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Bang&Olufsen Beogram CD50 repair and restoration
Bang&Olufsen Beogram CD50 repair and restoration
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Old 11th March 2020, 10:52 PM   #1
joydivision is offline joydivision  Portugal
diyAudio Member
 
Join Date: Feb 2007
Location: Algarve
Default Bang&Olufsen Beogram CD50 repair and restoration

Latest precious vintage CD player that I managed to get my fingers on (for a reasonable price, seller stated it wouldn't open tray, no more info): Bang&Olufsen Beogram CD50. Hadn't had the chance to touch any B&O stuff yet, great experience, nice design and really, really solidly crafted, hadn't seen anything similar before. The player arrived very well packaged and in very good cosmetic condition.
The problem quickly turned out to be much worse than just a broken belt (as I had hoped).

Upon opening (and downloading service manual), I quickly notice that the big cam wheel that is in charge of directing the movement of the tray (and the disc clamp), was completely off it's pinion, I mean it was just hanging in there in the middle of the other gears!

Turns out that someone before, in a desperate repair attempt, had broken the shaft loose off the base (of the metal framing), the pictures clearly show the damage. Probably because they didn't manage to get the brass holder off the top of the shaft, to be able to remove the gear and lubricate it. I wasn't able to get the brass thing off, either. Don't know why. Doesn't matter, as it was broke, so I thought a bit about it and quickly had an idea: I'd drill a hole into the shaft from the bottom, then cut a M2 thread into it (had some nice thread cutting bits and the appropriate hand tool lying around from the time I built an amp with custom metal case), and then fix the shaft with a screw to the metal frame.
My plan worked perfectly, wasn't even really difficult, and the gear was back in, sturdily fixed. One tiny problem (which later turned out to be important and I had to find a fix): the brass holder somehow was much too high, allowing the cam wheel do displace itself a couple of millimeters towards the top, so that the movement of the disc clamp would be different than intended. I later found a solution for that, as the last images show (a distance piece fixed by one of the holding screws which keeps the gear at the right position).

So, now that the mechanism was working modestly (obviously I took out all the old grease and lubricated everyhting, gears turning smoothly afterwards), but still stalling sometimes, I decided to try and see if the player would play CDs. After I got it to (supposedly) clamp the CD right, it started to try to play, but the motor would not move enough, the laser was trying to read (making noises), and the player then would stop and eject the disc.

So I figured laser was fine but tracking/focussing problems, whatever. Started with a good clean of the lens and measuring some signals. First, the power supplies. This was a problem as it took some time for me to understand that 90% of the circuits of the player would only get power when the main microprocessor would decide so. Then I learned that from the schematics and the power supplies (when turned on by the controllers), turned out fine. I then measured the focus signal - gave OK. Took out my oscilloscope and had a look at the eye pattern - looked nice, laser was great. What could be the problem? Decided to go through all the adjustments from the service manual. Took me hours. Last adjustments (which need a tone generator) didn't go well, I'm not sure if I misaligned some of the pots in the process, as it wasn't working like in service manual, although me following it all precisely. But basically I can say everything was perfectly adjusted (I had feared that someone might have "turned around a few pots", as the gear was broken and the player definitely had been in a tried service session or two, as the screws clearly were telling me....). But no, adjustment was fine. I managed to make it recognize a disc for two times and even play a few seconds, btw, by giving the disc motor a little kick start at the right moment. That left me thinking and I ended up measuring disc motor transistors and signal and turning the gain pot, with no success. Ended up calling it a night. But I was pretty sure the problem was somehow related to the motor.

Next day researched on the internet and found a great thread on beoworld forums, kindly the user Soren had made a nice tutorial to how to clean and lubricate the disc motor https://forum.beoworld.org/forums/t/628.aspx . I decided to test if this would really be a mechanical problem and applied quite a bit of silicone lubricant to the disc motor, letting it get into it. Then tried and woha!! the player started playing the disc!!! I heard music! So I was sure the problem was the disc motor being dirty and full of old grease, not getting right RPM at start so the controller would abort.

