HP 1652B scope/logic analyzer
Power Supply Repair
My HP1652B logic analyzer in use
had been using my HP 1652B scope/logic analyzer for many years, and it
was with sadness one day in 2015 that I found out it would not power up
and had a problem. I could hear a soft ticking sound and
a burning smell. I did some searching and found only this
of someone repairing theirs. In that case, it appears the
capacitors had gone bad. I hoped mine would be the same issue
started the repair.
With the benefit of hindsight, the best way to access the power supply
module is as follows:
- Remove the top cover by using a T10 hex driver.
- Remove the disk drive (A6 below) by unscrewing it (H5) and
pulling it out the front.
- The power supply module is held by two plastic pins near
the front and back (MP11). Pull those out.
- Disconnect the cables to the power module (one input and
- Slide module (a5) out to the (right) side.
- Remove top screws and the top lid on the module's case.
- Unscrew board and remove from case.
You should then see the image below.
Photo of the component
side of the board.
with the red connector in the top left and going roughly clockwise,
this is the function of the parts as far as I can tell (note
'left'/'right'/'top' etc refer to the orientation in the above image):
- The red connector is the input power. There are
two pins for the back panel switch and two for input power.
- The 5Amp input fuse is below it.
- The two grey blocks and the yellow transformer is the input
- The small black block below the bottom grey is the full
wave bridge on the input power.
two large caps in the bottom right are to hold the +V and -V main DC
power. When the scope is disconnected from the power supply,
these read + and - 166V. With the scope plugged in, it will
about 165V. To measure this, use the 'top' lead of R1
the two black capacitors) as the return. The bottom lead of
-V. R2 (visible here) is above the right most cap, and its
lead is the +V terminal.
- The big transistor (TIPL755)
with the heat sink in the middle is the main chopper of the DC bus.
large transformer in the middle with the CAUTION sticker is the main
transformer that is pulsed by the above transistor. It is the
output windings of this transformer that provide the five output
supplies: 5V, 3.5V, -5V, 12V, and -12V.
- I suspect the
transformer below the main one couples energy back to the primary side
to modulate the main transistor's duty cycle to regulate the output
board on the bottom is the main controller. The five pots
you to trim the voltages. I say that because these five pots
accessible through a slot machined into the top cover.
the small board on the left are all the filters for the secondary side.
You will see several metal 'fins' sticking out of the board,
diodes soldered to them. These are the rectified and filtered
output busses. So you can monitor the unregulated buses by
tapping into these. Typical voltages are from 3 to 16V, so
nothing particularly dangerous. The rectification is a single
diode, so half-wave.
- The chokes/coils with the red dots connect the quad
capacitor bank to the output section.
- The five TO-220 packages along the perimeter heat sink are
the linear regulators for the five outputs.
first decided to try and isolate the fault by disconnecting most of the
power diodes from their output heat sinks. With this process
was able to isolate the problem/short to the bus that is near the bank
of 8 identical filter capacitors. These are 1000uF
The vertical 'fin' near the transformer appears to be one of
output terminals, and four diodes bridge from this fin to two
horizontal ones adjacent to the octal cap bank. I removed the
bottom four caps in this octal bank and saw quickly that there was a
short between the case of C8 and a trace below. There was a
prominent burn mark on the trace. See below for an image.
The damage/short area on
the board is right by C8.
tested all four of the capacitors from C7 through C10, and found they
all had a good value (about 1100uF). Their ESR all tested to
~0.05 ohms, and their insulation resistance was in the MegOhm range.
a result, I judged them all undamaged and installed them back into the
On power up, the unloaded power supply board still
produced a soft ticking noise, but no burning smell. I
that the Switched Mode Power Supply may need a minimum load, and was
perhaps shutting down due to lack of load. The voltages I
measured on the output connector was as follows. Top reading
on the left most terminal:
- Return / ground
then powered down and plugged in the scope. On subsequent
up, success! The fan spun up and the passed self-test.
then reinstalled the disk drive and the cover and the test after that
the system worked normally.
analyzer has a 25-pin female RS-232 port in the back, and I decided to
see if I could transfer data gathered by the analyzer for further
processing on a PC. With some
experimentation I found out that I could capture the data, as many as
1024 samples with a single pod. Adapter cable information is below.
|Male 25 pin D (LA)||Female 9 pin D (PC)|
|6 looped back to 20|
Adapter to connect the serial output to a PC
My notes on setting this up:
- I/O -> I/O Port Configuration.
- Use 9600,8,N,1 speed
- In "Printer Information", