Beomaster 8000: Unexpected Burn-in Test Results

I successfully played music through the Beomaster for about eight hours. That felt like a good test and I put the Beomaster into Standby mode.

The next morning I wanted to check the Beomaster again. When I switched the Beomaster on (from Standby to TP2) the house circuit breaker for the outlet I was using tripped. What a surprise. Not wanting to take any chances I plugged the Beomaster into my dim bulb tester, reset the house circuit breaker and turned the receiver on again. Now it would try to switch on but immediately go back to Standby.

So trouble-shooting this power problem became the order of the day. This is why burn-in testing is necessary.

I suspected something with the output amplifiers so I unplugged the rail voltages. The Beomaster would now switch on and the ±55 VDC rail voltages measured okay (when not connected to the output amplifier boards). I will note that I also had to take the Beomaster off the dim bulb tester at this point because it interfered with the power up.

After some more trouble-shooting I determined that the left channel output amplifier board appears to be good. I was able to hook the left channel rail voltages back up and check the no-load idle current and DC offset. This was not the case for the right channel output amplifier. I re-connected the right channel rail voltages again and they measure correctly when the Beomaster turns on  However, there is no right channel output. The no-load idle current stays zero volts.

The problem is in the right channel output amplifier so I will need to examine that whole assembly.
Here is the right channel output amplifier assembly removed from the Beomaster chassis.

Here is the output amplifier with the components removed from the heatsink -

I discovered that TR208 (NPN) was not securely fastened to the heatsink. The mounting screw was loose and it was free to move. That doesn't mean it is the culprit but I probably need to make removing and checking this assembly part of my normal Beomaster 8000 restoration process.

The next step is to check the transistors and diodes of this circuit to see if there are any failed components.

Beomaster 6000 4-Channel: Trimmer Replacement in Output Stages

Following excellent advice from Jacques ('charz') at Beoworld.com, I decided to also replace the quiescent current trimmers with modern 25-turn types. The challenge is to mount them in a way to achieve 'trimmability' with the output board installed in its operational position. Luckily the space between the two big 100n capacitors (C1/2 in the LF channel)is perfectly dimensioned to take standard encapsulated trimmer. The only thing left was to extend the leads to be able to bend them in the right orientation to be able to replace the old single turn trimmer. Here is a pic of a new trimmer in comparison with the old after this procedure:

It is important to orient the trimmer in a way that the quiescent current adjustment is in the same orientation as specified in the manual, i.e. current increases in clockwise direction. Otherwise, there might be confusion and hot heatsinks the next time this is done (in another 40 years??...;-). So I made sure that a complete counter clockwise adjustment means zero resistance between the left and center leads (in the orientation shown in the photo).

By the way: I used 500 Ohm trimmers since I was not able to get multi-turn precision 250 Ohm types through my usual channels in the US...I could only find single turn types...so I decided to use 500 Ohm. A look at the circuit diagram shows that there is nothing to worry about this. The only impact is that the base of 11IC3 (LF channel) can be pulled up a bit harder relative to the emitter if one would turn the pot all the way to the high current end due to the larger drop along the trimmer...but this is nothing one would do when following the quiescent current adjustment instructions in the manual (turn all the way CCW then turn on and slowly go CW until desired current is obtained). The 500 Ohm value does not affect the gain of 11TR2 either, since the collector resistor of 11TR2 is provided via 11R10 and 11R16, and the gain is theoretically close to infinite anyway due to the lack of an emitter resistor...Furthermore, the DC network for the base of 11IC3 is 'locked in place' (voltage wise) by the drop across 11IC3, which is defined by the quiescent current adjustment to the specified value, i.e. should be the same no matter if there is a 250 or 500 Ohm trimmer...

Next step was pulling the circuit board out again...certainly not a procedure I like to do. I hope this was the last time!

Here is a pic of the RR channel with new trimmer:

Pretty, how it fits! As if Jacob and his friends knew that I would want to do this in 2014!..;-)

And here a shot of the other three outputs:

And finally: The old 250 Ohm trimmers (which are all in pretty good condition..):

A test of the amplifier with the tuner revealed happy operation. This hopefully concludes the output amplifier restoration!

Beomaster 6000 4-Channel: Output Amplifier Repair and Quiescent Current Adjustment

Well...I did something really stupid. It seems like a repeat of history. I got into this beoloving thing due to a smoking Beomaster 8000, and now I fried one of the four output amplifiers (RR) of the Beomaster 6000 4-Channel!! Exciting! Very!

