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 guys,
A while a go I got my parents old beomaster 8000 running after some time of hibernation. After three days of use the right amplifier went bust. I posted a topic about it here:
http://archivedforum2.beoworld.org/forums/t/3158.aspx
I took out both amplifier boards and completely rebuilt the right one with a kit Martin supplied me with. On the left channel I replaced the caps and trimmers. I reassembled the unit and powered it up. The machine now only displays 8's on all display units. So, it looks like my "repair" did more harm than good. Has anyone seen this problem before? Or maybe clues on where to look? Maybe a CPU or display problem?
I had some email conversation with Martin about this and since I don't have a lot of knowledge about electronics and I don't have a scope, they're is not much I can do when it comes to troubleshooting. I know how to work with my multimeter, but that's where it stops.
So, since this came to a dead end I called my local audio shop. They had a beomaster 8000 on shelf with defective amps. So I thought I'd just buy that and swap in my refurbished amps. Well, I went to the shop and we tested it there, it does not do anything at all. Only two relay clicks and that's it, no standby light, no nothing. I still bought it since he offered it to me for only 25 euros and it's exterior is in mint condition. When I got home I plugged it in a few times to listen for the relay clicks. At one time something in the thing started buzzing... Maybe bad caps?
So... Any ideas on what would be a good place to start? Maybe I could swap the display and cpu board from my "new" beomaster into the other?
No, don't start swapping parts over as you could potentially introduce new faults to your refurbished beomaster. These hifis are rather delicate with age and require a great deal of care when working... You would need to check the power supply and look for dry joints. More than likely the fact you have disturbed a marginal wire or board header plug with a bad joint. The board to board connectors also go bad, requiring bridging wires.You would normally need a scope to check for life in the micro, although the 5v power supply / reset line and the possibility of dry joints is where I would start!I actually have one on the bench today that I am refurbishing, I estimate there will be 12+ hours of work, but that does include replacing the led chips... I will take pics as I go along :-) (I really will this time!!!)
Olly
Olly, we are holding our breath then!
I agree with you about connections and solder joints. Those should always be looked into first!
Jacques
Allright, at the moment I got both machines opened up and I was about to swap the pcb's but decided to check this post first. The "new" beomaster which has a great looking exterior actually doesn't look very good on the inside, everything is dusty, the main caps look dirty and there is a lot of rust on the cover under the input panel.
It would be sweet if I can get my original unit going again and put the front panels of the "new" beomaster on it along with the tone control pcb (which was hardly working at all)
Problem is that I don't have a scope and checked all solder joints I can reach without removing the metal cpu cover. I'm just at loss on what to do next, so this seemed like a feasible option.
Where do I find the 5v supply?
It's great news for you Jacques cos I will finally be able to fully test the other cpu board recap and send it to you :-) Postponed today as I was underground until 3am this morning and I need to do some wall papering now :-(
Beomaster4 if you are not experienced with a soldering iron I would not recommend removing the screen of the processor, the board is very delicate and will more than likely get damaged in the process. The 5v regulator is on the heat sink under the display panel on the right wall of the machine. It is a 7805 I would replace that and its associated capacitor. The processor reset circuit is on the display panel you will need the service manual though to adjust that. Maybe send the boards off to someone (Martin?) to have them looked at? Then you can work on the rest of the machine yourself?Where do you live?
I live in The Netherlands.
I'm pretty handy with the soldering iron, so I don't think that should be a problem. I do have some basic knowledge about (dc) electronics, but not nearly enough to troubleshoot a machine like this. I do have a copy of the service manual.
Since the display is displaying 8's (volume: 8.8, channel: 8, freq: 8 8, balance: 8) doesn't this imply that the 5v supply is working? The display board only gets power from the CPU board, so that must be energised.
Is it really such a sin to swap the CPU board from the "new" beomaster in my original one? I already swapped the display boards just to rule that out, doesn't make a difference, also displays the 8's. I'm just very much tempted to swap the boards since I have 2 non-working machines here, so as far as I'm concerned it can't get much worse :P.
Just because you have got a 5v supply, does not mean it is clean enough for the micro! and Again, if the reset circuit is not working on both the display boards how has swapping helped determine where the fault is?Not being funny but this is quite possibly the worst piece of B&O you can pick up and try to fix without previously having experience with B&O gear, it really is a very delicate machine!By all means if you would rather try swapping the processor boards over and risk loosing a piece of hardware that can fetch £100 on its own on ebay, and you feel you have nothing to lose then do so, just make sure you at least do so in an anti static environment and be absolutely methodical in your approach, and you are happy nothing else external to the processor board has created the problem in the first place!Also check for dry joints and the connectors while you are at it!Let us know how you get on :-)
Hi Olly,
Thanks for your advice. I know repairing this machine is way over my head, but it's an old unit from my parents. I don't have the means to buy a new one or get it professionally rebuilt. Since I'm pretty handy with 12v car electronics and soldering, I decided to give this a go.
