Beomaster 8000: Reservoir Capacitor Replacement

I have all of the Beomaster 8000 restoration parts in hand now. The first step for me is to replace the electrolytic capacitors that are the most difficult to get to. Those are the four 10,000uF reservoir capacitors in the back. It takes the most disassembly to get to them so doing them first will get that out of the way.

Modern electrolytic capacitors of the same capacitance and voltage rating are often smaller in size than when this Beomaster was made (early 1980's). That means finding some filler for the capacitor cavity in the Beomaster chassis where the reservoir capacitors mount. Fortunately Beolover has solved that problem by designing 3D printed capacitor sleeves that allow modern replacement capacitors to be mounted as the original component.

Here is a photo of the four Beomaster 8000 reservoir capacitor assemblies I ordered from Beolover.com. This is my first time using these parts and they are very nice.

Here is the Beomaster 8000 with its original reservoir capacitors. As you can see they cannot easily be disconnected and removed. They must be de-soldered and care must be taken in doing that.

To get my de-soldering gun and soldering iron to the capacitor terminals so I can remove the wires I removed the back plate of the Beomaster chassis.

With the back panel off it makes de-soldering and removal of the capacitors easier as well as allowing some cleaning of built up dust around the big transformer.

Here are the new Beolover replacement capacitors soldered in place.

Finally, here is the Beomaster with the new reservoir capacitors installed and the rear chassis re-assembled.

The next step will be to recap the power supply board followed by the two output amplifier boards. Then I will do a test to make sure the power is functioning.

Beomaster 8000 Volume Attenuator Repair Left Channel

Before the Holidays a Beomaster 8000 decided to not be able to adjust the volume on the left channel anymore. The right worked fine, but the left channel had a constant, fairly high (maybe 3.0) output, no matter what the volume encoder wheel tried to do. I opened it up and hoped for bad contacts on the plug on the cable that connects the 6 control bits to the microcontrollers as it happened before in a different 8000 (see: http://beolover.blogspot.com/2012/12/beomaster-8000-left-channel-volume-cuts.html), but no luck: All 6 bits came through to the pins of the volume attenuator (board 4, IC102, this is the board under the control panel). I also checked the supply voltages, and they were all there. This meant the chip most likely died. Very unfortunate, since this Analog Devices AD7110 is not made anymore. Of course it would be possible to replace it with a modern chip, but this would have required some circuit design and most likely the fabrication of an adapter circuit board. Luckily I was able to find a handful of NOS AD7110KN chips on AliExpress. It took about two weeks to get them. I replaced the defect chip with a DIP16 socket and plugged one of the NOS chips in. And it worked again! I think this is yet another reminder to put these old machines on a quality uninterruptible power supply, which are usually pretty good at filtering power spikes and lightning related voltage sparks. They do not have much in terms of transient voltage suppression on their power supply board. The usually only way silicon dies is by too high voltage. The really only way this can happen in a Beomaster 8000 is through grid voltage spikes .
Here is a pic of the replacement:

It is strange that the original chips have different markings: AD13/002 8401, while the circuit diagram shows them correctly as AD7110. The replacements show the real names. Maybe 'in the day' they did not want anyone to know what hot state-of-the-art chips they put in there to slow reverse engineering. I guess we will never know...;-).

Beomaster 8000 Tuner Repair: Exchange of Prescaler U264B

***********************this is a follow-up to this post****************************

Yesterday the U264B prescaler ICs arrived from Germany and the UK. I bought two batches of Telefunken NOS ICs from to make sure I would have something that actually works. It turned out that both batches worked. I guess now I have enough U264Bs to last me for the next 40 years. This does feel good...;-).

I unsoldered the old chip (series TFK 208). Before:

The new one I put in using a DIP8 socket. I wanted to test all the U264Bs I received, and this enabled a fast swapping of the chips. Here are the pics of the empty socket and with a new old TFK 225 installed:

I also got some series 234 and they also worked. This puts the question to rest whether the series number matters...it does not. The tuner works very nicely now. However, I wonder why this IC died. Maybe it is time to consider putting some TVS diodes on the power rails to prevent over-voltages.

