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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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krais
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krais replied on Wed, Mar 25 2020 11:18 AM

Thanks Manfy

manfy:
Smoke coming from electronics is never really a good thing! There must be a reason for it and that GND connection you re-attached to PC4

Sorry I wasn't clear, I think this is a signal ground pin (P27-3).

manfy:
Which rectifier blew up? 30D1 or 30D2?

The left rectifier when looking from the front of the BM, so 30D1

manfy
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manfy replied on Wed, Mar 25 2020 11:52 AM

I just edited my previous post; please check.

The circuit diagram shows that 30D1 only supplies the +/-55V to the left output amplifier. Random component failure is possible, but I'd still check very carefully for any potential short circuit that could have caused this to the rectifier.

If you're certain that everything is fine you should replace the rectifier and verify the power supply voltages with all other boards disconnected before starting to reconnect the boards and testing further.

krais
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Apologies Manfy for wasting your time, I may have jumped to a wrong conclusion earlier. Did some more measuring out of circuit and the bridge rectifier seems intact after all (0.48V one way voltage drop across the diodes).

It's all rather confusing. I'm quite sure I saw smoke on the left side of the power up circuit board but there are hardly any components on that side of the board. The 2 fuses haven't blown and resistors R1 and R2 are within spec (5.6 ohm).


sonavor
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sonavor replied on Wed, Mar 25 2020 11:53 PM

krais:

It's all rather confusing. I'm quite sure I saw smoke on the left side of the power up circuit board but there are hardly any components on that side of the board. The 2 fuses haven't blown and resistors R1 and R2 are within spec (5.6 ohm).

Hi,

If you think you saw some smoke then you probably did. Something could be shorted somewhere and resulted in too much current through a component. There are two power relays on the component side of that BM8000 Start Up PCB. 

You can open that Start Up board some more so you can watch what happens on the next power up attempt.

Before that though, re-check all of the BM8000 wires. Because you rebuilt the output amplifiers and tested them already they should be good. Did you make sure the black ground wires are attached on the backside of the Microcomputer board?

P27-3 is a ground connection between the Filter Control panel and the Preamp board. If connecting it completed a connection that exposed a problem in the Beomaster then it could involve 15 VDC power. How that affects the Start Up circuit I can't guess right now. 7RL1 and 7RL2 must both engage to apply the ±55 VDC rail voltages. The rails must also have +15 VDC for the output amplifiers to turn on. I am not seeing the ground - Start Up circuit - smoke correlation but I believe there is some indirect thing going on there. 

If you don't find any wiring issues I would try the variac and dim bulb tester again as a precaution. I recently blew out a couple of TIP 146 transistors on a Beomaster 4400 because I didn't trust my dim bulb tester. Voltage was applied and measured fine at first but then started to drop. I took the dim bulb out of the power path and the two good Darlingtons were gone. I should have seen the dropping voltage as a sign that there was something wrong.

Smoke is another sign :-). Hopefully it wasn't fatal for some poor component. Check power on the BM8000 with the dim bulb protection and re-check what voltages come on (both in Standby and with an audio source selected).  With something like Tape 1 or 2 selected all of the power should come on and you should here the relays click on the Start Up board.  Check the +5 VDC regulator and the ±15 VDC regulators next to it.

One other thing...Double-check the Power Supply Board (PCB6) that 6P50 and 6P51 connectors are correct. Those two connectors have the same physical keying so they could accidentally get swapped. They also both play a role in the Start Up circuit.

-sonavor

 

krais
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krais replied on Thu, Mar 26 2020 7:52 AM

Thanks for the help Sonavor!

I'll do some more thorough checking/testing this weekend, making sure to use the dim bulb tester for the voltage checks (I don't have a variac at this time).

One other theory that came to mind: maybe the smoke was not coming from the startup circuit board but from the tone control board instead. The smoke could have been funneled upward between the pcb and the control panel and exited near the reservoir caps (making it look like it was coming from the startup circuit board).

I'll do a visual inspection of PC4 first before starting with the voltage checks.

sonavor
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Okay. A variac would be a great addition to your tools. I use the variac more than my dim bulb tester. My tester is designed to monitor the current and voltage with the variac as I apply power to the unit under test. The dim bulb portion of the test can be switched in or out of the path since I only use it when I am powering up an unit for the first time.

If the fault was on the tone control board then did you hear the two relays (7RL1 and 7RL2) engage when you tried to turn the Beomaster on?

-sonavor

krais
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krais replied on Thu, Mar 26 2020 6:52 PM

sonavor:
If the fault was on the tone control board then did you hear the two relays (7RL1 and 7RL2) engage when you tried to turn the Beomaster on?

The relays engaged when turning the Beomaster on but immediately the magic smoke appeared. Can't remember if the relays disengaged but the display stayed on.  

