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Beomaster 8000 help with preamp issue

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Lee
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Lee Posted: Tue, Nov 10 2020 12:53 AM
Hi guys,

My long relatively problem free Beomaster 8000 has developed an issue and I’m hoping someone can point me into the areas of the preamp board to focus on.

If I play through TP1, phono or FM I often get a static sounding noise, cut outs and distortion. None of these issues appear when listening to TP2. If I tap the unit on the side it plays normally for a few seconds and then goes back to static noise. It affects both L&R channels equally at the same time.

What components are shared with both channels on the TP1, Phono and FM but not TP2.

This is a later BM8000 with the TP2 inputs having the monitor function on them and not on TP1. I’ve had a look at the schematics but I can’t really follow them very well and work out which part of the board to concentrate on.

Any help would be hugely appreciated.

Thanks

Lee

sonavor
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sonavor replied on Tue, Nov 10 2020 3:47 AM

Hi Lee,

That is interesting.  FM, Tape 2 and Phono route through the first analog switch (IC202/IC102 - AD10/278) according to the service manual schematics.
The selection of one of those three continues on to the second analog switch (IC4 - AD 10/192) where the selection is between Tape 1 and whichever source is from the first switch.  Keep in mind that the Beomaster 8000 does just actually select one for the output to the amplifiers.

The first suspect in this type of problem is with the analog switch/multiplex devices on the preamp board that perform the source selection.
As I said, three of the sources are selected by one analog switch (two ICs) and then the fourth source factored in on the second analog switch (one IC).
What I mean by interesting is that in order for the analog switches to be the problem then it would seem like Tape 1 and Tape 2 are swapped. That would match up with your description of the fault. In that case the common part that is noisy would be the analog selection switches IC102/IC202.

Having said that...besides the idea of the Tape 1 and Tape 2 routing being switched, it would also be a odd that both IC102 and IC202 would fail in their switching (one is for the right channel and one is for the left channel). So that logic puzzles me. Perhaps the issue is with the source selection signal not making good contact on any of the control lines except Tape 2.

Can you test and see if the problem is on both the left and right channel?

If the problem is on both channels then this brings the issue back around to connection issues that are common place in the Beomaster 8000.

The selection control lines involved in the selected source are on P14 pins 1 (Phono), 2 (FM), 3 (Tape 2) and 5 (Tape 1). Check that connection cable and the board connectors it joins. The board connectors might need a reflow of their solder joints. 

If you have a DMM you can monitor the selection signals and compare their voltage levels. It would be better though to measure the control lines with an oscilloscope to observe if the control lines are noisy or intermittent. You can't really tell that on a DMM measurement.

I hope that helps.  

-sonavor

sonavor
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One more thing...just for reference.

According to the BM8000 service manual the source selection signal routing from the Microcomputer Board (which controls the selection) to the Preamplifier Board is like this -

Beomaster 8000 - Audio Source selection cable

From Microcomputer Board - IC4 (Slave Processor)     to      Preamplifier Board

Tape 1 - P82-1  --------------------------------------------------------->  P14-5 Tape 1

Tape 2 - P82-3  --------------------------------------------------------->  P14-3 Tape 2

Phono  - P82-5  --------------------------------------------------------->  P14-1 Phono

FM       - P82-4  --------------------------------------------------------->  P14-2 FM

Verify that the way yours is wired. 
I have seen these two different Beomaster 8000 source input boxes. The preamplifier boards themselves are the same. So if the Tape 1 and Tape 2 connections to the BM8000 preamplifier board were swapped, then the control lines would also have to be swapped.  

I have no idea if that is the case on yours. I am just trying to come up with a scenario where the point of failure you describe could be located at one source.

Here is a photo of two the different Beomaster 8000 source input boxes. There are obviously some wiring differences in that assembly regarding the connection for a tape deck via the RCA jacks. Whether that also means the wires on to the BM8000 preamplifier are different I don't know.  The next chance I get I will trace through one of those. However, you see that the panels also show different adjustments on the input level knobs for Tape 1 and Tape 2. So for that to be accurate the actual tape signals would have to be swapped where they connect to the preamp board...because those adjustment trimmers are fixed to the preamp board.


sonavor
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sonavor replied on Tue, Nov 10 2020 5:54 AM

This issue has my interest :-)

I did some more digging and while I cant' find anything in the Beomaster 8000 schematic versions I did find a service manual note that the tape monitor functions (via the RCA jacks) were swapped between Tape 1 and Tape 2. No diagrams of the wiring change though.

Looking through the Beolover Blog I found a post by Rudy back in 2015 where he came across a Beomaster 8000 where an owner had tried swapping the source input assembly boxes. 

