Solartron 7150 - Overload Issue

I already have a working one of these meters which I use all the time but I saw another one on eBay which was sold as spares fairly cheap (£35) which arrived yesterday;  so I thought I would try and repair it today.

When you power it on there are no indicated fault codes,  but the issue is whatever mode of measurement the meter is in the display will flash  "- OL -" or overload.

I checked the main voltage rails and they seem ok;  I briefly looked over the relays but I think I will need to look more closely at the analog input side of things.

...later that afternoon

Well, after a shocking afternoon literally with this unit (lol don't ask!)   I managed to narrow it down;  so I just reheated a load of pins around the suspect area and it started working!   The calibration looks not too bad at all;  the sticker on it says the last calibration was 2016 so it should be good!

Btw, if your TIL117 chip is labeled IC307 then you have a later model 7150 (serials 300921 onwards), and if it's labeled IC306 then you have the older version;  there are some very subtle differences in the power rail circuit.

All in all very happy! :)



[later that afternoon... ;) ]



Well, the story didn't end there! it looks like I was somewhat premature in my celebrations!!, later that day it started glitching out again with the same issue!   I initially started to think it might be a component acting intermittently - that would have been a real nightmare to find and fix;  but what I found was after tapping the board in various places I found that it would fix itself, then fault again.

So, I carefully tapped each component and narrowed the issue down to near the front edge of the board;  after flexing the PCB very slightly one way I could make it work and by flexing it the other way it would fail;   so,  I started looking for any broken tracks on the board;  the PCB looked ok, so I started tapping each component,  then, all of a sudden I noticed the leg on a resistor had separated from the body - so I used my desoldering tool and took it out and replaced it for a metal film one, I tested a few resistors of the same value to get a better even closer match to the original one,  but the broken resistor was certainly the root cause of the overload issue.

I left the meter running most the evening and it was fine so I will leave it on for a few days to give it a bit of a soak test.