[Glitch]

I cleaned-up the “blip” enough that it should be considered high from a TTL point of view. This didn’t appear to make any difference in the overall power-up issue.

[Glitch]

artig,

Thanks for the suggestion.

I checked the 50-60Hz signal fairly early in the debugging process. I saved a trace of the signal as a reminder to investigate an anomaly in the signal.

The extra blip in the signal is unexpected. My assumption was that the 50-60 signal would just be used for the timer functions and I would revisit this signal if the timers had issues.

I rechecked the 50-60 signal today and (initially) the signal constantly stayed low. I started to debug this and it “spontaneously” started to work again.

I ran an experiment where I decoupled the power supply (PS) from the CPU board by removing the pin P47-1 from the PS connector. I added a 10K pull-up resistor on the P47-1 connector side so I could check the PS circuitry. The PS circuit worked as expected except for the extra blip. I also monitored pin16 on the CPU. Sometimes pin16 would stay low, other times it would go high. The state of the signal seemed consistent with what was showing on the display. If the display got stuck at “:.” the signal seem to stay low. If the signal got to the “.” (standby?) display, the signal stayed high.

My interpretation is that pin16 on the CPU is pulled high as part of the CPU start-up process. However, I am having a difficult time determining if this is part of the CPU hardware initialization (standard output per the MK3870 datasheet) or if the software is controlling the pin state (direct output). This would only matter as a way of determining if the CPU is running. Pulling pin16 high makes the 50-60 signal “active” and the transistor circuit on the power supply will pull it low at at the appropriate time to make the 60Hz square(ish) wave. The CPU can then read the signal as needed.

I’m also trying to understand if the controller is faulted because the various signals are bad, or if the various signals are bad because the controller has faulted.

As a long-shot, I’ll try to get rid of the extra blip and see if that helps.

It would be interesting to see the source code for this. Then again, it is likely written in assembler and figuring out how it works would be a headache.

Glitch

 

[Glitch]

I haven’t made any real progress on the BM6000. I’ve resorted to replacing diodes and transistors that I didn’t think were bad. The removed parts tested OK. I’ve been able to slightly change how the circuit reacts, but it is just a different version of random behavior.

The good news is that I’ve been able to get my Beomaster 8000’s working. I have one fully working and a second that works except for the FM display. I have parts on order to fully restore/repair these.

My next step on the BM6000 project is to run some experiments on one of the BM8000’s to see if I can learn anything about how B&O handles the start-up procedure, especially when faults are detected. Ideally, I would run these tests on a working BM6000, but since I don’t have one, I’ll have to make due with the BM8000.

I’ll keep monitoring this tread to see if anyone has suggestions on things to try and report if I make any progress.

Glitch

[Glitch]

I like the idea of the broken turntable with a good cartridge. The downside to this is trying to explain this plan to my wife. The argument about selling the turntable wouldn’t carry much weight since my “buy-to-sell ratio” is overwhelmingly weighted towards “buy”. Also, she seems to prefer things in small packages even if they aren’t for her ;-).

Glitch

 

[Glitch]

Mark: Thank you for the suggestions. I’ve started watching eBay for used B&O MMC’s. I looked at the Sound-Smith website and the SMMC4 seems like it would be a good candidate (and likely what I’ll get). However, it is at a price point where I’ll have a hard time justifying the purchase to my wife. I’ll also keep my eye out for a used SMMC4.

Guy: I’m in the USA so the duty and shipping won’t be an issue.

Are B&O and Sound-Smith the only sources of compatible cartridges? I suspect that other sources are unlikely, but wanted to ask to be sure.

Thanks,

Glitch

[Glitch]

Christian,

Your reply above is consistent with what I’ve read about the B&O equipment of this era. B&O was pushing the bleeding edge at the time and reliability sometimes suffered as a result.

I’m hoping that I can get my BM6000 to show enough signs of life that it is worthwhile to spend the time and money to do a full restoration. I’m still hoping that the issue is related to an overly sensitive startup procedure and it is just a matter of figuring out what it takes to make the CPU happy enough to keep running.

Thanks again for your help,

Glitch

 

[Glitch]

Christian,

Thank you for the reply.

I just rechecked the STROBE (Digit Select) signals on pins 26-28. All three signals are always HIGH (+5v). The only time that they change is when I plug or unplug the receiver.

I’ve checked these in the past and expected to see them changing. My assumption is that all HIGH on these signals means that the display is either meant to be blank or that the CPU is not properly updating the signals. Since some of the other signals that I expect to change during power-up are not changing, like the RELAY signal, my theory is that the CPU isn’t running properly.

I think that the STROBE signals and the downstream circuitry is not broken since I was able to get the “P” display, as well as other LEDs randomly lit, when I ran the microprocessor board outside of the chassis with bench power supplies.

My interpretation of what I’m observing is that the CPU initially starts, then fails or stops at random times. What I see on the displays and other signals depends on how long the CPU runs before it has an issue. Of course, I might be having tunnel vision on this theory and would be happy to try alternative solutions.

What did you do to fix your STROBE issue on your BM8000?

Glitch

 

 

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