Since I'm only down by 25 euros so far, I decided not much would be lost by swapping the boards. I do understand that this may not be a wise choice, but I did it anyway. So, I tried the "easy shortcut" and it didn't work. The other cpu gives me the exact same behaviour.
The original cpu board I took out looks pretty good, the solder joints all seem to be in good condition. I can't really check the traces without removing the pcb cover. However, the fact that both cpu boards give me the same behaviour may point to a problem elsewhere, like you suggested.
So how exactly do I go about checking the 5v and reset circuit? I found it in some of the diagrams, but don't know where exactly to measure.
Check the reset line on both the display and processor board.
How exactly do I go about checking this?
I found it in the block diagram on page 2-3 of the service manual. The lead from P75-2 to where it terminates on the display board checks out fine for continuity, or should I check actual voltage?
I can't check continuity on the cpu board without removing the metal covers...
Anyone?
A storage scope, monitoring the reset line, triggered by the 5V supply or delay-triggered by itself (if it gives a signal at all).
Martin
I don't have a scope...
So, have I reached the point where I have to give in?
No, you've reached the point where more knowledge, experience and instruments are needed and wherediagnosing at distance becomes difficult.Don't get me wrong.I have the deepest respect for owners willing to give repairs a chance themselves and I liketo support and guide as far as I can.There is, however, a reason for the 3-4 years of education it takes to become an electronics mechanicand there is a bit of way to go to build up a good repairshop with tools, instruments, learn to use it all, let alone gain experience.
It would be difficult for anyone to go further without the proper instruments but if you find a skilled tech guynear you it shouldn't be too difficult for him to move on. Having a more or less complete Beomaster donoras backup should only make things much easier. He may even be able to bring both units back.
Thanks for your help and advice Martin.
I've brought both beomasters to a local repairshop, let's hope for the best.
BeoMaster4: Thanks for your help and advice Martin. I've brought both beomasters to a local repairshop, let's hope for the best.
Too bad that you couldn't fix it yourself, but come back and tell us how it went.
Collecting Vintage B&O is not a hobby, its a lifestyle.
BeoMaster4: I don't have a scope... So, have I reached the point where I have to give in?
If you need some help.
You're welcome.
All right, I just picked up the unit from the audio shop, it indeed was the 5V supply. So, it is up and running now!
Just one strange thing, I can't measure any voltage across the power resistors at TP200/201. On the left amp I did measure voltage and adjusted R226 to 18mV. I just ran the amp at full volume and it works fine on both channels... so seems a bit weird.
Also, how should I go about doing the offset adjustment? I measured the voltage at the speaker outputs and it just jumps around, 1 or 2 mV up or down from 0. Is that ok?
BeoMaster4: All right, I just picked up the unit from the audio shop, it indeed was the 5V supply. So, it is up and running now! Just one strange thing, I can't measure any voltage across the power resistors at TP200/201. On the left amp I did measure voltage and adjusted R226 to 18mV. I just ran the amp at full volume and it works fine on both channels... so seems a bit weird. Also, how should I go about doing the offset adjustment? I measured the voltage at the speaker outputs and it just jumps around, 1 or 2 mV up or down from 0. Is that ok?
Did you try adjusting the trimpot for the right channel to see if it increases the voltage, perhaps it's just turned down too low?
The service manual says 0V+-5mV on the speaker output, so 1 or 2 should be ok, I suppose.
/ Johan
Glad to hear it :-) Now have you replaced the trimmers? If not then do so next before you end up back to square one!Not being able to set the bias current obviously suggests there is another fault somewhere and you need to get to the bottom of that before putting the amp to use or even working in other sections.
Step1: Not being able to set the bias current obviously suggests there is another fault somewhere and you need to get to the bottom of that before putting the amp to use or even working in other sections.
Not being able to set the bias current obviously suggests there is another fault somewhere and you need to get to the bottom of that before putting the amp to use or even working in other sections.
Right. That's what I was thinking, so turning the trimmer a little to see if that changes anything might be a first step.
Replace the trimmer first. Then adjust.If the old one loses contact, the amplifier will run astray and end its life in a big bang.
I replaced every components on the right channel, including the trimmers R226 and R200. Or are you guys talking about a different trimmer?
