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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi all,
I've recently started restoring an old BM 8000 (type 1901) that I bought second hand years ago. The seller told me that the BM would not turn on (repeatedly clicking relays). I did not try to power the unit up to avoid blowing the output stages so not sure what the actual symptoms are.
After not being able to find a B&O technician in the Netherlands that is willing to take on a BM8000 due to the labor involved I decided to have a go at it myself. I know, I know, these are complicated amplifiers, not really stuff for an inexperienced hobbyist with little knowledge of electronics... I just cannot resist the challenge so decided to give it a try with the help of information available online (beolover website).
Any help along the way would be appreciated. I love this design so would be awesome to get it working again (or at least take care of the laborious tasks such as recapping and rebuilding the LED displays).
So far I have disassembled the BM and cleaned out most of the dust and debris. Here's the beast with all the boards removed and cables marked.
I'm now in the process of rebuilding the output stages (replacing caps and trimmers), cleaning and applying new thermal grease to the Darlington transistors. Here's the right output amplifier removed.
First question I have is on the trimmer replacements. I ordered Bourns 100ohm 12-turn 1/4W trimmers (PV37 Series). However they seems smaller than most of the trimmers most others are using. Would these be ok or should I really use 0.5W trimmers instead?
Use the good old black Piher PT-10. It'll be fine for the job.
But - I really would recommend diagnosing and repairing before making any "updates" and "improvements".The initial problem may not be capacitor or trimmer related (indeed I don't think it is in this case) - and by doing a lot of work there's a huge risk ofintroducing more faults, - including faults that have no "natural" cause, - and that could make diagnosing very difficult.
THEN - do one module at a time. Adjustments and all.Now, when you've messed with the idle current settings, you will have to set those right away (both of them!), when the Beomaster starts up (if it will start), leaving you no time to do any other diagnosing at start-up, which could have been of interest with regards to the initial problem.If you don't set the idle currents (and DC-offsets) right, the Beomaster may not stay powered on long enough for you to do the settings, or if set all wrong it may overrun and do enormous damage to one or both output stages.Keep an eye on the current monitor of your variac the first time you start up the Beomaster - and cut the power immediately, if the power consumption is not normal.
Martin
I would advise the same as Martin for the trimmers, PT-10 or PTC-10 for the cermet version for better thermal drift.
As for the capacitors, the red ones must go out, from past experience. Actually, one looks dirty on top. Might be a leaked cap or liquid with dust mixture...
I have to applaud your ambition....and the amount of work you have already done is quite impressive too. Myself I would have chased down the actual fault before going to town on the whole unit....but hey you are where you are....this is going to be an interesting project, keep the pictures and dialogue coming
Craig
Thanks Martin,
It was my understanding that it would be wise to change the trimmers before powering up. So figured I should start with the output amplifier boards first and it would make sense to replace the capacitors on those boards as well. In fact most of the caps I checked were either out of spec or completely gone.
I'm planning to set the idle current and DC offsets with the output boards out on the workbench and refrain from making too many updates.
krais: I'm planning to set the idle current and DC offsets with the output boards out on the workbench and refrain from making too many updates.
Great idea.- But how?
And I agree on replacing the caps, but they will not be the reason for the present fault.
Hi,
I use this method to do my initial restorations of the Beomaster 8000 output amplifier assemblies. I have also successfully reworked the assemblies while they are still inside the Beomaster but I have found that I like reworking boards outside the cabinet better. Besides making the desoldering and soldering work easier, a lot of the units I have come across have required a lot of cleaning (dust, dirt, debris, rust, etc.). In addition, more than one output amplifier board has required some repairs to burned/damaged traces.
-sonavor
Thanks Sonavor, that's exactly the guide I was planning to follow. Those blog posts are so incredibly helpful.
