Signal genarator Simpson 420

[Filippo52]

I have come into possession of a "Simpson 420" signal generator; the apparatus works but would need a good calibration.
I found the manual with the wiring diagram on the net but there are no instructions for performing a calibration and since there are various trimmers, it is tiring to go at random.
Does anyone have the calibration manual for this instrument or maybe they have one and have already done a calibration?

Thank you

[DeepLink]

Hi Fillipo

I had a Simpson 420D once. Found the manual online, but have never seen any "calibration" or "adjustment" manual
Mine was the "D" version, with internal NI-CD batteries (which leaked al over)

As I recall, it was not so advanced, so adjustment should be straight forward

I'll see if I can find the schematic tomorrow and guide you

Hans J

[MathWizard]

Do you mean this model
https://bama.edebris.com/download/simpson/420/simpson420.pdf

There's not many things to adjust in that, but they are labelled anyways. IDK what load size would be used to calibrate it, or any of that.

What kind of resistors and capacitors are in it ? When was it made, the 70's/80's ?

If you don't mind desoldering a few things, I'd check a few resistors to see how close they really are. Maybe some caps are wearing out too. But it shouldn't be like 1930's radio repair, where the resistor are way off and the caps are near dead and can't hold a charge anymore.

[DeepLink]

Found the schematic

These are the "trimmers" and their function
VR1   Frequency - Front Panel
VR2   Frequency   
VR3   Triangle   
VR4   Triangle   
VR5   Sine   
VR6   Sine   
VR7   Sine   
VR8   Sine   
VR9   Square   
VR10 Square   
VR11 Amplitude - Front Panel
VR12 Offest - Front Panel
VR13 Offset   
VR14 Charge Adjust "D" Version
VR15  +4,75V Adjust "D" Version
C5    High Frequency "compensation" x100K Range

You need at least an oscilloscope for the adjustment

Start by checking the power supply's.
They must be "clean" without ripple, otherwise replace the electrolytic capacitors.
They are quite old.
Anything will do - just same capacitance and voltage

If you have the "D" version check the batteries for leakage
and adjust VR15 for 4.75 V at the logic supply

Set AMPLITUDE to midrange
Select 0dB ATTENUATOR
Set OFFSET to locked (no offset)
Select TRIANGLE
Set FREQUENCY (VR1) to 5
Select x1k range
Adjust VR2 for 5kHz at the TRIG output

Adjust VR3 and VR4 for symmetry (50% duty cycle) at the TRIG output

Select x100k range
adjust C5 for 500kHz output

Select x1k range again

Select SQUARE
Adjust VR9 and VR10 for best square wave at OUTPUT
Sharp edges and 50% duty cycle

Select SINE
Adjust VR5, VR6, VR7 and VR8 for best looking sine wave on OUTPUT
(a scope with FFT or a distortion analyzer are handy here)

Adjust VR13 for 0 volt DC at the OUTPUT

The Simpson 420 is a Triangle generator, that can convert the signal into a Square or Sine wave
So don't start to adjust the Square/Sine, before the Triangle is working properly

This is a simple instrument, so don't expect world class performance
That said, I use mine for more than 10 years

Now check all ranges and functions to make sure everything is in order
To check the 30dB attenuator you need to load the generator with 600 Ohms (2 x 1k2 in parallel)

Don't go crazy with the trimmers, as you might get totally lost. So do note the position before you start (a simple photo is good practice)
(If lost - you can set  all trimmer to their midrange and go from there)

Hans J

[Filippo52]

thanks very much for you answer with instruction

However i did found from "Simpson" a tecnical document for calibrating

I have a copy , if you need one with more quality i can put it for you in a google drive.

P.S.
your instructions seems more easy touse then tecnical bullettin