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.

Beomaster 8000: No Tuning Above 93 MHz

A 'Beofriend' sent me an email about an interesting tuner issue, which is closely related to a problem I had last year with a Beomaster 8000. In my case, the tuner would not respond to the frequency setting with the rotary encoder, and when set to a frequency close to 91.7 MHz, I could hear the stations of the entire frequency band 'zoom by', either up or down, depending on being slightly below or above of 91.7 MHz. The relevant blog entries are these:

http://beolover.blogspot.com/2013/09/beomaster-frequency-counter-feedback.html

http://beolover.blogspot.com/2013/10/beomaster-8000-tuner-repair-exchange-of.html

Now, the problem described by Beofriend was that the Beomaster would properly respond to the frequency setting in the lower range (he said below about 93 MHz), while in the upper range the 'zooming' issue would occur. Very interesting!
It turned out that the cause for this was a broken flip flop chip, which is in the frequency feedback after the prescaler for a further frequency division by 4 to make the signal palatable for the ancient 6501 micro controller IC4 (which runs on a 1 MHz clock, i.e. anything above this frequency appears as a blurred whirl to it...hard to believe that there was a time when 1 MHz was considered 'fast'...;-).
Here is the relevant circuit diagram section:

After replacing IC8 everything worked again.
I really wonder how these ICs can fail (in my case the prescaler (IC5) needed replacement...In my opinion the most likely failure method for silicon is a too high operating voltage or too high signals. Probably another reminder to put our cherished Beomasters on uninterruptible power supplies (UPS) to put a buffer between them and the power grid/lightning strikes...

Beomaster 3000-2 Bulb Replacement

I recently repaired a Beomaster 3000-2, and the stereo indicator light bulb needed replacement. Unfortunately, these bulbs are difficult to come by. So I replaced it with a white SMD LED (LUMEX SML-LX1206UWW-TR 58K2313) that I mounted on a 1/8W 680Ohm current limiting resistor, which just fit perfectly into the plug that held the original light bulb. Bending the leads of the resistor created a reliable contact. Here are some pictures:

After soldering the LED across the 'bend' I cut the turn off, creating an in-series scenario for the resistor and the LED:

And fitted into the socket for the bulb. Proper fit needs a resistor that has the right diameter...you may need to try a few brands:

Powered on:

Inserted into the Beomaster 3000-2 (I wrapped it onto a couple turns of Scotch tape to prevent accidental short circuits in the. Looks like a real light bulb!

Beomaster 8000: Speaker Switch Replacement

Further inspection of the Beomaster 8000 yielded difficult to switch speaker switches. As in most 8000s of this vintage these switches are corroded and most fail within a short time if used consistently. We decided that the switches needed to be replaced with new ones. Here a couple pics of the old ones:

and bottom view:

Unfortunately these switches are not made anymore. Technology has progressed, enabling fully enclosed plastic packages. To fit new switches to the compartments of the Beomaster 8000, the old-style PCBs that formed the bottoms of the old switches need to be transferred to the new ones. I took the old switches apart by Dremeling the plastic 'buttons' off that hold the PCBs to the plastic  enclosures, and drilled holes into them to allow the pins of the new switches to protrude:



Then I glued the PCBs to the new switches:

And the final product:

Time to solder them in:

And installed in the 'compartments':

Testing yielded a smooth operation. Much better than the old ones! Time to give the Beomaster 8000 a spin!

Beomaster 8000: Right Output Working Again

In the previous post I diagnosed a failed IC206 in the right output amplifier. I just replaced it with a new TIP146. Here are some pics:
Old TIP146 extracted:

New one put in:

Testing yielded a properly working output stage. So far so good! This Beomaster is now ready for further inspection.

Beomaster 8000: Interesting Output Amplifier Failure Mode!

I received another Beomaster 8000 this week. Let the fun begin! I was told that the heat sink would quickly get hot and automatic shutdown would occur after a few minutes. So when I got it the first thing was to check the main fuse (which was o.k.), and then open it up. The two output boards (#5) looked normal, but the right channel showed slightly browned R236/7 resistors, a telltale sign for a  troubled quiescent current adjustment trimmer (R226). 

Here is the right and the left channel before rebuilding:

So the first order of business was to rebuild the two boards. I always do both, since these trimmers are always corroded and in danger of going open circuit, which causes immediate burn-out of the Darlingtons. I replaced the trimmers with 12x precision encapsulated trimmers, which give a much better current stability over time. The caps are 105C 2000hrs types...these boards get a bit warm during operation...

Here are the boards after refurbishing:

After this procedure, I usually start the boards up with current limited bench power supplies to ensure that no dramatic events occur. Here is the right board with the power jumpers. When doing so it is important not to forget to also apply 15V to the constant current source (TR208/211), otherwise the output does not open up.

I did this first for the left channel, and I got the characteristic 100mA per voltage rail after adjusting the quiescent current to the prescribed 18mV reading at test points TP 200/201, which showed that the amp was working properly.

On the right side I did the same, and it almost behaved like the left channel. However, after a few turns of the trimmer, I was not able to exceed ~14mV between TP200 and 201. Even when completely maxing out the trimmer (100Ohm), this value would not budge. Under normal circumstances, the current would dramatically increase when exceeding about 35 Ohms.

Something was wrong on the right side!

The question was what! I poked around and measured the DC grid around the Darlingtons. I found that the voltages around IC205 were much too low: -1.4V on the emitter (should be -21V), -2.8 on the base (spec: -22.2V), and -2.1 (spec: -38.8V) on the collector. Very strange! I initially suspected that IC205 was damaged, and removed the cooling tower, and the heat sink. Then I checked the ICs with the component tester of my trusted Fluke 97. Both IC205 and IC206 looked pretty normal, i.e. they showed tentative diode curves between all pins. I thought at that point that they might be o.k. after all. But something was wrong as the low voltages around 205 suggested.

Strange! I removed the left channel heat sink and did the same measurements. And indeed both IC205 and 206 looked somewhat different on that side, but still like diodes....not very conclusive. I suspected a problem with the measurement, since the components were connected to the circuit around them.

I decided to replace 205 with a pristine TIP146. I ramped the power supplies up, and nothing had changed...this meant that 205 was o.k. So I put the wires back on the original one, and replaced 206. 

Here is a pic with the temporary 206 replacement (note the TI logo on the chip - NOS from ebay!!):

This fixed the problem. All the voltages around 205 were restored. I checked the original TIP146 after removing the wires, and it turned out that CE was completely open circuit. This explains the low voltages around 205. If 206 is off, this drives the voltages around 205 up towards ground. This is the first time that I experienced an OC Darlington in a Beomaster 8000. Usually it seems they just burn out and go completely conductive through CE, which usually causes the fuse to blow.

Anyway...Enough for today! Next step is an orderly replacement of 206 and then some more testing.