Quick question on the dummy speaker load: I assume that for the DC offset adjustment procedure output power is limited so using two 50W 4ohm resistors in series would be ok? Thanks
That should be fine. The audio inputs to the output amplifier are shorted in my test setup so the audio signal is zero. After the output amplifier assembly is complete I recheck the no-load current adjustment and DC offset adjustment again when the assembly is back inside the Beomaster.
Finally found time to make some progress on the output amplifiers. Replaced the elcos and trimmers (used the Bourns after all as my local supplier did not have the Pihers in stock). Also, I swapped the 0.5W resistors for 1W carbon film versions to alleviate some of the heat issues (the boards had significant burn marks).
I'm currently working at cleaning the dried out thermal paste and mica insulators on the Darlingtons. However, I noticed that although most transistors had a single mica installed, some had multiple (thinner) mica sheets. Does anyone know if there is a logic to this (there does not seem to be consistency between the right and left heatsinks)?
The original mica insulators were probably flaking apart. Using mica and thermal paste only requires one mica insulator (plus the thermal paste). Use thermal paste designed for transistor heat sinks. Not the paste used for computer processor cooling.I like those Bourns multi-turn trimmers as well.The bank of resistors on the output amplifier board do get hot. The discoloration on the PCB from their heat is normal but shouldn't be black...as I have seen on a couple of boards. I don't think increasing the wattage rating will reduce the heat. Were the original resistors still in tolerance?
It's weird, on one transistor I even found 3 thin mica insulators, one of which fell apart when cleaning. Do you think it would be ok to reinstall the micas even when using the thin sheets (I'm using Wakefield 120 for thermal grease btw)? Or would it be better to use sil pads instead? .
The boards were quite dark but not black. Replacing the resistors was probably overkill although about a third of the old ones measured out of spec.
Replace the insulators with new mica insulators and grease. You can opt for sil pads instead of the mica/grease insulators if you want. I have restored using both methods without any problems. I just wouldn't re-use the old insulators at this point on yours.
Good choice on the Nichicons, I like them!
Personally, I'm using thermal pads instead of the micas, much cleaner work and one versus the other, a matter of preference. Now, if you had multiple layers of mica, I would give the edge to the pad for heat dissipation performance.
I'm also concerned about the cooking PCB. Mounting the resistors with a gap from the PCB is a good practice if you have clearance on top. Consider adding cooling for that transistor that seems to suffer from heat too!
"Also, I swapped the 0.5W resistors for 1W carbon film versions to alleviate some of the heat issues"The same amount of energy will be dissipated, regardless ot the components wattage range.You could do as Jean suggests and lift the resistors a milimeter off the board, though it's usually not a problem - unless completely burned - in which case something else will most likely be amiss too.Lifting components without fitting some kind of relief under them puts stress on their solder joints, which must now carrythe components weight - "upwards". The lifted components will easily be pressed down, forcing the solder pads from the board. Vibrations will also be much higher and will eventually cause cracking to the solder joints.Using radial capacitors instead of axials will also do something like that. Why not use axials?
Thanks for the great feedback all, I have ordered new mica insulators and will finish the output boards when they arrive.
Dillen:You could do as Jean suggests and lift the resistors a milimeter off the board, though it's usually not a problem - unless completely burned - in which case something else will most likely be amiss too.
Yes, indeed the resisters are raised slightly from the board.
Dillen:The lifted components will easily be pressed down, forcing the solder pads from the board. Vibrations will also be much higher and will eventually cause cracking to the solder joints.
Good to know, I will be more mindful of minimizing vibrations when replacing components. I'll see if I can add some stress reliefs here and there (on the radial caps for example).
Dillen: Using radial capacitors instead of axials will also do something like that. Why not use axials?
Using radial capacitors instead of axials will also do something like that. Why not use axials?
The reason I used radial capacitors for C205/C211 is that I couldn't find axial capacitors with -10%+50% tolerance as specified in the service manual. Maybe this is not a strict requirement?
krais: The reason I used radial capacitors for C205/C211 is that I couldn't find axial capacitors with -10%+50% tolerance as specified in the service manual. Maybe this is not a strict requirement?
