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 6000 4-Channel: Tuner Cord Replacement and Main Pulley Repair

I recently made some progress with my Beomaster 6000 4channel. Geoff's picture helped a lot to understand to repair the ripped out pulley issue. I bought some 5 mm external retaining clips from McMaster and this allowed me to put the pulley back into place. Here is a pic of the installed clip:

This allowed me to install the pulley back into the Beomaster and put a new tuner dial cord (50lbs braided fishing line) in there. I had to do it a few times to get it right...not too difficult, but there are a few important points to consider. I made a tutorial video about the tuner dial cord installation process (recently learned making videos with Camtasia2...cool stuff!....;-)...Here is the video:

Unfortunately, I had no info how a correctly installed cord looks like since the cord was already wrongly installed and the pulley fell out due to the missing retaining clip when I opened the Beomaster up...Luckily, I have the service manual, where the figure below is shown. It is important to exactly follow their details: Three windings around the main potentiometer wheel, and wrapping the cord two times around the pulley, coming in from the top and leaving at the bottom - this prevents jumbling of the cord when turning the pulley.

This is how it looked after I was done:

This pic shows why the pulley needs to be lower than the wheel...this keeps the cord that goes to the wheel on top, while the cord exits from below and goes to the guide pulley on the corner (se epic below), which is on the lower 'cord exit level' this keeps things nicely compartmentalized and the cords never want to cross.

The calibration of the tuning indicator belt was done with a local station of known frequency (I checked it with the digital indicator of a Beomaster 8000...quite a leap in technology between the 6000 and the 8000!). On to other issues...

Beomaster 6000 Quad: Speaker Jacks Replacement

Finally I received the package from Martin/Dillen of Beoworld (thank you!!) containing light bulbs and speaker jacks for my Beomaster 6000 4-Channel. The speaker jacks for the front channels were unfortunately broken out, i.e. I was so far not able to test the front channels for fear of creating short circuits etc.... Here is a pic of the broken out jacks before replacement:

The exchange is not too difficult. I removed the six screws that hold the panel with all the jacks. This allowed me to pull the panel up by a few cm. This made it easier to access the solder tabs of the jacks.
After unsoldering the connections the jacks came out, and I replaced them with the new ones. Here a couple of pics of old and new: These are the old ones:

The problem with these jacks is that they originally relied on some degree of stress between the plastic wings and the metal panel to seat them snugly. Over the course of 40 years unfortunately the plastic becomes brittle and the wings break off, which causes the jacks to be pushed into the enclosure when plugging in a plug. Anyway, here the replacements:

Beautiful! Here is a pic after inserting them into the slots:

And after attaching the filters and leads:

After this procedure I plugged in speakers, and lo and behold the front channels also seem to work!!
This is good news. It appears I really may not have to take the entire thing apart for now!