 

sonavor
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Okay, well when you fixed the wire in the connector that allowed a missing ground to make its connection on the Tone & Filter board so it sounds like there is just some problem localized on that board. Hopefully that will be easy to diagnose and fix.

-sonavor

krais
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I took a closer look at the left side of PC7 and did find a very dry solder joint on the AC trace (no burn marks, still continuity). Could that have produced enough heat to cause a small amount of smoke? It cannot possibly be that easy, right?


sonavor
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sonavor replied on Thu, Mar 26 2020 11:25 PM

What device does that lead go to?  That is the type of thing you need to look for when going over this Beomaster though.  Even if it isn't the source of your immediate problem it could become one in the future.

-sonavor

manfy
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manfy replied on Thu, Mar 26 2020 11:46 PM

No, I'm afraid not.
A cold solder joint cannot create smoke -- unless there is arcing, in which case you'd see burn marks and you could smell a dinstinctive "electrical" smell.

It's more likely that the smoke came from R1 and R2 because of heating up rapidly, but that would imply that there was a high current drawn on the output side of the transformer. Those resistors are 5.6 ohms/5W and the trafo fuse is 4A slow-blow. So if the input current was 3A the fuse wouldn't blow but the resistors would have to dissipate good 50W. Now, that can cause smoke.
Normally, relay RL1 would switch off those "soft start" resistors after 1 or 2 seconds and such a short overload shouldn't create smoke - heat yes, but not smoke.

Just re-solder that solder joint and since you're at it, check that RL1 makes proper contact. It should be well below 1 ohm when the contact is closed.
Relays do have a limited lifetime and if those contacts have deteriorated, this inrush current limiter is not completely switched off in normal operation.

[cross-posted]

krais
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krais replied on Fri, Mar 27 2020 11:23 AM

manfy:
Normally, relay RL1 would switch off those "soft start" resistors after 1 or 2 seconds and such a short overload shouldn't create smoke - heat yes, but not smoke.

Hmm I assumed that the interval between the two relays engaging would be much shorter. So what I thought was a double click was probably just RL2 engaging all along, not RL1. I'll investigate RL1 to see if that component is working and makes proper contact. When testing the relay out of circuit what voltage should I apply to switch on?

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?

Dillen
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Dillen replied on Fri, Mar 27 2020 11:42 AM

There should be apprx. 1/4 of a second between the two relays activating.

Close to a “double click”.

Martin

manfy
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manfy replied on Fri, Mar 27 2020 12:15 PM

I agree with Martin.

I don't have hands-on experience with the BM8000, but all the other high-end high-power amplifiers I've seen have a distinct double-click (usually in a 1 second range).

Looking at the circuit diagram I see that RL1, which disables the inrush current limiter, is driven by 40VAC via D21. So that's a half-way rectifier with an output voltage of approx. 18VDC. But you can check the printing on the relay; it almost always shows the nominal voltage there.

After the diode there's the RC circuit R27 + C20 and I calculated an RC time constant of 0.63sec. Without analyzing the switch-on circuitry with TR21 and TR22 in detail, I'd say that TR22 would probably switch the relay on after 0.5 to 1sec.  
At power-off RL1 should open immediately because the smoothing cap C21 with 22uF shouldn't be able to supply the relay current for more than a few line cycles, i.e. 100msec maybe.

The main relay RL2 is controlled by the uProcessor and it switches on the power trafo that supplis the output amps and this RL1 circuitry. I didn't see any other relay in the diagram - but to be honest, I didn't scan every little bit of it!

If there are really only these 2 relays in the BM8000 then it would be very strange to hear another relay click after putting the unit into standby (aside from RL1 and RL2, which should switch off simultaneously when the uP signals standby)!!

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

Oops! I may have to take my last statement back!

I just noticed that the main relay RL2 is driven by -24V from the standby transformer. So, this voltage is available as long as the unit is plugged in.
RL2 is switched off by a High signal from the uP. So if this signal drops to 0V, which should happen immediately after unplugging, there might be time before the -24V decay to 0V. In this case the relay RL2 may switch on for a short moment (until the relay current bled this -24V supply to below holding current. This would probably happen in an instant and it may sound like a single click.)

This scenario is thinkable but it needs to be confirmed by insiders!
Hmm, on second thought, I see that the standby signal is also swithing that supply circuitry via TR5!?? I'm not sure. Let's wait for Martin or other guys with BM8000 experience.

Dillen
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Dillen replied on Fri, Mar 27 2020 2:58 PM

There should be no click when mains is connected.
When mains is disconnected while the Beomaster is in standby it's not unusual to hear a relay click on/off.
Perhaps because the supply to the processor falls down faster than the supply to the relay driver etc. and a low signal from the processor is then "interpreted".
I never dug any deeper into this.

Martin 

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