At last I got my BM 901 home from Texas.

History: Bought on auction site from a welfare org. in Seattle, price USD 20.00, freight USD 27.00. Shipped to McAllen Texas, from there in my car to Mexico city. Came with a danish power plug, so I presume it was brought to Seattle from DK, stored in a basement for some years and then given to the welfare org. I am a mechanical engineer, normally working on printing presses in sizes from 3 to 60 long tons, I'm 6'2" and have accordingly big hands and fingers, electronics is not new to me, but radios and the like is, so consider me a totally dummy in this strange Beoworld. I will start taking the 901 apart. First I took a very close look at it, from all sides top, and bottom, got a cup of coffee, a set of small screwdrivers and some other (small) tools that I thought I may need, lighted up a smoke and started screwing. First thing to take of is the bottom plate, it may be attached with 7-8 M3 screws, mine had only the 4 screws for the rubber feet, 2 rubbers missing,

There is one more screw on the back panel, between cooling ribs and power cord.Lift the back and pull backward and it comes of easy. Uotside of the powercord there is a small plastic cover for the speaker jacks, 2 screws and pull to the outside, the other side is the same but the cover must be bent a lttle backward to come free from the phono and antenna jacks.

Turn the 901 around, on the back of the back panel you will see 2 M3 screws that holds the top panel, unscrew lift and pull back. Now the side panels, they are fastened with each 4 plastic pins pressed into holes in the side frame, carefully lever them out with a small screwdriver, a little bit on each one until they comes of.

More coffee, more smoke, the front panel, the volume control and the FM slider needs a carefull look before doing anything, the strings for volume and FM may be brittle so I went on with care, the strings are glued to the sliders, do not pull them of there, I took them of from the wheels and left them with the sliders attached. The pushbottons are easy to take out, check the photo.

They are attached with tiny plastick hooks, start at off/on botton position inn, with a small screwdriver press the rh clip to the right and turn out the botton, next one inn and so on.

The front panel looks like one piece, but is actually 3 pieces, upper and lower part is attached with each 2 plastic pins through the side frame, pull out the pins with a small plier or lever with a screwdriver. The top part comes out with the stereo and tuning lamps attached, lever them out and disassamble if needed (change of bulbs ??), the bottom part comes out with bass,treble and balance sliders attached.

All out, no problems, well maybe when I start assambling.

Buttom view, very clean as is and appears unharmed of bad repairmens.

Top view, a little dusty and some solder points was not cleaned properly, maybe from the factory.

Tonight I will start changing caps provided from Martin, as I've just done this on a BM 2400, this lokks easy. More to come.

I will start changing caps. cleaning, lubricating and assambling the 901.

For soldering and desoldering you will need good tools, I use a solder iron with temp. adjust, a small thin nosed pliers, a thin nosed cutter and a desoldering pump. Martins cap kit, description and solder, for solder I use a rosin core 60/40 from Steren.

There is alot of different stuf out there, but I found that this cheap setup is OK for me.

I have to change all the orange colored and 3 silver caps, all of them radial and fitted close to the PCB, normaly I prefere to cut of the caps and then desolder the pins, thats not posible here, so I turn around the PCB and check the position of the caps on the solder side. On the PCBs I worked on until now the position of each item was marked on the solder side, on this 901 board there is no marks, but its not to difficault to locate the solder points for the caps.

Located, I cut of overstanding pins, clean the solder points with a nylon brush and a little alcohol, then heat the point and suc of the solder with the pump, then heat the pins and push them down with the solder iron and the caps comes of easily. If the solder point looks dirty or ugly you can clean it with soder wick, but normaly its not necessary.

As I'm a novice in this job, I make notes of the polarity of the caps, before desoldering. The 901 is easy and I desoldered all caps before putting in the new ones, on other boards where its more crowded I do them one by one or 2-3 at a time.

Attach the new caps through the holes, bend the pins on the solder side 45 degrees to fix the cap in position. Soldering addjust the temp, you normaly need a little more heat desoldering than soldering, keep your iron clean. Put the solder tip to pin and solder point ad a little solder and see the solder magicly floating out on the point and a little up on the pin, a good solder point is shiny and has the form of a cone, ball form bad, not shiny bad, try again. Here new caps in.

