OK, my fault for relying on my poor memory. I don't have a BM6000 to try it on and I just remembered it as being like other products from that era which did use the 50/60Hz signal for power failure indication. It's now more than 40 years since I last worked on the BM6000.

Perhaps unrelated, but I noticed resistor R27 has been snipped out of my motor control board. Looks to be related to the volume control. Is this a factory thing, or should I install a 470 ohm resistor there? I noticed the resistor present in Sonovor's thread, here.

 

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The 50/60Hz signal is an example of the chicken/egg situation I was referring to earlier. It would be easy to interpret a missing square wave on the signal as the cause for the receiver not running. The lack of the signal could also be an effect of the CPU not setting up the I/O ports. The tricky part is figuring out if it is the cause or effect. In quattro's case, my test provides a clue to what is going on. Another test that could be done is to add a pull-up resistor to +5v somewhere on the 50/60Hz signal circuit and verify that the signal works as expected.

Glitch

One of my BM6000's has R27 present, it is "cut out" in the other two.

I don't think that anything related to the motor control will stop the receiver from initializing.

You mentioned that the motor runs continuously... This is usually caused by a dirty initial volume slider or the motor control trim pots being out of adjustment/corroded. I believe that you can perform the Preset Volume Adjustment from the service manual without the CPU being fully initialized. I know it would bug the heck out of me if the motor was constantly spinning while I was trying to debug the other issues.

Glitch

 

Adjusting R25 solved the motor always-on issue. The full calibration procedure requires the receiver to be on so the volume can be adjusted to various levels.

I'm probably getting desperate, but would 14.7 volts here cause a problem? I know this 15v goes to the standby circuit amongst many other things. And I'm not getting 5V on Pin18 of the CPU (st.by), though i'm probably running into that chicken/egg problem again :)

I'm getting 14.7volts at pin 3 of IC2 (board 16).

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I understand that desperate feeling...

14.7v seems to be a bit low, but not so much that I would expect it to cause a problem.

It is a fully regulated voltage so I would expect it to be closer to 15.0v. A quick test to see is something is dragging it down would be to pull P42 and measure the voltage on pin P42-5. The relative difference between no-load and loaded is of most interest.

Glitch

 

 

 

 

I had to go back and check my notes...   There is a good chance that the Standby pin is setup/initialized at the same time as the 50/60Hz pin.  So whatever is keeping one of them from working is likely keeping other ones from working.

Are you sure that BOTH +5v power supplies on the CPU board are close to 5v?

Glitch

Same 14.7V on P42-5 with P42 wires pulled. Interestingly, I get "P" in the display when P42 is pulled. It never displayed anything before save for the colon, dot, and radio LEDs.

Definitely have both 5V supplies. I get a perfect 5V at the CPU pin 40 and the other ICs on board 2 are all getting 4.9v from the 2TR1 supply.

I am getting some sort of signal from the CPU's 50/60Hz pin (16). The waveform looks exactly like your did on page one of this thread. Even has the same blip on the right side of the square.

 

Regarding the 14.7v instead of 15v... Check that the heat sink compound isn't dried up. Otherwise, the regulator is likely just "off a bit". Swapping it with a new one is a easy job and the part is commonly available (but probably isn't needed).

I believe that the "P" display in this case just means that the CPU board is getting 6.5v to the LED displays, but the CPU isn't pulling up the FAULT line. Getting a "P" is pretty easy. For example, there will be a "P" display if the CPU is removed. I think a "P" can also be an indication of a genuine fault in the main amp.

I mentioned the second 5v supply because it was the most recent cause of a ":." display on my machine. I was testing pre-scalers chips and I bumped TR1 or TR2 and opened up a cold solder joint that killed the secondary 5v supply. I was pretty nervous for a while since I thought I might have damaged the CPU while swapping chips.

It is interesting that your 50/60 signal has the same blip. I'm not sure why this happens. I think I decreased my blip by changing 16TR4.

I'm not sure what to suggest next. Are you sure that you aren't just "looking at it the wrong way"? Sometimes it seems that "looking at it the wrong way" is all it takes to break it. ;-)

Let me know if there are any other signals that I can check on my working BM6000 for you to use as a reference. I should have one one of them on the bench in a few days to replace a burned-out bulb.

Glitch