Here is what happened: I used an only partially insulated screwdriver to adjust the quiescent (quiet) current of the output stages, and when I got to the rear right channel, I slipped with the screwdriver out of the (bent down) trimmer and I very briefly touched the exposed lead of the resistor to the left of it (i.e. 11R96). At the same time, I touched the metal U-shape that holds the output printed circuit board with the upper, non-isolated part of the screwdriver (this is definitely a good moment considering buying a set of electrician screwdrivers...;-). A brief spark at the resistor, and the formerly 7.2 mV across the collector resistor of the PNP Darlington in the output turned into about 150mV, followed by a rapid heating of the respective heat sink (and a dramatic raise in my pulse plus developing sweat...;-). However, the main fuse of the Beomaster did not blow. If you think of it, the 150 mV across 0.12 Ohms correspond to less  than 2 Amps, i.e. this makes somewhat sense. At that point I did not understand why there was so little current, despite both output transistors having been fried (as it turned out when putting the Ohm meter to them via the access granted through the heat sink ribs). Later it became clear that I also fried the emitter resistor of the npn Darlington (11R102)

Oh well, after the initial panic subsided, I discovered that it is actually not too difficult to fix an output stage of the Beomaster 6000 4-Channel, despite the relatively poor service friendliness of the 'everything soldered together' design of this early 1970's construction.

I was not able to find the matched pair of MJ2501 (pnp) and MJ3001 (npn) Darlingtons...it seems they only kept the MJ3001 on the active device list, but dropped the pnp version. Since it is best to use matched pairs in push-pull amps, I decided to use the MJ11015 (pnp) and MJ11016 (npn) pair, which are slightly beefier versions available in the same TO-3 package and with the same DC current gain values. I also replaced the driver for the output, 11TR8 (a BC332, also not available anymore), with a 2N2222A with similar characteristics due to the possibility that this transistor was exposed to maximum rating-approaching voltage levels. On a general note, due to the feedback based design, it is not very crucial what transistors are used as long as they can take power and voltage, are fast enough and have sufficient current gain.

I made a video about the repair procedure:

 Here are a few high res pictures of the process:

After removing the two screws that hold the transistors in place the heat sinks can be pushed a bit back and then upwards to reveal access to the transistors.

Moto MJ3001 and MJ2501 say hello after their extraction...can one make cuff links from them??

Here is a picture of the pulled circuit board to allow access to the amplifier circuit for replacing the two output resistors. If you do this, do it slowly and deliberately making sure that the wires are all free to move... The right red 'can' on the bottom (hidden underneath the wires) was the resistor that went open circuit (and prevented the fuse from burning by limiting the total current to below 2 A after the transistors died):

Here is a shot of the old and new resistors (Newark 73M8331, 0.12 Ohm, 3W). I decided to replace both of them to make sure there would be no later surprises:

Here is a shot of the amplifier after replacing the resistors and also the electrolytic caps with 105C models (I did that for all four outputs - I also measured the capacitance and ESR for all the extracted caps for the fun of it: not one was out of spec!):

This is how I adjusted the quiescent (quiet) current (shown for the left rear output). It is convenient to simply clip the probes to the emitter leads of the Darlingtons. This measures the voltage across both output resistors (in this case 11R48 and 11R49), i.e on needs to adjust for 2x 7.2mV = 14.4 mV (now, please, do me a favor: use an insulated screwdriver...and put some cardboard sheathing into the U-profile to prevent accidental contact between circuit and ground!...;-):

That's it! Back to the actual restoration tasks!

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 Left Output Stage Rebuild

The summer is coming to an end. Back to B&O! Today, I finally recapped the left output stage and also fitted it with a 12x precision trimmer for the no-lad current. As stated earlier, the original low-cost trimmers are an Achilles heel of these beauties. Corrosion in combination with vibration can easily result in the destruction of all TIP transistors in the output stage. It is a multi-hour repair effort if that happens. Hence, whenever a recap is done, one should also immediately replace these trimmers!

Here is a picture of the left channel output before (power rail plugs already pulled - Some advice here: make sure that the big reservoir caps are completely discharged before messing with these plugs. One thing is that there are almost 110V DC between them, which can be painful, the other is that touching components with the unplugged cables during rebuilding the board can easily result in damage, especially if the circuit is still grounded. To discharge the caps I usually put a 10R power resistor across them for maybe 10-20 sec - measuring the voltage at the terminals confirms completely discharge):

The picture above shows one of the two100uF caps of this board...cracks are visible. A sure sign that this capacitor would have soon died.

This is how the board looked after restoration:

Beomaster 8000 Rebuilding the Power Amps

Today I rebuilt the right output stage. I put in new electrolytic capacitors and replaced the no-signal current trimmer. It was a good idea since a look at the old one revealed that it was intermittent when turning it. Adjusting the voltage between TP200/201 to 18 mV according to spec yielded a nice cool heat sink, even after a 1hr run. Here are before and after pics:

after:

At the end I adjusted the offset of the differential amplifier input with R200 to 0mV at the speaker jack. On to the left channel!