Trials and Tribulations of a Geriatric Electronics Specialist

February 6th, 2010 by k8gu Leave a reply »


I have an old radio problem.  Make that three old radio problems…err…problem radios.

My first used radio was the TS-930S you see in this picture.  It’s been good to me, and I’ve tried to reciprocate, although I am a contester.  It hasn’t quite been the same since I put the third set of PA drivers in it and repaired the power supply.  Something deeper must be wrong.  I found out tonight that something deeper was indeed wrong, in the form of not one, but two, exploding electrolytic capacitors.  At least I had one of them in the junk box, but I figured I was pushing my luck and came up here to the shack to blog instead of blowing up another one.  The power supply pass transistors were assuredly toast since they were riding high at 40 volts—got that fixed.  But, it still pops caps.  Crap.  Something is amiss in the final PA bias circuit (MRF-422).  I don’t think that the ratings of the MRF-422’s have been exceeded.  So, it should be a matter of diagnosing the bias circuit and getting back on track.

The TS-700S at right is my most recent used find.  It’s in practically perfect cosmetic condition.  But, during the ARRL January VHF Sweepstakes, the output power started fluctuating before finally going to zero.  The T/R relay driver transistor failed, but there’s still something else going on—it looks like an ALC issue of some sort.  I did just finally find a service manual for this guy and that should help the diagnosis.  Although, the service manual is unusually vague about the purpose of some of the control signals that are passed between functional blocks.  This might take a while, too.

The other TS-930 actually works, except only on 40 CW.  On the other bands, the PLL unlocks.  KA5IPF suggested that I tweak the master oscillator to keep it off the ragged edge of unlock.  I’ve done that a few times and can’t quite hit the sweet spot.  My next plan is to pull the PLL unit out (a real pain) and reflow all the solder joints on the board since it comes from the early era of the TS-930, when cold solder joints were common.

I’ve pulled the FT-840 out to be the second radio for bands other than 40 CW.  And, I’m missing the Sprint tonight to spend time with my wife since I spent the better part of the afternoon tracing through the TS-700 and hooking up the FT-840.  Fortunately, I have the FT-840 configured as a drop-in replacement for a TS-930—this is in no small part due to the fact that the PIEXX boards use Yaesu-style BCD band-decoders—so, I can plug right in with the appropriate adapter.