Sierra Frequency Selective Voltmeter model 126B

This 14 tube modular piece covers 8 to 1620 KHz in two bands, made by Sierra Electronics division of Philco Corporation in Menlo Park, Calif. A note on the front panel says it was last calibrated 18 January 1973. The crystals indicate May 1965 on the date code and the voltage regulator pass tube date code hints at 1966. Has switch positions for AM and Upper and Lower sideband. Has two switchable modular crystal bandpass filters for 250 and 2500 Hz. The 2.5 KHz pass band makes for rather limited fidelity on AM, but the main purpose of this device is for measuring signal strength and it does that very well.I measured the oscillator at 2.215 MHz higher than the received signals. An IF at 2.215 MHz makes good sense since the IF frequency would be higher than any received signal. The alignment was off by a consistent 2 KHz or so. I pulled the lower shield off the oscillator section, and found two piston capacitor trimmers. Replaced the shield and adjusted those to correct the alignment for both bands with the front-panel fine trimmer and dial pointer adjustment knob at their center positions.

The 2500 Hz and 250 Hz positions are quite sharp for selectivity. I listened to AM broadcast signals using a matching transformer connected to the headphone jack and a cheap powered speaker. At such a narrow bandwidth, AM is obviously a very limited signal. My attemps at bypassing the crystal filter resulted in a bandwidth that was much too wide leading to a conclusion that the crystal filters are the chief contributors to the selectivity in the set.

It would seem that this set could be used for listening to SSB or CW signal from a boatanchor by way of its IF, typically in or around 455 KHz. Has anyone ever tried that?

Sierra listening to oldies at very limited fidelity on 1530 KHz



The modular setup inside the Sierra 126B.
The large tube is the voltage regulator pass tube.
The crystal filters are at the center with blue labels.

Repairs
As obtained, the Sierra was in working condition in that I could tune and listen to broadcast band stations with an antenna attached to the 600 ohm input module. Cosmetically, it is obvious that the set has been used and is shop worn. I decided it was not yet in need of new paint. One of the rack handles was broken off. Will check my "boxe de junque" to see if replacements can be found.

I cleaned the switches and controls with contact cleaner. The crystals for upper and lower sideband (1.5 KHz higher and lower than the IF) were out of their sockets and rattling around under the module shield. The contacts in the crystal sockets had widened. I crimped them a bit to assure a good tight fit.

I had no schematic or other documentation but, thanks to Kevin T., I have received a scanned schematic for the similar Sierra 125B which has been useful in troubleshooting the crystal calibrator.

Let me know how you used the Sierra and what value it may have as an SSB attachment to the IF of a BA receiver lacking a product detector.

Here's a note received from Stan W4SBA:
"Your Sierra Selective Level meter was used in the telephone industry to demodulate either telephone calls or else to monitor multiplex and radio (carrier or microwave) pilots. That is the reason for the 2250 position which is a CCIT standard.And the unit will work fine as a receiver, but it will also work wonderfully well as a tunable IF for whatever you wish to feed it.The only drawback is that you may not have an external if or baseband output in the event that you wish to make a different demod for it such as FM video, FM radio, wideband am and the like."