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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hey guys,
My first post here at beoworld. My parents had an old Beomaster 8000 set in the garage (beomaster, beocord, beogram and beovox ms150) which has been unused for 8 years or so. We brought the set back to life two days ago. All the devices have their problems, but the beomaster was working ok (apart for half of the buttons on it). We have used the beomaster for about two days now and some time at full volume, all was well.
Today we were just watching TV and had the volume set about half way when suddenly it made a loud pop on the right channel and smoke came out on the right side of the heat sink.
I disassembled the unit right away to look for anything obvious. No visible damage can be seen, but a very distinct burned smell comes from the number 3 column of metal plates on the heatsink (numbering the columns from left to right, so 3 and 4 being the right side). I have no idea what these things do, but apparently they get hot...
I hope you can advise me on what to do next. I guess these machines have common problems. I already read about the right channel running hot. It's strange I can't see any damage, while it produced a fair bit of smoke and smell.
Where do you live, its easier to help if we know at least your country
Collecting Vintage B&O is not a hobby, its a lifestyle.
I live in The Netherlands
We have some well experienced and respected Dutch members such as Blackrix, Ipaul and Beobuddy who can help you. Try the Dutch forum en ik weet zeker dat je antwoord zult krijgen...
Brengen & Ophalen
Leslie: We have some well experienced and respected Dutch members such as Blackrix, Ipaul and Beobuddy who can help you. Try the Dutch forum en ik weet zeker dat je antwoord zult krijgen...
Hey hey, dont run away to the Dutch forum, we like to know what happens, and if we have to use the Google translator it gets kind of problematis
Hi,
Sometimes when a transistor gives up the ghost, there is a tiny crack on its surface and you will really have to look with a magnifying glass to see it!
Probably bad caps and idle (quiescent) current trimmer caused that...
Jacques
And that crck might be towards heatsink, invisible to front. Power resistors (0,18ohm) has possibly burnt too.
blah-blah and photographs as needed
...and obviously all depends how experienced you are with electronics :)
Probably indeed (unfortunately) some short in a power output stage which causes a smoking resistor :(, which will than need some soldering, replacing and probably adjusting.
Maybe a magnifying glass will help, sometimes it's just hard to see.
Nice set though, too good for a garage :), hope it was dry and not cold there, as that may cause trimmers to go bad etc...
Typical case of an open idle current trimmer.The output stage heats up and makes whats called a thermal runaway with the output stage pulling maximum from the power supply.The power supply will deliver a good 700 Watts so the output stage, however beefy, becomes the weakest point.It all ends in a blown output stage, most often dragging driver transistors etc. with it in the fall.
Replace all components in the output stage of the affected channel, preferably all components on the board plus those on the cooling finand replace trimmers and capacitors in the other channel to prevent it from happening here too.
This is a job for a skilled tech guy but worth it for an otherwise good Beomaster 8000.
Martin
Thanks for all the advice so far.
The Beomaster also has some other problems, the tp2 channel is not working and pretty much all the controls on the controls panel are non responsive. (bass, treble, filters etc). So... maybe it is not worth rescuing anymore?
I do have a fair bit of experience soldering electronics, so that shouldn't be a problem. On the other hand, I don't have a lot of knowledge about stuff like this.
Martin, I also just ordered a cap kit for my Beogram from you. Do you also sell components to fix up the beomaster? Or do I have to start disassembling the output stage and source new components locally?
Is there a guide or something to the PCB's and it's components on this machine?
Yes, there are some kits etc. but, honestly - and don't get me wrong, this is not a job for you unless you are more than fairlyskilled and educated with electronics in general and powerful DC coupled soft-clipping complimentary output stages in particular.Several, and even some authorized, repairshops would reject this repair simply because they don't have the training and experience neededand the Beomaster 8000 is a rather complex beast and definitely not a machine for beginners.If you don't know where to locate and access the components mentioned above, I dare say you don't have the skills to perform this repairand I suggest you bring it to someone with a good experience in these machines.A good repairer will not be that costly and he can fix the other issues too which sounds like caused by lack of use etc.
