HP 9915B Repair Help Request - now OK!

From: John S <john_a_s2004_at_hotmail.com>
Date: Sun Feb 13 15:42:03 2005

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

J> I have a faulty HP 9915B (the industrial version of the HP 85B). I don't

T>I guess I should respond :-). I don't know anything about the 9915B, but
I have a reverse-engineered schematic of the 9915A, and from what yuo say
below the PSUs are very similar.

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?

J> pin 9 = 0.5V (see below)

T>This is called 'comp' on the 3524 datasheet. It's the output of the error
amplifier and what's worth knowing at this point is that it can be pulled
low exernally to shut the outputs down.

J> I traced pin 9, and this is connected to C20 which in turn is connected

T>That's the first compoent reference difference (or I've made a mistake).
my diagram, C20 is the dcoupling cap on the V+ (pin 15) supply input to U30.

I'm not sure if is is C20 actually, the labels are under the components so I
sometimes guessed!

J> then pin 1 (similar to 85). pin 9 is also connected to diode CR4 which
>into transistor Q2 (which in turn is connected to 0V). It looks like pin 9
>is being pulled down by CR4 and Q2 as these are both forward biassed. One
>question is what does CR4 and Q2 do? Are they part of some protection


T>Q2 is the overcurrent protection transsitor. It's turned on by U31 (LM311
comparator), which is driven from T1 (9100-0456 transformer) which
measures the current through the chopper. If that current is too high,
then U31 triggers, turns on Q2, and shuts the supply down.

I found a similar circuit in the Texas Instruments datasheet for the SG3524.

T> Watch out!. Q5 is not a simple transsitor (at least not in the 9915A).
It's a PIC645, which seems to be some kind of darlington with the flyback
diode, etc, in the package. It's roughly a PNP transistor, emitter to Vin
via T1 primary (current sense), collector to pin 3 of T2 (12V supply
transformer primary tap), base to the output of U30 via that 820R resistor.

Thanks, I found the datasheet for this too on the web.

T> OK, what I'd do next is look at U31 (assumeing it's a 311 comaparator).
The -ve input should be at about 2V (potted down from Vref again), the
_ve input should be 0 (it comes from the sense transformer, but with no
AC current flowing through the chopper, it can't be doing much). Then see
if Q2 really is turned on, or shorted, or what.

Yes, U31 / LM311 was faulty. It had:

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
component (glad the PIC645 was OK!) and this part of the circuit sprang into
life, U31 now has:

p2 (+in)=19mV (may be just noise)
p3 (-in)=2V
p7 (out)=130mV

U31 is no longer disabled, and is driving Q5 with pulses. I checked T3 and
this matched the 85A diagram, I now had +12V, -12V and -%V working fine, but
still around 0V on +5V and +6V rails.

I looked at T1, and this has the same circuit to provide +12V to +5V
conversion as the 85A. I found +7V DC on CR14 (equv to CR12 on 85A). I
looked at T3-4, this had a square wave pulses but going from only +2V to
-20V so something was wrong. The nearby capacitor was war,, and when I put
my probe on diode CR12 (eqiv to CR7 on the 85A) it was so hot the solder
melted and the diode popped out! I quickly powered off, and found that C45
(eqiv to C9 on the 85A - the label C45 btw is totally hidden by the
capacitor in situ) was shorted internally. I replaced it with a modern 470uF
cap about a quarter the size (but 105degC rated), powered on and now have
+5V and +6V = hurray!

I plugged the front panel in and ran the self test and extedned self test
(took about a minute, wonderful feature of the 9915), now all I have to do
is reassemble everything (eg the tape nd video output and keybioard modules
and try this beast out - if you don't hear back then everything worked OK!

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.


>Any advice gladly received, eg source or equivalent for a SG9496
>controller, copy o9f service manual would be great too!

Hope that helps


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Received on Sun Feb 13 2005 - 15:42:03 GMT

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