ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Back onto my Beomaster 5000 restoration project, and an interesting find...
After having gotten the 5000's processor running, I went on to examine the rest of the system. Several more electrolytic caps were high ESR, so I ordered up a bunch and some trimmer pots that would be most likely to cause significant trouble, particularly the idle current and 5v regulator pots.
Put it together, set the 5v regulator, go to set the idle current and... Nothing! It seems there is no -40v on the board, though it is present at the -40v 10,000uF reservoir cap.
Pull the board and... ah-hah! a broken -40v circuit trace. Inspect for likely causes, and found both Right channel output transistors shorted. So, I obtained some new output transistors, bridged the broken trace, checked the solder joints in the vicinity, including on the BF587 transistors, replaced all of the mica insulators and dried silicone grease with modern heat transfer silpads, put it all together, set the idle current and all seemed well... using headphones.
Put the system onto some speakers and it sounded great! For about three minutes, then a low frequency 'BZZZZZT...POOM!" followed by shutdown and smoke.
A more thorough dissection followed. This time, both Right side output transistors had shorted and the .22 ohm current limiting resistors blew. A check of surrounding resistors and the BF587s all came up OK.
Moving on, testing of things like snubber capacitors al tested OK. Then I noticed something odd.
The Bridge Rectifier 2D1 for the +40v / -40v main power is marked "B80-C5000/3300" Now that's odd.... doesn't the "B80" designator indicate an 80v part? On an 80v circuit? Wouldn't that be pushing things right to the limit for a rectifier with no heatsink? Check the parts list and... Ah-Hah! 2D1 should be a B250-C5000/3300! There is an incorrect part here! Or maybe the schematic reflects a later update?
Added 16 Nov. 2020 - Ah hah! I have discovered that the B80 designator might indicate a bridge rectifier with a maximum 200v reverse voltage. That makes a lot more sense! The replacement has a different designator with a "D" suffix, which also indicates 200v reverse voltage.
So my questions are:
Could a breakdown of the 2D1 bridge rectifier at higher load account for shorted output transistors? Could the low frequency noise (sounded like power line frequency) through the speaker just before destruction of the output transistors be clue? The rectifier does seem to pass diode tests on a Fluke meter OK.
Could the Left channel have been spared because the Right channel is much closer to the bridge rectifier , and the slight resistance of several cm of PCB trace have allowed the Right channel to breakdown first?
As an aside, 2D2, a B80-C3700/2200 which runs the 5v standby had darkened the PCB beneath it. As the 5v systems had several exploded electrolytic capacitors I decided to replace both bridge rectifiers. Also, in the course of repairing the processor, I had already replaced the main power relay, which had high contact resistance.
Thanks for any ideas before I re-assemble and try again...
¡No entiendo Español!
NEIN! Nicht Versteh!
Я немного говорю по-русскии но не очень хорошо... и
I'm not very good at English either! Maybe someday I'll find a language I'm good at?
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....