So Lee, first check the source wiring on your Beomaster 8000 unit and see where the tape audio signals route to.
If it turns out that your Tape 2 signals are managed by the IC4 switch then that could very likely be the source of your problem. The IC102 and IC202 switches could also be a problem but if they are then I would think that just one of them would fail at a time...not both. So the problem would only be in one channel. That is just a guess though and it is possible that both switches could be failing.

I will be interested in what you find out in how your Beomaster 8000 source inputs are wired.

-sonavor

sonavor
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sonavor replied on Tue, Nov 10 2020 3:43 PM

Okay...hopefully others can confirm this but I think I have the answer where Tape 1 and Tape 2 input wiring was changed by B&O.
From the previous investigation I found that the Tape 1 and Tape 2 swap with the RCA in/out jack pairing would have to mean that their positions in the Beomaster 8000 preamplifier board would also swap. The preamplifier board did not change versions on this change so the source input adjustment trimmer position also changed with this wiring change made by B&O on the connection from the source input box to the preamplifier board (connector P20).

The confusing part was with that change how did the Beomaster 8000 handle the control line for the source selection?
The wiring for those control lines (the cable from the Microcomputer board P82 to the preamplifier board P14) did not change and again, the preamplifier board did not change.

The change was made to the firmware of the Beomaster 8000 IC4 Slave device. It outputs the control signals for the selected source.
Going through my collection of photos on all of the Beomaster 8000 units I have restored I found that I had a mixture of Beomaster units where some had the original Tape 1 / Tape 2 configuration and others had the swapped configuration.

Here are the corresponding Microcomputer board firmware versions -

BM8000 with Tape 1 having the RCA monitoring Jacks - Microcomputer Board IC4 firmware: R1021-11

BM8000 with Tape 2 having the RCA monitoring Jacks - Microcomputer Board IC4 firmware: R1021-13

So Lee, check what configuration you have and if it is the one with Tape 2 and Tape 1 switched then your failure scenario points to the possibility of a failure in the preamplifier board analog switch(s)...or a connection involving them.

-sonavor

Dillen
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Dillen replied on Tue, Nov 10 2020 4:01 PM

Bang & Olufsen Tech info BM8-10:

Monitor TP 2

==========

In order to move the monitor function from TP 1 to TP 2 the following has been changed:

Position Designation New Order No.

91C4 Slave uC 8340265

22 module Socket panel 7219001

3080 Cover for socket panel 3112005

Preamplifier PC3 is changed:

R115/215: is removed

R1l6/216: changed to 4.7 kohms no 5010048

The changed PC3 is ordered under the usual number 8002399. The

changes were introduced in the current production from serial no.

2554003.

The above spareparts can still be ordered for sets produced before

the change under the usual number.

“Old” and “new” spareparts are flot compatible.

 

Martin

sonavor
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sonavor replied on Tue, Nov 10 2020 4:12 PM

Very nice!  Thanks Martin.

The firmware change was required for the change.
To go with the IC4 version numbers I should have included the IC4 part numbers.  Here is the updated information for that -

BM8000 with Tape 1 having the RCA monitoring Jacks - Microcomputer Board IC4 8340239 ver: R1021-11

BM8000 with Tape 2 having the RCA monitoring Jacks - Microcomputer Board IC4 8340265 ver: R1021-13


Interesting also about the changes to the resistors on the preamplifier board.  I never noticed that difference so that is something to check for when doing a restoration/repair where the preamplifier board requires replacing. 

-sonavor

Lee
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Lee replied on Tue, Nov 10 2020 5:50 PM
Thanks for your advice.

So after changing IC6 and reflowing every solder joint on the preamp board it turns out the issue was the connector on the CPU card to the preamp. I should’ve known. Almost every issue I’ve ever had has been the CPU card!

I’ve got another issue now though...

All the filters (loudness etc) have stopped working (the filter light comes on but they don’t do anything) and I no longer get mono or manual lights on the screen when I switch them on Sad

One thing after another. Any help appreciated.

Lee
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Lee replied on Tue, Nov 10 2020 6:00 PM
I’ve tried a completely different filter board and it exhibits exactly the same symptoms. I’m hoping it’s not the CPU board again but it’s not looking good.
Lee
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Lee replied on Tue, Nov 10 2020 6:07 PM
In my haste to reassemble I’d got p83 on the cpu card plugged in one pin offset to the left. So pin 1 was going to pin 2 on the cable etc. Plugging it in correctly and the issue remains. I assume I’ve damaged something :-(
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sonavor replied on Tue, Nov 10 2020 6:52 PM

Not necessarily. P83-1 supplies +5VDC from the Microcomputer board to a couple of other boards (Filter & Tone Control and Preamp).  P83-2 is ground so you would have just supplied ground so no problem there.