It's not that critical. 20% tolerance caps will be fine.
Worked this weekend on replacing the mica insulators and performing the no-load current and DC offset adjustment procedures. Here are the completed output amplifier boards.
I followed the adjustment procedure from beolover.com (link). Here's the setup using a small collection of vintage linear power supplies (one PSU each for -55V +55v power rails with 150mA current limit, one for the +15V line).
Setting the no-load current test voltage to ~18 mV
The +55V/-55V power rails were drawing around 100mA.
I was a bit nervous when trying this procedure for the first time but it all worked out. The no-load test voltage was a little difficult to get right as the voltage was drifting. It steadily increased from ~15mV to around 18mV after a few minutes. However, the voltage kept creeping up to 19.5-19.7V after 45 minutes as the components were getting warmer.
No issues with setting the DC offset voltage. After warming up the voltage kept drifting between -1.5mV and +1.5mV which I assume is ok.
I prefer adjusting when everything is back in place into the BM.
Do check your voltages when the amp board are back in place.
Jacques
Moving on to the power supply. Here is the board just after removal from the chassis.
Notice that C40 has bursted? This was one of 3 capacitors along with C39 and the one on the board with the voltage regulators that are not original. The guy who replaced those caps made a mistake and put in C40 with reversed polarity which caused it to fail.
Here's the recapped board. I also reflowed all connector solder joints.
Very nice.MKS is probably not the worst choice here, but I really would have chosen tantals for the two in the remote receiver amplifier.B&O chose to use tantals here for a reason.We talked about tantalum capacitors vs. electrolytics in a thread here some time ago, but I can't seem to find it right now.
EDIT:Found it
https://archivedforum2.beoworld.org/forums/p/25190/202897.aspx
I measured the -15V voltage regulator IC6 that is connected to C40, which seems to be ok. Any other components I should perhaps check?
Dillen:MKS is probably not the worst choice here, but I really would have chosen tantals for the two in the remote receiver amplifier.B&O chose to use tantals here for a reason.
Thanks Martin, very helpful. I'll order some tantals to replace the old ones on the various boards.
Dillen: Very nice.MKS is probably not the worst choice here, but I really would have chosen tantals for the two in the remote receiver amplifier.B&O chose to use tantals here for a reason.We talked about tantalum capacitors vs. electrolytics in a thread here some time ago, but I can't seem to find it right now. EDIT:Found it https://archivedforum2.beoworld.org/forums/p/25190/202897.aspx Martin
WOW! Thanks a lot for that Martin! However I always fitted tantals where tantals where used before except for the signal path where I would replace them with films and I will certainly rectify that. But to add you your outstanding tantal application 101, I do not hesitate to replace other type of capacitors around IC's and controllers with tantals to keep those chips healthy and stable (depending on their purpose of course). I've seen a lot of people here replacing tantals on the FM circuit boards of the reputable BM4400 with electrolytics and I believe this is a bad pratice. Thanks again!
Quick update: I finally found some time to work on the Beomaster and continued with the replacement of the reservoir caps. As I don't have a 3D printer I had to come up with another way to secure the capacitors (the new caps are considerably smaller compared to the old ones). At my local hardware store I found some thin walled PP tubing that fit in the cap holders rather nicely. Here are the 4 sleeves cut to size.
New vs. old
Here are the new caps installed in the chassis.
Looks great. Nice work.-sonavor
The plan is now to connect all boards and do a power up test. However, it looks like someone made an 'improvement' to the preamplifier and tone control board. First of all the filter out and filter in RCAs have been bypassed with jumper wires on the preamp board connector.
From the RCA panel a cable is connected to the tone control board (I assume with the purpose of a PRE-IN without the need of U-cables).
Here's the connection to the backside of the tone control board (P24).