Beomaster 8000: Standby Issue - Beomaster Does Not Turn Back On During a 50 sec Window

Today I looked into a strange phenomenon I came across in the last Beomaster 8000 I restored. When no speakers are connected, or the speaker switches are in 'off' position, then the Beomaster would not turn on the output stages (i.e. no relay-click audible) if a turn-on occurs in a time window of 10-50 sec after turning it off.

This is not much of an issue in 'normal' operation, where one listens to some music, then turns it off, and comes back an hour later for more. But on the bench, when one tries out a few things and the Beomaster gets turned on and off in quick succession, and often without speakers attached, this phenomenon became quickly apparent.

This is the first Beomaster I encountered with this problem.
So I thought I would try to get to the bottom of this. The first thing is always to do some Googling these days. What came up was a Beoworld.org 'Vintage Products Forum' entry by Dillen/Martin where he states that:

 "It's not uncommon to see that a Beomaster 8000 won't switch the amplifiers back on right after being put into standby. It will light up the displays but not power on the amplifiers.
Put back into standby for 15-20 seconds and it will power up fine again. This is quite normal."

In the particular case discussed here the Beomaster does come on during the first ~10-15 sec, and then not until about 50 sec have passed. After 50 sec it starts up again. The phenomenon is completely reproducible, but the speakers must be disconnected/switched off to experience it. When the speakers are on, everything is fine.

Anyway, I opened it up again today and did a few measurements with my oscilloscope (since this is a transient phenomenon, we need a time dependent measurements...). I fairly quickly identified the cause for the phenomenon: Pretty much the only way to prevent the outputs from turning on in a reproducible fashion is via the fault circuit:

Both right and left channels have a 'fault output' that triggers the 'Fault Switch' on the power supply board to cause a shutdown of the outputs in case a DC voltage is present at the speaker output, or if the temperature in the heat sinks gets too high. In either case a non-zero voltage occurs at the voltage divider between 5R24 and 23 (on the left...that part of the circuit is on the #5 PCBs (output amps), which is used as 'fault output'

I measured the voltage at that R23/24 point depending on the time after shut down with speakers connected. The yellow trace in the scope shot below is the positive 54 V rail which was used to trigger the measurement at 48.8V, i.e. a short while after turning off the Beomaster as the rail started to decay. The blue trace is the voltage at the fault output. So we see that there is a ~600-700 mV negative voltage spike that decays within about 10 sec to zero. This is not enough to trigger the fault switch because of  6R62, and everything is o.k.

An entirely different picture presented itself after disconnecting the speaker from that channel:

Note the now larger 1V scale for the blue trace, i.e. the initially still present small negative voltage spike seen with speakers connected in the scope shot above was completely overwhelmed by a ~4V positive potential when the speaker was disconnected. This potential boost was reset after about 45 sec and zero voltage remained at the fault switch at that point. This corresponds nicely with the ~50sec I measured that I had to wait to get the Beomaster to turn on again. It seems that after 50sec everything resets. This correlates with the delay when the microprocessor turns off the 15 V rails after shut down has been initiated.

It may be that there is a potential build up due to an inconsistency in the decays of the positive and negative 54V rails after turning the Beomaster off. This is no problem as long as there is a speaker connected, which provides a path to ground if there is some DC transient on the output during shutdown. Without speaker, the potential builds up, and the fault switch is triggered.

The real question is: Why do only some Beomasters exhibit this phenomenon, while others do not. I am aiming to do similar measurements on one that does not show this phenomenon, maybe we can learn something from that...

Beomaster 8000: 3D Printed Control Panel Linkage

This is the continuation of an earlier blog entry at http://beolover.blogspot.com/2014/06/beomaster-8000-3d-printed-controls-lid.html

I started reassembling the recently recapped Beomaster 8000. It was time to try out the 3D printed control panel cover linkage that I had printed a few weeks ago after finding that the original linkage had cracked and lost one of its hooks. 

I put in the 3D printed part (Makerbot II, PLA, highest density), and it worked nicely - for a few openings and closings - then it popped out at the damper. This meant that my part had a slightly too wide gap on that end, which would allow the damper arm to come loose. I redesigned the part in Inventor with a 0.2 mm smaller gap, and had it printed again. This time it worked very well. The part clicked in solidly, almost too tight. I lubricated it with Ballistol, which resulted in very smooth operation. 

I made a short YouTube video that gives some more info and shows the part installed and in action:

The STL file for the linkage is posted on my website at beolover.com:(http://beolover.weebly.com/control-panel-cover-linkage.html).

Beomaster 8000: PCB 1 and 2 (Tuner) Recapping

Today was Tuner Day of the current Beomaster 8000 restoration project. The tuner worked basically fine, but there was an intermittency on the right channel, seemingly depending on the moon phase...;-). Tracing the signal with the oscilloscope, revealed immediately that one of the contacts in plug P6 was poor. I pulled the boards apart (they come out together...), and cleaned the contacts with a fiberglass pen and bent them a bit for added contact pressure. Then I added a coat of DeoxIT D100. As usual, this seemed to do the trick. Wiggling the boards did not cause problems in the right channel anymore! Here is a pic of the contacts before cleaning...some black oxide spots are clearly visible where the female headers made contact:

Such black spots come easily off when using a fiberglass pen on them. I forgot to take a picture, but one can achieve as-new looks that way. The DeoxIT further cleans the contacts and provides a protective layer against future oxidation.

