Keithley 168 "repair"

Yes. that is right. I have a Keithley 168 scope meter
that is in unknown condition.

I had found the old manual here, which is very helpful in my attempts to figure out the connections for the battery header, but got me absolutely nowhere.



Because according to the schematics (manual above), there are 7 1.2V battery connected from pole D to pole C (with pole C supposedly at 8.4V), and two more connected from pole A to pole B (with pole B supposedly at 4.8V)
However, the section “how to check batteries” invalidate the previous assumptions because the test pad B is connected to pole C (which is supposedly 0V, connected to the negative lead of the two BA-29 batteries), as shown on the picture of the PCB.

Which means that either figure 8 is wrong, or Table 2-2 and the description is wrong.

Which lead to my attempt to figure out which battery lead is connected to “ground”, so I start to probe the battery pins and the “Lo” pin (or GND).
So far, I have found that pin A is connected to GND when the device is NOT in “CHG” mode. Which is supported by the fact that when charging, the battery is connected in series (manual). Which suggest that the two BA-29 battery should be connected from A to B (B being the positive end of the BA-29), rather than the other way around and also validates the fact that the battery probe is on pole B.

So to summarize, this is my current conclusion. Black means confirmed, and blue means unknown.

Let me know if you have any thoughts over the orientation of the batteries (marked in blue).

The voltage requirements require a bit of finesse. I have 9V batteries and Li-ion batteries, as well as AAA batteries and USB input.
My current idea is to use a 2S lithium battery, one across D and C (supplying 4.2V, which is below the recommended 4.8V but way above the “low battery” of 2.5V) and another one across B and A (supplying another 4.2V, which is 8.4V). The two batteries at normal discharge sits at 3.7V, which when in series give 7.2V, still above minimum.
However, I currently have only one … nevermind. I can “borrow” the one from my Ti calculator. Let me know if you think this idea (of a 2S li-Ion) will work or not.

I found it in the schematics. Helpful? Maybe.
However, over here it also mentioned that the batteries are to be 8.4V and 16.8V respectively, which isn’t what that is displayed on earlier sections of the manual or the bottom of the case.
There is this regulator-like thing, but the schematic is too blur and raises more questions than answers.

There is a blown fuse on the input, the “mA fuse”. Rated at 1/100A 250V, I am pretty surprised they can make a fuse with such a rating. Guess just a bit of really thin solder wire.
If that’s the reason this device is thrown away, well, that’s a good chance that the device is working when it is thrown away.

so last night I tied together my 4.2V Li-ion and a (group of) 2.4V Ni-Mh batteries. I tied the Li-ion from D to C and the Ni-Mh from A to B.
Yes, the voltage is very very low (6.6 combined) but I was expecting at least something to light up.
Nothing happened.

Should I try 9V? I am worried that the 9V might damage the board. Where should I try it?

I regret not buying a Li-Po from Adafruit. That said, I do have two laptop packs lying around. I can gut that Dell one and extract the cells, but I hope I don’t have to because I am planning to restore my Dell at some point in the future.

Why worry about batteries for now? It’s a bench meter. The battery pack is optional, for portable use.

Get it working when powered from AC line voltage first and then worry about adding batteries later, if you need it to be portable.

Yes but I fear that it might “blow up”. Not because I am unable to wire the AC correctly (if I have a cord to begin with) but because of the antique circuitry (and possibilities of electrocuting myself)

If I can get it running via batteries, I don’t need to worry about the AC part or having the risk of electrocuting myself

However I think the steps must be taken in order to calibrate it (as it is required to connect Q107 to -12 on case)

Then don’t use LiPo batteries with it. Something that old would be designed to use 1.2V Ni-Cad cells (not even NiMH cells). If you use LiPo, you risk fire or explosions.

I think his concern is that it will blow up using AC.

@CDR_Xavier sounds like you need to take care!

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Yes, I realise that, but using LiPo batteries instead of AC is another way of possibly “blowing it up” or causing personal injury.

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Sure is … those LiPo batteries can be nasty.


Ugh I don’t like where this is going …
(touches battery contacts)
Hm. No spark. Probably good sign.

To be fair, everything about it looks … wrong. But it lit up, and it didnt deviate from the schematics.
I wonder what is the battery life of this …

Does anyone know the name of this type of connector (the type it supposedly has)