1. RADIO FILTER. An improved radio filter can be added to all amplifiers that use the LM 391N integrated circuit. There have been reports from many areas of the country about radio pickup. Inaudible transmissions can be picked up by audio circuitry and cause unstable operation. We feel this simple mod will reduce the possibility of damage to the power output stage and/or power transformer that can result from pickup of satellite, radar, microwave, or cell-phone transmissions. (This is especially applicable to units that are used on ships at sea.)

Procedure: Locate pins 1 and 2 on the LM 391N IC. Tack solder a 220 pF, 50-volt ceramic capacitor to pins 1 and 2 on the bottom side of the board.

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2. BIAS STABILITY. The quiescent bias of the power amp output stage may sometimes go out of range due to the effects of shock and vibration, over-driving, extremes of temperature, or physical damage to the unit. The problem sometimes appears to be intermittent.

Procedure to correct or prevent potential problem: A circuit that includes a Tip 31 transistor (Tip 29 on some later units) sets the bias point of the output stage. This part is located in the center of the row of three TO-220 transistors on the main heatsink. The leads are connected to the board by a connector, which is soldered to the PC board. Over time, the point of entry to the PC board of the connector pins may develop bad connections, or ring voids. This usually happens on the TOP side of the board. Re-solder the connector from the BOTTOM of the board, adding a small amount of solder to assure that there is re-flow of the solder joint at the pad on the top side of the board. (This pad is just visible under the plastic body of the connector.)
Check to see that the solder joint is smooth and shiny, and that there is a meniscus, or curved solder fillet, between the pad and the lead. If you do not feel certain that the connection is good, simply melt away a small portion of the plastic housing with a soldering iron, and re-solder the joint to insure a good connection.
To be thorough, locate the other components associated with the TO-220 transistor, including the bias adjust trimmer and two º watt resistors, and re-solder those on the TOP and bottom of the board as well.
Re-check the bias setting. Adjustment of the bias is done by connecting a DC voltmeter between the emitters of the MJL 21194 and MJL 21193 transistors. (MJ 15003 and MJ 15004 on earlier units) Rotate the trimmer until a value or 2 to 3 millivolts is read on the meter. The setting should be made with no load connected to the output point.
These steps will insure reliable long-term operation of units that are exposed to radio transmissions, or which are subjected to vibration from hard use or transportation.