[volt-nuts] 3420B chopper drive problem

[David C. Partridge]

Interestingly the PCB is marked to suggest the schematic is correct!  And while the connections to the neons have been reworked, it would appear the diodes never have.

Reversing one diode gave me a +ve going square wave at TP1 from 0V.

Restoring that diode and reversing the other gave me a -ve going square wave again.

That didn't makes sense, so I checked the wiring, from the edge connector on the chopper amp board to the neon PCB.  Turns out it was wired quite unlike the schematic, and someone had reworked/moved the wire to board connections at both ends.  I disconnected everything, and reconnected as per schematic.   With diodes in the original position it was still wrong. 

When I then reversed CR2, I got a nice square wave that was asymmetric about 0V and I could adjust to +150mV/-75mV.

I think that this is the correct waveform for a 3420B, and that all the waveform diagrams in the manual for TP1 on the A2 board are correct for the 3420A, but totally wrong for the 3420B!!!

The specific figures in error are: Figure 5-8 (page 5-8), Figure 5-18 (page 5-18).  Figure 5-20 on page 5-19 is correct as it says correctly that it only applies to the 3420A!

Cheers
Dave
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Dr. Frank
Sent: 07 November 2017 16:32
To: volt-nuts at febo.com
Subject: [volt-nuts] 3420B chopper drive problem

Hello Dave,

in the schematic, the polarity of the diodes in series with the neon bulbs is definitely wrong.

If you compare with the 3420A on page 7-7 (29), but also with the Fluke
845 which is similar, both neons have to be switched at 180°.

Maybe someone else has erroneously reversed one of the diodes in your unit before?

If both neons are really firing in phase, instead, that could explain this non-symmetric waveform, which you observe.

Frank

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