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
sonavor: Is the hum you are hearing the 50Hz line voltage frequency? If so then it sounds like there is a grounding issue somewhere. Check that the shielding grounds on the source input box are all connected. From there check the grounds to the chassis ground points. -sonavor
Is the hum you are hearing the 50Hz line voltage frequency? If so then it sounds like there is a grounding issue somewhere. Check that the shielding grounds on the source input box are all connected. From there check the grounds to the chassis ground points.
-sonavor
Sorry for the delayed reply. Finally had some time this weekend to start troubleshooting the speaker hum.
I'm not sure it is 50Hz line frequency, sounds more like a higher pitched noise to me. Tried to do a measurement with my oscilloscope but difficult to draw any conclusions.
I did check the shielding grounds on the source input box and chassis ground points (the ones at the source input box, near the transformer housing and on the front plate next to the tuning wheel) and found no issues. Also, measured resistance between ground points on various boards but could not find anything out of the ordinary.
So to recap the symptoms:
I have to say I'm quickly running out of ideas so any tips would be very welcome.
Try disconnecting the the audio signal cables from the left and right output amplifier boards and see if the noise is still present. That will tell you if the noise is coming from the output amplifier circuitry. That won't necessarily mean the output amplifiers are the source of the hum but should eliminate the preamplifier and tone control sections as sourced of the problem.
On the flip side, if the noise goes away with those connector disconnected then the problem must be coming from one of the stages prior. If that is the case then I think the next step would be to put the audio connectors back on the output amplifier boards, then disconnect the connections between the preamplifier board and the tone control board.
I haven't seen this before but I wonder if there could be noise coming from the Beomaster transformers. We see it in the older Beomaster 4000 transformers. But, as I said, I haven't heard of the problem in the Beomaster 8000 before.
Offhand I can't think of anything else.
Also, you might post some internal photos of your Beomaster. Specifically where the black audio ground wires have connections. I often find it good when more sets of eyes are on the problem.
Thanks Sonavar!
However, the BM is disassembled once again. I'm in the process of replacing the board connectors from PC9 to PC8 but ordered the wrong parts... Once the new parts are in I'll put the boards back together and post some pictures.
sonavor:Try disconnecting the the audio signal cables from the left and right output amplifier boards and see if the noise is still present. That will tell you if the noise is coming from the output amplifier circuitry.
The noise disappears when disconnecting the right output amplifier. I guess that's good news as this would probably eliminate the transformers as the source.
When disconnecting the signal cable from preamplifier board to the tone control board, the noise remains present. So it looks like there is a problem with the tone control board PC4 or a grounding connection issue through that board.
Here are some pictures...
Preamp connections + ground point
Tone control board closeup
Microprocessor board / ground point
From your investigation it sounds like the problem gets introduced in the Filter & Tone Control board.Connector 4P24 of that board sends the audio signals and ground on to the left and right channel output amplifiers.That connector is on the left side of the board next to the FM signal strength meter.
Unfortunately that cable is a more complex cable in that it has a single connector to the Filter & Tone Control board but from there it splits into separate left and right channel cables to the respective output boards. In addition, the cable assembly has black ground wires to ground locations on the output amplifier boards and to the grounding point of the BM8000 chassis. I would suspect that harness and the connections involved with that harness.-sonavor
sonavor:In addition, the cable assembly has black ground wires to ground locations on the output amplifier boards and to the grounding point of the BM8000 chassis. I would suspect that harness and the connections involved with that harness.
I did a bunch continuity checks on the various connections from 4P24 (with other cables disconnected in order to eliminate connections through other boards). However, it definitely looks as though those signal and chassis ground connections are good.
Wondering if a noisy component on PC4 could be the problem... I may yet again remove the tone control board, power with external lab power supplies and trace the signal path with an oscilloscope to see if I can discover something.
A noisy TL072 OpAmp could do this.
Martin
It looks as though the problem is not with the tone control board after all...
