Analog Oscilloscope repar - Page 1
Forum section: electronics / repair

author: npkolb
author_rank: Newbie
timestamp: January 29, 2020, 02:16:21 am
content: I have an old 1970s 2 channel scope, but ch.2 has no trace at all. Ch.1 works fine and so does the timebase. I’ve confirmed that the attenuators are fine so I’m at a loss for an issue. Herehttps://drive.google.com/open?id=1tjrIy9NrdqkZD2vdLiUm-qig_l13v0znis what I believe to be the main vertical amplifier schematic. If y’all could point me in any direction that would be great. Thanks.

author: bob91343
author_rank: Super Contributor
timestamp: January 29, 2020, 04:27:12 am
content: That diagram is almost impossible to read.Measure the voltages in both channels and see how they differ, one from the other. That ought to point you to the trouble spot.

author: goaty
author_rank: Regular Contributor
timestamp: January 29, 2020, 08:10:47 am
content: Also check if pots VR241 and VR242 are good.Best compare voltages of IC201 to IC101

author: npkolb
author_rank: Newbie
timestamp: January 29, 2020, 02:54:30 pm
content: Will do thanks!

author: fcb
author_rank: Super Contributor
timestamp: January 29, 2020, 03:07:09 pm
content: Need more infomation (make/model/pix), full schematics,It might be as simple as the Ch1/Ch2/ALT switch/lever/button needing a clean.

author: npkolb
author_rank: Newbie
timestamp: January 29, 2020, 10:25:53 pm
content: Its a beckman industrial circuitmate 9020 20mhz oscilloscope.https://drive.google.com/open?id=19emhL9rIXPaoqhMWqZaXyxsXk316LlHFthere is a full user manual of it with all the schematics in the back. Any pics in particular you are looking for?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 29, 2020, 11:34:48 pm
content: I have repaired several old scopes. The common problem is that someone put too much signal into one channel. These scopes usually have a dual FET amp on each input and this blows the FET. Some of the really early scopes had two separate FETs rather than a combined dual FET.This is what I would look for as a "most likely failure" AKA: The Usual Suspects. The FET should be in the front of the board close to the input and attenuator.Maybe just measure the ohms from lead to lead on one FET and compare to the other good channel without removing the component.Also look at the input and make sure there is nothing blown, like maybe a resistor. Measure ohms from input to the start of the vertical amp (if you can find it) or to the first FET.Good thing is you can compare resistance measurements from good channel to bad.You could also compare voltages but start with resistances with unit turned off.Wally KC9INK

author: npkolb
author_rank: Newbie
timestamp: January 29, 2020, 11:39:12 pm
content: is thishttps://www.ti.com/lit/ds/slfs027b/slfs027b.pdfthe FET you're referencing?

author: npkolb
author_rank: Newbie
timestamp: January 29, 2020, 11:55:03 pm
content: I have confirmed a difference of resistance on the separate ICs through pin 1 to pin 14. I ordered a new IC hopefully that has the fix. If y’all have any other suggestions I’m open to trying and I will keep y’all posted once the chip arrives.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 30, 2020, 12:39:33 pm
content: The scopes I have worked on have discrete components in this stage. Usually 6 pin dual FETs.However, looks like this IC is your amplifier, even though it is not a Field Effect Transistor.Good work. Let us know what happens when you get the new chips.Wally