There is another 10/192 analog switch on the Filter & Tone Control board (PCB4) that is responsible for engaging or bypassing the filters (tone controls). Investigate the connections related to that. The control line for that switch goes from the Microprocessor board through the Display board and over to the Filter & Tone Control board where it arrives on connector P30-1.  So there is a connection between the Microcomputer and Display board to check at P75-1. Then the cable from the Display board P71-3 to the Filter & Tone Control board P30-1.

Connections and solder joints can be a real nightmare on the Beomaster 8000. 


-sonavor

Lee
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Lee replied on Fri, Dec 4 2020 2:33 PM
So I’ve reflowed all the connectors on the CPU board. The filters are working fine again so small victory. But I’m still getting the issue with the sound becoming very quiet and distorted in PH, TP1 and Radio. TP2 never has any problems.

If I tap the unit lightly the sound goes back to normal so I’m thinking that could be a dry joint but could it also be a component?

And ideas on which connector or components could cause this issue? I’ve changed the switching IC on the preamp board so I don’t think it’s that.

Thanks again

Lee
sonavor
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sonavor replied on Fri, Dec 4 2020 4:20 PM

Where on the board are you tapping?

To recap...
On yours we resolved that your BM8000 model has the B&O change between TP1 and TP2, right?
So TP2 gets switched in via the AD 10/192 analog switch. Is that the switch you swapped (with a spare you have)?
Both left and right channels are switched by one AD 10/192 analog switch.
The other sources get switched by two AD 10/278 analog switches (one switch per channel). The selection there gets routed to the single AD 10/192 switch (which has TP2).

From your decscription though I would tend to agree that it behaves like a connection issue. But it has to be at some point where the three affected sources have been combined. Maybe somewhere between AD 10/278 IC and the AD 10/192 IC. Anything before the AD 10/278 switch point would likely just affect one channel. Anything after AD 10/192 would also affect TP2.

-sonavor

Lee
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Lee replied on Fri, Dec 4 2020 8:05 PM
Hi Sonavor and thanks again for your help.

Yes mine is the later version

And that is the chip I swapped.

It doesn’t seem to matter where I tap... I’ve really tried to narrow it down with the tap trick but the issue is so sensitive even a light tap fixes it for a few seconds.

Sometimes tapping on the CPU board fixes it and sometimes tapping on the preamp board fixes it.

I’m beginning to think it’s a faulty ribbon cable?

I’m going to fully disassemble the beast and have it all laid out on the bench to see if I can narrow it down further.

Thanks again

Lee
sonavor
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sonavor replied on Sat, Dec 5 2020 2:12 AM

Hi Lee,

Have you ever changed out the Op-amps on the Beomaster 8000 preamplifier board?
If not you might try swapping 4IC5 with 4IC7...or better would be to change out 4IC5 with a new Op-amp.
4IC7 is for the source pre-out to the Tape 1 (Tape 2 in your case) record out ... so it isn't being used in the problem you are seeing.
I confess that I don't believe an Op-amp problem would be solved by tapping on the board but 4IC5 is in the signal path of the audio sources you are having issue with between the AD 10/278 switches and the AD 10/192.  
The only cable I can come up with that would relate to your symptoms would be the cable that goes from the Microcomputer board to the Preamplifier board 4P14 connector. That is the one that selects the audio source on the analog switches. However, it seems a little odd that a problem selecting the audio source would turn into the problem you are seeing. Still worth checking I think.

-sonavor

Lee
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Lee replied on Fri, Jan 8 2021 8:17 AM
Hi Sonavor,

I changed all the OPAMPs a while back now so I don’t think it’s those.

I’ve invested in a new oscilloscope to do some testing to find out what the issues are. I’m just concerned about connecting the thing up as I don’t have an isolation transformer. Any advice on floating voltages and where I should attach the probes to ground etc.

Many thanks

Lee
sonavor
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sonavor replied on Fri, Jan 8 2021 6:31 PM

Hi Lee,

I connect my scope probe using the 10X attenuation most of the time. I connect the scope probe ground to a ground point in the Beomaster 8000.
Just be careful to set up your probes with the Beomaster unplugged to avoid the risk of shorting something. Once your measurement probes are in place you can plug in the Beomaster and turn it on.

For your reference here is a youtube video that covers scope probes. The guy in the video is kind of a character but he covers the topic pretty well.

-sonavor

Lee
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Lee replied on Mon, Jan 18 2021 12:52 PM
Just to follow up on this. Replacing the CPU sockets cured the issue. I’m still not sure how it caused the problem but hey ho. I think the preamp was being instructed to play the radio on top of the other inputs (apart from TP2).
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