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Taking on the BM8000

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krais
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krais posted on Sat, Dec 28 2019 11:56 AM

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).


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sonavor
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krais:

Also, what I noticed is that if the Beomaster is turned on, switching to standby will cause a relay click, but when subsequently unplugging from AC there is another relay click. Assuming that a relay cannot switch off twice, would this also indicate a problem?

When the Beomaster 8000 is in Standby and you press a music source you will hear two relays click on very close together like a "tick-tock".  Pressing Standby when a music source is selected will dis-engage both relays but they will almost be in unison. So it won't be "tick-tock" but you can still hear two relays. Unplugging the Beomaster 8000 when it is in Standby will result in the relays cycling. It will be the "tick-tock" sound.

-sonavor

Spassmaker
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Hi 

Maybe you have a closer Look to the big AC Mains Capacitator wich is located  on the Relay PCB.

My BM 8000 was playing music, suddenly I heard a crackling and sparkling Noise of the Amp with a lot of bad smelling Snoke comming out of Direction of the Right Amp Board...but the BM was still playing Music and after shutting down and opening the BM I could´nt realy find anything burned.

Checked some Voltages, No Load Current and Offset, everything fine.

So I put back to AC and starteted the BM and he was still working proper.

Final Investigations sortet out that the AC Mains Cap was totaly kind of "exploded"

Mounted a new one and everything is fine aggain.

 

Best Regards

Christian

krais
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krais replied on Fri, Mar 27 2020 8:16 PM

Spassmaker:
Maybe you have a closer Look to the big AC Mains Capacitator wich is located  on the Relay PCB.

Thanks Christian, I already had proactively replaced the Rifa safety cap as these are infamous for catastrophic failures due to old age.

krais
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Update: I figured out why RL1 would not engage. Another loose connector pin, this time P51-3 on the power supply board that controls the relay.

So this would have caused a continuous current through R1, R2 potentially causing the smoke to appear. But as that happened immediately after switching on the Beomaster, in this scenario there could still a fault somewhere that is causing a high current to flow through the resistors as Manfy indicated.

So some more troubleshooting left to do... 


sonavor
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With your Beomaster 8000's recent history of connector problems I would check every connector again. ;-)

-sonavor

manfy
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manfy replied on Sat, Mar 28 2020 10:23 AM

Yes - thumbs up I second sonavor's advice.

It's not really normal for contacts to slip out of their housing. It happened to me several times but only because I had taken those contacts out intentionally and later was careless when putting them back.
In any case, since it happened to you twice, carefully check every single connector. Loose contacts can make you go in circles when trying to narrow down the source of a problem.

Once the connectors are confirmed good and in their right socket, check for potential short circuits (particularly in the path of preamp, tone control, output amp). You can get a good idea of whether the resistance values on supply points are good by comparing left to right channel. It's unlikely that both channels show the very same fault.

If all that looks good, use the dim bulb tester and start with power-on tests.
You might be lucky and all is good now. The fact that the inrush current limiter relay did not engage is a good explanation for the smoke, but of course it's not a guarantee that this was the only problem.

Good luck with your troubleshooting!.

 

krais
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krais replied on Sun, Mar 29 2020 10:49 AM

I double checked all connectors - no more loose pins. I did find that P84 on the uProcessor board was not plugged in (probably slipped out when fiddling with the cables). Probably not important as this cable connects to the FM board.

The P37 ground pin on the left output amplifier might have been disconnected as well (not 100% sure as this may have come off when checking resistances on that board).

Resistance checks on the signal and power inputs of the preamp, tone control, output amp boards did not show any short circuits. The only issue I found that resistance for the left and right signal lines to the tone control board (P27-1/4) were not consistent. The cause for this was a dead tantalum cap C128 in the volume control circuit.

PS: Also checked PC4 for burn marks just in case, but could not find anything obvious. 

Spassmaker
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Hi there

I´ve a question to the BM 800 in generally:

Did someone measured the Standby and On Powerconsumption?

Mine is taking about 10 to 15 Watt in Stby and about 70 Watt in On State.

Is this OK or not?

Thanks for reply.

Berst Regards

Christian

manfy
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manfy replied on Mon, Mar 30 2020 8:28 AM

Well, I don't have actual measurements, but your values look ok.

The manual specifies a power consumption of 20-700W, so that matches your standby consumption of 10-15W. This value seems reasonable considering that the standby transformer is always powered on when the unit is plugged in.
70W as idle consumption (i.e. unit is powered on but without input signal and volume set to minimum) is fairly low for a class G amplifier and it shows that the designers did pay attention during the design process.

Let's see what values krais measures on his BM8000.

rgds, manfy.

krais
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krais replied on Thu, Apr 9 2020 8:22 PM

Following up on Sonavor's advice on using a variac for the startup test... I spotted a used high quality combined variac / isolation transformer (Germany, 1980s) on Ebay and managed the win the auction with a very reasonable bid.