Instead of using oscilloscope I bought a €10 external soundcard on Ebay and fed the input signal to REW (Room EQ Wizard) to create a poor man's spectrum analyzer. Here's the setup I used: PC4 powered with +15V/-15V/+5V external power sources and the signal inputs shorted to ground.
Here are the measurements at the board outputs (left/right) P24-1 and P24-6.
Seems pretty normal. The slight peak at 50Hz is caused by the power supply I believe and the peaks around 3/5K are some artefacts from the sound card itself. Unless there is an issue with the measurement setup it looks like the problem is somewhere else...
cool idea
Found it!
@Sonavor you were right, the P24 connector was the culprit. Even though I measured continuity on all pins apparently some contacts had some corrosion. After removing the contacts from the connector housing and cleaning them, the humming disappeared.
Yes, that type of thing happens a lot on the BM8000. I have had a similar problem with one of the connections between the Microcomputer board and the Display board. Every time I pulled the boards and checked continuity everything appeared good but the failure persisted every time I connected things back up.I finally just replaced those board to board connectors with new ones and everything worked great again.I have also had a few BM8000 units where that P24 connector caused problems.Glad you have that working again :-).
krais: The volume is not fully muted at level 0.0. I can hear a clear signal coming through the speakers and headphones with the volume wheel dialed to minimum level. This is apparent on both the left and right channels and on all input channels.
The volume is not fully muted at level 0.0. I can hear a clear signal coming through the speakers and headphones with the volume wheel dialed to minimum level. This is apparent on both the left and right channels and on all input channels.
Does anyone have any thoughts on this? Is this normal behaviour for a BM8000 or could this be another fault?
Not on all Beomaster 8000s - but many.Particularly when Loudness is on.
Thanks Martin!
Another build update: display LEDs and indicator light bulbs replacement. Here's the display board removed from the chassis and displays removed.
Following instructions from beolover.com I scraped off the old LEDs with an X-acto knife and soldered in new SMD 0603 LEDs making sure of precise placement so the housings could be reinstalled without issues.
Testing resoldered display boards...
I decided to secure the inner housings with blobs of ABS slurry (ABS dissolved in acetone). Hopefully this will create a strong bond with the plastic pins of the housing but will not stick to the PCB itself so it's still possible to disassemble the housings if needed.
The outer housings were secured with black hotglue (also reversible). Not very elegant but effective.
Final assembly of the display board with replacement LEDs for the indicator bulbs. I also changed the old connectors that were acting up with new Molex KK connectors.
Looks good. You might have to remove places where the black hot glue is if it interferes with the display mount clips but that should be easy.Remounting the displays in the display mount and into the Display Board holes can be a little frustrating sometimes. :-)
I made my post before you posted the final, completed one...so it looks like you had no problems.Well done.
Just reinstalled the board and adjusted 8R15 to lower the intensity of the new leds (I had already replaced 8R14 and 8R16 with 3.3K and 1.5K resistors to adjust the voltage range). Initially there was a problem with the BM not going into standby mode with FM active but after increasing the voltage over R15 to 3.85V this was fixed. Oh and all LEDs are working fine!
Interestingly also the problem that the volume decimal digit was displayed in the balance display (with very low brightness) has now been magically resolved. I guess this was also a connection issue with one of the board connectors.
sonavor:Remounting the displays in the display mount and into the Display Board holes can be a little frustrating sometimes. :-)
Tell me about it ;-) Took me a full hour to get all those pins aligned.
Brave man. I have one to do but I just can’t get started on it.
How did you solder the leds?
Jacques
chartz:How did you solder the leds?
I just hand soldered the leds under a microscope. Already had some experience replacing the leds on a Beocord 9000 a few years ago so it was actually pretty straightforward. Just apply some solder on one pad and then place the led with tweezers in the correct position and tack it in place. Then apply solder to the remaining pad (making sure not to use too much solder so the housings still have a tight fit).
