Keithley197 low voltages

Thread Starter

DarthVolta

Joined Jan 27, 2015
521
I'm checking the voltages on ebay DMM score, it' was working, but I'm modding it and had it open, then noticed low DC voltages. It's not the buzzer mod tho, I even unhooked it, since I don't understand the whole DMM.
http://web.mit.edu/8.13/8.13d/manuals/keithly-197a-microvoltmeter.pdf

And with 114Vrms in, I get 14.5Vrms out, w/ +7.3V and -10.3V at the FWR. I found the -V_rail zener had a bad connection, I took it out, cleaned it all, it's a 15V zener and it tested fine on my PSU. I put it back and the still, the voltage is down on both rails.

The -V_rail is moving a lot, from -10.3V up to -7V then -4V (hence I thought bad connection, which there was). There's not much 120Hz ripple tho. It's supposed to be 15V +/-10%

The +V_rail is 6.45V, but steady without much ripple. It should be 10V+/-10%

I had already checked the ESR on the electrocaps, before I knew about the bad voltages...I found the 250uF -V_rail cap was high, so I put in a 330uF just now, and there's no real difference, still only at +6.45V, and -10.35V (when it not moving by volts) But this zener barely turned on at all until 14.5V on my PSU. So maybe that's part of that.

The other caps seemd ok, 0.07 on the 1500uF



The trans. 2ndary windings read about 17 and 18 ohms, for the upper/lower windings (IDK which was which, I could put the LCR meter on it too)

I checked the other TP in the service manual, and at a glance, they seem as expected, given that the total voltage is down.

R131 is 10R, but reads 11R, which is just at it's 10% limit. The current is 85mA. R132 is 330R but is 310R, which is ok for these old style brown resistors. The current was like 10mA.


It's working tho...unless some solder ball went flying this weekend, it should be as I got it, and it all seemed to work great.

So now what ???


If it the transformer, its small, so should be easy to find a suitable replacement. It's also built to run off it's 12V battery pack, so that's nice too if I wanted t try making/modding a low noise PSU.
 
Last edited:

Thread Starter

DarthVolta

Joined Jan 27, 2015
521
I think it's just the transformer, I pulled it ,and with +/-15.5V into the rectifier it's back within spec

Hmm actually it's just the 240V switch. The only time I looked at it, I mixed up the background and the switch. It is indeed set to 240V, it was sold from the US too, so I never checked a 2nd time.

Well it was a good excuse to clean the old solder and make a nice set of +V/GND/-V cables for once, w/ banna plugs.
 
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