Products > Test Equipment
Yokogawa WT1600 Digital Power Meter
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capt bullshot:
Just out of curiosity, the voltage input channel:



See the small copper areas above R72, R73? These are opposite to the resistive input voltage divider string and most probably do some frequency response compensation magic here.
The rest of the circuit looks different from the current channel, too. One thing is in common - there's a shift register (BU4094) driven through some slow speed optocouplers controlling all the analog switches and muxes for the range switching.
capt bullshot:
So, here we go with some insights:

This is a partial schematic I've drawn on the input structure, in particular the switching and protective circuit for the BNC and the shunt.
The BNC enters the schematic top left - signal goes through a resistive divider with some frequency compensation caps.
The unknown (K52 marking) SOT23 devices look and feel like J-Fets used as switches. The OPA27 buffers the downscaled signal.
Then some DG444 analog switches route either the scaled down BNC input or the voltage across the shunt to the main amplifier (to Q101).
The shunt input circuit features some protection diodes plus a not yet investigated additional circuitry along a tap into the protection diodes.



So clearly both signals (BNC and shunt measurement) are routed through the same amplifier / gain switching circuit. The BNC input signal is scaled down to the same level as the shunt signal before input selection takes place.

As all ranges at the BNC input work just fine, it's an easy conclusion that the main amplifier / gain switching is working fine, too. So the troubleshooting can now continue along the area of the protection and the uninvestigated circuitry.

BTW - the 51R in the shunt signal path had been replaced by the former repair attempt. So I'd guess, the unit has seen some serious overcurrent event.
As far as I did test the shunt, this important component still looks OK.

capt bullshot:
Continue with live probing - at the moment by soldering short wires to interesting points and putting the board back into the unit (I dislike this method).





So what can I see now:
- no influence of that unknown circuitry around the protection diode on the shunt signal (by probing that 51R resistor).
- The JFet / LT1222 amplifier has switchable gain and appears to work in BNC input mode as well as in direct shunt measurement mode
- as for the largest ranges there's no visible signal at this amplifiers input and output, there might be another signal path

Interestingly, while switching the lower ranges (while the signal is routed through this amp), one can see this amplifier behave exactly the same in both input modes. No gain or offset errors visible.
This points to a conclusion that I don't want to draw right now, so I'm trying to find some more useful test points now.
capt bullshot:
Moving on, next test point is the primary winding of the isolation transformer.

The unit isolates its signals by the means of transformers, as one can see from the block diagram, too. There's three transformers per channel: One does the supply job, next transfers the carrier frequency from the digital circuitry to the front end amplifiers and the third transfers the modulated analog signal to the ADC circuits.

So I'm tapping the output transformer, using two channels of the oscilloscope as a poor mans differential probe. Works nicely here.
On the scope screen the modulated signal, it's a very simple scheme: the signal gets inverted with each half period of the carrier. So there's no DC across the transformer, and demodulation is easily done by an synchronous demodulator.



This is just one screenshot, I did one at each input range and input mode for comparison: This signal is absolutely as expected, switching gain through the ranges, no gain error, no offset error.

Conclusion: The isolated side of the current measurement path works fine, both in BNC and shunt measurement mode.

Still there's some large errors (gain and offset) displayed at the instruments screen when switching to shunt mode. What might be wrong now?
I suspect, there's no difference in the analog signal path on the non-isolated side between BNC and shunt mode. One thing left: Digital calibration constants - without knowing how to access these or how to adjust (calibrate) the instrument I'm at a road block now.

For the brave, one might check the ADC input once more, I'll see if I can find a suitable tap.

Edit: Confirmed, there's a perfect signal at the ADC input, no offset or gain issues. So it's a matter of adjusting / calibrating the unit. I couldn't find any calibration menu, nor description of how to do this through the GPIB, so I'm at the end of this journey for now.
Anyway, depending on the method to calibrate the unit, it can be quite challenging to provide the necessary signals.

TERRA Operative:
For service mode, try switching on the instrument while pressing display button. After start up, press shift and then the display button.

https://www.eevblog.com/forum/metrology/yokogawa-wt1600-power-meter-calibration-mode/
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