AM-65/GRC
AF Amplifier AM-65/GRC
According to the Technical Manual TM 11-5039 the AM-65/GRC is a "lightweight, compact, three channel a-f (audio frequency) amplifier and electronic mixer". Apart from the first two adjectives, appropriate perhaps in the 1950s, the description omits to mention that the apparatus included its vehicular power supply that could also power the RT-70 tactical transceiver when necessary. The AM-65/GRC was in fact a component of the AN/VRC-7 Radio Set. It was used to mix, amplify and distribute the audio coming from two radio sets and an interphone, in particular when mounted on tanks or other vehicles. Of the two radio sets, the RT-70 was the default Set 2, whose audio signal was routed to the AM-65 via a special interconnection cable that also carried the power. For vehicular use the apparatus got its power from the vehicle's battery, which could have a voltage of 6, 12 or 24 V. To operate on one of those three voltages the operator had to install inside the AM-65 one of three vibrator power supplies (see below.) A fixed installation was also possible, with external A and B+ supplies. In this latter case the internal power supply had to be removed. The apparatus is completely watertight and four hooks are fixed to the case to allow the secure mounting of the RT-70 on the top of the case.
- Dimensions mm, 200 (w) x 327 (d) x 108 (h), weight kg. 7
- 8 tubes, including two gas regulators and one current regulator
- Power: 6/12/24 V DC (depending on power supply installed) or external 135 V B+ and 6.3 DC filaments
- Provision to supply DC power to the RT-70 transceiver
- Audio output power: 1.2 W maximum
- Output impedance: 600 ohms for all three channels
The relevant Technical manual is TM 11-5039. Click here for the 1956 french edition (50 MB) which is more complete than the 1951 one. If the link does not work please email me.
Block diagram
The block diagram shows the various internal components and interconnections. The three channels correspond to radios SET 1, SET 2 (by default the RT-70) and INTERPHONE. These three channels are mixed together and amplified by V1,V4 and V5. Two separate amplifiers are provided for SET 1 (V2) and SET 2 (V3), each channel being mixed with the interphone audio. So the interphone cannot be defeated.
The front panel with the various connectors. The 12V tag on the left is a reminder of the voltage the apparatus is set up for.The AUDIO connector carries the audio output from the main channel. The connector RT-70 POWER is used for the interconnection with the RT-70 (output audio and power.) The Cable assembly CX-1213/U (click to see) is used for ths purpose.
My AM-65 comes from the italian army and apparently was built in Italy by ELMER S.p.A.. Externally and internally it is in very good shape: it does look like not having being used much, perhaps not at all. It is set up for 24V battery power, so it uses the PP-282/GRC internal vibrator supply.
Preliminary Tests
To perform the preliminary tests I unplugged first the PP-282/GRC power supply. Since a typical problem of radio gear having been inactive for years are the electrolytic capacitors I started by applying a low voltage on the +135 V line (contact D of J-6). This tests sections B and C of C-1. I could not test section A. C-1 is the only electrolytic capacitor in the apparatus. Since C-1 is a plug-in type I could have removed it from its socket but I preferred not to disturb it. After several hours at +30 V the current dropped to less than 3 μA, which I considered acceptable. The fact that C-1 plugs into an octal socket is very convenient, in case that the capacitor should fail.
Then I measured the resistance of the LV line (contacts A/B of J-6) finding 17 ohm with S-2 in position "INT" and 36 ohm in position "RT-70" (the RT-70 was not plugged in.) I then applied +24 V to contacts A/B of J-6 having set the S-2 switch to "INT" position. The current was about 0.6 A as expected (see Fig. 10 of the TM.) I took note that the Amperite regulator (R32) was heating but not glowing, with a voltage drop of 5.7 V across it. The datasheet says that the current should vary between 0.58 and 0.625 A when the voltage drop across it varies from 4.3 to 9.7 V. These measurements indicated that the tube filaments and the Amperite tube were OK.
To complete the test it is necessary to build a dummy load to be plugged in the RT-70 socket J-3. A plug for J-3 is difficult to get, so I made a round PC board where I soldered 9 pins of rigid copper wire. The dummy load consists of two 33 ohm power resistors in parallel connected to pin F and a 1500 ohm power resistor connected to pin J (albeit the TM says 1200 ohm). It is also necessary to connect pin D and F together to ensure that the gaseous regulators V6 and V7 have their cathodes grounded. This short is already wired in the interconnecting cable CX-1213/U in actual operation. The 33 ohm resistors simulate the filament load of the RT-70 transceiver. For correct operation of the RT-70 it is also necessary to connect pin J to pin H. This is done by a jumper between J and H on socket J-4. Whith the dummy load plugged in the current on the 24V line stayed constant at about 0.6 A when S-2 was switched from "INT" to "RT-70" position, as expected.
 |
Measuring the filament voltage on the dummy load resistors. The J-H jumper on socket J-4 is also visible. |
Inserting the previously tested PP-282/GRC supply the B+ voltage (pin D of J-6) in the "INT" position was 160 V, which dropped to 132 V in the "RT-70" position, as expected. The RT-70 B+ voltage was 92 V. The current drawn at 24V was 1 A in the "INT"position, and 1.5 A in the "RT-70" position. So everything was ready for the test of the RT-70.
Vibrator Power Supply
 |
 |
To power the AM-65 from a storage battery three separate compact plug-in power supplies are available: PP-448/GR, PP-281/GRC and PP-282/GRC, for 6,12,24 V respectively. A synchronous vibrator is employed, thus dispensing for a rectifier. The output under full load is 135 V @ 117 mA. In my case a PP-282 was present. Unfortunately, despite the vibrator reed oscillated, the switching contacts were oxidized and did not conduct. So I had to open the can and clean the contacts with fine-grained sandpaper (#1200.) After this treatment the vibrator started working perfectly. I tested it with a load resistor of 1200 ohm as recommended in the TM and the DC voltage was 135 V.
Click here for the PP-282 schematics. .
It is very convenient that the filter capacitor C-3 plugs into an octal socket, simplifying the repalcement in case it should fail.
Despite the vibrator was fixed and worked perfectly its loud buzz was quite annoying. Perahps the vibrator made more noise than normal since it was impossible to put the can back into place correctly crimped. So I decided for a transistor replacement. The schematic is below (click on it to enlarge.) The frequency is determined by C1 and R2. I measured 119 Hz and I considered it within tolerances, since the mechanical vibrator oscillates at 115 Hz ± 5%.
The oscillator was mounted on a piece of perforated board and wired to vibrator plug. It is also possible to insert the can that stays in place just by friction. No heath sink is needed for the two power transistor. The replacement performs exactly like the mechanical vibrator - of course with zero noise! Two silicon diodes were necessary to rectify the AC voltage (see picture below right) and this required a little rewiring. The power for the 4001B comes through pin 4 of the vibrator socket that in my PP-282 was left unconnected. If one wishes,the mechanical vibrator can still be plugged in and used, with the rectification occurring via the diodes and no longer via the synchronous contacts.
 |
 |
To be completed
| Data | RT-70A (1) | RT-70A (2) |
| gennaio 2022 | 0,58 W | 0,57 W |
| aprile 2023 | 0,60 W | - |
| luglio 2023 | - | 0,55 W |
Schemi
