2014. október 1., szerda

Multimeterles 2. - Story of my stupidity

For today the power sourcing of my HP benchtop multimeter has been finished. Here is the finished 110V transformer:

Connected the multimeter to it. Switched on everything. Nothing.
I looked under the hood. There is a TO-3 cased 5V regulator in it.
On the output I measured a little more than 4.4V:

Because the Fluke just arrived and it is used I checked it with the Agilent also:

This is  ~12% minus. Doesn't look good.
On the regulkator's input:

Less than 6V.
The regulator itself is an:

HP 1826-0536. There is a National Semiconductor logo on it. On the internet I found a cross reference what states that is a LM340AK-5.
This is a normal regulator and not an LDO. And the dropout is:

2V. Based on this the regulator's input should be at least 7V.
What is on some miracle this is not a 120V unit.
I found a service manual on the internet. My gratitude to HP/Agilent/Keysight. On the actual website you can find correctly scanned the service manual of such an old unit.
In the service manual I found this:

And after some searching in the board also:

And what was the original setting? Of course 240V.
110V transformer removed, unit switched on, works.
If we are here, lets check some accuracy. My 5V-os reference:

Some reference resistance:

What is this reference resistor?

So I've a working bench multimeter and an unnecessary 110V transformer. It will be good when I got a real 110V something.

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