komrad2236
author_rank: Contributor
timestamp: May 06, 2023, 12:49:42 pm
content: Where should I look for a problem if my oscilloscope is showing wrong readings ?I have a 9v battery, measuring it withfew multimetersshows 9.6v reading, but with oscilloscope the reading is 13v which should indicate there isfault with oscilloscope.I have no idea where to start looking.This fault is present in both channels btw

author: wasedadoc
author_rank: Super Contributor
timestamp: May 06, 2023, 01:10:02 pm
content: Surprising that someone with 21 previous posts here did not think to provide any information about the scope except that it is analogue, has two channels and is "vintage". Whatever than means.

author: komrad2236
author_rank: Contributor
timestamp: May 08, 2023, 12:19:37 pm
content: Sounds fair, its analog CRO, two channel 20 MHz oscilloscope, model Iskra MA 4049 -https://drive.google.com/drive/u/2/folders/1-KAHHziQhGj40ROpt0AJ1LJmDQhn_cz9As far as symptoms, I mentioned those in first postNot sure what else info I should provideSo, @wasedadoc, where should I look for a problem ?What stage malfunction can cause an CRO to give a wrong reading ?

author: wasedadoc
author_rank: Super Contributor
timestamp: May 08, 2023, 12:42:54 pm
content: Is the X axis timebase sweeping as expected? Look at a known frequency to check. Low voltage between cathode and final anode can cause both deflection axes to swell.

author: JimmyJo
author_rank: Contributor
timestamp: May 08, 2023, 12:45:36 pm
content: First make sure the Var knob is all the way CCW, i.e. in the cal position.

author: Chris56000
author_rank: Frequent Contributor
timestamp: May 08, 2023, 08:17:43 pm
content: Thank you for providing a share to your Google Drive Folder it helps us enormously!Always remember that most oscilloscopes for normal hobbyist and laboratory use are only calibrated to a vertical deflection accuracy of ± 3% at maximum deflection and 3% of 9 V is 0.27 V.To check the deflection accurately you need a multimeter with an accuracy of at least ten times better than your oscilloscope amplifier, i.e., 0.3% accurate or 27 mV at 9 V.Providing you're using a reliable meter, connect your battery or d.c. power supply to the input of your Iskra oscilloscope, and adjust the "Var" knob to the anti–clockwise "CAL" position. Once you've determined the exact voltage from your battery or power supply, set the coarse volts/div switch to give you a suitable amount of vertical deflection on the screen, (2 V per div for your 9 V battery), then if your voltmeter indicates "9.6 V", you can preset the vertical gain using internal adjustment R47 according to the instructions in your manual to give a vertical deflection of 4.8 divisions (4 large squares plus 4 small subdivisions) – the vertical gain of your Oscilloscope will then be adjusted to within specifications!Chris Williams

author: wasedadoc
author_rank: Super Contributor
timestamp: May 09, 2023, 10:29:15 am
content: Quote from: Chris56000 on May 08, 2023, 08:17:43 pmThe A and B channels have their own separate R47 gain adjustment pots. Why would both channels be equally and so far out of calibration?If the problem really is a vertical gain only fault then the fix is much more likely to be in the vertical output amplifier and deflection plate driver on page 5 of the schematic.But I still await an answer to my question about X-axis accuracy.

author: 807
author_rank: Frequent Contributor
timestamp: May 09, 2023, 10:59:11 am
content: Quote from: komrad2236 on May 06, 2023, 12:49:42 pmDoes it show 13v on all vertical ranges (2v, 5v, 10v/div)?

author: Greybeard
author_rank: Frequent Contributor
timestamp: May 10, 2023, 09:26:27 am
content: Have you checked CRT acceleration voltage?If it is too low, x- and y-scale would be stretched.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 10, 2023, 09:46:03 am
content: Quote from: Greybeard on May 10, 2023, 09:26:27 amThat is not easily checked as it should be more than 2000 Volts. I have asked twice if the X-axis is also stretched but no answer has been forthcoming.Checking the X-axis is very easy. Just connect about a metre of wire to one of the vertical inputs to act as aerial/antenna to pick up AC mains hum. Do the number of divisions on the graticule match 20 msec period?