Megger MIT320 faulty low Ohms continuity range FIXED!

[Signal Simon]

I'm trying to repair my Megger MIT320 which has developed a fault on the low Ohms continuity range.
The high continuity range works correctly.
Insulation ranges also work correctly.

The instrument outputs a test voltage just under 5V.
The sensing circuit sees that the leads are connected to a circuit and operates the internal relay.
However, on low Ohms, the test current is only 1.5mA, but should increase to about 205mA if the applied resistance is low enough.
With null not active, negative zero is flashing.
With leads nulled, displayed readings are a fraction of the applied value, presumably due to the low test current.

Any hints or tips, maybe a schematic would be gratefully received.

Someone did say this was a common fault caused when the instrument was applied to a live circuit. But that is supposed to affect the K Ohm range too.
However this wasn't the case, I was in the middle of testing a whole lot of cables which weren't connected to anything other than a row of wago terminals each end, with no links in.
Also the instrument is supposed to be idiot proof, inhibiting the test and automatically switching to voltage range of any voltage is present.

TIA,

Simon.

[coromonadalix]

there is a thread here : https://www.vintage-radio.net/forum/showthread.php?t=112071 

It may help you ??

[Signal Simon]

Thanks for the info.
I had found that post a couple of weeks ago, but it relates to the FET that drives the transformer for the HV insulation ranges.

There is another FET with 5 paralleled 20 ohm resistors adjacent, which looks like it will be the 205mA current source.
I will try and do some reverse engineering later....

Simon.

[Signal Simon]

I think I'm getting somewhere.....
That FET with the 5 paralleled resistors next to it was part of the current source.
I shorted out the FET and the test current rose to over 500mA.
I traced the gate drive back via some resistors to the LMV324 op amp pin 7.
Pin 7 was high at 4.96V, so I checked the inputs.
Pin 5 (non inv) was also at 4.96V, which looked wrong.
From the circuit layout there was an obvious voltage divider.
After checking the resistors connected to pin 5, I measured between pins 4 and 5 of the LMV324.
17 Ohms either way round. Not good.
Removed the LMV324, the short is within the IC.
Hopefully a new LMV324 will restore to full working order.

Simon.

[Signal Simon]

Replacing the LMV324 did make a difference....
I now had a test current just below 500mA.
Further testing found that I had damaged one of the adjacent resistors during repair.
This was replaced and the low Ohms test current now regulates correctly at 204mA.
The transistor I referred to as a FET is actually a PNP bipolar, NSB9435.

I'm happy now, saved myself a lot of money!