Dillen:If you don't know where to locate and access the components mentioned above, I dare say you don't have the skills to perform this repairand I suggest you bring it to someone with a good experience in these machines.
And that's why I recommended these Dutch techies who might be able to help you in your own language, simple as that!
Thanks for your advice. But I'd like to try to fix this myself if at all possible. Part of the fun of something like this is fixing something yourself rather than paying someone to do it for you. I've put together a fuel injection system for my car recently, which was a lot more hairy than having to solder a board like this together. I understand you advise not to do it, but you only challenged me to do it myself :)
I removed the output stage today:
I was hoping that removing the board would reveal anything visually obvious, but it didn't. Only C212 has a body that has slightly cracked on the side. Besides that there is a very distinct burnt smell from one of the left three power transistors on the cooling fins.
So, what step should I take now? I was thinking I may want to remove the left power transistors and replace them. After that also rebuild the boards (both left and right) as described here: http://beolover.blogspot.nl/2012/08/beomaster-8000-left-output-stage-rebuild.html
Do you think this would be a good idea? Or do you suggest a different approach?
The blown transistors have blown because the others have shorted.Replace all components on the cooling fin and the board.Adjust idle current and DC offset to factory specs and it will be playing again.
Do B&O use matched pairs?
By all components, do you mean I also have to replace all the resistors? Even if they are within spec?
I would say yes.Some will have seen a serious overload, even if only momentarily but experience has it thatsome may show correct ohmic value unloaded but drastically change characteristics under load.Some may even have physical scars, often found underneath the component (warmest spot).
Besides, they are cheap components and easily replaced so why risk a full set of expensive output transistors ?
Don't feel tempted to fit metal oxide resistors. Use good old 1/2 watt coal types.
Dillen: I would say yes.Some will have seen a serious overload, even if only momentarily but experience has it thatsome may show correct ohmic value unloaded but drastically change characteristics under load.Some may even have physical scars, often found underneath the component (warmest spot). Besides, they are cheap components and easily replaced so why risk a full set of expensive output transistors ? Don't feel tempted to fit metal oxide resistors. Use good old 1/2 watt coal types. Martin
I dont know the, 8000, but the failures that appears only when the unit is warm, I have had in my 4400, a more complicated unit, and the board more crowded, I ended up getting from Martin, spares for 2 complete channels. I recommend to you, to do the same, I dont know if Martin can provide the kits for the 8000 but if he can provide it, buy from him, you will get the correct spares, and you dont have to go looking for them yourself, to get the right good quality today is very difficult, Martin has the experience and knowledge after years of repairing and refurbishing these units, if there was a better, faster or cheaper way to do it, he would recommend just that, as he always do.
Don't get me wrong in that I would like to source all the components myself. If there is anyone selling rebuilt/remanufactured boards or a kit with the required parts I'd be more than happy to buy that.
So, Martin, if you have anything available that I could use, please let me know.
Just posting a follow up, to keep everyone who is interested up to speed.
Martin is sending me the components for the right output stage, so I can replace everything on that board. He's also sending me a recap kit for the left board, so I also removed that from the beomaster. Removing the left side really is a pain in the ass compared to the right side. Only after about 10 minutes of wiggling with the board I read you have to rotate it 90 degrees clockwise for it to slide out. Even rotating it 90 degrees it's still quite hard to extract. But both boards are out now, awaiting their new parts.
I'll post another update when I'm done rebuilding both amp boards.
Dillen: Don't feel tempted to fit metal oxide resistors. Use good old 1/2 watt coal types. Martin
Hi Martin,
Can you expand on the replacement of resistors. I have heard that in the case of tuner circuits that it is best to stay with carbon resistors but in other amplification circuitry I hadn't heard that it made any difference. I have had a couple cases where I used metal film resistors in repairing a power amplifier and didn't notice any problem with the result. I would like to know the difference in using metal film versus carbon.