Time to start cleaning, lubricating and assambling. I use a silicone oily kind of lube, its acid free and dont harm plastics or disolve glue. I start with the last piece I took of, the middle section of the front panel with the sliders for volume and radio skale attached, to clean it I took of the sliders with the strings attached

Cleaning the sliders with alcohol, the plastic screens came of, someone did a bad job gluing them back on, there were old glue everywhere, I cleaned with a cutter and a small screwdriver that fits into the small slit in the aluminium part, taking care not to screw everything up, cleaned I glued back the screens with Crazy Glue, nice and clean. Applied lube to the feltpads and the guides.

The strings came loose in the process, I cut of old glue with a plier taking care not to touch the string, glued them back in exactly the same place with Crazy Glue gel. Put the middle panel in place with the 2 M3x6 screws. Now to the fun part, getting the strings put in the right places, As I didnt make notes or take close up pics, I had to go by trial and guessing, I got them in, but it took some 2 hours.

The volume string is the easiest, the radio scale is a ***, I used a tooth pick to locate the strings on the wheels.

the lower part of he front panel looked like this before assambly

Took of the sliders cleaned all with alcolhol, lubricated and put them back, take care not to rub to much on the red stripes they tend to disapear, before attaching the panel to the frame you will have to attach 2 small clips to the frame below the middle piece.

Here shown from the upper view. To attach the lower and upper panel you will need the pieces shown below, the bigger part is inserted in the sides of the panel with the hole facing out, place the panel and push the pins into the hole through the side frame.

On the slider the pin, inserted the plastick piece. Placing the panel take care to fit the slider to the controls.

Now clean and attach the push buttons. Just fit in one side and turn in the other, the polished side facing up.

Yes the lower left is the clean one The off button has a small slit on the outside end, the rest are all the sames.

Now put the lamp holders into the upper part of the front panel and attach the wires to the lamps, of course you marked the wires dismanteling, attach the the panel with the plastic pins same as buttom panel.

Ad a little lube to each pin on the side panels and place them.

The top cover has a net attached between the wood and the metal part, the net was unglued from the metal part, cleaned of the glue with alcohol and knife. added some tacky glue and positioned the net.

I cleaned the wood with alcohol and left it like that, later on I will sand and treat it with Linseed oil, but I have some Teak speakers that needs the same and I want to do all of it at the same time. (dusty).

One of the small back panel had some glue rests from a label or something, I cleaned very carefully with alcohol, afraid that the text on the panel would disappear, but the text takes a lot of abuse from rubbing and alcohol, no problem.

Attach the back panels in the slits and with the screws, on this one there is missing some, but I will look for some nice black screws, until now I only found blanc or electrogalvanised M3 and 4 here in the worlds biggest city.

Before I put on the buttom panel I check that all my tools and cleaning stuf is there and accounted for, a last look inside checking that nothing is left inside, lifting up the 901 and turn it around to hear if anything is loose or skipping around.

Put on the panel using screws with the wrong color and at last attaching 4 glue on rubber feet.

Front view, what a beauty

Back view

I connect it to the power and turns it on, nothing happens, its ok the lamps are bad, connect a speaker to the left channel and turn the dial with FM selected, still nothing, I forgot that I've turned down the volume to "0", Slide it to 3, white noice, turn dial to 92.5 my favorite station, a sport commentator is speaking, not my station, start searching for my station, find it at 98.9 the dial scale is working backward.

I told you I'm a dummy, connect the other speaker, checking volume, bas, treble and balence, all working. Turn tha dial a little away from the station and push the AFC button, station comes out clear emidiately, turn the dial 5 Mm to both sides stays clear.

One of the tuning lamps are working, the 2 other ones are not, but I knew that. Connect my BG 2404 and put on Mexicana with Louis Cobos and the london Philharmonics, comes out nice and clear, no hum or other annoing noices.

I will have to get the lamps, screws and feet, opening it up again to change the lamps I will change the dial to the correct way of working and sand and treat the top and side panel, then this beautiful Beomaster 901 will play perfectly for the next 20 years.

Will post when all is done, will take some time though, as it takes time to get the parts.

Soren Mexico

Very nice job, Søren

I have just bought a fully functional 901 (i just have to replace the lamps). Maybe one day I'll replace the caps -so i will bookmark this post.

Good luck with the final details.

Excellent stuff! A couple of detailed photos on mechanical parts are often a much better practical reference than the blowup diagrams from service manuals.

A couple of hints on desoldering:

Those hand operated pumps can be very handy as long as you have a well working model. It often helps if you apply a little fresh solder and melt the joint through before trying to remove the solder. Finish off with solder wick ("Soder-Wick" brand is the only one that works well without added flux).

