WB testing problems

[bcroe at juno.com]

Steve, 

Sounds like the meter is OK.  

With 7.8 V on the +4 side of R38, there must be leakage 
on the other side pulling it down to 6.75V.  Check first for 
solder splashes to that wire.  Assuming no shorts, there 
are 3 parts that could cause the leakage.  Those are
ISO1 at pin 4, C12 at plus, and U3B at pin 5.

As a check against reversed parts, move the probe from 
R38 to those pins.  Count from the dot on the case to pin 
4 of ISO1 and expect the same 6.75 V.  Do this at pin 5 
of U3 and the PLUS terminal of C12.  If you see a different 
voltage, the part might be reversed.

If that checks out, certainly do BobRs test of shorting ISO1
pin 1 to pin 2.  If this fixes the problem, Q1 may be leaking.
Try shorting Q1 pin 1 and 2 together; if that gives you 8V at 
R38, Q1 is leaking.  I would suggest reducing R1 to 33K 
and see if it is cured.  

If all that fails, you could remove the 3 parts one at a time 
until R38 goes to 8V.  As BobR suggested, disconnect 
C12 first as most likely suspect.  I would disconnect 
ISO1 next, and then U3 pin 5.  Sometimes you can just 
cut the pin, then bridge the cut with solder when the bad 
part is found.  Its a good idea to check each cut with an
ohmmeter to make sure its really cut, or use your volt 
meter to see a different voltage on each side running.

Bruce Roe

On Sat, 13 Apr 2002 21:46:26 -0400 Stephen Andersen 
<SAndersen at advan-tek.com> writes:
> I replied to Bruce Roe privately and don't 
> know whether I said it here, but I replaced 
> C12 and ISO when I found the probelm.
> I am now getting nervous, as I lifted the pads
> off the board when removing the ISO with too
> much heat.  I think I have repaired things 
> effectively, but there was no change with the 
> replacement parts.  I will do as you suggest
> and see if that at least isolates the problem.
> 
> Are you implying that even with this voltage
> "deficit" at this spot in the circuit that I 
> may be okay?
> 
> I will try to perform these checks in the next few
> days, and continue with the checkout of the circuit
> and report back.
> 
> Thanks!
> Steve
> 
> 
> > -----Original Message-----
> > From: owner-diy_efi at diy-efi.org 
> [mailto:owner-diy_efi at diy-efi.org]On
> > Behalf Of rr
> > Sent: Saturday, April 13, 2002 6:29 PM
> > To: diy_efi at diy-efi.org
> > Subject: Re: WB testing problems
> > 
> > In this case the cap C12 may be a little leaky. To check that
> > lift one of the cap pins and re-check. If no change then short
> > together pins 1 & 2 on the ISO1. May be some leakage in
> > that portion of the heater control circuit.
> > 
> > Most definately run through the rest of the checks.
> > 
> > BobR.
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org