Thanks,Sonavor
sonavor:I would like to know the difference in using metal film versus carbon.
Me too
It's a fairly well-known fact that metal film resistors are no good for high power output stages (with a few exceptions).They are basically good components and usually preferred to carbon (coal) resistors in many applications but the way they are made can causethem to introduce a bit of induction (coil function) to the circuit in which they work and that, in turn, can result in unwanted feedbackand make an amplifier circuit go into self-oscillation.I'm not saying that it will always happen, but the risk is far greater if using metal film resistors.Even more so in a circuit that wasn't specifically intended for metal film resistors.This self-oscillation typically ends in the 100KHz range so is not audible to us. Self-oscillation usually goes on atfull power, you don't notice it until the smoke comes, the cooling fins run hot and/or the tweeters blow in speakers connected to the amplifier and,eventually, the amplifier destroys itself in a loud bang.
When B&O designed the Beomaster 8000, the engineers were specifically told NOT to compromise. And they didn't. They fittedcarbon resistors (with a single or two exceptions in circuits where stability is a greater issue and self-oscillation a lesser).
There are other issues regarding metal film resistors, one of them being the fact that in rare cases they canactually fail short where a carbon resistor always fails open, soft or high.They are physically slightly more brittle (think ceramic) and they don't survive high current spikes as good as their carbon relatives.If spiked, they can be found having a melted pinhead sized spot, typically on the side facing the printed circuit board (out of normal view)where the heat buildup was greatest.
They are, however, far more exact and will normally be found in +/- 1% types where carbon resistors usually are no better than 5%.They also have a better temperature stability but these matters are not always as important as it may sound. It all depends on the circuit.
edit: reaction to paired transistors.:
Probably not, but preferable.
With these amps, you should use a variac. Just replacing caps and pots without some knowlegde can cause a faulty amp again. And endless soldering on these boards isn't also preferable.
Dillen: It's a fairly well-known fact that metal film resistors are no good for high power output stages (with a few exceptions).They are basically good components and usually preferred to carbon (coal) resistors in many applications but the way they are made can causethem to introduce a bit of induction (coil function) to the circuit in which they work and that, in turn, can result in unwanted feedbackand make an amplifier circuit go into self-oscillation.I'm not saying that it will always happen, but the risk is far greater if using metal film resistors.Even more so in a circuit that wasn't specifically intended for metal film resistors.This self-oscillation typically ends in the 100KHz range so is not audible to us. Self-oscillation usually goes on atfull power, you don't notice it until the smoke comes, the cooling fins run hot and/or the tweeters blow in speakers connected to the amplifier and,eventually, the amplifier destroys itself in a loud bang. When B&O designed the Beomaster 8000, the engineers were specifically told NOT to compromise. And they didn't. They fittedcarbon resistors (with a single or two exceptions in circuits where stability is a greater issue and self-oscillation a lesser). There are other issues regarding metal film resistors, one of them being the fact that in rare cases they canactually fail short where a carbon resistor always fails open, soft or high.They are physically slightly more brittle (think ceramic) and they don't survive high current spikes as good as their carbon relatives.If spiked, they can be found having a melted pinhead sized spot, typically on the side facing the printed circuit board (out of normal view)where the heat buildup was greatest. They are, however, far more exact and will normally be found in +/- 1% types where carbon resistors usually are no better than 5%.They also have a better temperature stability but these matters are not always as important as it may sound. It all depends on the circuit. Martin
Thanks for the information Martin. Very useful to me. I always leaned towards metal film resistors because of the precision and temperature factors. I am glad to know the downside relative to carbon resistors. This information will change the way I do things.-Sonavor
Little update; I replaced all components on the right channel amp and also replaced caps and trimmers on the left side.
Martin already warned me this repair could very well prove too much for me and it did... I've introduced a new problem and decided to start a new thread for it since it seems like it is not related to the amplifier problem.
http://archivedforum2.beoworld.org/forums/t/3796.aspx