On a general note, I always run my B&O on uninterruptible power supplies to keep them safe from the all to often occurring brown outs and voltage spikes in our 3rd world style power grid.

One more comment about the defect U264B: When I measured the voltage at pin 1, the meter showed only about 1.2V instead of the value of 3.3 in the circuit diagram. The replacement chips showed a value of 3.25V. Maybe this can be used to identify a broken chip if one is far from an oscilloscope.

Beomaster 8000 Frequency Counter Feedback Issue - Dead Prescaler TEF U264B (?)

I was just about getting bored about Beomaster 8000 #4...it seemed it only had 'standard issues', which is nice as a soldering exercise, but nothing beats the 'detecting' if there is a new previously unknown problem! After fixing the displays and the output stages everything seemed great, until I turned on the tuner. While the frequency rotary encoder properly dialed in the desired frequency, the tuner itself did not really care about this setting. Basically, all I got was white noise no matter what frequency I selected. Oh well...a quick measurement of the tuning voltage yielded that it was either at the low end or at the high end but never in between...then after some playing around I noticed that when I set the freq close to 91.7 MHz I could hear some radio stations 'zooming' through.

Strange! Monitoring the tuning voltage showed that if I set the freq slightly above 91.7 the tuner would more slowly ramp the freq to 108 MHz, and when I set the freq slightly below 91.7 it would reverse course and go down to 87 MHz. In fact, after a while I was able to actually dial in stations with some stability by just very slightly changing the frequency above and below 91.7 and thereby driving the tuner slowly up or down.

Reading the "Product Description" about the tuning mechanism revealed that the microprocessor rams the tuning voltage up or down depending on whether the dialed in frequency is higher or lower than the actual tuner frequency. This is used to stabilize the tuner, but also to change the frequency. Such a mechanism requires feedback. In this case this feedback is provided from the tuner board via plug P84 to the u-controller board (#9). Here is the relevant circuit:

The orange tags are my measurement points (see below). After the transformer L2 the signal ( arrives at the preamplifier input of the prescaler chip U264B. I do not have a fast enough oscilloscope for a proper 100MHz measurement, but by 50MHz Rigol gave me a blurry 10mV sine wave at Pt.1. A better way to detect the presence of the signal was to use a signal analyzer. I hooked its 50 Ohm input up via a 10x oscilloscope probe to reduce the load on the signal. This is what I got:

The U264B chip is necessary to divide the 100MHz range tuning frequency down to something the slow 2 MHz microprocessor (IC4) can actually count. It also amplifies the signal with its preamplifier. After the 1/64 prescaler the frequency is in the 1.5MHz range. This signal comes out at pin 7 of the U264B (Pt.2). This signal now has a ~600mV amplitude, but a ~4.3V offset. Here is a oscilloscope shot measured on fully functioning Beomaster 8000 #2 (on the presently open Beomaster #4 this signal was absent suggesting that the U264B is fried. Judging from the above described behavior it also suggests that IC4 is programmed to 'think' that an absent frequency feedback signal represents 91.7 MHz):

The PNP transistor TR5 has its emitter hooked up to 5V, i.e. the 4.3 to 5V output signal of the U264B is able to turn this transistor on and off. This results in an amplification of the signal at the collector of TR5, and now we have a standard 5V square wave signal, perfect for the flipflop CD4013. Here is an oscilloscope shot from the collector:

This signal is fed into the flipflops in the CD4013, which are in series to affect a further 4x reduction of the frequency, i.e. now the signal is about 400kHz. This is how the CD4013 output looks like:

This is the signal that is sent to pin 21 of IC4, the frequency counter feedback input.
Now I am waiting for some NOS U264B chips from Germany where someone is selling off his hobby kit. Lucky me! I will report back once I put in a new chip. I hope this will fix it.

******************This post discusses the solution of the issue.**********************

Beomaster 8000 Output Transistor Replacement

Today I checked out the output stages of my 4th Beomaster 8000. The blown fuse indicated some trouble there. Indeed a visual inspection of the two output amps revealed two darkened R236/7 power resistors, a telltale sign that the output burned out at some point. The right channel seemed to look pristine. I removed the heat sinks with the output transistors.

