Beomaster 6000 4-Channel: Positioning the Indicator Bands

I finally had some time to continue the LED-ification project of my Beomaster 6000 4-Channel (several of the lightbulb fixtures had broken out tab-receptacles, and I did not want to use tape to fix them in place, so I decided to do it with LEDs)

I made some major progress on this today, and it seems everything is finally working. More about that after I do some more testing of this set-up.

I was confident enough to re-install (for the 4th time, I think...patience is a great asset with the Beomaster 6000...;-)) the indicator bands of the motor unit. I realized that I previously adjusted them slightly wrong in their position. So I wanted to share here how to do it right: 

The small white dots in the indicator panel are meant to adjust the bands relative to the potentiometer position. This is how I aligned them this time before pushing in the clips (see here for details on the entire procedure and a video):

That's it for today...

Beomaster 6000 4-Channel: Rebuilding the Motor Unit

Oh well, another setback!!... While messing with the output amplifier and doing the tuner dial string etc...I had to witness the degradation of the Buna N O-rings that I installed a month back into the Beomaster 6000's motor unit. I could literally see how cracks in the Buna N rings developed that increased in depth every day in a noticeable way. I think in another month the rings would have broken and fallen off by themselves. Here is a picture of the damage:

Pretty uncool! Anyway, this meant I needed to redo everything, and I already had the indicator bands installed.

I studied up on O-rings and the materials they are made from. It turns out that the best material for 'drive belt' uses under tension is EPDM rubber (ethylene propylene diene monomer rubber). I found the "Parker O-ring Handbook", and guess what, it has a chapter on "Drive Belt Applications" (section 3.17). There it suggests material "E0751-65" as ideal for drive belt applications. A bit of internet sleuthing then yielded that this is EPDM...unfortunately it is very difficult to buy Parker products as a private person...it seems they have a distribution system that is loth of modern purchasing methods, i.e they want you to go to a local handyman location to pick up the O-ring...crazy! So I went to the O-ring store, and ordered six of their E70032 model, which is a 1-7/8 inner diameter 1/8" thick ring made from EPDM. I also went through a learning curve with regard to the inner diameter - the original rings measured 2" diameter, and I installed the E70033 model first, which has 2" diameter, only to learn that two of the potentiometer drives slipped with these due to a bit higher resistance in these pots...so I had to do it a 3rd time with the 32 size...live and learn!

Anyway, I was able to finally complete the repair, and I made another video about the process. This video shows the entire process: Removing the indicator bands, putting in the new rings, and putting the bands back in. There is also a demo of the rebuilt unit in action at the end. You can find it on YouTube:

********Please, note that the video does not make clear how to properly position the indicator bands relative to the potentiometers. See this post for details.**************************************

Another interesting issue about this repair procedure was the brittleness of the V-shaped plastic clips that hold the indicator bands to the purple drive pulleys on the potentiometers. The original clips immediately broke when I tried to squeeze them a bit when reinstalling the bands:

So I took measurements of one of the still intact ones, and made a model in Autodesk Inventor. Then I exported the finished model as an STL file and printed it on a Makerbot II in yellow PLA (I thought the yellow would work well with the purple drive wheels, adding to the colorful interior of the Beomaster 6000....;-).

Here are the new clips:

Pretty cool! But I had to manually Dremel the grates off at the side that connected to the plate of the printer to prevent damage to the indicator bands when sliding the clips in.

This worked very well. They can be installed by squeezing them together with needle nose pliers and pushing them in from the top while bunching the band ends together into the 'crack' (sounds more complicated than it is - watch the video posted above - it shows this process in detail).

In a second step, the clips are pushed into the wheels until they seat against the wall:

And that is pretty much it. I really like the yellow-on-purple theme...;-). Watch the video for a demonstration of the rebuilt unit and the clutch mechanism.

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 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!

Beomaster 6000 Quad: Rebuilding the Motor Unit - Some Progress

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Note: This entry is 'historic'...it turned out that the Buna-N O-rings mentioned below did not work out. Please, use rings made from EPDM. I used model E70032 from the O-Ring Store (theoringstore.com). The Buna N rings cracked after only one month, and I had to replace them with the EPDM rings. Check out the update on this repair procedure and a detailed repair video at: http://beolover.blogspot.com/2014/05/beomaster-6000-4-channel-rebuilding.html

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I received the belts for the motor unit of my Beomaster 6000 Quad from McMaster. They are 'metric Buna-N O-ring, 2mm width, 50mm ID' (9262K208). I measured the broken belts as good as I could, and it seemed these dimensions would fit the bill. The original belts had only 1.8 mm diameter, but 2 mm is all they had.
So I set out to put them in. Not a task for the faint of heart. It is immediately evident that the indicator tapes need to be removed. Sweat started building at the thought. But there is only one way now with this Beomaster: forward!
The critical moment is when one takes the V-shaped retaining clips out that clamp the tapes to the purple wheels. Per service manual one is to push them out laterally, which works fairly well (three of them had sort of bonded with the tape/wheel, and it took a bit of force until they started moving). It is best to move the grooves in the wheels a bit away from the display element to obtain a clear path for the clips for moving them out. Here is a pic:

