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artig wrote:

The signal on pin 16 is definitely required for a normal startup.

I disagree, at least for my BM6000. Here is a picture of the the receiver running with pin P42-1 on board 16 disconnected (i.e. the 50/60 Hz signal).

The BM6000 does have some power drop protection, but it is implemented as part of the RESET circuit.

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From the schematic, it looks like the CPU supplies the +5v for the 50/60 Hz signal (likely via a pull-up resistor on pin16). 16TR4 then pulls down pin16 in sync with the line voltage. Not seeing a signal could be that the CPU is not supplying the voltage to pin16 or that 16TR4 or 16D5 is bad.

What happens on pin16 when you power-up?

My best guess is that the 50/60Hz signal would NOT be needed to initialize. The only thing that I could think of it being used for is for the timer functions (which aren’t critical for operation). Then again, there could be a few lines of code that check for the signal changing and fault the processor if it isn’t as expected. I would expect the display to indicate a fault if this happened.

What is insidious about trying to debug this are the possible “chicken or the egg” situations that could be happening and not knowing if one is dealing with chickens or eggs ;-).

What does you display do when you power up?

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DanteM: A couple of other things to consider are texture paint in a spray can or Plasti-dip spray. I don’t know if these would be available where you are at, but they easy to find in the US (except for maybe California).

These would not be as nice as the powder coating, but would be easier to remove if you didn’t like the results.

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The signal coming in on pin38 shouldn’t make any difference for the BM6000 starting up. It is related to the FM frequency display.

Does the voltage across R95 change when you adjust R89?

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Like this?

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Yes, remove P5 completely. I believe that you are correct that removing P5 breaks the ground path back to the BM6000 power supply, so you will need to connect the bench power supply’s ground to Board 2’s ground. I usually either clip on to either the CPU shield or one of the ground vias.

I suspect that they call for a using an oscilloscope due to the high impedance of the o-scope probe (as well as a fast response for any readings). I would expect that a high quality multimeter would also work, but I haven’t tried it myself.

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Your results do seem to be unusual.

IIRC, the voltages should jump around quite a bit with small movements of R89. Are you sure that R89 is OK? At the very least, the original trim pot should be cleaned to ensure smooth, reliable contact. Better yet, a new trim pot may save you some headaches. The adjustment is sensitive enough that installing a 10-turn pot can be helpful. Be sure to exactly follow the procedure in the service manual. This is an adjustment where “close enough is good enough” usually doesn’t work very well ;-).

The only other thing that might give you more clues is to try the RESET adjustment with the CPU removed. I would do this as a last resort since the mechanical stress of removing the CPU might damage it.

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I would expect about 0.7v forward and 2.7v +-0.1v in reverse (since it is a zener diode).

You might get a misleading reverse voltage reading depending on your tester. Testing with a series resistor, voltmeter and a power supply can be used to verify the reading.

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At this point I would just pull TR5, TR6 and D6 and test them. There is not much else to the RESET circuit if the voltages are all OK.

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quattttro: Do you have 5v on the secondary 5v circuit? If not, check the edge connections on the Microprocessor board for cracked solder joints, especially 2P2. Then check 2C65 and 2C2 for shorts.

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quattttro:  I think I was able to prove to myself that the CPU was indeed dead. I ended up swapping the bad CPU into another BM6000 and the main issue followed the CPU. I still have a mystery to solve since the original machine does not work properly with the “good CPU” from the other machine.

Have you checked the 5v-cpu and 5v voltages on Board 2?  Have you replaced 2R89 and readjusted the RESET circuit?

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My assumption was that you were experiencing something different than with your other/previous B&O receivers, otherwise you wouldn’t have asked the question.  If there is an issue, there are multiple things that can cause a speaker thump.

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lollo:  You might want to put a multimeter across the speaker outputs and check the DC offset voltage. This should be very close to 0.0V. Sometimes power-on “thumps” are normal/unavoidable, other times they are indicative of something else. It is hard to judge what you are actually experiencing from the information that you provided.

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I think the black line looks better. Now every time I see one of these I’m going to notice the seam ;-).

Could you put a piece of black electrical tape on the lip? That would give you the “black line” look and would be reversible if you don’t like how it looks.

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You guys are lucky. The closest B&O showroom to me is 300 miles away.

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chilibt wrote:

I thought they may be connected on the trace but it would need to be confirmed with removing the board from the heat sink

You can verify the connectivity between points on the front side of the board by using a multimeter in the “ohms” setting.

Yep, the wire on the coax cable to the FM board is precarious. I had to repair this connection on all of the BM6000’s that I have. The trick for this repair is to remove the pins from the white connector for soldering and adding strain relief when reassembling.

Also, care should be taken to not jostle the light bulbs on the display board. The wires to the bulbs are very delicate/brittle. Just closing the “lid” hard from the service position can provide enough of a shock to break the connections.

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chilibt wrote:

What ground do I use, the chassis, the removed connector, of the Mains ground?

For something like this, I setup the bench power supplies using a floating ground. I use multiple supplies (in series) to get the voltages needed.  Googling something like “lab power supply grounding” will get you a better explanation than I can provide as well as providing safety tips and identifying pitfalls.

chilibt wrote:

I am attaching photos of the different boards and spots. Am I correct, the error is on the PCB?

The markings on the PCB might be a bit misleading, but are actually correct. If you look carefully, the small resistor is connected to the large green one via the traces on the circuit board. Clipping the test leads on the green resistors is the most convenient location. The positive lead goes on the right resistor if I recall correctly.

