FG501 repair

Kuba Ober <ober.14@...>

I've got a FG501 S/N B127250, ex-University of Louisville EE dept. Here's what
I did to get it up and running (interspersed with service manual calibration

0. The thing looks pretty beat up (ex EE student lab), had to get all the
knobs off, clean them with a toothbrush and windex, and clean the front panel
in the same fashion.

1. Power light out. +20V checks out OK, bulb out, so change R495 to 1.02k,
DS495 to a green LED.

2. Visually inspect: broken connecting wires to R27 (frequency vernier), also
loose mounting nut, resoldered and retightened.

3. Check out power supplies, stupidly enough missed DCPL -20V. Adjusted +20V
and current ref. 13.8V, all supplies (miss one) are fine.

4. No output, so check current sources and diodes with a multimeter,
everything is fine (current sources source/sink current, diodes are not

5. Remove U80, feed 4V/1kHz/450Ohm cal signal (from scope cal output) to
U80:13 to get diode bridge current switch to flip back and forth.

6. Set frequency to 10^1 x 10 to get a triangle on TP6.

7. TP6 shows 2Vpp triangle @ -6V, same at source of Q120 and emitter of Q125.

8. Noting is coming out of the following transistor, so check biases and DUH
-20V DCPL is dead. U420 (ua741) turns out to have the output pin (6) at 0V.
Had no 741s handy, so replaced U420 and its companion with OP177.

9. U195:3 <4> seems to be low, but methinks it's OK as it's below B130000 and
waveforms really apply to B130000 and up. Output span checks out OK at
+/-7.8V (compared to minimum +/-10V for B130000 & up).

10. Output [amplitude] and offset pots are very noisy, heating them up just a
bit with the heat gun and rotating them a few dozen times back and forth
seems to have gotten them back into shape.

11. Could not get 10^3x10 R20 in range, frequency is 4% high, so change R18 to

12. After calibrating 10^3 and 10^5, check of various ranges shows 10^2 range
to be consistently 2% low in frequency.

13. Suspect 10^2 timing capacitor C74 0.1u/100V (bottom of board). Desoldered
it, and replaced with a mechanically larger capacitor from a junker 5B12
timebase plugin. The x1 and x10 frequencies are now within .5%.

Replacement BOM: two resistors (what a luck: they were adjacent!), one
precision cap, one LED, two opamps.

All in all, I think that I'm pretty lucky as at x1 and x10 all the frequencies
check out well within +/-1%. This is a nice, simple instrument and it was a
joy calibrating it.

Cheers, Kuba

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