Newbie question on the variac startup procedure: I understand that the 7RL1 and 7RL2 relays must be bridged before the voltage on the variac can be slowly increased while watching for any excessive current draw. But at what point should I turn on the amplifier (microcomputer)? I assume the uprocessor will not turn on if the supply voltage is considerably lower than 5V but needs to be switched on at some point to control the 15V line going to the output amplifiers.

manfy
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manfy replied on Thu, Apr 9 2020 9:27 PM

Good question!
I don't have much experience with using a variac, but based on the circuit diagram and based on theory alone I'd say it doesn't matter.

Once you jumper 7RL1/RL2 the output amp is connected to the variac as power source and if there is a shortcircuit after the main transformer, a high current will flow once you increase the voltage. The uP now only selects the input source - and what I remember from last time, the uP doesn't do any other power switching. (But I'm not totally sure about that!)

Better wait for a binding answer from Sonavor!

-----------------------------------

[edit:] I just checked the service manual and you're right. The slave processor 9IC4 pin 14 does also switch on the +/-15V, which is the supply for tuner, pre-amp, volume control and tone control.
But even if there is a short circuit or a problem on one of those boards, it should not affect the power drawn from the main transformer. The 15V regulators are normally short circuit safe, i.e. the 3-pin regulator will decrease the voltage and limit the current.

Concerning the processor, I'm not quite sure how it will handle a gradual low-volt startup. It doesn't have an internal hardware watchdog, which means it can be in an undefined state in such a brownout condition. There is some circuitry controlling the reset-pins, but in low volt condition that circuitry might be in an undefined state too.
I don't believe that this type of startup will cause any damage to your system, but the actual startup behaviour of the processor might be unpredictable. The chip is specified to need a supply voltage of 5V +/-5%, so it might not start operating while the supply voltage is too far below that.

I hope Sonavor will be back to share his experience on variac startup on uP-controlled units.

krais
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krais replied on Sun, Apr 12 2020 11:01 PM

Before doing the full power up test, I thought I would do some basic voltage tests on the PSU board with P51 disconnected (connection to the relays that I have not yet bridged). So I hooked up the variac and dim bulb tester, raised the voltage to see if there was any excessive current and measured the 5V, +15V and -15V rails.

Well, there is now an issue with both the +15v and -15V rails (which were fine in my previous tests). The voltages at the C33, C34 capacitors are fine (26.6V, -26.4V respectively). However, the regulated voltages (measured at 6IC5, 6IC6) increase steadily when raising the input voltage but at some point (Variac set at ~120V) the regulated voltages drop all of a sudden to almost nothing (60mV at +15 voltage regulator, -0.4V at -15V regulator).

At first I suspected the voltage regulators but I measured almost no voltage at the input pins of the regulators. So there must be a problem somewhere between the filter capacitors and the voltage regulators... to be continued

manfy
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manfy replied on Mon, Apr 13 2020 6:08 AM

krais:

....but at some point (Variac set at ~120V) the regulated voltages drop all of a sudden to almost nothing (60mV at +15 voltage regulator, -0.4V at -15V regulator).

Hmm, I'd say that suggests that at 120VAC the 5V rail has risen enough so that the uProcessor can start up and initialize its outputs, and that's when the 15V rails are being switched off by the uP.
When you try to switch on the BM8000 now (via selector buttons), the uP will switch the 15V rails on again.
Can you see the standby indicator at 120VAC? If yes, then you can take the standby indicator as a sign for a working uP.

---------------------------------

[edit:] Come to think of it, the same thing should happen with 7RL1 when P51 is connected. The relay would be switched off once the uP is ready (and provided that the rail that feeds the relay has reached a voltage level that can drive the relay). And when you power the BM8000 on via the selector buttons, the uP will switch 7RL1 back on.

Why don't you just connect the multimeter between Variac and BM8000 to measure the input current? Set it to a 10A range and monitor the current while cranking up the variac voltage. If it exceeds normal idle current too much, you can interact and off the unit. Spassmaker talked about 70W in on-state, i.e. about 300mA as normal idle current.

Just make sure that you switch on the BM8000 as soon as the uP is up and ready, or else you're only measuring the standby current.

krais
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krais replied on Mon, Apr 13 2020 9:29 AM

Thanks! Indeed when I switched on the BM8000 with the selector buttons the +/-15V lines measure ok. I was thrown off because the voltage drop off points are a little different (100VAC for +15V line and 120VAC for -15V line) and the standby indicator comes on only at 180VAC so I did not realize that the uProcessor was active.

manfy:
Why don't you just connect the multimeter between Variac and BM8000 to measure the input current?

There is a built in AC meter on the variac, so no need to use a multimeter. 

manfy:
Just make sure that you switch on the BM8000 as soon as the uP is up and ready, or else you're only measuring the standby current.

I'm not sure that's going to work. Even with the variac at 180VAC the BM8000 switches off immediately when pressing one of the selector buttons. Only when I first increase the variac to 230VAC and then press one of the selector buttons the device stays on. Wondering how others are able to set the no load current trimmers with the variac power on procedure, perhaps power the uProcessor board with an external 5V supply?  

 

chartz
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chartz replied on Mon, Apr 13 2020 10:35 AM
In my experience a variac can produce false positives—ie problems that don’t exist. I never ever use one, unless it’s an old valve radio.

Jacques

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