Decided to follow Soren's guide. But there was a problem: I couldn't get a 1,3mm Allen key easily. Ended up buying a nice kit of precision bit tools, really cheap and nice, which contained the 1,3mm Allen bit. BUT: it's a bit, so it's not that fine. Which means: as the space between that tiny screw which secures the disc clamp to the motor shaft and the metal frame is really tight, with that Allen bit, the platter hight adjustment would always be limited to the width of the Allen bit. Does that make sense? I had recorded obviously the originally set distance . Was about 5,2 mm, much more than in service manual. The bit allowed that, was about the minimum distance possible. So, I did all the process, motor was indeed full of dirt and old grease, was quite difficult to get off the brass ring, managed with two flat screw drivers at each side between the bell case and the screws of the PCB (so to not damage the PCB), this made the ring come off slowly, this indeed is difficult, but can be done.
Long story made short: disc motor was well cleaned and put together again, now turning really lightly, much better than before.

Put everything together and tried and.... bucket of ice cold water: the laser wasn't focussing anymore, no Focus OK signal, laser making noises, disc motor not turning, disc ending up ejected again! Why?? Started investigating, thinking maybe I tinkered too much with disc motor adjust pot yesterday and now that it's clean, doesn't work anymore, but now. Then by luck I discovered that the disc wasn't clamped correctly!!!
I clamped it and.... player played the disc nicely!!!
So, I now knew the problem was the platter hight adjusment. But I did adjust to the same hight as before??? Could this have been tinkered with, in another desperate attempt, after breaking the pinion of the cam wheel?? In fact the screws showed signs of having been turned and the 5,2 mm distance was strange. So, I ended up grinding the little Allen bit so to make it thinner in order to be able to adjust a smaller distance... very bad when one can't get the right tool. But my dad learned to make tools when he was a teenager after WWII, so I guess it's in the genes ahaha... Now that I had a working tool, I adjusted the distance. The problem: couldn't get it well, and then I discovered why: the cam wheel of the loading mechanism had too much play, as already reported above, due to brass holder being to high. So I improvised a spacer. Done! Now, I could adjust the platter hight. Took some trial and error, as it easily gets too short as well. Ended up nailing it, around 4,9 mm, just like service manual, disc well clamped and spinning perfectly!
Tried it out and YES!! Player is now reading perfectly all kind of CDs, even CD-R and badly scratched ones, quick initial reading, good track changes, plays until the end of the CD, every time, hasn't failed once! Disc loading mechanism also improved with the spacer, hasn't stalled anymore, although a little slow still. But as I'm going to exchange the electrolytic caps, maybe it will get better if voltages get better filtering and more stable? If not, let's see, maybe the belt is worn (haven't changed it as it looks fine and seems to work).

So, now I'm ready for exchanging caps and the rest of the service.

I do have a question and would like to hear opinions: shall I go again through all these (really daunting) adjustment steps according to service manual after recapping? As I possibly messed up a couple of adjusments yesterday when the player wasn't working well and that tone generator adjustment thing didn't go well or should I simply leave it alone if it continues to play as nicely as now, after changing the caps??

As I'm not sure if I can do these last adjustment steps which envolve the tone generator...

Two more things: I noticed this player uses PCM53 which is a mono DAC and is mutiplexed by switching at high frequency between left and right to obtain stereo. This, from point of view of audiphile, isn't nice at all, but the DAC still seems to sound nice (have read a lot on the net), and there doesn't seem to exist an alternative (except for using two chips, which I don't have), but I thought: maybe I can get a better multiplexer chip than 4053?? So I ordered DG303. Also OPA2134 as replacement opamps for output filters. Does anyone have some advice about implementation of these new and better pieces of silicone into the player? I think I'll figure it out, but if anyone has done this before...

Will let you know how it goes.
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Old 19th March 2020, 08:23 AM   #2
Turbowatch2 is offline Turbowatch2  Germany
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Join Date: Aug 2013
Location: Northern germany
If you want better sound, install a digital out and use an external DAC. Just a small intrusion, not really making it "not original", as this one is more a collectors item than a high end sound machine. That way you get the best from the player, itīs solid digital data and can use a modern high quality D-A section. You will be surprised how well even €20 external DACīs sound, compared to the analog out of an old CD player. There has been quite some evolution, making ultra cheap DAC-chips sound better than quite expensive constructions from 30 years ago. Just some switcher and an amp IC will not do it, you need re-clocking, better supply etc. Stuff that, today, is build into a matchbox sized gadget.

