Vintage Products

Re: Beomaster 900K & a novice needing help

beaker — Thu, 21 Jan 2010 06:12:55 GMT

Just thought I would say what a great thread this is. Well done to all involved and it may well inspire me to recap the 900 in my Compact.

Re: Beomaster 900K & a novice needing help

tournedos — Sat, 16 Jan 2010 12:43:38 GMT

sebastian:

I'm not even sure this voltage difference really matters in real life, as it works just swell, but i've done already so much on it that it'd be nice to get it just there, to close it and never worry about it ever again.

Remember those old resistors have a 10% tolerance (some might even be 20%). So you can allow about that much on most test point voltages, as well... of course those that are directly adjustable should be accurate.

Re: Beomaster 900K & a novice needing help

sebastian — Sat, 16 Jan 2010 12:36:24 GMT

You know, that's funny because i was just about to come and declare that whatever it is, it sounds too good to worry about, but of course you're right. I've spent the day listening it, and it's just amazing how nice it sounds after the recapping. I mean, i can appreciate the "better" systems, but it just has this nice and warm old sound. It was this that i was expecting when i first listened to it and was so disappointed. I can just picture it already with an old turntable and listening some records with it (note : next project : turntable Another note : must find RIAA board). And i know that it'll just get better while the caps burn in.

So it's mainly done. But i kind of would tend to wish to finish it proper, even though i doubt you'd be able to make a tech out of me no matter what happens So, yes, i'll need to get to it, and just measure every damn thing on it that i can, without expecting that someone holds my hand every time. I'm not even sure this voltage difference really matters in real life, as it works just swell, but i've done already so much on it that it'd be nice to get it just there, to close it and never worry about it ever again.

So thanks for the pointer, i'll go through all the resistors & zeners when i will recap the stereo decoder next week

Sebastian

Re: Beomaster 900K & a novice needing help

yachadm — Sat, 16 Jan 2010 11:26:11 GMT

The fact that you're concerned and want to correct it, means we'll make a tech out of you yet!

Definitely start checking resistors, without a doubt. On some of them you may have to desolder a leg to get the actual reading. Anything more than 10% deviation (and I would aim for 5% max) is definitely cause for replacement. Use metal or carbon film resistors for replacement, not metal oxide types - they are not good for audio.

And I would check any zener diodes in the circuit. If the circuit diagram specifies BZX types, don't replace them with the more common 1Nxxxx types - I learnt that through experience.

Menahem

Re: Beomaster 900K & a novice needing help

sebastian — Fri, 15 Jan 2010 11:01:45 GMT

yachadm:
Check those carbon resistors which are in the Power supply circuit, especially those whose function is to drop voltage - you'll probably find a couple out of spec.

Did you do a voltage check across the power cap, as before?

I'd be interested to see the difference in measurement results.

Well, in fact they all seem to be just a bit above the supposed values, or close enough, so nothing to report really.

As for the power cap, no real change in values, i just noticed that depending on the input voltage it's more or less close to specs (normal, i suppose). So at 220V it's 34,5V, and at 240V it's 32V. Given that 34V is aimed at, i guess that operating it at 220V it is, then.

Dillen:
Worth checking the 1,5Kohm resistor that sits right next to the cap with the severed pin on the stereo decoder.

It looks chipped but may still be OK.

It measures 1.55K, so seems ok as well. Looks like these dinosaurs of resistors do resist to time too.

So far, after the recapping and a few measures at same points around the power supply, things seem just a tad more balanced. I managed to adjust to a spot where i get around 24V/34V where needed, but the same problem than before remains at below voltages. This is to say that i have 34,6V on the power cap, it goes to 24V & 24,2V after the 2N555, from there it goes to the AC128 (24,2V / 24,4V / 34,6V), and it's after that, at AC126 that it goes a bit weird, and i don't know what it means ; i get 24,4V / 8,6V / 8,7V where i should have 24,4V / 9V / 9,4V ?

To the same effect, the amp board gets exactly 24V at arrival, but for example at the first input transistors i measure 7,9V / 8V / 16,5V where it should be 6,8V / 6,9V / 18,5V. And so it goes, everything below 24V is a little off.

Any idea what is to be looked for ?

[EDIT] I also get a somewhat high "bzzzzz" from the transformer which was already there from the beginning, but maybe just a bit louder now. [/EDIT]

Sebastian

Re: Beomaster 900K & a novice needing help

Dillen — Fri, 15 Jan 2010 03:40:20 GMT

Nice job, I agree.
Worth checking the 1,5Kohm resistor that sits right next to the cap with the severed pin on the stereo decoder.
It looks chipped but may still be OK.

Martin

Re: Beomaster 900K & a novice needing help

yachadm — Fri, 15 Jan 2010 01:05:29 GMT

Well, I don't see any substandard work - looks like you did a good job!

Check those carbon resistors which are in the Power supply circuit, especially those whose function is to drop voltage - you'll probably find a couple out of spec.

