Author Topic: Hameg HM408 calibration methods - question  (Read 128 times)

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Offline sordid_resistor

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Hameg HM408 calibration methods - question
« on: June 19, 2021, 04:35:21 pm »
Hello all - I have a Hameg HM408 that my kind neighbour gave me, we're both are pro electroincs engineers and he had to leave planet earth (RIP).  :(

I've used it for general fault finding and it all seemes to work fine. However I come to do some measurements and notice it's out of cal. I feed it an accurate 1kHz and it's off, and so on for most of the ranges. Looking at the circuit diagram it seems this scope is hybrid where the Rs used in the X timebase are switched in like a binary counter from the digital part. This means each "bit" has it's own weighting and therefore each bit needs to be calibrated! (hand select resistors) Is this correct? Is this how Hameg expects us to calibrate the thing?

I have set the X-Timbase Cal preset using 1kHz test signal and that is now correct but changing X-timebase still makes all other readings wrong and I can't find any other way than to hand select resitors for each of the "bits". Bit of a faff.

I also notice the on board calibration oscillator is off freq, it looks just to be a simple CMOS gate osc so changing the Rs is easy enough.

I have the circuits/manual, given them a good looking over and there's no clear instruction on how to calibrate the X-timebase.
 

Offline tautech

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Re: Hameg HM408 calibration methods - question
« Reply #1 on: June 20, 2021, 12:04:34 am »
As with any older instrument you should first be checking its PSU meets service manual specs exactly.
So many old bits of kit fall back closer to spec when the PSU is reworked and adjusted to spec however scope componentry does age so in the absence of trimmers somewhere they can be difficult to align correctly.

I'd be surprised if Hameg made no provision for accurate alignment however for the probe compensation output no provision for adjustment is not uncommon as what only matters is a fast edge and flat top so its absolute values only matter as a sanity check not a precision output.
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