With kit from the hand assembly era, the component legs are often bent flat on the PCB. No amount of desoldering will completely free those, so you usually need to carefully pry the legs up with a small screwdriver while melting the joint with the iron.

NEVER excert force on the joint until it has melted, as the copper track easily separates from the base PCB material, making for highly annoying repair work or difficult to find hidden cracks. But don't heat too much either, as the same thing will happen. Invest in a temperature controlled iron if you can.

Two-legged components like radial caps (the "standing up" kind) are often easy to remove by melting the legs alternatively and rocking the component so the melted leg is pulled out a bit at a time (careful with force again!). If there are more legs, you should try to remove enough solder so they stay loose when the joint freezes again (wiggle the leg a bit with a screwdriver to confirm it can be moved, or push it away from the hole edges while the solder freezes so it won't get stuck again).

If you know you won't be reusing a component, cutting the legs and removing them one at a time is the safest way, if there is room for cutters.

Make notes on the orientation of the components before removing them - the silk print on the PCB is sometimes incorrect!

Sorry I haven't posted to this thread -an excellent one! Great pics and write-up.

I'm back in the saddle, got the lamps from Martin, and I only had to change them and change the direction of the mhz scale, an easy task.

But as I in the mean time since I changed caps I have done my homework read al the forum threads about BM 901, got good advise from Martin and Eugine about how to and surfed the net, I, as the 901 was open, wanted to do the, in the manual described, addjustments, I started with the 32 V supply V, no problems, then onto the no signal current in left channel, got it, right channel, got it. Checking the manual I saw that it was needed an oscilloscope and tone generator, so I left it there because the 901 did good before.

I connected my speakers and left the 901 on for an hour or so, end checked right and left channel again, no problem, connected just for the fun of it my BG 2404, played an LP and checked again.

And thats where the hit the fan. I shorted between the 2R142 and the aluminum frame, smoke, puff, puff an a fuse bluw.

I didn't throw the item in the corner, I didn't throw it after my wife, I mantained my normal cool appearence, went to the carport and kicked my emergency power generator. (nobody got hurt). How can one be so friggin stupid and careless.

Today after a month, I opened the 901 again and started looking for damaged component, I found a lot of interesting things that I did not see before. In the end stage there are 3 pieces 3000 My caps, one of them is not 3000 my but 4700 My, and in the left channel circuit I saw a different cap in the same position as in the one in the right channel, check the pic

In front of the 2 grey 3000 My caps you can see one white 0.47 My and on the other side a green 0.47 cap, the green one is an ERO, the white one is an EVOX but both is sitting in the same point, one left one right, thats not original. And someone changed the black cap in the middle, thats supposed to be 3000 my and is 4700 my.

I went through the diodes in the right and left channel, seames ok, checked all the resistors and found differences to the schematics, but this without desoldering, tomorrow I will desolder and check again.

I desoldered the 4 Darlingtons and checked them, the 2 from left channel is gone, the 2 right ones seames ok. Going to change all.

Check in the down right corner of the pic there you see 3 white and one green, as far as I can see from the spare list it is 2 and 2 eguals so it should be 2 green and 2 white, or ?? 4 green, the green ones is EROs and renowned as very good audio caps.

I also desoldered the powerbridge, seems ok, the 3 ERO caps after the bridge and before the bridge seems ok.

I intend to change the 4700 cap, powerbridge, Darlingtons and the resistors and diodes where i find differences to the manual, but what more can be wrong here, I checked the power transformer without load and its ok.

Man I'm in over my head, and afraid to do more damage, just because i'm so frigging strupid

So guys, HELP,HELP

Hi Søren, (had to steal the "ø" from your post)

Those BM901 are very different machines.

Fisrt, which end-darlingtons do you have? There are Motorola's BD697/BD698 which are mounted on the component side with a plastic strip. Those are virtually unable to find - they have an uncommon, strange housing.

B&O changed to TIP120/125, mounted on the solder side. Those are available.

I had a lot of machines with burnt power amplifiers. The 0R47 (e.g.R143 left) is the smaller one of the 0R47's and most likely burnt.

Looks like they use it as some kind of fuse here...

Check also the diodes D19-D26, TR21/29 MPSA13 and TR20/28.

And you'd better set the voltage selector to 240V as the power voltage seems to be critical (was learning by doing; forgot one time and had one immedialely burnt amplifier channel . It states 48V but I measured some 50V there. As the cap is a 50V only - danger...

