Looking for someone with experience on adjusting the sine-wave shaper for lowest THD.
I recently obtained a good deal (eBay) on a 3311A function generator for about $50 "as-is".
It was in good physical shape, originally from a university laboratory.
At the low price, I was not surprised that I needed to repair it.
I was surprised that both 7818 (TO-3) regulators were blown, but I found some on eBay and the unit functioned well after replacing both 7818s and spraying all the switches (no useful output before cleaning the switches).
The adjustments went well (hard to read PDF from the manufacturer's site), but I have a minor problem with the distortion adjustment.
Before THD adjustment, following the manual, I adjusted the triangle amp offset, square wave symmetry, and tuning calibration (in that order) to reach specification.
I have an -hp- 339A THD analyzer, that reports the percent distortion directly, independent of the input level. Before adjustment, the maximum output at 10 kHz (10 x 1 kHz) was 3.6 V at -25 dB THD (4.25%). The manual warns that adjusting R149 can result in low output, and I found that to be true. I was able to achieve -30 dB THD at 3.5 V rms output (into 600 ohm load); lower distortion was possible, but the output fell below the 10 V p-p (3.53 V rms) level. The strange thing is the other two (positive-peak and negative-peak) pots: on both pots, minimum distortion was achieved at full CCW setting of the pot, applying zero bias to the diodes.
Should I have started with the peak-adjust pots? That seemed only to increase the distortion.
The manual only directs to adjust the three pots for minimum distortion, being careful with R149 not to fall below 3.53 V rms.