Beomaster 8000: Rebuilding the Outputs, A New Interesting Output Failure Mode, and First Startup

I started working on the Beomaster 8000 that I recently acquired via eBay on behalf of an B&O enthusiast in the UK. The 8000 is definitely one of my most favorite restoration projects. An absolutely awesome design. The Beomaster 8000 was sold under the premise to be fully working but to have had some 'amplifier trouble', which supposedly had been 'professionally' repaired. Oh well, I heard such claims before, and especially on eBay the unsuspecting buyer needs to be aware that there are not many repair outfits left who can or want to tackle these units. Instead there is a lot of amateurish effort going on with the aim to make a fast buck and turn these sought-after units around for a ransom 'in restored condition'.

With that in mind I approached this Beomaster with caution and gave it my usual 'fix outputs first and then turn it on' treatment. So I opened it up and found a fairly clean situation. However, three screws are missing under the speaker switch cover:

And the cardboard shield that is supposed to keep the solder points on the display board from shorting against the toroidal transformer was installed incorrectly:

An interesting feature is the late-series microprocessor board that has soldered on EMI can lids and a sticker that says 'Sealed unit. To be returned to the distributor for service'...we'll see to that...;-)

After I lifted the display/uProcessor boards into service position and took the input socket panel tray out I looked at the output PCBs. And I made an interesting find: The quiescent current trimmers were replaced on both channels with 47 Ohm resistors. A very creative, but questionable fix to the corroding trimmer problem that commonly leads to frying the transistors in one or both outputs when the trimmers go open circuit. Here is a picture of the left channel board:

The resistor in question is in front of the third electrolytic cap from the left. Very crafty! I hooked the right channel board up with three external power supplies providing the ±55V rails and the 15V control bias:

The multimeter shows the voltage across the two emitter resistors R236/7 as 4.7mV. This is much lower than the prescribed 18mV. Well, at least the 47 Ohm resistor errs on the 'better side' by running the output Darlingtons IC203/204 at a too low operating point. Had the 'expert' who did this elected to use the next standard resistor value of 56 Ohms, most likely the unit would have run pretty hot. The problem with running at a too low operating point is that class B amplifier cross-over distortion is introduced in the output signal since the two half-waves are not 'connecting' smoothly at the cross over point. Since all TIP141/146 pairs are slightly different, trimmers are needed to find the perfect operating point for each unit independently. 

I installed as usual 25 turn encapsulated 100 Ohm precision trimmers

in both of the output boards and also exchanged all the electrolytic capacitors with 105C level quality Japanese units. This shows the left channel:

And here is the right:

Then I ran the right board with my external supplies and adjusted the trimmer to yield 18mV across the emitter resistors:

The power supplies showed the usual 0.11A (-55V), 0.1A (+55) and a close to zero current on the 15V control voltage pin

Very good! Then I did the same for the left channel, and I was surprised: No matter to what value I would set the trimmer, the multimeter would show zero voltage across the emitter resistors. Something was not working!

I poked around and measured the voltages across the TIP141 and TIP146 'columns' in the output. The voltages seemed reasonably close to the values shown in the service manual. Very strange! Then I measured the voltage at the base of the npn Darlington (IC203) while turning the potentiometer. Nothing happened. It stayed constant at about 1V. This was strange, since the potentiometer is supposed to adjust this voltage to about 1.2V to set the operation point of the Darlington. I did the same at its pnp match (IC204) and this base changed the voltage depending on the trimmer setting. A bit of head scratching lead me to the root cause of this phenomenon: The yellow lead between the emitter of IC203 and the PCB was open circuit. I unsoldered the lead and found that it was only hanging on by a shred of insulation, but that the wire had disconnected:

I took some insulation off and soldered the lead back in, and then everything was as it should be.

Time to plug this baby in and finally turn it on:

Allright! First light! This marks the start of another Beolover restoration!

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.

Third Beomaster 8000 - First Life Signs

This is turning into a fully blown addiction. I bought a third Beomaster 8000!!! Of course broken...gotta have some fun! Today, I finally opened it up with the plan to bring it on line slowly. I disconnected the 45V power supply from the output stages (pulled red and black cables on L and R output boards). The purpose was to avoid a potential burnout of the output transistors in case the trimmers have contact problems. After disconnecting, I checked the main capacitors on board 6 (power supply) to see if they still work. They showed reasonable values. Then I connected the Beomaster to the mains. Without pressing a button, it immediately showed some random segments on the displays:

No response to any buttons. This made me think and pull out the circuit schematics. After some study I came to the conclusion that it is time to check the voltages relevant for the microcontroller and the displays. I measured on P48 on board 6 that the pins that should carry 5V only measured 2V. This means that the microcontroller is starved below its minimum voltage of about 2.7V. The circuit diagram shows that these pins are directly connected to the output of the 5V voltage regulator. That left practically only one possibility (aside from an (unlikely) short somewhere on the microcontroller board): C35 must be bad. Indeed a measurement with my capacitance meter indicated a capacitance of 2.3 nF instead of 10 uF. I immediately replaced the capacitor with a new one. Plugged the Beomaster back in and it started up normally. After pressing a button I got this:

As usual, several segments are dead and these displays need to be refurbished. A worrisome phenomenon: No response to turning the volume encoder...well, it would be boring if they always had the same problems. More about this in the near future.