The General Radio 1232-A Tuned Amplifier and Null Detector was primarily intended for use as a detector for bridges. It is a low-noise amplifier and a filter that can be tuned from 20 Hz to 20 kHz, or set to 50 kHz, 100 kHz, or a flat frequency response.
The manual notes that “the Type 1232-A can also be used as a detector of high-frequency modulated signals (with a crystal demodulator), a wave analyzer at audio frequencies, and a preamplifier for transducers. Usable to 10 Mc by addition of Type 1232-P1 RF Mixer.”
Here’s the block diagram from the manual, which says “[t]he Type 1232-A consists of a low-noise preamplifier, a frequency-selective stage (feedback amplifier and null network), an amplifier-compressor stage, and a meter-rectifier circuit (see block diagram, Figure 2). The total gain of the amplifier is about 120 dB. Full-scale meter sensitivity is 1 microvolt or better over most of the frequency range.”
There are a few scans of various versions of the manual for the Type 1232-A scattered around the internet. The oldest manual I’ve found is from April 1962, and the most modern is from 1992. Here’s my list of URLs, if anyone knows of other scans please let me know.
- April 1962
- August 1963
- 1988
- 1992
Here’s the internals. As typical for General Radio, it has all high-quality construction: film capacitors, germanium transistors mounted in sockets, 1% tolerance capacitors in the two Twin-T fixed frequency filter networks, good shielding, etc.
The Type 1232-A was introduced in the July 1961 issue of the General Radio Experimenter. The 1 µV full-scale sensitivity was apparently a big deal back then. The vacuum-tube predecessor to the Type 1232-A, the Type 1231-B Amplifier and Null Detector, specified a sensitivity of 25 µV for a 1% indication on the meter in semi-logarithmic null-detector mode, and 8 µV for a 1% meter indication in sensitive linear amplifier mode.
The tunable filter was quite an advance. It uses a then-new R-C notch network invented by Henry P. Hall that can be tuned by a single potentiometer. However, that potentiometer is something pretty special – it must have an exponential taper of over a 100-to-1 range! Of course one of General Radio's specialties was precision specially-tapered wire-wound potentiometers, so it was no problem for them to produce it. The potentiometer is the gray cylinder in the upper left of the above photo.
Here’s a view of the bottom. The rotary switch on the left is the filter frequency selector. The input preamplifier is contained in its own shielded compartment at the lower right in the photo.
At the center of the right-hand side of this photo, you can see the phone jack for the insertion of an external filter. The dark tube is the battery compartment.
Note the external filter phone jack.
The predecessor Type 1231 Amplifier and Null Detector didn’t have any built-in filtering, but instead used external plug-in tuned circuits: the Type 1231-P2 and -P3 fixed-frequency filters and the Type 1231-P5 switch-tuned adjustable filter. The -P5 could be tuned to other frequencies by the addition of an external capacitor. These external filters can still be used with the Type 1232-A.
The GR1232-A was designed to be powered by nine M72 mercury cells. The Mallory type number was RM-4R. On newer versions, General Radio offered a rechargeable NiCd battery instead of the mercury cells as a special-order option.
Due to environmental concerns, mercury batteries are no longer available. IET still sells refurbished Type 1232-A's, but they now fit them with with eight AA-size 1.5V alkaline batteries. IET apparently also sells a kit to retrofit older instruments to use AA batteries.
Conrad Hoffman has refitted his GR 1232-A with NiCd button cells. The GenRad group on groups.io also has a PDF in their files archive that uses three cylindrical three-AA cell holders and a modified battery compartment cap.
I wanted to maintain the original appearance with no internal modifications, so I created an adapter to fit modern CR123 lithium cells to the 1232-A battery compartment.
The adapter is made from various sizes of model rocket body tubes and a pair of model rocket centering rings and a few other odds an ends.
I also printed a wrapper on Kodak ink-jet sticker paper.
The positive end is a 1.25” diameter #10 size zinc-plated steel fender washer. I turned it down to 1.20” diameter using my drill press and a file. I also used a center drill to bevel the inside edge of the center hole, so that the button-top of the CR123 cell self-centers in it.
The adapter consists of 5.75” lengths of Semroc ST-11, ST-7, and ST-6 body tubes. I bought them from eRockets.biz, along with laser-cut centering rings for ST-7 to ST-11.
If I had it to do over again, I’d make the tubing a bit shorter, maybe even as small as 5.625” (5 5/8”). Nine M72 cells should be 5.9” long, and I sized the tubing to make the overall assembly that length. However, the Type 1232-A battery compartment is also tightly sized for the 5.9” length of the battery, and so doesn’t leave much margin for error. My examples are a tight fit and fully compress the spring on the door of the battery compartment.
The ST-6 is a slide-fit inside ST-7, and brings the inside diameter down close to the diameter of the CR123 cells. I glued the length of ST-6 inside the ST-7, then glued centering rings on the two ends of the ST-7. It’s important to use white glue (Elmers) instead of yellow carpenters glue, as the yellow glue shrinks when it dries and the shrinkage will constrict the internal diameter of the tubing. Finally I glued the centering rings on the assembly inside the ST-11 tubing.
I used 5-minute epoxy to glue the fender washer on the positive end of the adapter.
The button-top on the CR123 centers in the hole in the fender washer.
I made an assembly from a 10-32 x 1/2" brass machine screw, three 10-32 brass nuts, two 1/4” brass washers, and a 5/16” brass washer to act as a spacer to fill out the negative end of the adapter.
I soldered two of the nuts to the washers, but really the only one that needs to be soldered is the 10-32 nut on the 5/16” washer.
The spring on the door of the battery compartment bears on the 5/16” washer.
I finished up with a wrapper that I printed on sticker paper. I made three adapters. The first one ended up with a wrinkled wrapper, but I got the hang of it for the last two.
Update:
It didn’t take much to restore this 1960’s-vintage instrument back to working order. Other than providing battery power, all I had to do was exercise the filter selector switch a few dozen times to clean the contacts.