HP 9915B Repair Help Request

[John S]

Hi,

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

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.

Signals on the other pins are:.
.
pin1 - OV DC (would be around 3V if ok)
pin2 -  3V (ok, derived from %V)
pin 9 = 0.5V (see below)
pin 7 = 0.2V to 4.5V ramp as per 85 service manual
pins 12 & 13 = 32V DC

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

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

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

Thanks,
John

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