Did you do a voltage check across the power cap, as before?

I'd be interested to see the difference in measurement results.

You're very lucky that the structure is in such good condition without corrosion.

Enjoy it for many years to come!

Menahem

Re: Beomaster 900K & a novice needing help

sebastian — Thu, 14 Jan 2010 18:29:56 GMT

Thanks Mika,

yes, for it's age it's pretty pristine, just old but no structural deterioration, not one bit of rust. Same for the speaker elements, not the smallest hole. Except for the fuse holders that i changed before anything else, it was just a question of lubricating and fixing the component problems. It would indeed look like it has spent it's life indoors in the dry, like it sure deserves.

It's pretty amazing actually that you own another that is as close a cousin of my one Here's to them

Sebastian

Re: Beomaster 900K & a novice needing help

tournedos — Thu, 14 Jan 2010 12:42:59 GMT

Good work, and good pictures! Thanks for taking the time to post them.

Seems like your Beomaster has had a pretty good life. Mine have surface rust on the metal parts of the connectors, screws and on the chassis, probably from years in the attic or something. Especially the oldest which is a S290 as well, only 1390 serial numbers away from yours

Re: Beomaster 900K & a novice needing help

sebastian — Thu, 14 Jan 2010 08:01:53 GMT

The next chapter ;

i thought to report on how it went, so that maybe one day it can help someone else, in any way, as my good Karma payback.

So, got the recap kit (thanks Martin !), nicely documented, and attacked the job.

Luckily this is old circuitry we're talking about, because my soldering equipment isn't what you would call the most advanced kind (nor in the best condition, to top it all). But it went well. Well, i'd say so, and i won't show you any too-close-up photos so that you could tell me the contrary

Turns out that this was indeed a version of the 900K that was still partly undocumented in Martin's information vault (just how unlikely is that ?!). So photos were taken. Notes were written. PM's exchanged. And here's a résumé of all that. A recapping of a Beomaster 900K, subversion S 290, export model with a six input voltages selector, and whose wooden cabinet has a stamped date of 8 JAN 1965.

This is an extremely image-heavy post, so my apologies to those who it might derange, but personally i would have loved to find one like it when i first encountered my radio, so here goes. The first few photos are for identification, of the untouched unit before the recapping. You'll notice (well, i just did) that the volume and radio dial buttons are reversed (the volume button has the black point) for the moment, during the project. It goes pretty much in the following order ; the original unit, the recap kit, and then the recapped unit.

Front

Back

Power unit ; 1 cap to change

Tuner ; 1 cap to change

Bottom side ; in my model version there are 12 caps to change (+ the two 800uF output caps that are fixed to the back wall)

Top side (on topmost there are the amp board on the left, and the stereo decoder on the right)

Amp board top ; in my model version there are 17 caps to change

Stereo decoder top ; in my model version there are 6 caps to change

Stereo decoder bottom

The recap kit from Martin

Bottom after recapping

Amp board after recapping

Power unit after recapping

Here are all the original caps from the main board, with their respective designations on the schematics that i have. After discussion with Martin, i've come to learn that these designations may well vary between model versions and/or schematics, so it just might not be the same ones for you. But this is the complete set on my specific model/version.

And here's the same thing for my amp board, to be considered with the same grain of salt.

And here's something i discovered only while taking the photos, even though i had manipulated the board frequently during days before (i've bent the cap slightly to make a point, i hadn't noticed it at all before). A severed + leg on a cap on the stereo decoder board. Just for the curiosity factor. It'll be fixed by the recapping of the stereo decoder board, coming in the next chapter.

So there, for now. What remains is the recapping of the stereo decoder, and the single one on top of the tuner box, both to be done in the coming days. I've also completely redone the wooden cabinet and given new luster on the gray speaker grilles with some polishing. It's starting to look good

Sebastian

Re: Beomaster 900K & a novice needing help

yachadm — Wed, 06 Jan 2010 13:30:27 GMT

Yes Mika, I'll second that - I also learnt something new here. Actually on the Fluke 187, it's so stable, that if I see fluctuations on the AC or DC scale (decimal points as appropriate), I'll suspect ripple, and connect my scope to confirm it. If the Fluke is rock stable, I know there's no ripple.

Those Uni-trend UT models are no longer suitable for our type of audio work. They changed their models, and I couldn't find anything over 100uF, let alone the minimum 10,000uF I'd want.

Perhaps check new Amprobe, as Mika suggested. But I can tell you that plenty used Fluke 8x or 11x go for well under $100 on ebay. You don't need a 187 like mine.

Menahem

Re: Beomaster 900K & a novice needing help

tournedos — Wed, 06 Jan 2010 12:31:44 GMT

sebastian:

Martin has, in his infinite kindness, proposed to send me a recap kit free of charge, in exchange of some detailed informations about my specific subversion of the 900K, which apparently isn't the most common kind (well, uncommon enough for him to not have encountered one).

