good afternoon, ebclr and all the friend that are following this thread!
Problem (partially) solved. Some resistors were replaced, with wrong values, in the PA board, that was not working correctly (no AC output). Also, the reference diodes in the AC Converter board, were replaced by ordinary zeners (1N4735), so the reference voltages varies continually, the same occurring in the AC output. The same problem occurs in all frequencies. I am using an HP 34401A DVM to monitor the AC voltage in the output of the instrument, and for example, in the range 10V, the variation begins to occur in the 4th decimal.
Besides, the IC4 (in dc reference amplifier), the DC offset of this IC cannot be zeroed. As this IC is custom made by EDC, and has a different pin out of normal OPAMP ICs, it will be hard to find it to replace. Probably the most indicated replacement will be an OP07 OPAMP, but I´ll have to modify the original circuit.
I send an e-mail to Krohn-Hite (sucessor of EDC), and they kindly send me a PDF copy of the operation and service manual of this model. There are some differences, however it was possible to follow the manual in most part. I will ask their permission to upload this manual to a site like BAMA/Edebris, to make it available in the internet.
I thank you all for the help. But I intend to keep trying to maximize the performance of this instrument. To begin with, I have to replace the zener diodes with the originals, I found it in the irnternet, but it is very expensive. Maybe it will be better option to replace it with more recent IC´s, like, for instance LM329, that is a 6.9V V, with changes in the original reference circuit. Any suggestions will be appreciated.