Need help fixing IO-102 Oscilloscope
- I bought a Heathkit IO-102 oscilloscope at a pawn shop for $15. Unfortunately, it doesn't seem to work right.
Here's a link to the manual
I have followed the instructions in the "Tests and Adjustments" section starting on page 61. The 100V adjustments were extremely sensitive and I was only able to get them to around 90-110V.
Using the "without signal generator" instructions on page 65, this is the closest I can get it to look like a square wave.
The display holds steady with what is shown in those pictures unless I fiddle with the controls.
When I set the vertical control to either 10x or 100x, it pretty much just shows a flat line even when the vertical gain is all the way up. There's a slight "blip" but it doesn't look like a square wave, more like a triangle going up and then back to just slightly above the baseline. I couldn't get this captured well on camera.
The trimmer capacitors don't seem to change the display at all.
The 40uF capacitor (C7) on the vertical amplifier circuit board was puffy so I replaced it. That didn't make any difference though.
I checked all the voltages on the voltage chart for the vertical amplifier circuit board on page 90 and they are all pretty close.
- Zener diodes look like a conventional diode in the forward direction and an insulator in the reverse direction until the applied voltage exceeds the zener potential when they rapidly start conducting.You can test zeners using a DC supply greater than the zener potential and a series resistor to limit the maximum current to safe value.73
Bob F Burns G3OOU
C Eng, FIET, MSE
Crystal Palace Radio & Electronics Club: www.g3oou.co.uk
Technical web site: www.qsl.net/g3oou
@BobFBurns on Twitter: http://twitter.com/BobFBurnsIn a message dated 05/04/2014 00:44:42 GMT Standard Time, riley@... writes:
So I finally got it working. I'm not sure what exactly fixed it though.
I tried hooking the signal generator directly to the gate of Q1. When I did that square waves displayed correctly. When I hooked it back up to the regular input on the front though, it didn't work at all anymore. I realized I must have blown out something in front of Q1.
So, I remove C1, R1, D1, D2, and C401, and tested them all. Both D1 and D2 tested bad so I replaced them with 1N5239B zener diodes. The other components all tested fine. After I soldered everything back in, it started working correctly.
I had tested D1 and D2 before and they tested fine. I just tested that they were working as diodes. I don't know how to test the zener voltage. Is it possible that their zener voltages were off but they were still working as diodes? Would that have caused the problems I saw?
Thanks everyone for all your help. I've learned a lot and I have a working oscil loscope now to show for it too.