Red channels died; please, what now? [resolved]

[rsmiii]

I've been running the BeePre 2.0 (original, no upgrades) successfully for a few days.
Sounds great! No buzz or hum, and the balance control worked perfectly.

Now, all the red input channels appear to have died, and the red output channel is dead, too.
Still sounds great – so long as I only use black inputs and the black output.

Please, how do I deduce the answer?

[Paul Joppa]

The usual starting point is to re-do the resistance checks in the manual, and if those are OK, then re-do the voltage checks. Post the measurements that differ from the manual.

[rsmiii]

Paul Joppa: Thank you.
Page numbers are reference to BeePre 2.0 Manual dated 5-24-21:

Page 34: Black 33L & Red 31L reads 188VDC, off from the target 250VDC.

Page 52: Black 4 / Red A4 reads no resistance; off from target 445Ω
Page 52: Black 4 / Red A1 reads no resistance; off from target 445Ω

Page 53: Black 14 / Red B4 reads no resistance; off from target 445Ω
Page 53: Black 14 / Red B1 reads no resistance; off from target 445Ω.

All other electrical checks are on target, per the manual.

[rsmiii]

Paul Joppa: To embroider my earlier post above:
Page 34: Black 33L & Red 31L reads 188VDC *with* 300Bs installed.
Page 34: Black 33L & Red 31L reads 265VDC *without* 300Bs installed (and therefore within spec of ±20% of target 250DVC)

So too, page 52 and page 53, *without* 300Bs installed, the resistance checks are easily within spec (close to 445Ω)

Okay – so all formal voltage and resistance checks are good, per the manual.

[Paul Birkeland]

What are the DC voltages and pins 2, 3, and 4 of each 4 pin socket?

[rsmiii]

Paul: Thank you – you asked, "What are the DC voltages and pins 2, 3, and 4 of each 4 pin socket?"

Please help me – attach black lead to 4 and red to A2, A3, and A4 – and then black lead to 14 and red to B2, B3, and B4?

[Paul Birkeland]

You can use terminal 4 as your ground (where the black probe goes) for all these measurements.

[rsmiii]

Paul: replying to your question, "What are the DC voltages and pins 2, 3, and 4 of each 4 pin socket?"
Measurements with 300Bs *installed* :

Black 4 & Red A2 = 94 VDC
Black 4 & Red A3 = 0 VDC
Black 4 & Red A4 = 10 VDC

Black 4 & Red B2 = 94 VDC
Black 4 & Red B3 = 0 VDC
Black 4 & Red B4 = 15 VDC.

[Paul Birkeland]

Electrically your preamp is working, so the problem is with the input or the output wiring. 

If you temporarily install a wire between 6 and 16, that will sum the outputs of the preamp.  If that temporary wire (this is NOT any kind of fix for your issue) provides output to your right channel, then you have an issue with the wiring from the RCA jacks up to the balance pot.  If this temporary wire does not provide any right channel output, then you will need to look for issues related to the output of the preamp feeding the volume pot, and the volume pot feeding the RCA jacks.

[rsmiii]

Ah, please mark as resolved!

The story:
The 6/16 temporary wire did not resolve anything, so I re-examined everything forward from the DC Filament Supply printed circuit board.
Examining the bottom of the printed circuit board, a trace had lifted near the +Reg and -Reg on the left side.
I removed that lifted trace and replaced with a short "jumper" cable.

No other connections outside the PCB looked suspect, but I re-soldered many for the good of the order.

Whatever the culprit(s), all channels and the balance pot seem to work perfectly now.

Paul Joppa and Paul Birkeland: I'll "thermal cycle" for a few more days before I declare victory but THANK YOU VERY MUCH for your patient counsel.

[Paul Birkeland]

I hate to say it but the DC filament board could not cause your issue.  If half of that board was dead feeding your problematic channel, you could get very high voltage at pin 2 on that 4 pin socket, and 0V at pins 1 and 4, which is why we had you measure those voltages in the first place.

The issue you have is most likely a loose wire, flaky solder joint, or debris that has gotten into a place it shouldn't be, so don't be too alarmed if this issue ends up returning.