[R-390] R-390A low audio level with limiter off

[Leo Reynolds]

I’m trying to get my 1967 EAC R-390A back in service after audio level with limiter off dropped drastically. With limiter on audio level is normal. With limiter off audio is down 28 dB by my calculation. Audio level at speaker output (8 ohms) is only 20 mVp with local gain at max when the limiter is off. Audio is not distorted just way too low to be useful.

It looks like to me the limiter diodes both should conduct hard with limiter off, bypassing the limiters. Voltage at pin7 at the limiter switch is about 135V and pin 5 is zero, so the diodes should be on. But pass-through does not seem to be working.

What have I tried so far? I’ve replaced all the tubes. Resistor values in the limiter circuit are within tolerance. I thought it looked like C531 might be a little leaky (with tube out, limiter off and power on plate of V507B was about 38 volts while plate voltage of V507A was about 124 volts.) I replaced C531, C536 and C549 with 400V polypropylene caps. This did not change measured voltages or resistances and audio level did not increase. The hermetic West-Caps were not the problem. I have not tried to replace the mica caps in the circuit. I actually want to avoid unnecessary parts replacement. I can’t duplicate the original workmanship in these assemblies. I don’t understand why DC voltage at the two limiter plates should be different. With the tube out there should be no current in the plate resistors?

Has anyone here solved this problem before? Can anyone suggest the most likely productive way to proceed? What to look for? I have signal generators, oscilloscope, multimeters, etc. I have the 21st Century manual and the military manuals. I don’t have experience with these radios. I picked it up at a hamfest about a month ago. It seemed in good shape. The first oscillator and crystal oscillator didn’t want to work for a while, but after getting some newer tubes it came to life. I really want to get that audio back.

Thanks,

Leo