I grabbed myself an EDC 520A a couple of weeks ago and found a dry joint (well, entirely separated from the pcb joint), and a magic smoke precision resistor on the current circuit (caused by high voltage cause by the former joint) and once replaced and calibrated (thanks to the video from feedback.loop) everything appeared to be fine.
However....
1. The dry joint was on a quite bizarre resistor arch setup (photo attached) which I can't believe is original.
2. The stability was perfect once it had warmed up for an hour or so, then after two or three hours all went a bit wild (+/- 400uV on 10v range)
I initially suspected the zener, but it's actually rock solid ... I've checked with a heat gun and it's well within spec even with quite a wide range of temperatures.
So I then got out my dodgy Chinese thermal camera (which although it's not a Flir, is actually quite handy) and I discovered two different places where resistors were getting hot, and I mean 120 degrees C ... which to me seems wrong (interested to see if people think this is ok.)
Firstly there is a 10R in the 12v supply (but not on the schematic I have) prior to the 7812 that's dropping about 3v, so that's best part of 1W, it's looks fairly chunky (photo attached, it's between the two caps) but still seems like it's not coping.
Secondly there is the bizarre resistor archway, these are three x 4.3k, so just over 12k total, but they are dropping nearly 200V, so 3.5W or so ... but they are also well over 100 degrees.
I can't find a proper 520A schematic, but the 521 manual (which has quite a few differences) shows a 10K 5W resistor (although actually located in the power supply section) but schematically the same. I am really struggling to understand the workings of this section of the schematic (R307, Q301, Q302), but in all ranges I'm seeing nearly 200V dropped across that resistor arch with very little variation. I've ordered some new resistors and some MJE350's just in case.
I've watched feedback.loop's video, but his board is slightly different and doesn't show this 10K, so I presume it's in the power supply section, there are a few differences in the board so it's probably a different revision.
Then there's also a very bizarre set of mods on the back of the board, this looks like a strange attempt to deal with a different capacitor setup (photo attached) ... it's obviously working reasonably well, but I'm really struggling to understand what was going on.
And the other thing is that the schematic says +/-150V, the PCB says +/-130V, but I'm actually measuring +/-200V ... I did think it might be to do with the 110/220v switching, so I ran it back on a 110v supply (obviously with input switch set appropriately) and it was exactly the same.
So ... some questions I'd love to see if anyone has answers to...
- surely running components at 120 degrees C+ isn't right?
- does anyone actually have a 520A schematic?
- any ideas why I'm seeing 200V rather than 150 or 130 .. or is that right? That's definitely not going to help my hot resistor problem!
- any other help would be great!
I'm going to replace the resistor arch with a single 10K 5W component as per the schematic, 200V (measured) across 10K is 4W, so hopefully this should be ok.
I'm also going to beef up the 10R in the 12V supply ... I'm seeing 21V at the regulator, so I'm presuming the 10R is there to drop the voltage prior to the regulator, so I've ordered some 20R higher power versions as well.
This is a really nice bit of kit and very accurate, but for a limited time, so it's slightly frustrating. Any help would be greatly appreciated.