Electronics page 2 (started December 2013)

Bumblebee and Black Beauty capacitors are known to be unreliable so I removed all of them from both the preamp and servo amp PC boards and replaced them with new polypropylene and polyester capacitors (orange caps in the pictures below). I tested the old capacitors and found three bad ones. The electrolytic capacitors in the servo amp (bottom right in the second picture) were also replaced with new electrolytic capacitors. I'm also using a 6AL5 dual diode tube instead of the dual silicon diode plug-in (V203) that was in the instrument when I got it. Tube diodes turn on a lower voltage compared to solid state diodes, also using the 6AL5 seems to provide better sensitivity when used.

preamp; the components above the PC board on the left is the twin T network discussed below

servo amp

Twin T networks

I tried putting a 13Hz (rotating sector mirror/chopper frequency) twin T network in the second preamp triode (V101) circuit arranged to provide negative feedback through the twin T network to convert the stage to a narrow bandwidth 13Hz bandpass filter. R103 was removed and the twin T network was connected between the triode grid (pin 7) and the low voltage side of C106 (connected between the triode plate and gain control R201). Click here to see schematic. This reduced noise significantly, improved stability and resulted in better resolution when scanning spectra. Smaller IR bands are now visible, especially when scanned at slow speed. It was found that the preamp had higher gain with C101 (across input transformer T301) removed so this was left out. This might be because the capacitor and transformer combination wasn't adequately tuned to 13Hz. Two 60Hz twin T networks were originally provided in the servo amp circuit, one between gain control R201 and V201 and the second one between V201 and V202. This provides two notch filters to reduce 60Hz hum in the instrument.