Beomaster 8000: Replacing the Reservoir Capacitors

Today was the day for the reservoir capacitors of the Beomaster 8000 on my bench. Always a pleasure to replace these big 10000 uF cans! After all it all begins with them! I replaced them with the 105C ECES1JA103EZ Panasonic type. They have almost the same size (35 mm diameter vs. original ones at 40 mm and same 80 mm height) and clock in with a solid ESR of 10-11 mOhm at 100Hz for an unimpeded flow of power. They are also rated for 105C, i.e. promise to last for a while, even if it gets a bit warm back there. Unfortunately, this type was discontinued not too long ago...it is getting difficult to find this form factor...electronics parts continue to get smaller. The 5 mm diameter difference was corrected with a wrap of thin corrugated cardboard. That way they fit snugly in their compartments.

Here are some pictures:

Before:

Pretty dusty back there...(sorry for the slight blur...)

And after replacing them:

Almost sexy!!...if you know what I mean...;-)

And here a detail shot of the left channel:

On to the speaker switches, while the heat sink cover is off...

Beomaster 8000: Occasionally Missing Display Segments - PCB #8 Rebuild

Today I worked on the display PCB (#8) of the Beomaster 8000 that I am currently restoring. While I worked on the power supply board I noticed that the display segments flickered occasionally when touching the display board. The contacts between the processor board (#9) and the display board is really one of the weaker points of the 8000 internal design (no weak points on the outside, in my opinion!) and prone to issues. So I took the display board off and cleaned the contacts with a fiberglass pen and then coated them with DeoxiT D100. I also bent every second pin of the headers slightly to increase the pressure between the male and female contacts. This usually does the trick. Here is a pic of the contacts - it really does not take much bending:

I also replaced the two electrolytic capacitors on the board (there are only two - I'll let you find them...;-).

Before:

And after:

Detail (C7):

After I put the board back in, wiggling it would not produce missing segments anymore...hopefully this is now stable!

It is noteworthy that this display board features fiberglass PCBs for the volume, input and frequency displays - note the green color on the display in the upper right corner of the photo - previously, I only saw these boards used for the balance display, and these usually then had no missing segments. Maybe this explains that this Beomaster has not had display issues so far.

On to the inner sanctum, the processor board (#9)...problematic vias may need some attention there!

Beomaster 8000 Power Supply Restoration - More Dead Caps!

Today, I rebuilt the power supply of the latest Beomaster 8000. Nothing very surprising. Two of the reservoir caps were out of spec. C31 was at about 2000uF instead of 4700uF and it also measured a slight current when hooked up to my bench supply (this was the one that felt slightly warm when I tested the outputs the other day)...C35 was disconnected due to a broken lead, but it also tested 20nF (instead of 10 uF), i.e. it did not really matter whether it was connected or not. Since the Beomaster worked fine even with these caps in place, it appears that they are not essential...but the 5V supply would definitely have shown an out-of-spec ripple. Her are a couple of pics before and after rebuild:

And after:

I also cleaned all the pin headers on the board with a fiberglass pen and then slightly bent the pins alternatingly to increase the contact pressure. 

On to the main reservoir caps!

Beomaster 8000 Output Amplifier Restoration - Most Capacitors Out of Spec

Another Beomaster 8000 arrived for restoration. As usual, I opened it up for rebuilding the output stages before turning it on. This is a preventative measure against burning out the output transistors due to potentially corroded quiescent (quiet) current trimmers. And I like my 8000s with their original Texan transistors...;-).

I put in closed 12-turn cermet quiescent current trimmers, new differential input offset trimmers and new 105 C electrolytic capacitors. It definitely needed new caps. All of the 'red caps' except one of the 220 uF and the two 4.7 uF signal path caps were dead. Here is a capacitance and Equivalent Series Resistance (ESR) measurement on one of the 220 uF cans:

413 nF and 149 Ohm ESR is definitely missing the mark! This suggests that this Beomaster definitely needs a full recap. It is great that these red caps at least seem to die open-circuit instead via short circuits...Interesting to note that the axial leaded 100uF caps were all o.k....it seems it is always the red/orange types that go.

Here are the pics of the right and left channels before and after:

Right channel before:

and after restoration:

Left channel before:

and after restoration:

After putting in the new parts I measured the quiescent current trimmers, and indeed one of them had a too high resistance between swiper and track (~200 Ohm). This would have caused at least a pretty hot heat sink if not fried the transistors. But first things first. I hooked the output stages up to external power supplies and fired them up with the quiescent current trimmers turned to 0 Ohm (all the way CCW), thereby initially turning off the current through the outputs. See my post at http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details. Then I carefully adjusted the trimmers to yield the prescribed 18 mV across the emitter resistors (the big green wire wound resistors in the center of the photos, R236/7 in the diagram)

The currents into the the power rails were:

+55V --> 0.10A

-55V --> 0.11A

The +15V control voltage for the constant current source (TR208/11) yielded 0.01A.

After the above soft-ramp-up, I turned everything off, and reconnected the power rails, and the signal and control connectors to the output PCBs, and turned the Beomaster on. It came on without fuss and the radio immediately tuned in a local station. Both channels seem to work, and all other functions seem to be working, as far as I can tell at this early stage. Rather remarkable in light of the dead caps in the outputs. It will be interesting to see if many of the caps on the other boards are also dead, or if the ones in the outputs lived a more stressful life...they probably did experience higher temperatures than those on the other boards. At any rate the next step is to rebuild the power supply board...it seemed that one of the lower voltage reservoir caps got a bit warm during the above test, indicating a potential near-short in it.