After I removed the five tapes, I went on to get the shaft out. All one has to do there is to remove the plastic bearing at the end of the motor unit and wiggle it out (see below a photo where I put it back in - that should clarify the process). It is a good idea to put a box into the cavity of the motor unit, to lift it out a bit, to create a better working situation.
Here is the shaft after I had it out...a pretty interesting design. The clutches are just metal pieces that are pulled into place by coils (OL1,2,3,5,6 in the schematic).

Then I loosened the potentiometer nuts. Of course my Beomaster 6000 Quad did not come with the appropriate Band and Olufsen supplied special ~1mm thick 10mm wrench. But I am a man of the Dremel, and so I had the 'collet wrench' that came with the set. This wrench has the perfect thickness, but is slightly to small (it may be a 9/32 - not sure). I used the Dremel itself with a grinding tool to widen it up a bit to a 10mm opening:

This fit perfectly:

Note: this pic was taken after the job was done.

All one has to now is to loosen the nuts and wiggle the pot is out. I forgot to take a pic when they were out - the heat of battle...I then put the belts on their groves on the purple wheels. They just stayed snugly put in them by their own tension. Then I put the pots back in and tightened the nuts.

Now came the second hard part: To get the shaft back in while pulling the o-rings into position. I did this in a 'caterpillar' like motion, moving the shaft a bit pulling the o-rings one ager the other on it while moving it a few millimeters forward, moving the rings and so forth. Here is a when I was about halfway through:

And here after putting it all the way through:

After installing the shaft all the way, I tried to turn the wheels by turning the pulleys on the shaft, and everything felt pretty solid and in good contact, while having enough friction to reliably turn the wheels. It seems the belts may do the job.

Now I needed to replace the motor coupling, which was completely rotted away. Here is is before:

And with the new piece of a 3mm ID poly tubing piece:

Since the shaft can only be stuck into the coupling when the motor is installed, I needed to pull the shaft again a bit back, install the motor back into position, and then press the shaft into the coupling. This worked very well:

So far so good. The next step is to put the bands back in. This is probably the biggest problem since the retaining clips have degraded and lost their elasticity. I tried to bend one a bit together to be able to get it into the wheel in a 'dry run' without a tape (not sure what I would do if I damaged them...)
Anyway, this is what happened, as you may guess:

It immediately broke after applying very little force. I guess the others will not be much different. So a solution needs to be found. I have a few ideas, but this needs to 'percolate' a bit more...aside from this damage, a few wires came off of board 12 (motor control) since the board chafed quite a bit on the cardboard box while I was fighting with the shaft and the o-rings. But this is a minor problem. I thought that would happen and I took some nice pics before...Enough for today. More later.

Beomaster 6000 Quad: Tuner Pulley Mystery Solved!

Just received a picture from Geoff from his Beomaster 6000 Quad showing the bottom end of the tuning wheel assembly:

He suggested that there is just the external retaining ring missing. I had a closer look at my bearing assembly, and it turned out that the bearing was just jammed at the bottom end of the assembly, rendering it inaccessible for the pulley shaft. His photo encouraged me to push the bearing a bit, and indeed it moved further into the housing. Now the pulley shaft comes through far enough that I could put a clip on it if I had one...

I guess in a couple days I will have an appropriate retaining clip from McMaster.
On to the other issues!

Beomaster 6000 Quad: Tuning Wheel Pulley Damage

Today I removed the bearing that (normally) holds the pulley that is driven by the tuning wheel to have a more diagnostic look. After removing the three screws that hold it to the bottom of the enclosure, it came out. Here are some pics:





















And from the bottom:

While it appears that the pulley bottom-end may have been press-fit into the bearing, this cannot be since the pulley groove would be much lower than the the string level in that case. This suggests there is a part missing in between the bearing and the pulley. Not sure at this point what to do about it.

