By way of additional information, there's a good description of the neon chopper circuit in the 419A manual which I include verbatim below (T2 in the 419A = T1 in the 3420B):
4-43. Assume that DS1 lights when the input is applied to T2. Capacitor C1 charges until the oscillator switches the input, and DS1 goes off. When the oscillator switches again, the charge on C1 insures that DS2 fires, and DS1 stays off. This cycle continues with DS1 and DS2 firing as long as there is output from the oscillator. CR1 and CR2 prevent the capacitor from discharging through R1 and R2 This is quite different from the Fluke chopper circuits which do have the diodes wired for anti-phase operation (i.e. facing in opposite directions). Dave -----Original Message----- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of David C. Partridge Sent: 14 November 2017 09:37 To: 'Discussion of precise voltage measurement' Subject: Re: [volt-nuts] 3420B chopper drive problem In off-line discussion with Dr. Frank, it turns out that his proposal that the diodes should face in opposite directions was incorrect. The below is therefore wrong. Dave -----Original Message----- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of David C. Partridge Sent: 08 November 2017 14:08 To: 'Discussion of precise voltage measurement' Subject: Re: [volt-nuts] 3420B chopper drive problem 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-boun...@febo.com] On Behalf Of Dr. Frank Sent: 07 November 2017 16:32 To: volt-nuts@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 _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