Again, the biggest challenge with this is refitting the display boards on the PCB, that was much easier on the Beocord.
krais: Just reinstalled the board and adjusted 8R15 to lower the intensity of the new leds (I had already replaced 8R14 and 8R16 with 3.3K and 1.5K resistors to adjust the voltage range). Initially there was a problem with the BM not going into standby mode with FM active but after increasing the voltage over R15 to 3.85V this was fixed. Oh and all LEDs are working fine! Interestingly also the problem that the volume decimal digit was displayed in the balance display (with very low brightness) has now been magically resolved. I guess this was also a connection issue with one of the board connectors.
Yes, when you adjust the brightness for the LEDs it affects the +5VDC power supply. That in turn will messes with power to the Microcomputer board and affect the Standby mode you saw.
Planning to do the last tests and adjustments before starting reassembly.
One last issue that I noticed: there seems to be a spark coming from PC7 when switching on the Beomaster (not always but I have seen it multiple times). So I removed the rear cover and did some additional testing and it seems that RL2 is producing the sparks. Even when just plugging in the power cord I have seen sparks coming from that relay.
Is that expected behaviour or does it mean the relay needs to be cleaned or replaced?
I haven't ever watched the 7RL2 startup circuit relay when plugging in the power cord. However, that relay is switching in the AC line voltage for the +-55VDC rails when coming out of standby so there can be a spark there. Especially as the relay ages and the contacts start to get dirty. If the relays are original I typically replace them along with the 7C1 capacitor (330nF). If you do that make sure you replace it with the same type of safety capacitor.
sonavor:Especially as the relay ages and the contacts start to get dirty. If the relays are original I typically replace them along with the 7C1 capacitor (330nF).
Would you happen to have a part number or specifications for the 7RL2? I noticed that RL1 and RL2 have different B&O part numbers.
sonavor:If you do that make sure you replace it with the same type of safety capacitor.
Thanks for the tip, I already had replaced the safety capacitor with exactly this part as a precaution.
Today I did some sound tests and adjusted the volume control offset. I also checked the amplifier output boards with an IR heat gun. The darlingtons run nice and cool, the temperature around the 0.5W resistors settles around 50-55℃ after warmup. The BF857s get pretty warm (~50℃) but nothing crazy.
The part that runs noticeably warmer is TR210 on the right output amplifier: around 70-75℃ after running the amp for about 30 minutes. Should I be worried about this?
I am also in the middle of restoring a BM8000 and keenly following your posts and benefitting from the inputs.
I am facing the same connector related challenges. Can you share the model no. for the connectors you got ? (trying to avoid the buy/return/buy cycle )
Thanks !!
dahiyas:I am facing the same connector related challenges. Can you share the model no. for the connectors you got ? (trying to avoid the buy/return/buy cycle )
For the connections between PC8 and PC9 I used Molex KK connectors from these series:
- KK 254 series 4455 (PC board connector female)
- KK 254 series 4094 (right angle header male)
Be aware that the pitch for these is slighly different than the connectors used by Bang & Olufsen (2.54mm vs 2.5mm). Not a problem with connectors up to 7 pins but the 13 pin connectors did not fit. So I used 2 connectors (6 and 7 pins) instead of the 13 pin connector and sanded down the housing material a bit to make them fit.
For the 4094 headers you’ll have to readjust the pins with pliers so they have the correct height and the boards are aligned verically. Hope this helps...
Molex (and some other manufacturers) do make board to board connectors with a pitch of 2.54mm though.
sonavor:Molex (and some other manufacturers) do make board to board connectors with a pitch of 2.54mm though.
Yes the molex connectors are 2.54mm pitch but the original board connectors on my BM8000 are 2.50mm pitch. Not sure if this is true for all board versions.
Thanks !! Found them on digikey. Should have before weekend.
krais:The part that runs noticeably warmer is TR210 on the right output amplifier: around 70-75℃ after running the amp for about 30 minutes.
This weekend I checked voltages on the right output amplifier but could not find anything out of the ordinary.
Any thoughts on this? Is it recommended to replace TR209/TR210? Or are these temperatures within the max. temperature range and nothing to worry about?