AN/TIQ-2 /A/B Public Address Set manual TM11-2586
CAD Schematic for Power Supply PP-31C/TIQ-2 (not included in PA system manual)
The power supply converts 6 or 12 volts DC vehiclular power to the high voltages required for the amplifier and record player.
The following notes and some simple, invisible, and reversible modifications to the AM-20 amplifier are worthwhile. While the system is intended mainly for voice, a good turntable and a radio input jack are included in the AN/TIQ-2 system for a reason. Unfortunately, the amplifier does not reproduce music well. No disrespect to David Bogen.
Note: If you are not comfortable working with 400 volts DC, 950 volts AC, and objects heated to 175 degrees C, don't go poking around in the amplifier when the power is on.
1. Adding a 3.3 megohm resistor from pin 4 (the 250 ohm output) of the output transformer to pin 4 of V3B the 6SL7 phase inverter will provide a small amount of negative feedback to improve the sound quality of the amplifer. No kidding, the amplifier has no negative feedback at all and it needs some. The amplifier can be unstable under certain conditions and can oscillate at supersonic frequencies. Placing a 10 to 27 picofarad capacitor across the 3.3 megohm resistor can help this by slightly reducing the high frequency response above 20KHz. The devout enthusiast can measure the response by setting all gain and tone controls to midpoint and using a sweep generator connected to the grid pin 1 of V3A the 6SL7, an oscilloscope, and a 250 ohm 25 watt resistor as a load. These tweeks are useful because the output transformer is rated for 50Hz to 10KHz, and depending on the reactance of the load, it can act a bit strangely above 25KHz. So, let's clean up the overall response.
2. A 40uF/450V capacitor can be added across C17 to reduce distortion at high power levels, especially following a loud percussive sound that might drain C17 the 4uF capacitor. When this happens, the whole amp starves for power and it takes a short time to recharge the capacitors for the low voltages used by the preamp and drivers. The result is a 'chopped off' sound following the crescendo, or in the case of a sustained note, an annoying modulation by power supply hum.
3. While you are at it, add a 40uF/450V capacitor across C18 too.
4. Adding a 10uF 450volt capacitor to pin 4 of V5 the 6L6 screen grid will help reduce distortion at high volumes.
5. Don't add anything across C19, or the high voltage will increase too much. The amp is designed for 115VAC, and we have 120-125VAC these days, so don't try to increase the voltages on the old components, they are already high enough.
6. Use 6L6GC tubes. The high voltage is 400VDC, a little too much for the 360V 6L6G. In place of the 6L6, The 5881 may also be used. 7027's and 6CA7's should not be used. Try to match the tubes because they use a common cathode resistor R20 for bias and you don't want one tube hogging the current.
7. Some of the capacitors use PCB oil, and may have leaked. Just replace them, clean it up, and place the capacitors and the cleaning materials in a plastic bag. Go wash up, and do a good job of it. Then, you may cheerfully present the bag to your local waste center during your municipality's Hazmat Collection Festival. Once you tell them it is a bag of "small PCB capacitors" (thereby invoking the EPA term), they should in turn cheerfully accept it from you.
8. Lastly, and if you do nothing else, make sure the blower is working. If it is not, the amplifier will overheat. If you have nothing else, you may be able to mount a small 3.5" AC-operated fan behind the exhaust grille at the top left corner of the front panel to blow the hot air out of the cabinet. Use internal brackets you make yourself, please do not drill the panel, ya heathen! Don't forget to remove and clean the air filter intake at the bottom of the front panel.