Hi,
Just wanted to share my attempt on repairing a Voltech PM300 since there is very little information on this bit of test gear.
For just over a year ago I posted a question about a
weird fault I had on my PM300. Dave's teardown of similar unit is in
. I even posted a similar post on voltechs
own forum. After this I got in contact with Voltech support service and got a quote for repair and calibration, which was way to expensive for me to warrant the cost for hobby usage. (I guess the price was resonate at a professional level but not for a hobbyist) (It is also a nightmare to ship stuff from Norway to the UK and back due to Norwegian customs...)
The unit has just been sitting on the shelf for around a year and until I found time to have a go at fault finding on it by myself. After a lot of searching on the internet I kinda gave up on finding a service manual or a circuit diagram for it. But since it has three identical channels it means that I can measure on a working channel to get a known reading to look for.
I must say that the design does make fault finding a lot harder, it it really hard to probe inside the unit when all PCBs is mounted together. The fault was also on the channel on the bottom card, which makes it even worse to get to.
After a bit fiddling with reading the data sent over the three optocouplers (Seems like one is for clock, and the two other is TX and RX) I found out that the controller was not receiving anything from the isolated channel. Probing around on the ADC and the shift registers, I quickly realized I had forgot rule number two: "Thu shall measure voltages." 5V rail was dead. The 2-stage transformer gives out (one one of the healthy channels) approx 10.8VAC at 166 kHz. but on my dead channel; nothing!
To be continued.
JAH
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