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Test main power supply

1. [PLUG OUT]
2. Remove P41 & P42 from Board 16 and P34 from Board 9. Leave P40 connected on Board 16.
3. [PLUG IN] Measure the 6.5v signal on pins 1 & 5 on P41. This may read high (~10v) since there is no load on it. Measure the 15v signal on pins 9 & 6 on P42. This should be very close to 15v. [PLUG OUT]
4. Pull P19 on Board 6. Energize the rail voltage relay by connecting B16-P42-9 to B6-P19-8 and B16-P42-6 to B6-P19-10 (i.e. supply 15v to the relay)
5. [PLUG IN] Measure the +50v and -50v rails at the capacitors. Measure the +32v signal at B16-P42-3 and B16-P42-9. This will also read high (~35v) since there is no load on it. [PLUG OUT]
6. Reconnect P41 and P42 on Board 16. Leave P40 on Board 16 disconnected.

Test CPU power supply

1. Remove P2 and P5 from Board 2
2. [PLUG IN] Measure the CPU +5v on P5-1 and P5-3. This should be very close to 5.00v. Measure the +6.5v signal on P2-2 (it will still read ~10v) [PLUG OUT]
3. Reconnect P2 and P5 on Board 2
4. [PLUG IN] Measure the +6.5 signal on B2-P2-2.  This should be close to 6.5v now. Measure the “other” +5v signal on B2-P2-3. This might read a few tenths of a volt high. [PLUG OUT]

Test Main Amp with Bench Power Supply

1. Connect to power supply

• B9-P34-1    -50v*     * Start at about +-10v and work your way up
• B9-P34-2    GND
• B9-P34-3    GND  (I can’t remember if pins 2 & 3 are connected on the board)
• B9-P34-4    +50v*
• B9-P31-3    GND**
• B9-P32-3   GND**

** The ground signals on P31 & P32 are not connected to the grounds on P34 on Board 9. I think B&O wired things this way to avoid a ground loop within the chassis.

2. Set power supply voltage to +-10v and the current to about 0.10A

3. Connect voltmeters to measure the Idle Current and Offset Voltage per the Service manual

4. [POWER ON]  Adjust R110/R210 for an idle current “voltage” of around 5mV and R134/R234 for an offset voltage near 0.0v. The actual voltages are not critical now as long as they are below the 22mV and near the 0.0V that you are trying for at full voltage.

5. Increase the +- rail voltages in steps of say 10v at a time while watching the idle current and offset voltage and adjusting the trim pots as needed. You will need to increase to current limit as you go to about 0.3A

6. Work you way up to +-50v while keeping things in spec.   [POWER OFF]

7. Wait for things to cool off to room temp

8. [POWER ON]  Do a final adjustment per the service manual [POWER OFF]

At idle, all of my BM6000 draw about 0.19A on the -50V rail and 0.17A on the +50V rail.

For the light bulb wattage, I use a 150w bulb for most things. Other sizes should work as well. You won’t (hopefully) be drawing anywhere near the bulb rating since you aren’t trying to run the amp at full power. The bulb should stay very dim except when the the relay kicks on and the rail capacitors charge (it takes less than a second).

I just ran through the power supply steps on one of my BM6000’s to verify that I remembered what I did correctly. Please double check the procedures against the schematic (and common sense) to be sure that I didn’t make any typos, etc. The amp check-out procedure is from memory, so be more careful with that. Don’t hesitate to ask if it looks like I forgot a step or if my description isn’t clear.

BTW, I will sometimes hook up the main amp to a music source and speakers as part of the bench test. This can help provide confidence that all is well with the amp. It also saves the steps of install/remove if something else on the amp needs to be fixed.

Be sure to post results of your progress.

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A few months ago I did a similar project of getting several BM6000’s (that were put away “broken”) working again. I have a Variac, but didn’t bother pulling it out of storage for this project since I didn’t think there was much risk at destroying anything irreplaceable in the power supplies. However, when I did power-up the main supply, Board 16, I used a rig that has a light bulb in series.  The order I did things was:

1. Test the capacitors
2. Test the main power supply (Board 16)
3. Test the CPU power supply (Board 6)
4. Test the main amp (Board 9) with a bench power supply. I brought the voltage up slowly with current limits in place. This is a good time to adjust the no-load current and offset voltage.
5. Reassemble everything except the power connector to the main amp and test with the light bulb rig. It is hit or miss that the receiver will start in this configuration since the RESET circuit will likely trip.
6. Test with the main amp power connected and the light bulb rig.
7. Try power-up connected directly to the mains (only if step 5 & 6 looks good)

I recall that there was a signal or two that used the edge connectors as a safety circuit which kept the receiver from powering-up if the connector was off. These can be found by tracing through the schematics.

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In addition to the above suggestions, there is also the option for testing the various boards or subsystems separately. For example the power supplies can be tested with the other boards disconnected. The main amp can be tested using bench power supplies, etc.

My main goal in this situation is to avoid damaging parts that are hard or impossible to replace. For a BM6000, the microprocessor would be the main concern. Make sure that the 5v and 6.5v supplies to Board 2 are in spec.

I would also remove and test the large capacitors on the power supply before applying any power.

The “best way” to power-up an old piece of equipment depends a lot on your goals (how serious you are about preserving the machine) and resources (what test equipment you have to work with). For some people, all they need to do is plug the machine into an outdoor outlet if their main goal is simply not to stink up the house with electronics smoke ;-).

It would be easier for people to provide detailed help if you provide more information about YOUR goals and resources.

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