Here's one of my (sl*ughtered) ones but that does show exactly the same strange cap difference with the white / green one as yours.

Those on the lower right are 4 same ones, though.

That one was one with the BD697/698's

The older ones also seem to have one speaker pair output only.

I wouldn't worry about the midddle cap not being 3000uF; Impossible to find nowadays. As it's a stabilizer and not the output cap - those should be at least the same cap for both stages.

Good luck,

     Gunther

Hi Gunther

I have the BD 697/8, but they were solder side and with plastic strip.

I dont find the 0R47, but the 2R143 left 0.39 ohms, as i have the Darlingtons and the big caps out i checked all 4 0.39 ohms and they give me 0.40 ohms so they should be OK.

Diodes 2D19 to 2D26 measured on print seemes Ok, but will desolder one side and check again, will desolder TRs and check, same with the resistors where I find differences to manual values.

What is MPSA13 ? Main power supply ??

Will it be OK to put 3 x 4700 caps in it ?

As for the power bridge it seemes OK, but in my BM 2400 I put 4, 3amp 600V diodes insteed of the PWbridge, and they run much cooler, can I do the same here ?

And forget the ø, use the o , and as I'm in Mexico I have 110 volts, but it goes a little up and down, yesterday had 39 V on the power transformator without load.

Thank you for your help it comes highly appriciated

You are not alone Soren, I blew up a BM 5000 last summer making silmilar trimmer adjustments to the no load current. I my case it was laziness. the alligator clips I normally use were downstairs and I thought I could handle it alone with my steady hand.

The MPSA13 is a small output darlinton (looks like a common transistor). You will find them on the heat sink adjacnet to your bigger output drivers. You will have two one for each channel.

Eugene:

The MPSA13 is a small output darlinton (looks like a common transistor). You will find them on the heat sink adjacnet to your bigger output drivers. You will have two one for each channel.

There are two on the component side, bend through a hole in the PCB and "connected" to the darlingtons with heat grease, that must be those.

Hi Soren

I was wrong with the 0R47, they are 0R39. I took the burnt unit out of my storage so I couldn't read it; and I obviously recalled it wrong

But looking at my gathered spare parts those are definitely 0R39.

MPSA13 (or MPS-A13 as sometimes written) is the thermocoupled Darlington Eugene already mentioned. That one is easily available.

I'm afraid I can't say that about the BD697/8; If you can get some take them, you might need some in the future...

I personally would set the voltage setting to 120V then; In Europe they raised the 220V to 230V, in UK they lowered it to 230V.

I'm sure that's on paper only to fulfil the normes as there's a tolerance of +/-10% which allows the former values as well.

I've never changed the rectifiers; If they run hot it might be a good idea to use Schottky instead; higher AMP Diodes might even have a higher drop-out voltage and run even hotter

There should be no problem using 4700uF's on the output stage; could even give a little more bass...

And the mentioned 50V are measured at C126 (smoothing cap) where the diagram says 48V=.

Did you find the schematics in that envelope tagged at the top cover or was it already gone?

Good luck again,

   Gunther

The voltage in the UK whilst it was lowered on paper to harmonise with europe was actually left at 240v, but remains within stated European tolerance.

Would the lower capacitance on the output not actually reduce bass slightly...

Good thread Soren shows how a little bit of determination, mixed with attention to detail can do wonders for these hifis.

Only one small thing, try not to ever apply to much force on your precious soldering iron tip, that includes pushing component pins out etc... You might damage the coating on the tip and it will bend too. I found out the hard way many years ago!

Step1:

The voltage in the UK whilst it was lowered on paper to harmonise with europe was actually left at 240v, but remains within stated European tolerance.

exactly what I expected

Step1:

Would the lower capacitance on the output not actually reduce bass slightly...

If 4700u is lower than 3000u

  Gunther

@Gunther and or who wants to comment, jokes are welcome.

Got a little confused, you are using component values in your good explanations, I was thinking in schematic numbers, but I got it all clear now, but will not be able to do more before Friday. I didn't find the TR697/8, what I can get here without problems is TIP122 and TIP127, will there be sound difference, the 122/7 is max. PE 65 W (BD697/8 70W), Ueb 100 V (60V), Uce 100 V (60V). I will probably change all TRs in the end stages, and for the rectifiers, if I find one bad they all will be changed, same for the resistors.

