Pulsetek 232 AC Calibration Source teardown and easy fix

[rigrunner]

I picked up a broken Pulsetek 232 AC Calibration Source. Initially it powered on but had no output. The meter pegged hard right and the fan isn't any more.
The fix was very simple. A new fan, replace an opamp and clean things up.

As i can't find any internal pictures, schematic or manual for it i thought i'd take pictures and post them.

[rigrunner]

Under the top cover are two large transformers and enclosed shunt resistors with a cooling fan.

Inside the cooled enclosure are 7 power resistors.  6 x 75R in parallel, 3 per heatsink, and one 130R in series mounted on the lower aluminium plate.

[rigrunner]

The function switch on the front panel is a 6 layer rotary wafer. The large blue  390K resistors seem to be switched in for the voltage ranges.

Output level is controlled by 3 controls. Course is a 22 way rotary with a string of 100R. Medium and fine are single turn potentiometers.

The AC frequency oscillator is inside its own aluminium box.

[rigrunner]

Somebody has viciously oiled the fan and the oil has made its way down to the main board.
Isoprop cleaned the oil off without any trouble.

Main board has Bradley electronics on it rather than Pulsetek.

The TO-3 transistors on the rear heat sink are 2N3055 dated 1977.

And the meter goes up to 11

[cnqhdszq]

The photos is good,I had never  seen before  . What the accuracy is ? it is 40 years old now.

[Vgkid]

Interesting that it uses 22 resistors in the course adjust, instead of the expected 10(or 9).
Thinking about it, I know that in audio they use 22(or is it 23) as a divider.

[rigrunner]

I fired it up set it to 3V and logged some data on a 3457A to get an idea of how stable it is. (3V range is the least noisy on 3457A)

Given its age and that it has single turn trimmers for adjustment i wasn't expecting much. I've nothing to compare it against AC wise but it looks better than i thought it might. The frequency was set to 50Hz on the dial and it gave 58Hz out.

On the 10V scale each of the course settings should give .5V steps and they vary +/-2% or so