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Electronics => Repair => Topic started by: enut11 on December 21, 2019, 08:43:40 pm

Title: Intermittent Current Switches HP 3466A DMM [FIXED]
Post by: enut11 on December 21, 2019, 08:43:40 pm

I am in the process of restoring an old HP 3466A DMM (1974 manufacture I think).

Most of the DC amps switches are intermittent, especially the 2mA one.

Measuring the 2mA shunt resistor gives wildly varying results. It should read 100 ohms and only does so if I short the appropriate switch pins (see yellow crocodile clip).

The switches seem to be sealed so no easy way to clean them.

Can these switches be disassembled without de-soldering from PCB?
enut11

Title: Re: Intermittent Current Switches HP 3466A DMM
Post by: enut11 on December 21, 2019, 08:54:16 pm

HP3466A main board. Some of the amps push button switches behave intermittently.

Title: Re: Intermittent Current Switches HP 3466A DMM
Post by: wn1fju on December 21, 2019, 09:56:14 pm

My 3466A was even worse than yours - none of the switches worked correctly.  I surmise they tarnished from sitting around unused for decades. 

What I did was to drill very small holes at the top/back of each switch.  That allowed me to flood it with contact cleaner.  No problems at all after that.

In my case, the 3466A wasn't a piece that I was too excited about, so the prospect of actually ruining it by drilling holes wasn't upsetting.  You may
feel otherwise in your situation.

Title: Re: Intermittent Current Switches HP 3466A DMM
Post by: enut11 on December 21, 2019, 11:55:03 pm

This thread (post #14 and #16) shows how to repair sliding push-button switches after removing them from the PCB.

https://www.eevblog.com/forum/testgear/hpagilentkeysight-34401a-2w-problem/ (https://www.eevblog.com/forum/testgear/hpagilentkeysight-34401a-2w-problem/)

However, I am still hopeful of finding a solution that does not involve un-soldering switches or drilling holes in them.
enut11

Title: Re: Intermittent Current Switches HP 3466A DMM
Post by: enut11 on December 22, 2019, 12:05:46 am

a better view of the recalcitrant switches.

Title: Re: Intermittent Current Switches HP 3466A DMM
Post by: enut11 on December 27, 2019, 10:56:55 pm

I eventually removed one of the switches to see if it could be disassembled and cleaned.
Sadly no go, no way to get into it without breaking the top cover, so it was back to plan B?

Title: Re: Intermittent Current Switches HP 3466A DMM
Post by: enut11 on December 27, 2019, 11:03:59 pm

This involved drilling a small hole at the top back end of the switch and flushing it with contact cleaner.

Thanks for the tip @wn1fju  :-+

I found that if I pushed each switch fully in while drilling, this minimised any plastic swarf entering the switch.

My HP 3466A multimeter now works well on all ranges.

Happy ending