HP 9915B Repair Help Request

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Feb 9 17:03:04 2005

> Hi,
> I have a faulty HP 9915B (the industrial version of the HP 85B). I don'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.

(Incidentally, the 9915A schematic is on the HPCC scheamtics CD-ROM,
should you want it)

> have the right circuit diagram for it, only the ones for the HP 85A and 85B
> from their service manual. Looking at the 9915B it seems to have a similar
> circuit, but some of the differences I can't understand.
> So far I have checked the following:
> rectifier output = 32V DC (OK)
> but none of the regulated voltages are working (eg the +12V line is at
> +1.4V, similarly +5V, -12V are around 1V).


I am going to assume it _is_ the same PSU as the 9915A. From what you say
there, it's clear that the crowbar (Q4) hasn't fired, since this shorts
the 32V (Vin, on my diagram) line to ground.

> The power supply is based on DC-DC converters running at around 30kHz. I
> found the PWM regulator (U30, equivalent to U1 in te 85B), and this is
> generating pulses on pin 3 and has a 5V reference on pin 16. This part is
> labelled SG9496, the 85 uses a SG3524 (datasheet on the web), the pin-out
> seems to be the same.

My diagram shows U30 as an SG3524, but that may be because it had a
custom HP number on it, and I found it was pinout- and functionally-
identical to the 3524...

> Signals on the other pins are:.
> .
> pin1 - OV DC (would be around 3V if ok)

Should be a potted-down version of the 12V line, If that's missing, then
pin 1 will be olow.

> pin2 - 3V (ok, derived from %V)

Potted sown from the Vref ouput (pin 16

> pin 9 = 0.5V (see below)

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.

> pin 7 = 0.2V to 4.5V ramp as per 85 service manual

OK, so the oscillator is running (that's the timing capacitor pin). And
the reference output is also present. U30 may well be good.

> pins 12 & 13 = 32V DC

Outputs to the chopper (collectors of the output transistors, the
emitters (pins 11 and 14) are grounded). It's clear the chopper is not
being driven.

> I traced pin 9, and this is connected to C20 which in turn is connected R23

That's the first compoent reference difference (or I've made a mistake). On
my diagram, C20 is the dcoupling cap on the V+ (pin 15) supply input to U30.

There is a compenstation network, 0.1uF in series with 8.25k between pin
0 and pin 1, though.

> then pin 1 (similar to 85). pin 9 is also connected to diode CR4 which goes
> 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
> circuit?


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.

Whatever you do, don't disable this circuit. If there is a short, the
results will be spectacular and expensive.

> The output on pins 12 &13 has no negative going pulses, so the psu never
> gets started up. pim 12 connects to an 820 ohm resistor (number hidden)
> which then goes to transistor Q5 (the main switching transistor, Q1 on the
> 85). This is different to the 85 which uses an intermediate driving

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.

> tranhsistor. I guess this means the U30 PWM has to drive more current.

No, Q5 is more than one transsitor.

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.

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

Hope that helps

Received on Wed Feb 09 2005 - 17:03:04 GMT

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