talking something analog.
that Krohn Hite 4100
is a Wien-Robinson Generator, made with interesting parts,
so the resistors in the generator filter network are selected to 0,1%, C to < 0,15%.
Some C have Krohn Hite Brand, selected to +0 -4% in parallel with a little C to get ±0,15% there also...
The square comes from a comparator following the sine oscillator. We find 2 "IC" , black plastik blocks full of glue.
The instrument like high current amplifications, so it is warm and need 55W from AC
I have found it in the bay, "for repair". 27,-€ was enough to get the fun on the table.
"wrong frequencys, very distorted sine, square OK". Open Case and making some test.
picture1: distorted sine. true.
picture2: For the regulating of the generator this instrument use a opto coupler.
In the (wrong) application, it came with papers from 4100A, is this part made from a LED and a LDR. So this cannot make problems,
but we have the 4100 here, not 4100A... here the LED is a little bulb, and this bulb was RIP = this is the problem.
Get a saw and make operation of that opto coupler, full with transparent glue. Cutted the bad bulb also, the rest of its glas is still in the glue, and nside is the spare bulb ,
there was some experiments to do to find the right voltage and current , then I got from Conrad some little bulbs and repair it.
picture3: 10kHz
picture4: the full 1MHz , there was nothing before.
cal.: the distortion have to be < 0,02% , up of 100kHz <0,1% up to 1MHz <1%.
DC Offset must be set very exactly bec. it have DC coupling to the output, otherwise a spectrum analyzer can be damaged by connect.
Case sheets can be washed in the dish washer machine.