HP9915B with PSU problems

[Tony Duell]

> 
> 
> Hi,
> 
> I have returned to my HP9915B that blows its fuse as soon as you
> power it up.

I am going to make some assumptions : 

1) The fuse holds if you unplug the transformer connector from J6 on the 
mainboard (if not, then you've got shorted turns in the transformer or 
something like that.

2) You have schemaitcs. I only have the 9915A ones, which I think I gave 
you a copy of. If not, you can get them (reverse-engineered as always) 
from HPCC.

> 
> I followed Tony's advice and checked the two large capacitors (C46
> and C58), and the PIC645 and they are all OK. I then checked the
> U30 (3524) in the chopper sense circuit and its OK as well.
> 
> I checked the parts after removing them from the PCB to ensure that
> there was no interference by other components.
> 
> So now I am really at a loss and wonder if anybody has any ideas.

OK, what I'd do next : Pull JP6, JP3, JP5 (disconnect the main PSU 
outputs from the logic). Fuse still fails?

Check CR19 (1N5401, connected form pin 1 on T2 to the +ve side of C46. If 
that's shorted, the crowbar wull necessarily fire and blow the fuse. 
While you're about it, check Q4 (crowbar SCR) and the zener connected to 
its gate. You'd feel really stupid if a short in the crowbar was the 
cause of all this.

Fuse still blows? Remove the diode between pin 6 of T2 and the -12V line 
and the diode between pin 4 of T2 and the V+ line (RH side of sheet 1 of 
my CPU board schematic). With those lifted, the supply is reduced to a 
simple 12V one driving no load. If the fuse will hold now, I'd supsect 
(a) a short in one of the output regulators _and_ (b) a problem with the 
current sense circuit (round U31, LM311).

Do you have an adjustable bench supply? If so, remove the transformer 
connector from J6, conenct the supply across C46, and slowly ramp it up. 
See what the +12VH output does (across C58). Does it just track the 
input, even when the input is just over 12V? Is U30 oscillating at all 
(look at TP2)? 

You removed Q5 (the PIC 645 chopper). Did the fuse hold with that out? If 
not, then you know there's a short either in the crowbar, or between the 
windings of T1 (current sense transformer), or CR19, but you should have 
checked that by now. If the fuse will hold with Q5 out, you could see if 
U30 is oscillating. Provide a temporary pull-up to TP2 (9V battery and 
resistor?), do you get drive pulses here?

-tony