[Boatanchors] National HRO-500 Intermittant

[Jim Wright]

I've got a HRO-500 with a strange problem and hoping someone out there has seen something like this before.

If used every day it works fine.  But if it sits for a few days, then when it's turned on it operates for only about 30 seconds.  For that 30 seconds or so the SSB or AM stations come in normally, and then the signal strength (and the S Meter) start to slowly decay until there is no signal received and the S Meter gradually goes to 0.  During the final few seconds the audio distorts - sort of like AM splatter  sounds like.  Problem occurs on all bands and filter positions, and both SSB and AM. The radio PLL remains "synchronized" (no red light from synchonizer) and the 12, 11, 10 volt power measures okay.  After the signal goes away there is some audio hiss but that's all.    

Turning the set off and on again a single time repeats the same thing.  However, after turning the power off and on four or five times the radio resumes normal operation and runs without problem.   

At first, I thought it must be a bad cap but it seems like if that was the case it wouldn't ever operate normally after the series of turning it off and on.  I wiggled transisters in their sockets without any changes but haven't replaced any.  It acts like the problem is somewhere in the RF path up to the audio amp.  

Anyone experienced like this with a HRO-500?

Thanks.