I tried adjusting R226 to get a reading, but all I get is 0V across TP200/201. Is this even possible if the amp works? I just played one track with it and it sounds just fine... (actually more than fine )
The measurement on the left channel (on which I only replaced the trimmers and caps) did give me the correct reading after a little adjustment.
I also swapped the tone control PCB from the "new" unit to my original unit, so now I've also got my bass and treble controls working once again, sweet!
Dillen: Replace the trimmer first. Then adjust.If the old one loses contact, the amplifier will run astray and end its life in a big bang.
Those were already replaced according to the first post, hence my suggestion.
Johan, I tried making incremental adjustments to R226, but couldn't get a reading, no matter what position it was in.
BeoMaster4:Johan, I tried making incremental adjustments to R226, but couldn't get a reading, no matter what position it was in.
Turn off the Beomaster and check if you have a reading in resistance scale on that test point. Sometimes resistor leads oxidize so that test probes don't make contact at all until you really pierce through to the copper.
--mika
Across TP200/201 it measures 0.8 ohms. (I checked the left amp for reference, gives the same value)
Could it be R225 that's the wrong value? I had a BM5500 where someone had put in the wrong value resistor in that (corresponding) place and I had similar issues.
That's about where I stop, and leave it to Mika, Martin or Olly.
Nope, R225 is just fine at 100 ohms.
But, does the resistance vary when turning? On the middle leg that is.
Does R200 affect the offset voltage? Are the correct voltages on the output stages present?
Thanks everyone for the input, it is much appreciated!
Beobuddy, I guess you mean the resistance on R226? That does vary, when looking from the front of the amp, I measure the same value between the right 2 posts as between the left and lower right, this value varies between about 2 and 118 ohms. The resistance between the left and upper right post is nearly zero, about .3 ohms, independent of it's setting. I presume this has something to do with how the thing is wired on the PCB.
Just for comparison I also checked the donor beomaster I got, it has pretty much the same behaviour.
Just out of curiosity, I also checked out the old pots from my own beomaster for both left and right channels. The left channel (which did not blow up) showed about the same values. The right channel pot has very large resistance between A and B, about 1.8k. It's behaviour is very erratic, so maybe this thing caused the initial problem.
I did not change R200 on either the left or right channel, since the readings on the terminals are in spec...
Where should I check the output stage voltages you are referring to?
On the board the midldle leg is shortcircuited with one of the outer ones.
But the between these 2 and the other outer leg, the resistance has to vary. As you already mentioned.
It needs more measuring, thats obvious.
Probably also mentioned earlier, what voltage do you measure across the (emittor) resistors in the outputstage?
There has to be measured an idle current. A zero or to low current results in distortion. Anyway imho anyone aming to achieve a lower idle current than given, doesn't understand the meaning of it.
But that's another story.
Beobuddy: Probably also mentioned earlier, what voltage do you measure across the (emittor) resistors in the outputstage?
The emittor resistors are R236/R237, right? The problem is the voltage measured across TP200/201 is 0, these test points are across R236 and R237, so I can't measure any voltage at all, that's the entire problem.
I'm at loss as to where to start looking for potential problems.
Am I risking blowing up the amp again if I put it to use? We're throwing a little party by the 31st and I really need to get it going before then.
Where are you located in Holland. Utrecht an option for you?
Sure, I'll send you a PM so we can get in touch.
Thanks!
Hi BM4, hope you had a good Christmas. Regarding the fault with the amplifier, have you tried taking DC measurements with respect to ground? Also have you double checked your work? It is so easy to make a mistake that is not at all obvious until you go over everything with a fine comb, and then sometimes for a second time!!!However if you can come to an arrangement with Beobuddy then I think that would be best in this instance, amplifiers are unforgiving at the best of times, and you don't want to take a step back at this stage!
Hope you sort it out :-)
One working BM8000 again.
With all respect to the TS.
Just some advise for the one who's trying to change a set of components, be carefull with (de)soldering.
TS had several points where the traces on the circuit boards where broken. Some broken joints which weren't visible by eye.
With these heavy amplifiers I would advise to make use of a variac. Shorten RL1 and RL2 and start with a few volts on the main leads.
This way you can correct the trimmers and measure voltages in a early stage preventing damage on the output stages (TIP142/147).
Yes the print is very delicate with these amps, which is why I suggest not to remove the processor can unless absolutely necessary! I guess he is very lucky things were not worse!Well done Beobuddy :-)
Thanks to Beobuddy the BM is working beautifully once again.
I think Martin was right in telling me in the beginning not to try repairing this machine myself. Rebuilding the amp is one thing, but there are just so many other things that can go wrong or break. Without the proper equipment and knowledge it is pretty much impossible to fix it when that happens, so you're better of leaving this kind of job to a professional.