EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: ToHu on August 24, 2023, 08:41:32 pm
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Probably this is one of the cheapest option for isolated scope. So I bought few of those and spend some time to repair/calibrate them. The common issue with all of them are the A10 board electrolytic capacitors, they all leak and is quite an ugly picture. Based on my experience 5 of 6 cases the issue is related only to the capacitors and the leaks. Replacing the capacitors solve the problems, but in most cases the leak is all around so you need to remove all the components around and clean under. I try in my US bath it didn't help, also because of the leak a 0805 capacitors become a 30 Ohm resistors. So don't be lazy, take off all parts around, wash firmly and put them back.
The main problem after that is to calibrate the channel, the service manual is pretty much useless, since it miss a very important steps. There is a test point TP300, and you should set the offset and the gain based on that point, not the isolated output. Let it work for at least 20-30 min before you set the offset and the gain. You should let it work also 20-30 min after you assemble the AMP unit. At that point use R430 to set the isolated output offset, and finally if required to do small gain adjustment. R430 set the LED current, respectively the offset for the output. If you try to compensate a large offset with R210 as described in the manual, you will bring your preamp close to one end and it will start limiting there, also the LED will start work in a region that probably is not adequately compensate on the receiving side, so you will lost the linearity.
Attached is A10 schematic with some notes on the calibration and control bits meaning. Not all values are measured, some of them are guess values, especially the capacitors, but the schematic itself is more important than the components value, you can measure them from the board you have. Unfortunately there is no any information about the LED used, the only marking i have is BM and BH on the bottom side. The channel that is with BH diode do not work properly, you may pass initial calibration, but will fail at range calibration. Initially I was thinking the led was the issue but more and more I am thinking is not the proper type, probably a different bin. It works, but at some of the ranges the calibration didn't pass.
p.s. The BH led was scratched on the bottom side, so I suppose someone replace it. It can be from a different device, but i suppose if that the case they are some other differences as well in the LED driver and may be the receiver side amp.