Both of you!

To tell the truth, I didn't know about this kind of problem in AC measurements myself before this, as I've always used a scope to check for ripple... So this has been a highly educational thread all over

Re: Beomaster 900K & a novice needing help

sebastian — Wed, 06 Jan 2010 12:21:27 GMT

Magnificent.

yachadm:
A good (used) Fluke DMM and an ESR meter is a very comprehensive toolbox for troubleshooting electronics.

Menahem

Thanks Menahem, but i saw the Fluke's while browsing for meters on eBay just now. I could never afford one, not for the use i would have for it. Not today anyway. If i would really use it at least every week, i'm sure that it could justify itself, but as of now it's just once or twice every 6 months. I just couldn't. However i saw that you recommended an UNI-T UT70A in another thread, and that i could do.

For the ESR meter, i've seen it and sure it would be nice too, but it'll be for another day also. I'm in shock already finding myself repairing a 45+ years old B&O radio, you must leave me a few minutes to adjust before convincing me into believing that i'll be able to do it daily and need pro equipment But i've written the references down for the future, so one day...

tournedos:

I actually just opened up my 900M (please don't praise me for that effort, I was going to do it anyway ) and took a similar measurement across the main filter cap - I'm not at my "lab", so I only have an el cheapo multimeter here, not unlike the one you have.

Guess what, it shows 77.5 V on the 200V AC scale, and there's definitely nothing wrong with the power supply in this Beomaster.

When measuring through a series cap, the jump of the reading when connecting is normal as the cap is being charged to the DC voltage. I guess the zero reading is about right - as I recall, the accuracy of your multimeter on the 200V AC range was given as 1.something% of the reading, +/- 10 digits! So if there was 2 volts of real ripple, the meter might show anything between 0 and 4V and still be within specs.

Yeah the Mastech is rated at 1.2% for AC. This is great news, and really the only confirmation needed then. It was the multimeter. Whew. Thanks so much for the trouble, even if you meant to service your 900M anyway. That was very nice of you.

Dillen:

I was away for a couple of hours and you guys solved the mystery meanwhile.

I agree on the multimeter and instrument suggestions above, I use Fluke meters myself,
but I take it you are not planning on setting up a complete and stylish repairshop
but rather deal with the things the "kitchen-table" way.

But recap the poor thing and let's see what happens then.

Martin

Pretty much what i was replying before seeing your post. But i guess i could really use one a little more evolved and reliable...

So i guess it's back on track, then.

Martin has, in his infinite kindness, proposed to send me a recap kit free of charge, in exchange of some detailed informations about my specific subversion of the 900K, which apparently isn't the most common kind (well, uncommon enough for him to not have encountered one).

So i'd like to take profit of this unusually calm moment on my "restoration" project, where everything seems once again going well (before something else will come up), to repeat my sincere thanks to everybody around here, and in particular those who have helped me directly. And of course in first place Martin, who really has shown a patience out of this world.

This said, i won't say more, because it's still not finished

Cheers guys.

Sebastian

Re: Beomaster 900K & a novice needing help

Dillen — Wed, 06 Jan 2010 11:53:42 GMT

I was away for a couple of hours and you guys solved the mystery meanwhile.

I agree on the multimeter and instrument suggestions above, I use Fluke meters myself,
but I take it you are not planning on setting up a complete and stylish repairshop
but rather deal with the things the "kitchen-table" way. Correct me if I'm wrong,
in which case, as Mika suggests, the cheaper multimeters will do fine in most cases.
That's also my experience and most will easily tell you if something is wrong or right,
no need to go full steam ahead at this point.
There's also some meters that don't like measuring 100Hz AC or AC that is
not a nice sine wave, even more expensive ones.
A scope may be the better investment if it comes to that.

But recap the poor thing and let's see what happens then.

Martin

Re: Beomaster 900K & a novice needing help

tournedos — Wed, 06 Jan 2010 11:21:16 GMT

I actually just opened up my 900M (please don't praise me for that effort, I was going to do it anyway ) and took a similar measurement across the main filter cap - I'm not at my "lab", so I only have an el cheapo multimeter here, not unlike the one you have.

Guess what, it shows 77.5 V on the 200V AC scale, and there's definitely nothing wrong with the power supply in this Beomaster.

When measuring through a series cap, the jump of the reading when connecting is normal as the cap is being charged to the DC voltage. I guess the zero reading is about right - as I recall, the accuracy of your multimeter on the 200V AC range was given as 1.something% of the reading, +/- 10 digits! So if there was 2 volts of real ripple, the meter might show anything between 0 and 4V and still be within specs.

I'm afraid I can't recommend any particular meter... Fluke or Meterman (currently Amprobe) should be safe bets, although quite expensive for sporadic use. Don't get rid of your current one anyway, it has the transistor test function which is sometimes very useful, and not too common in even more expensive meters.