Sorry I haven't been watching more actively this evening. Um.. no, let's not do that. I think the supply may be going overvoltage on that 5v rail. It's important to know
where you're measuring this however and what's your minus reference.
I need to look at the system schematics to be of any further help on it. I'd like to follow how the lamp is driven and what the monitored voltages and criteria are. Seems the easiest way to continue. Perhaps someone else will
already know without having to research it.
Studying Schematic Pg 41, CONSOLE INTERFACE I see a series of Window comparators monitoring -12v, +12v, 5v, -15v and +5VB ... My thinking is to trace back through these to determine which is unhappy.
A little complaint here - whichever DEC engineer designed this damn thing should have included trace points to add blocking diodes and LEDs to this panel. Would make it real easy to see at a glance who's unhappy. As it is, all the outputs are ganged so it's impossible to tell who's firing except by their inputs. Grrrr!
Me be changing this puppy if it were my system.
The circuit seems to be depending on VB+ being at 13.8 Vdc (float charge for a lead acid or Gel Cell battery) to develop a reference for other comparisons. If VB+ is not above 13.3v it appears the circuit's conclusions will be invalid.
This has only to do with the generation of the DC ON signal (DC OK?) not the actual regulation of voltages or BUS signals.
I would also like to know the condition of BUS AC LO L (BF1) and BUS DC LO L (BF2) (description Pg 6-33 thru 6-35) It appears this may be describing your CPU halt.
More study...
I've just read the entire thread from the beginning... I'd like to return all the way back to your failed C4 on the BIAS Board...
Now I saw you got lots of excellent troubleshooting advice about tooling, clip leads and absolutely dangerous voltages. I'm a bit hesitant to have you go poking around there, but there are some fundamental things that bother me about those photos of the bias board. (scorched quadrant in the area of R2 the 15K carbon composition 2W power resistor - used in LTC gen)
BTW - Do you know what mod is on this vicinity?
Back to measurements - I'd particularly like to verify voltages across D12 the 16V Zener (although I think someone said 14v) and across C4. (two readings)
C4 doesn't look to me to have been the source of that heat, just a victim. (Q6 maybe?) I'd like to figure out who was the hot guy and if maybe C4 was fed overvoltage somehow, perhaps through the BIAS XFMR FDBK (Bias Transformer Feedback) in which case D34 is suspect.
J1-1 and J1-A on the LOGIC INVERTER BOARD appear to be the 5V remote sense lead inputs. Can we follow these to be sure they're hooked up?
Still need to find the original source of the VB+ mentioned above. Like to know it's value though.
I know it will be difficult for you to access all these readings. You should probably let me finish my read before diving in, unless one of those is easy.
Think I'll print out this schematic tomorrow. Helps me to scribble notes on them.