[Techtoolslist] Help with BK 545 circuit analyzer

[Chris Rinewalt]

Sorry, this is a bit long....

I picked up a BK 545 circuit analyzer that appeared to be operating OK, but
did have issues. For instance, upon first power up instead of displaying a
horizontal line, it had an asymmetrical oval shape distorted along the neg
Y-axis. It would display somewhat correct V-I curves for resistors and
diodes, but rather than a single line, signatures were ovoid an distorted.
Oddly, when I turned it on the next day, this issue was gone, but there was
a fair bit of noise along the curves.

I am still searching for a manual for the unit, but I was able to get the
schematic and service sheet from BK, so I tried to calibrate it. It was a
royal pain and some of the procedures I could not get to work properly to
meet the indicated specs (more on this below), but I have it operating
fairly well with two issues that I think are related.

First, the 60Hz sampling frequency cannot be adjusted to spec. I followed
the procedure and it just bounces all over the place (though if I engage
the low pass filter on the frequency counter it does settle down). To get
the clock, a TLC555 is used, with a fixed 20K Ohm resistor (R141) and a 50K
ohm pot (VR105) between the output and threshold (http://imgur.com/2Ez8C6N).
At 60HZ, F1 and F2 are lo, so it's just the resistor, pot and 470 pF cap
(C105) in circuit with the TLC555. To attain 60HZ, VR105 is adjusted to
attain an approximately 30.72KHz square wave from the output. I can adjust
the pot just fine, but it only outputs an signal of approximtely 30.2Khz
which gives an frequency of about 58.2Hz. Not bad, but the odd thing is
adjusting the pot one way has little to no effect while adjusting the other
way introduces a lot of noise into Q, which causes a noisy frequency output
that bounces around. It's odd, the signal is fine, turning the pot makes a
little difference for some travel and then at a certain point the square
wave gets all out of whack and the output sampling frequency bounces all
over the place. This happens up to about 48KHz or so (I forget the exact
freq) from Q when the square wave clears up again, causing the output
frequency to be about 75+ Hz, too far off. So, why is it that the Q output
of the 555 is 'clean' for only two small portions of the pots travel,
neither of which align with the required 59-61Hz called for in the service
sheet?

Then there is a second issue with the 60Hz signal. As I stated above, I had
a heckuva time getting this thing spec'd. I came to realize that the 4
other pots in other circuit sections used in the calibration are pretty
much dependent on  one another. The noise @ 60Hz was originally so bad I
figured it had to be an op-amp (TL084) or one of the other CMOS ICs. So, I
pulled and tested every IC in that part of the circuit (and then some),
along with the 50K pot. All tested fine, but I replaced them anyway. And
still there is noise. Even if I adjust VR105 to reach a stable 30.2KHz from
the 555, getting ~ 58Hz sampling freq, it still has so much noise that my
counter can't lock onto it without a low pass filter as noted above. When I
switch to one of the other sampling frequencies (400Hz, 1Khz, 2KHz) there
are no such issues.

So, here is where I am stuck. I did read somewhere that ceramic disc caps
should not be used in a 555 astable oscillator circuit, but I have to
assume BK engineers knew what they were doing and that is not an issue. I
did test the 22uF electrolytic (C104) on the +5v line and it is OK, but I
don't have anything to test the 470pF disc cap. I do know it is not
shorted. R141 is in spec (19.7K). There is some noise on the triangle wave
on pins 2/6 of the 555, which clear up when any sampling frequency other
than 60Hz is selected. I replaced the LS390 that Q feeds into but still no
change. So other than that what else in this circuit could be affecting it?