Signal genarator Simpson 420 - Page 1
Forum section: electronics / repair

author: Filippo52
author_rank: Regular Contributor
timestamp: April 10, 2023, 09:59:59 pm
content: 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

author: DeepLink
author_rank: Regular Contributor
timestamp: April 11, 2023, 09:12:51 pm
content: Hi FillipoI had a Simpson 420D once. Found the manual online, but have never seen any "calibration" or "adjustment" manualMine 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 forwardI'll see if I can find the schematic tomorrow and guide youHans J

author: MathWizard
author_rank: Super Contributor
timestamp: April 12, 2023, 05:59:53 am
content: Do you mean this modelhttps://bama.edebris.com/download/simpson/420/simpson420.pdfThere'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.

author: DeepLink
author_rank: Regular Contributor
timestamp: April 12, 2023, 06:45:55 am
content: Found the schematicThese are the "trimmers" and their functionVR1 Frequency - Front PanelVR2 FrequencyVR3 TriangleVR4 TriangleVR5 SineVR6 SineVR7 SineVR8 SineVR9 SquareVR10 SquareVR11 Amplitude - Front PanelVR12 Offest - Front PanelVR13 OffsetVR14 Charge Adjust "D" VersionVR15 +4,75V Adjust "D" VersionC5 High Frequency "compensation" x100K RangeYou need at least an oscilloscope for the adjustmentStart 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 voltageIf you have the "D" version check the batteries for leakageand adjust VR15 for 4.75 V at the logic supplySet AMPLITUDE to midrangeSelect 0dB ATTENUATORSet OFFSET to locked (no offset)Select TRIANGLESet FREQUENCY (VR1) to 5Select x1k rangeAdjust VR2 for 5kHz at the TRIG outputAdjust VR3 and VR4 for symmetry (50% duty cycle) at the TRIG outputSelect x100k rangeadjust C5 for 500kHz outputSelect x1k range againSelect SQUAREAdjust VR9 and VR10 for best square wave at OUTPUTSharp edges and 50% duty cycleSelect SINEAdjust 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 OUTPUTThe Simpson 420 is a Triangle generator, that can convert the signal into a Square or Sine waveSo don't start to adjust the Square/Sine, before the Triangle is working properlyThis is a simple instrument, so don't expect world class performanceThat said, I use mine for more than 10 yearsNow check all ranges and functions to make sure everything is in orderTo 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

author: Filippo52
author_rank: Regular Contributor
timestamp: May 12, 2023, 05:52:39 pm
content: thanks very much for you answer with instructionHowever i did found from "Simpson" a tecnical document for calibratingI 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