The Workbench

Re: Beomaster 901, cleaning and repair for dummies like me

Søren Mexico — Wed, 25 May 2011 13:43:38 GMT

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

Re: Beomaster 901, cleaning and repair for dummies like me

Søren Mexico — Tue, 24 May 2011 13:30:29 GMT

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

Re: Beomaster 901, cleaning and repair for dummies like me

Rich — Tue, 24 May 2011 12:55:28 GMT

Dillen:

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

That must be some dungeon.

We only have garages here in Florida.

Re: Beomaster 901, cleaning and repair for dummies like me

Dillen — Tue, 24 May 2011 11:50:39 GMT

I also found this :

Martin

Re: Beomaster 901, cleaning and repair for dummies like me

Dillen — Tue, 24 May 2011 11:44:25 GMT

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

Re: Beomaster 901, cleaning and repair for dummies like me

Søren Mexico — Mon, 23 May 2011 20:38:40 GMT

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

Re: Beomaster 901, cleaning and repair for dummies like me

hemenex — Mon, 23 May 2011 10:25:17 GMT

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

Re: Beomaster 901, cleaning and repair for dummies like me

Søren Mexico — Wed, 11 May 2011 17:52:21 GMT

@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.

Re: Beomaster 901, cleaning and repair for dummies like me

hemenex — Wed, 11 May 2011 16:17:50 GMT

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

Re: Beomaster 901, cleaning and repair for dummies like me

Step1 — Wed, 11 May 2011 05:26:37 GMT

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!

Re: Beomaster 901, cleaning and repair for dummies like me

Step1 — Wed, 11 May 2011 05:23:56 GMT

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...

Re: Beomaster 901, cleaning and repair for dummies like me

hemenex — Wed, 11 May 2011 02:25:59 GMT

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

Re: Beomaster 901, cleaning and repair for dummies like me

Søren Mexico — Tue, 10 May 2011 20:30:21 GMT

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.

Re: Beomaster 901, cleaning and repair for dummies like me

Eugene — Tue, 10 May 2011 17:40:00 GMT

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.

Re: Beomaster 901, cleaning and repair for dummies like me

Søren Mexico — Tue, 10 May 2011 13:24:13 GMT

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