This is a follow up to my recent post about the redesigned Beogram Commander remote control board, which now works in both (DC-motor) Beogr...
Saturday, October 10, 2015
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!