Maybe I will wait until I get everything listed and ask Martin to supply, its easier and he always send the right stuff. The problem is the postal time, the feeling waiting for your Beoparts must same feeling as pregnant woman feel the last two month before giving birth

Which cap do you recommend in exchange for the ERO/EROX 0.47 caps.

I'm using the online manual schematics, and there I found 2 different schematics, they are nearly identical, but in one there is shown one L/R speaker and no headphone, in the other one L/R speaker and headphone, my 901 has two L/R speakers and headphone, exept for that the schematics seems to be identical, also in the component values.

I'm worried about my beloved 901, but it will come out nice and like new, also if I have to buy one for spares. I'll never give in.

Hi Soren,

Any progress?

I think the TIP122 / 127 will be fine, their Umax specs are better than the 120/125 that I used. Don't think they will sound different.

And the 1N4148's and rest of the semiconductors are not a real price factor.

Those ERO caps are not that critical; cap problems are mostly related to electrolytics.

And you can use the schematics of BM1100 which is very similar (but black & FM only) and the on-site one has the 4 speaker / headphone output IIRC. The later BM901's had the 4 speaker outs as well; maybe Martin can step in with some serial numbers...

Did you look at the housing top for that schematic envelope?

Hope there will be another BM901 saved - I like their design

   Gunther

Thanks Gunther

No, I didn't find the diagrams inside the 901, but I'm using the onside manual with the headphone connect, until now it seames correct.

As my son graduated from High School and my wife (on wall speaker locator and fabric selector) had birthday ( for 30 times in a row she got 30 years old) I didn't get to much done the last week.

What I found out was the transitors in left end stage was gone and a couple before the end stages even so, I went through all resistors in the end stages and found differences of more than 50% between parallel resistors in L/R stages, I've decided to change all of them and I will go further into the circuits and check for more.

All trimmers measure Ok but a pair of them looks ugly so they get changed too.

Funny thing is thas all the small rectifiers survived, going to change them anyway, the 0.39 Ohm Rs ceramic (sand) behaves funny, I have them out and measures with a digital multimeter, they go up to 8 Ohm then down to 0.1, and then settles around 0.4 Ohm.

Checking with the suppliers here, I cant get the 0.39 Ohm resistors so I will order everything from Martin, but as I have coming down from McAllen TX, a BM 4000, a pair Bvox 3702 and Bvox flat panel 3000, (end of May), I will wait untill they are here and, after checking those, get everything ordered from Martin.

I love Mexico, but when you want something just a little special you have to run around in a 30 million people big city for days, because they dont have the costum of using the net to sell, they got it somewhere, but where, so its just easier to order it from outside, and then wait for it to come.

Will be back with more when all the party dust settles, still some graduation partys to attend.

Saludos Soren

Do you have the version where all four output stage transistors (plus a regulator) sits
on one long plastic bar with threaded bushings ?
That's the version using the uniquely countersunk BD697/698 transistors.

If so, I may have found something in the dungeons.

Martin

I also found this :

Martin

Dillen:

If so, I may have found something in the dungeons.

That must be some dungeon.

We only have garages here in Florida.

Will come back with photos and list of items I cant get down here

After the disaster I desoldered the transistors, the cooling ribs and the 0.39 My Resistors, The BD 697/8 shows left channel bad, same with MPSA13, the BD 236 evenso, checked the small rectifiers R/L, all good, Checked resistors R/L and found differences up to 50%, the right channel showing mostly correct (according to manual) readings and the left channel showing up to 50% less resistans, rectifiers and Rs was measured on PCB.

To be sure about the Rs I desoldered one of them, a 560 K Ohm shows me only 323 K Ohm.

The 0.39 Rs, all 4 of them, acts funny when connected to my multimeter, goes up and down from 0.1 Ohm to 0.5 Ohm and never settles.

The power bridge measures OK, all caps OK (the E caps was all changed before the disaster).

After the power bridge there are a couple of Transistors I want to change too.

Of The 3 big 3000 My caps one has been changed to a 4700 My, I want to change these too, lets see what Martin can recommend.

As I want to get everything correctly working, I want to change everything, thats bad in left channel, also in the right channel.

My 901 is with 4 speaker and Headphones connect, type 2602 Ser.No. 132031

I will check further into the circuits and when I get my other equipment Martin will get a detailed order of it all, and whatever he recommends.

Pic of component side after desoldering

Pic of desoldered components