To make sure I used my multimeter to measure the resistances in the 6 output transistors of both channels.

On the right I measured high resistance values (several 100k) in the emitter-collector circuit, while on the left the resistance was less than 1 Ohm, indicating a full short circuit between the power rails.

Next I rebuilt the left output board with new electrolytic caps and a 11 turn 100Ohm trimmer for the quiet current adjustment (see post http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details on this procedure). Then I replaced the 6 output transistors (3x TIP141 and 3x TIP146). This is one of the more painful Beomaster 8000 repair procedures since one needs to extract the old transistors and then squeeze the new ones in under the springs with some heat sink compound, which always turns into a mess. I recommend to use vinyl gloves for that. Here are the dead transistors after extraction:

Interesting to see that they were made in Italy...the good old days. The new ones that went in were from Malaysia.
The next step was firing up the left channel with bench power supplies (see again http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details). By the way, there is no problem with first turning up the +15V supply, and then slowly ramping the ±54 supplies to their max voltage, while watching the current (I usually keep the current limiters on the supplies close to turnoff to make sure that nothing adverse happens during this test)

The currents into the board (at 18mV across R236/7) were a bit higher this time:

+54V ---> 0.15amp
-54V --->0.16 amp
+15V ---> smaller than 0.01amp

Not sure why this difference...at any rate it seems the output is working again properly. After 30min the temperature on the heat sink was just slightly above ambient...like it should be.

Beomaster 8000 Suddenly Cuts Output Stages Power (Continued)

Again my Beomaster 8000 cut out! I opened it up again, and a quick tracking of the pin 16 voltage revealed that IC4 still did not connect properly to the relay driver. It turned out that the via I thought I had fixed yesterday was still intermittent. The bad contact was on the top side of the via, not at the bottom where I resoldered it. The pic shows the three vias for pins 14-16 from the top after removal of the top part of the shield can.

I resoldered the vias from the top side and now the Beomaster seems to run stable. Note that there is a tendency of the solder to be pulled down below when soldering the via from top on a flat laying board, i.e. the solder bump below gets larger, while from the top it appears that the solder 'vanishes'. This means that the soldering should be relatively quick, just long enough that the bump reflows properly.

Beomaster 8000 Suddenly Cuts Output Stages Power

It has been some time that I had to post something noteworthy of my B&O efforts. I guess this means that all my Beos are in a happily restored state. However, like with classic cars, one needs at least two of each no matter how perfectly restored they are. This showed last night when we indulged in one of the new Longmire episodes and suddenly in the middle of the show the Beomaster 8000 output died. A quick on/off cycle indicated that the ramp up relays on the main transformer did not switch anymore: The Beomaster was quiet like a lamb, but still turned the display on and off in an orderly way. This suggested that there was an issue with the communication between the microcontrollers and the relays. I switched out the Beomaster with one of my other units and we continued watching.

Today I opened the Beomaster up. After plugging it in and turning it on, the relays clicked like never anything happened. An intermittent problem...don't we love them?? Well, on the other hand this suggested immediately that there was a simple contact issue somewhere in the path between the responsible microcontroller (IC4) and Relay 2 on board 7. I slightly bent the microcontroller board back and forth, and promptly the relays cut out again. I followed the path from the plug to the microcontroller with my multimeter (IC4 pin #16 should pull the line to the relays down to ground when the Beomaster 8000 is on). What I found was that the via that connects the trace from pin #16 through the board right above the pin of the microcontroller had become an intermittent contact. The photo shows the three vias (in the center of the pic between "9" and "C97"). Please, note that getting there requires removal of the bottom part of the shield can that limits the RF emissions from the processors - this can be done by carefully inserting a screw driver in various positions around the can and turning its blade, thereby prying the lid off without bending it.

I had a similar problem before with the first Beomaster 8000 I restored. That one would out of a sudden go into stand-by, and not wake up anymore. In that case the pin #14 via was the culprit. I think the lesson here is that a complete restoration of a Beomaster 8000 should include the preventive measure of resoldering these vias.

Fixing was quick: I put small dabs of flux paste on the vias and then simply heated them until liquid with my soldering iron (at 380C). This restored the Beomaster to its original reliability and it is now happily back in our living room.