Beomaster 6000 Quad: First Time Power Up

Today, I thoroughly scrutinized the Beomaster 6000 Quad for 'obvious issues'. I found two disconnected wires and figured out where they needed to be connected. Then I measured all output transistors and found them giving reassuring resistance values between collectors and emitters, i.e. there was hope that they were not burned out. Then I moved the quiescent current trimmers a few times forth and back in an attempt to make sure that they had contact. I positioned them exactly where they were before this, hoping that they had been adjusted to a reasonable value. It seemed they were all more or less in the same position, so there was hope that they dilettante who messed with this unit before did not get to them.
I definitely was concerned that there is a danger to fry the transistors like in the Beomaster 8000 when they go open circuit. So I simulated the DC network around the transistors in iCircuit. The result is that an open circuit swiper of a trimmer should not lead to a catastrophic current avalanche since it appears that the biasing Darlington IC3/6/9/12 opens up when the swiper disconnects, effectively shortening the two bases of the output transistor pair. I put in a sheet of cardboard between the output amplifier board and the metal bracket that houses it (per advice from beocentral.com to avoid potential short circuits due to PCB warping)
Then I connected my bench speakers to the rear outputs (one of the jacks for the front channels is broken) and plugged in a 300 Ohm antenna.
A deep breath, and I flipped the switch in the back to 'ON'. Nothing happened except that the power LED came on. A good sign! Then I pressed on the FM button, and the radio came on! I was able to tune several stations. Playing with the volume potentiometer in the 'motor unit' produced a decent sound in my little workbench speakers. Then I played with the other potentiometers and I was able to change the sound, and the balance in all four directions. It appears that the Beomaster may be largely intact electronically (hopefully also the front channels work as the transistors seem to be in good shape). Not too bad for a dubious ebay purchase!

But there are a number of issues:

1) It seems the field strength meter of the tuner is not working
2) All potentiometer belts are broken:

3) The coupling between the motor and the potentiometer array is cracked and the motor spins freely when a button is pressed:

4) Several lamps are not working
5) The broken tuner pulley (see last post)
6) The string that pulls the frequency indicator band has a knot in it, and needs to be replaced:

I tried to operate the frequency dial with the knot in it, but it got stuck as soon as it went under the band...so it needs to be replaced.
7) Two broken speaker jacks (for the front channels):

The two wings that keep the jack from 'falling through' the opening in the panel are sheared off. The other channel is barely hanging on, but the wings are about to break off, if one would pull out a plug a couple more times...I wonder if one can still get these...

The big question is whether to try to recap the entire unit and risk to break something. It appears quite a chore to get the boards out for access. I am actually wondering how they put it together in the factory. Probably like they insert wiring harnesses into cars, everything in one piece.

Beomaster 6000 Quad: First Contact

I finally found a Beomaster 6000 4-Channel (2702 Model) on ebay. The usual seller description implied not working and 'have not the right cables to test'...very 'funny'. This usually means someone messed with it and was not able to get it to work. In this case my B&O addiction in combination with the rarity of this model in the US won and I put a high bid in. The exterior is not too bad, but there are a few scuffs on the metal panels and especially on the frequency dial, as well as on the plexiglass cover. Well, I figured, better learn to work on one that is not so perfect, and then get a perfect one to serve as "beolover's masterpiece"...;-). So here we go: This is how it looked out of the box:

There is not much information about the technical aspects of this Beomaster out there except an excellent thread on Beoword.org by Geoff, who heroically fixed one up and let the world know how it went. Reading this post a couple years ago immediately made want one of these beasts, and try to cut my teeth on it...;-). His issues with the output transistors suggested not to turn the Beomaster on before having had a good look inside. Here is how I opened it up:

I followed the advice on the bottom:

I removed the bolts marked B (and A...not sure why they labeled them differently, they need to come off to be able to remove the wood trim). Here we go:

The bolts have two functions: To hold the trim to the enclosure, and hold the enclosure together. this means, that at this point, the top parts of the enclosure are loose, and one needs to hold the housing together when turning the Beomaster around after this step.

The final thing to do is to remove the nut that secures the frequency dial shaft in its bearings. I was a bit confused about this. Geoff sent me some advice (thank you very much!) and I forged ahead. This shows the nut in detail:

I used flat nose pliers to turn the nut while holding the dial on the other side against the turning motion. The nut came off easily, but then I was in trouble:

The dial came out and I slowly lowered it from underneath the housing (the Beomaster was hanging over the edge of my work bench for this procedure, so I could turn the nut from above and get the dial out from below). Unfortunately, the pulley that advances the string that moves the frequency indicator tape also emerged. Here is a picture:

The pulley is the brass part, and I think it is supposed to stay on the bearing as shown here as part 158 in the service manual:

Now I turned the Beomaster around, and removed the top parts, i.e. the control panel, the plexiglass cover, and the heatsink cover (after removing the four screws on the back and the two hex (2mm head) screws that hold it near the push buttons). The control panel cover is permanently attached by wires, but can be flipped out towards the front. This shows the state of affairs after removing the panels:

This is a close up of the frequency dial area where the pulley came out:

It is obvious that this is not how it should be where the pulley is clearly still seated in the bearing (the part in the center of the picture between purple capacitor pulley and capacitor to the right). Geoff sent me this pic from his Beomaster 6000:

Obviously his pulley is in place and actually exerts some tension on the string while staying straight. Well, that is what one gets from ebay. I will need to deal with this later.

I will post more about my initial inspection in the near future.