Ah-Hah!

While preparing to re-assemble the Right channel amplifier with new Output transistors and current limiting resistors, I re-tested all of the parts I intend to re-install and.... 

One of the BF857s, TR208,  has turned into two resistors and no longer tests as an NPN transistor.  That's odd, it seemed to test OK upon removal.

This sounds like a really good time to get ahold of some BF857s! 

I can imagine the rectifier passing AC or doing some other things it's not supposed to do, when working on the limit, especially after all those years!

Replacing it with a beefier one is imho a good thing.

success with finding that BF857, I had to order some from Greece!

Progress on the Beomaster 5000 which raises new questions...

After replacing the right channel output transistors and BF857 transistors as well as the bridge rectifiers the poor old Beomaster started up OK. 

Setting the idle current resulted in the right pot almost at midway, and the left pot remaining at roughly 2/3 -3/4 of travel. So, I replaced the left channel output and BF857 transistors, which brought the left channel pot to almost exactly the same position as the right one.  A quick test of the BF857 transistors revealed high hfe (DC current gain) of 130-177 on the three that still functioned as compared to the four new ones that all came in between 63-70.  The left output transistors had good readings, but I thought it best that they match the newer type ones on the right channel.

Q- Does the relatively high hfe readings indicate impending failure? Could it be an indicator of damage caused by problems elsewhere, or is it a likely root cause? Or is this a normal  variation of readings of well-used parts relative to the New Old Stock BF857s I obtained?

Fortunately, the schematic has several published voltage readings at various points in the final stages of the amplifier, and they all seem ok.

So, with the Beomaster hooked up to speakers for a couple of hours, all seemed well at moderate volume!  No exciting noises, explosions or smoke this time.   The main heatsink gets just a bit warm, and the new bridge rectifiers remain cool. The sound seems really good.. a little bit better, I think, than the Beocenter 7700 on the same speakers and input source. Something I was curious about, the performance of the FM tuner seems excellent, with a little better reception than the 7700 and better sound, leaving me wondering if the Beomaster tuner is just better, or does the 7700 need some further tuner work?

 I'm going to hold off on high volume until I complete the remaining adjustments.

So far as the root cause of the catastrophic failures, I would like to figure it out, or at least hear from someone who knows the Beomaster 5000.  In particular does my reasoning seem sound?  Other than the obvious, that the system runs hot by design and nearly 35 years have taken their toll on components, and ignoring the multiple exploded and dead electrolytic capacitors on the 5v systems from either age, heat and stress or an unknown catastrophic failure of regulation....     Does my theory of bridge rectifier breakdown destroying the output stages make any sense?  The strange mismatch between the specified rectifier (250v) and actual part (80v) leads me to believe that one power rail or the other was possibly dropping low on voltage or passing ripple... I'm sure the high contact resistance of the main relay didn't help!

Thanks for any insights, and hoping someone finds this thread useful....