[AMRadio] AM-864/U

CL in NC mjcal77 at yahoo.com
Fri Aug 12 10:00:35 EDT 2016

Anybody familiar with this audio amp?    Was used in studio applications to maintain a constant audio level output placed between studio equipment and the TX and is apparently still highly sought after when you see what they want for it on ebay.

I have owned one for over 45 years, getting it in a govt. surplus sale in 1971 at NAS JAX,  and have used it on an off for different things, but now want to use it in earnest in a homebrew TX project.  Dragged it out and went through it, replacing all the coupling caps as they were all leaking, and performed the alignment procedures.  Putting in -35db (.066 volts) at 1Khz,  it is only supposed to deliver 0 DB  into 600 ohms, and mine gives +10 into a lab audio power meter set to 600 ohms.  It functions exactly buy the book in its regulation function maintaining 0 out over a 10db input swing.  The only thing I found was the plate resistors on the input push pull amp tube were 12K, while the schematic called for 47K. There is no evidence those 2 resistors were ever changed out.  Every other component is by the book, and the output is exactly 10DB higher as measured on the power meter, and by calculation from the RMS reading on a HP AC voltmeter. There is no reference to any authorized changes to the equipment.  Could those 12K's be the source of my extra 10DB?  I would think 12K would lower the output from 47K's.  I never did this test before, never had a manual.

Here's another question for those who ever went to an actual soldering class like those given in the military.  I was taught that on terminal boards, wires went to the bottom of the terminal, and component to the top to facilitate repair.  Alot of my military gear is just the opposite, including this amp.  Was wire on bottom/component on top not really a requirement of manufacturers?

Charlie, W4MEC in NC

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