HP 9915B Repair Help Request - now OK!

[Tony Duell]

> 
> Thanks Tony, your earlier reply pu tme on the right track, machine now up 
> and running self-test. Story included below:

Excel;ent....

> T>(Incidentally, the 9915A schematic is on the HPCC scheamtics CD-ROM,
> should you want it)
> 
> Are you sure the 9915A schematic is on the CD-ROMs? Which one please?

Yes I am sure, I am not refering to the MoHPC CD-ROM set, but to the HPCC 
(Handheld and Portable Computer Club) one. Go to http://www.hpcc.org, 
click on 'club', then on 'CD-ROMs' then I think it's the first thing on 
the page.

> Yes, U31 / LM311 was faulty. It had:

OK, that explains it.

> 
> p5 (Vin)=5V
> p1 (GND)=0V
> p2 (+in)=0V
> p3 (-in)=2V
> p7 (out)=0.7V
> 
> It seems strange that the output is directly connected to Q2 without a 
> current limiting resistor in series. Anyway, I replaced this very cheap 

Remember it's essentially an open-collector output. The output of U31 is 
pulled up to Vref via a 2.21k resistor, which is what actually supplies 
the base current to Q2. When U31's output is low, it effectively shorts 
the base of Q2 to ground.

> Thanks again Tony or putting me on the right path. BTW my tip (which you 
> probably do anyway) is to power the PCB from a bench 32V DC supply whilst 
> testing it, I made alead that plugged in the board in place of the mains 
> transformer output, pin 1 is +32V and pin 2 ground.

Actually, the outside 2 pins are 32V AC input (to a bridge rectifier of 
diodes that feeds C46), the middle pin is ground/0V. The 9915's 
transformer has a single secondary between the outside 2 pins. But a 
bench supply between eitehr outside pin and the middle pin should be fine 
for testing.

-tony