While the boards were out, I also recapped them with quality 105C caps. An uneventful procedure. Here is a picture of the boards before the procedure:

And after:

And a detail shot of the region around the IF stage (CA3189E):

Well, almost done!! On to the preamp board (PCB #3)...

Beomaster 8000: Rebuilding the Filter and Tone Controls Board (PCB #4)

Today I took the filter and tone controls PCB of the Beomaster 8000 out and replaced the capacitors. This required unsoldering the signal strength meter leads from the PCB, and unscrewing the plastic panel that holds the switches, buttons and sliders. Here is a pic from the bottom of the PCB before taking it out:

The meter in the lower left corner needs to be unsoldered to be able to remove the PCB from the plastic cover. Five screws hold the PCB to the plastic cover. It is important that these screws do not get tightened too much when re-installing the board. The plastic standoffs can easily crack.

Here is a photo of the meter after unsoldering the leads:

This is a pic of the board after removing it before replacing the capacitors:

And with new caps:

Several of them needed to be tilted to accommodate for the low clearance between the plastic cover and the board. This PCB also had one cracked ceramic capacitor (C207).

I replaced it with a new 10nF type:

After screwing the plastic cover back on (it is a bit difficult to get all the buttons and sliders slotted at the same time - patience helps...;-) I soldered the meter leads back onto the board, and tested the Beomaster...seems all works fine. On to the tuner boards.

Beomaster 8000: Replacing the Speaker Switches

Time for the Beomaster 8000 speaker switches while the heat sink cover is off. Another 'standard rebuilding task'. These switches are corroded in most 8000 units, and when used consistently will fail soon. I often switch between headphone and normal speaker operation, and these switches failed in all three of my personal 8000s after a few dozens switchings. Hence, they must go. Luckily, there are almost identically looking modern encapsulated replacements available. See here for more details on part numbers etc...:

http://beolover.blogspot.com/2011/10/beomaster-8000-speaker-switch-repair.html

I used pretty much the same procedure in the current restoration project, except that I directly drilled holes into the base plates, instead of messing with the Dremel tool...can't beat experience! Here are some pics:

Original switches from bottom:

After I removed the bottom plates from the old switches and drilled holes into them to permit the pins of the new switches to protrude:

After glueing them together. This establishes the original form factor that they fit into their compartments:

After soldering them in:

And in their compartments: Like it never happened...;-)!

The control panel is next! Slightly dicey affair due to the often cracked plastic receptacles for the screws, and the buttons and sliders that need to be refitted in their proper places during re-assembly. I always wish I had four hands to do that!

Beomaster 8000: 3D Printed Controls Lid Linkage

I just received a 3D printed part for the Beomaster 8000 I am currently restoring. When it arrived I pressed "Programming" to access the controls, and the brushed aluminum cover popped open at an alarming speed, indicating that the damper was disconnected. Once opened the Beomaster up, this was confirmed. Sadly the plexiglass link lost its bottom end and therefore lost contact to the damper that prevents the cover from snapping open. Here is a picture of this sad state of affairs:

So I recently drew up this part in Autodesk Inventor, guesstimating the missing bottom end (I didn't have another 8000 open at that point to take measurements of a complete part). Then I had it 3D printed on a Makerbot II using black PLA: Here is the result:

I designed the holes a bit smaller, and then drilled them out with a 9/64 (right, lid) and a 11/64 (left, damper end) bit. The Makerbot has a hard time to get hole dimensions perfect on this size scale, and it is better to drill the material carefully to get a more machined feel for rotating joints like in this case.

I tested the part on the damper and the lid joints, and it seems to click into place fairly well...when I assemble the Beomaster again, we will see if this really worked (I am a bit concerned that it might disconnect from a joint when using the cover in a routine fashion - I will post an update on that).

Anyway,my fun with 3D printing continues!

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: Recapping the Caps on the Processor Board (PCB #9) and Via Bypasses

Interesting! When I opened the shield around the processors on PCB #9 of the Beomaster 8000 I am currently restoring, I found this:

Note the green, blue and orange jumpers. They bridge traces that had vias in them on older versions of this board. These vias were not through plated but soldered in instead on those boards and prone to generate open circuits as the solder joints aged. A well-working remedy is to simply re-soldering them. However, I looked into my Bang and Olufsen Service Bulletins folder for the respective bulletin, which actually recommends to bridge them directly between the solder points at the ends of the respective traces. Here it is:

Anyway, this allowed me to directly move on to replacing the two decoupling capacitors on this board. Here is a shot of the board after replacing them:

Next steps are the main reservoir caps and the speaker switches.

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.