ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi everyone,
I'm hoping that someone can help me out. My lovely old Beomaster 1200, which has served me faithfully for well over 12 years now (after buying it for £4!) is now all but unusable. I have noticed a slight buzz developing if it is on for more than an hour or so over the past few years but it's now getting worse.
I have a confession to make though. Yesterday, I got so frustrated that I opened it up and started fiddling with the trim pots on the PCB in the base, without any scope or voltmeter attached. I know! I'm sorry! I'm technically quite competent but I just got so frustrated with the noise. I only did this because I seem to remember seeing a post on the Internet about this very fault and that adjusting some bias resistor would alleviate the problem and I'm sure that I tried it in the past, with some success but I can't find that original post now.
Anyway, today the buzzing seems louder than ever and I notice that it's getting very hot underneath, too hot to touch in fact. So, I know that my fiddling hasn't helped but I'm sure that it's only made worse a problem that already existed. I replaced the caps in the base some years ago, couldn't get ones that were physically the same size but they match the specs of the old ones.
Any help would be appreciated and if you want to admonish me for irresponsible tweaking then that's fine too!
Cheers
Welcome to Beoworld !
Your Beomaster is in need of a more elaborate overhaul, new capacitors, trimmers, calibration etc. They all are by now.You fiddled with the idle current and probably also the voltage regulator settings. These components are known to be fragile by now due toaging (oxidation) and shouldn't in any case be touched without monitoring using some form of instrumentation.I suggest you don't power it up again before it has been repaired, the chances of a more drastic failure is high now.Would you consider yourself a fairly skilled DIY'er with some electronic knowledge, skills and tools or are you looking forsomeone capable of doing it for you ?In case of the latter, maybe you could let us know where you are based.
Martin
Thanks Martin!
To my shame and embarrassment, I'm an ex-Audio Visual engineer who used to fault-find to component level so I know better than to go tweaking without proper monitoring! I'm afraid my ego got the better of me!
As I said in my post, I replaced the caps a few years ago but don't mind doing so again if you think it's necessary. Would you be able to tell me exactly what I should replace and then how to calibrate?
Michael
I cannot tell you exactly which component(s) are causing the buzz - a scope might tell.Did you also replace the large can to the right of the amplifier board ?Trimmers in general, replace and readjust.
Yes, I replaced that one and, if memory serves, another two nearer the front. Are the values of the trimmers on the circuit diagram in the brown envelope? I'm assuming I can just buy off the shelf items for these, they're not specific to B and O?
Yes, it's all in the schematics.And no, the trimmers are standard components.
Thanks, one last question! Is it possible to buy caps that physically fit in the old retaining straps and is there a place that people on here use to buy components? (Okay, it was a two-pronged question!)
Modern capacitors are typically smaller in size than what was used in the 1960s.For the large can-type capacitor I like to fit it using two crossing cable ties and short loopedleads for strain and vibration relief.
I think one big problem is that people servicing these machines replace pretty high quality only slightly worn thirty year old capacitors with modern aluminium/liquid ones designed to last five years in consumer electronics and bought off ebay manufactured by who-knows.
I would advise anybody to choose top-notch over-specced replacement devices and buy them new from a reputable outlet (R.S., Farnell etc.). Also higher voltage ones will last longer and are might solve the too-small problem. As I did with C66 in my Beomaster where the modern replacement electrolytic had failed as well as the replacement I stuck an extra tantalum one on the bottom of the board.
In my case it was NOT the big caps obvious though they are. Don't assume. Unless the ripple on those is volts that's not the cause.
Here's the kind of thing I went through. Is the buzzing affected by the volume and balance controls (assuming they work) ? That'll narrow things down. Both channels ? Does it go momentarily when you change input sources ? Does the 1200 have a myriad of power rails like the 1900 ?
Look at the shape of any ripple waveform. That should indicate if it's a failed rectifier or capacitors.
For caps (except the big ones) I worked on the bottom of the main P.C.B. poking around with a scope then holding new electrolytics to bridge suspect ones. If you find the bad one it's immediately obvious. Luckily the 1900-2 has silk screen on its bottom.
In your case get the output bias current down first if that is what you upset before something really goes up.
Jon
Hi Jon,
Thanks for your reply.
The capacitors that I bought were from RS or Farnell, I can't remember exactly now and I'm also fairly sure they were slightly higher voltage than the old ones. I'm sure I kept the old ones too.
Regarding the buzzing, it's not an audio buzz i.e. it's not coming through the speakers. It's more of a mechanical buzzing, if that makes sense, and it's coming from the transformer area. I have a piece of tubing that I use as a "stethoscope" and, when I find it, I'll have a listen to see if I can trace the source.
I actually forgot to mention that this buzzing has been there almost since I bought it, although it only starts after 2 or 3 hours operation. I initially thought that it was the transformer when I first had a look some years ago but it's a sealed unit and I was unable to even prise the end cap off. Following my ill-advised tinkering recently, the unit has been buzzing louder and getting extremely hot underneath within a very short space of time, so I have now stopped using it completely.
The only thing I have to work from is the circuit diagram found in the brown envelope inside, is there a more detailed diagram anywhere? Also, can you tell me exactly which trimpot adjusts the output bias current?
I just haven't had the time to have a proper, long look at this yet, complete with 'scope, but I'm hoping to be able to set aside a day soon where I can really get to the bottom of this fault. In the meantime, I'll order some new trimpots and fit them at the same time.
At a guess the transformer humming is along with the heat a symptom of the thing drawing far too much power. Is it the power devices getting hot ? Do you have a mains power meter you could use to quantify this and work out which way to twiddle things to reduce it (before using the proper procedure for setting it of course) ?
I don't know that machine but dare say schematics and other information you need are hereabouts.