Keithley 178 DMM Repair (Fixed!) - Page 1
Forum section: electronics / beginners

author: Hannibal_Hector
author_rank: Newbie
timestamp: August 28, 2020, 01:03:07 am
content: Hello,I purchased a "parts only" Keithley 178 DMM fully expecting it to be broken and attempting the repair as a learning exercise. Downloaded the Service Manaual and after checking it wasn't shorted, I built a power cord and it powered up on the first try.Day 1:- Resistance portion totally not working: Random numbers on all the ranges, no matter what on the input.- Voltage portion appears to be working, but while sweeping the voltage it seems to skip numbers and then comes back to showing the correct readout. Does this all the way up to my max of 30V.- Decided that it was most important to first track down the display problem (skipping numbers) before the Resistance portion.- Checked voltages per the Service Manual, all good- Checked clock (good), signals on the ADC, and then took a break after an odd waveform check, as compared to the Service Manual.Day 2:- This meter uses a BCD to 7-Segment driver, and while studying the schematic I realized the skipping must be a result of the ADC or the BCD not sending or acting on one of the bits.- Verified the hypothesis: All digits are able to show segments for 1,2,3,4,6,7 and skipping 8 and 9, so bit 4 is not going high.- I decided that if it was a failed BCD driver IC, I could replace that, and if the ADC chip was not sending the bit 4 high, I could replace the whole ADC portion with an Arduino Nano paired with a 16-bit ADC module (ADS1115), to drive the BCD converter. Keep in mind, I'm a noob, so this is less about getting a trusted/working meter, and more about the process to get there.- To determine which has failed, I tried to get the oscilloscope channel 1 to display the signal that enabled one of the digits, and channel 2 to display the state of bit 4. Reasoning, that if I see channels 1 and 2 going high at the same time, the ADC was good and the BCD driver is bad. If I don't see channel 2 do anything, bit 4 was not being sent by the ADC. I could also check the other bits as a sanity check.So after powering off, I traced IC pins and attached probes, I power back up. I got square waves on the scope, but observed a totally buggered display, where I got random things like this:_.9___9_.9__9The display is reacting to input, but its all messed up, so I power off, removed the oscilloscope probes, and the display is still buggered! Ooops...I am 99% certain I didn't short anything, not 100% sure, but pretty sure. So the questions I have are:a) Was my though to watch the signals of the ADC/BCD on the oscilloscope wrong?b) Any clues as to where I went wrong?c) Assuming I need to replace the ADC/BCD, am I smoking crack thinking that it is feasible to do so with a microcontroller?Thank you!!! (Having so much fun with this, btw!)

author: Shock
author_rank: Super Contributor
timestamp: August 28, 2020, 02:24:38 am
content: Welcome to the forums, we also have a dedicated repair section on the forums for more repair specific questions.This vintage of Keithley also has problems with tantalums caps, dirty switch contacts and I had one with a faulty voltage regulator that was killed by capacitors.With it powered off from the wall you could try disconnecting and reconnecting the display board a few times, or check if there are intermittent solder joints as you may have disturbed some connections. But you probably shorted something or created a short through different potentials when connecting both ground clips.The decision to reverse engineer in replacement parts isn't really worth the time or money in this case so that leaves only the educational benefits. When it comes to spending a load of time or expense on something it also pays to be absolutely sure. So often testing and breadboarding components out of circuit or testing the circuit with components removed (such as "ohming out" traces) adds confidence that you have sniffed out the issue fully.

author: Hannibal_Hector
author_rank: Newbie
timestamp: August 28, 2020, 05:41:29 pm
content: Thank you for the welcome, Shock! I debated if I should post here or there, and if I got it wrong, a mod would move it. I did watch a few teardowns of similar DMMs including Dave's glitchy switches. I didn't know about the tantalum caps on this one.I am going to follow your advice and check for joints, because I don't know if it stopped working after the probing to find the correct test points, or the actual connections to those points. I did use the same ground for both channels though.I will have to look for the technique of "ohming out" traces, and keep in mind building and testing in sections. I do recognize the possibility of frying the meter further if I am not careful attaching components they never intended to be there. Fun either way!Thanks again!

author: Shock
author_rank: Super Contributor
timestamp: August 29, 2020, 01:25:56 am
content: It's just measuring the trace resistance, then between different nodes and between nodes and ground. If you have contamination problem or an electrically leaking component the low DC resistance provides clues.It's useful to compare two similar signal lines as well to see if the input/output of a device is defective. The polarity of your probes matters when resistance and diode testing but that is to your advantage as it's more information.

author: KMoffett
author_rank: Regular Contributor
timestamp: August 29, 2020, 07:24:18 pm
content: We had few of those in a Physics Student Lab. Crappy "radio" push buttons. Both contacts and switching mechanism. Went to a recycler. Hope you didn't get one.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: September 02, 2020, 01:50:35 am
content: Clean switches first, deoxit may bring them back to life. The switches are open on the back. Spray contact cleaner in them and switch them on and off several times. See if anything changed reading wise.In my opinion, excellent choice. Great display and really reliable... when working.

author: Hannibal_Hector
author_rank: Newbie
timestamp: January 30, 2021, 11:36:36 pm
content: This Keithley 178 DMM is now FIXED!!!!Here's what happened several months ago... Purchased as parts only, and powered, but the display would display random numbers and no good readings. While trying to determine if the problem was the ADC or the display section, I made the display worse and all I got were 8s and 9s and blank digits... way worse than when I started.I'm a noob, so I felt a bit over my head and went to play and learn with other things. Eventually found myself with renewed confidence.Today, I picked this back up and I hooked up the oscilloscope to the four lines of the 14511 BCD-7 driver. All lines had data, and I also had signals from the multiplexer on all digits, yet the display was still garbled. Changed out the 14511 and the display worked correctly for the first time ever!Stoked, I tested DC on all ranges and worked perfectly. I tested resistance and nothing worked. Even shorting the two inputs yielded a weird number. At least I had DC working, which proved the ADC and display were ok.I cleaned all the switches, but this did not improve the resistance measurements.I went through the test procedure for ohms, from the manual, and had a problem when measuring the voltage across the inputs... weird, super low, micro volt readings. Spent hours studying the schematic and the block diagram, and focused my efforts on understanding how the ohms circuit worked, where it got voltage, and how it was connected to the op-amp. This led me to really focus on transistor Q103, and the diodes and resistors directly connected to it.Both the diodes, and the resistor array tested ok. I removed transistor Q103, an NPN 2N3439, and it tested completely open. This is a beefy transistor and part of the ohms protection mechanism(?). Since I did not have a direct replacement, I surmised that I could try replacing this with a 2N2222, and while I would not have the protection offered by the 2N3439, I might prove the original transistor was indeed the problem.This WORKED! The 2N2222 has fixed the resistance measurements, but I'm looking for a source for the 2N3439 to restore the protection circuit. I am super elated that I was able to learn enough and deduce both faults. Flying high right now.Thank you to all who pitched in to help me out!!!