Scullcom Hobby Electronics #2 - Keithley 175A Multimeter teardown

[@klystron44]

Its so nice to have a site in a calm voice. Good topics too, wasted time on other sites before finding this one.

[@scullcomhobbyelectronics1702]

+klystron44 Thank you.

[@priestblood]

Fantastic tear down and explanation of the parts and what they each do .This has to be one of my favourite tear downs .Great Job. Thank you & as always stay healthy

[@cashewmilkfan]

this was an excellent teardown video.

[@deterdamel7380]

Excelent videos. I Hope he is doing well - he inactive for some years now.

[@AntoniousAutodidacticasaurus]

Would you mind explaining more about the ADC? So that's a semidiscrete SAR? How many bits is it? This is a 4.5 digit multimeter, and you swap leads to measure negative voltages, so 10^4.5*2 = ~63.245k, so it's a 16-bit ADC maybe? (did I do that right?) What do you think about doing a project to make a simple ADC in the future? Anyway, thanks for this video... this is quite a beautiful old thing (as all Keithleys are) :)

[@fabiotrevisan8922]

Hello, even though I'm reading your comment five years after, and I'm not the author, I think I can answer your question... The ADC converter of pretty much most meters of that era, is of the "Integrating" type, usually a dual slope or modified dual slope kind. Basically it applies a known voltage (the reference voltage) to a previously "zeroed" analog integrator, which will ramp up its voltage up to a certain point in a very linear fashion, then it switches the input over to the unknown voltage, which will make the integrator "run-down" until it crosses zero volts (this is where it all started). While it's running down it's counting how much time it's taking to get to zero, and the reading is proportional to that counting (because the run-up took a determined period of time that is related to the internal reference voltage). This particular model may not be exactly like that, but the overall idea is there. It may differ in details. The number of bits is basically the immediately higher power of two that can count to that number that is the maximum count of the meter so, for instance, for a 4 1/2 digits meter, it counts up to 19999 so it's a 20 thou count meter. 2 to the power of 14 is 16384, and 2 to the power of 15 is 32768... so it's a 15 bit counter that is not allowed to count all the way... Or it may count already in decimal (a BCD counter) so it will be a 17 bit BCD counter (16 bit for 4 digits of 4 bits each digit) and one additional bit to just count 0 and 1 for the 1/2 digit (the tens os thousands digit).

[@ResistanceIsUsefull]

The only thing I didn't see that you did not cover was what the display back-light looked like when it was turned on, unless I missed that, but I watched the whole video & I think you skipped that part. Does the back-lighting work? What color is it? Does it make it easier to see. Also, besides the increase in the number of inputs, can you tell me what the difference is between the 175A and the 197A are? I've searched internet forums, but have not found an explanation. Great video minus the back lighting.

[@scullcomhobbyelectronics1702]

The back-lighting is white which is OK but not very bright. It does however make the display easier to read. The main differences between the 175A and 197A is as follows: 175A is a 4½ digit display and the meter has an accuracy on the D.C. range of ±0.03%. The battery pack on the meter is the model 1758 which uses a 9.6 volt Ni Cad rechargeable battery. 197A is a 5½ digit display and the meter has an accuracy on the D.C. range of ±0.015%. Accuracy on the other ranges are also better than the 175A model. The battery pack on the meter is the model 1978 which uses a 12 volt Ni Cad rechargeable battery. This model also has OHMS Sense HI and LO for four wire resistance measurements. Maybe we can address the back lighting with another video. I also have the 197 model which is the same as the 197A but without the back-light.

[@scullcomhobbyelectronics1702]

I have just done a short teardown on the Keithley 197 compared to the Keithley 175 which may help. The direct link is below: kzbin.info/www/bejne/mGKpYpljls-soLs

[@klystron44]

If you can do calibration by storing data on board, and there was a external interface bus it would be so easy.

[@petereggimann]

Well made teardown 👍🏼 I got two of these DVMs and both are faulty :-( Hence I'm looking for the schematics of the 175A. The schematics of the 175 is available in the user manual and I wonder how much it differs from the 175A which (afik) has only the backlight and the updated cover as difference. Can you confirm this? Have you access to the 175A schematics? Cheers Peter

[@mk6595]

Schematics available in the manual directly from Tek: download.tek.com/manual/175_901_01A.pdf