Beomaster 6000 Power-On Issue

[Glitch]

Hello,

I have a Beomaster 6000 Type 2253 that will not power on. I’ve been debugging this receiver for a few weeks now and not sure what to try next. I’m hoping that one of the experts on this forum can provide some sage advice.

A bit of background… I bought my first B&O equipment back in the 80’s, a Beomaster 8000, Beocord 9000, and Beogram 8002. I thoroughly enjoyed this equipment for many years. Unfortunately, I experienced my share of equipment failures. At the time, I didn’t have the time to repair the various equipment. Instead of paying someone else to do the repairs, I would just buy a used replacement on eBay. Some the replacements also had issues and I repeated the process.  I now have a pile of broken, vintage B&O equipment and the time to repair it. I’m starting to work through the pile, piece by piece, repairing/restoring the equipment.

My current project is the BM6000. I can’t recall the exact details of how it failed. My best recollection was that the actual failure was much less spectacular than some of the other equipment (no smoke or loud noises). One day, it simply did not power on.

What I’ve done so far:

1)      Replaced the pots and bad caps on the main amp module (board 9). Adjusted the no-load current and output offset. Tested the amp board out of the chassis powered by bench power supplies. The amplifier seems to be working very well.

2)      Replaced the caps on the main power supply (board 16). Resoldered all of the connector pins. Adjusted the 6.5v output. Verified that all of the output voltages are the correct value. The main power supply seems to be working properly. The voltages are all close to the specified values except for the +-50v rails which are a couple of volts low.

3)      Resoldered all of the connector pins on the motor control module (board 6). Checked the output voltage for the CPU power supply. The CPU power supply is close to the 5v specification. I have not been able to test the motor control portion of the board beyond verifying that it will reset the volume to an initial position if I manually move the volume pot out of position.

4)      Replaced the pot and caps on the microcomputer board (board 2). Resoldered all of the connector pins. Adjusted the CPU reset circuit via R89 per the procedure in the service manual.

5)      Checked the remainder of the board for any obvious broken wires, cold solder joints, or visually failed parts. Other than a (now fixed) broken wire for the FM frequency counter(?), P7-board2 to P26-board8, everything appears to be in good shape.

6)      Read or skimmed every thread in this forum (and archives) related to the BM6000 to see if anyone else had a similar problem.

I believe that my problem is with the microprocessor not starting properly. When I plug in the unit the display will usually show a “:.” (colon, dot) on the frequency display, a blank source display and the FM tuner lights will light. The receiver is nonresponsive any to button presses except for changing the tuner mode which changes the tuner lights. Occasionally, the tuner and source displays will have other random LEDs lit. Less occasionally, the relay for the +-50v rails will be enabled. When this happens, the receiver remains unresponsive to button presses, but will power down after a period of time.

I’ve tried to run the microprocessor board and button board (board 15) outside of the chassis by powering via bench supplies. I’ve been able to get it to progress to a “P” display by changing the timing on when and how fast the 5v CPU, 6.5v and 15v power supplies power up. The results of the bench power-up sequence are not reliably repeatable. What shows on the displays varies and the relay signal is inconsistent. The board appears to remain unresponsive to button presses.

I’ve check the CPU clock signal on pins 1 and 2 on an oscilloscope and it seems strong and at the correct frequency. Same goes for the FREQ COUNT and TIME BASE signals.

The next thing I tried was to capture the startup sequence of various signals. The Fig 1 plot below shows the power signals related to the microprocessor board. All signals are measured on board 2. The power signals are reasonably clean once fully powered on. Sometimes there is noise on the signals when the plug connection is made. This presence or absence of this noise doesn’t seem to affect the power-up results.

Fig 2 below shows various signals related to the microprocessor board. The CPU based signals start reacting after about 2.5ms after the power is applied. The STANDBY signal goes low and the CPU clock (pins 1&2) starts, feeding the TIMEBASE circuitry and the TIMEBASE signal is fed into the CPU. The RESET signal reacts as expected when the 5Vcpu signal goes high enough for stable CPU operation.

Fig 3 below is similar to Fig 2 except that the TIMEBASE signal is replaced by FREQCOUNT.

Fig 4 below shows the results of an experiment where I installed a smaller capacitor on the 5Vcpu supply to see if a faster voltage rise time would make any difference (it didn’t). The time scale of this plot is smaller than that previous ones. A couple of things that I noticed was that the time from power-on to the STANDBY signal going low is a consistent ~2.5ms. The STANDBY signal going low doesn’t seem to be related to the RESET signal. The RELAY signal tracks the 15v signal until the STANDBY signal pulls it low. Not shown in any of these plots is that the STANDBY signal also pulls down the FAULTSW signal when it goes low.

I’ve run many other experiments trying to determine if the microprocessor is actually running. I can’t tell if the CPU has failed or has detected some other fault and has halted. At this point I’m not sure what to try next. I would appreciate any help, ideas or suggestions.

One specific question that I have is whether or not the microprocessor board CAN be run outside of the chassis? In some of the archived posts, it appeared that it may be possible, but it wasn’t clear how it was done.

Thanks for reading this so far, hopefully any follow-up posts will be shorter.

Glitch

 

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