Doing deeper modifications will most probably ruin the player, as I fear that is all what you can get as a result from another multiplexer chip. Never heard anything about improving the sound that way.
Same goes with OP-amps, not any chip is the right one in any place.

Just use the best part of your old timer... the solid mechanics.
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Old 19th March 2020, 03:08 PM   #3
joydivision is offline joydivision  Portugal
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Join Date: Feb 2007
Location: Algarve
Thanks for the advice, I actually quit the mod after trying it, wasn't working well probably due to small differences in timing when using different multiplexer (sound was distorted). I normally avoid to do "destructive" mods, so it was fairly easy to get the circuit back to original. But I did change the opamp and the output caps, opamp I put is opa2134 and high quality electrolytic output caps (they have high value, so other kind of cap was not an option): sound improved a LOT! Not sure what made the most of a difference, but between the recapping and the new opamps, it got much better. The mechanism is reading pretty well, also, practically all CDs I put in it, no skipping, although I had to go through the whole readjustment process in service manual after recapping (it wasn't reading TOC anymore), the most sensitive adjustment turned out to be the disc motor gain Pot. I think I got it right. Tested it a couple of hours, playing great.

But I still have a problem with the loading mechanism - I teared the whole mechanism down, cleaned and relubed, with lithium grease and silicon lube, the belt feels ok to me, but still it loads and closes much too slow, sometimes stopping if it's not helped by hand (pulling the external metal flap open or closing it, when it loads). I even decided to reduce the tension of the spring of the metal nose that is controlled by the large cam wheel, so now it doesn't stop in the middle of the process, but still sometimes stops in the beginning of loading or at the end of ejecting. Disc gets always clamped, fortunately. I'm not sure if the belt might be bad after all? I'm also going to check the voltage of the motor and the controller, if necessary, haven't done this yet...

But all in all I'm close to finishing this restoration. The player is in really good optical shape and the laser and related components are in great condition, so it will be worth fixing the problems with the loading mechanism (as I recall, it was completely destroyed by careless attempt of someone who broke the pinion of the cam wheel). Then I'm going to sell it, I already have other players (better sounding and with a PAUSE key and track backwards, for me that's the biggest problem of this one, I don't have a remote and it doesn't have enough keys).
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Old 19th March 2020, 03:12 PM   #4
joydivision is offline joydivision  Portugal
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Join Date: Feb 2007
Location: Algarve
Btw, I measured some of the old electrolytic caps that I changed - almost all 10uF was down to about 7uF and a couple of 100uF were down to 25uF... None of them was shorted. But this shows how important it is to recap old equipment, even if it's still working, as many of the caps loose capacity over the years.
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Old 20th March 2020, 12:40 PM   #5
joydivision is offline joydivision  Portugal
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Join Date: Feb 2007
Location: Algarve
So, it's done!
I measured voltage that arrived at loader motor - only 4V! So I took a look at the circuit and checked the datasheet of driver IC and also noticed a label on the loader motor stating 12V, so I decided to try the motor with more voltage with the cable disconnected - the mechanism was too fast when operated at 12V, but with about 8V it would work really swift, without any hickups and quite fast. So I decided to raise the voltage that goes to driver IC: I installed a 7808 and feed the 8V to the driver IC. I had to cut one trace for that on PCB, but it's still easily reversible if needed one day (though I highly doubt that it would be needed), it works perfectly, the tray opens and closes so fast and swiftly now!
As the player's working so nicely now and it's such a nice (and quite rare) device, I made a small video and put on youtube, here's the link: YouTube

I also attach a couple of pics of the boards with new caps.

Ah, before I forget: I also exchanged the X2 cap in the power supply section, as they have a tendency to blow up spontaneously when very old (happened to my oscilloscope, for example). And that weird "golden" electrolytic cap next to it (100uF) was also changed - it was down to 20uF!!!

So, now it's ready to find a new and caring owner!
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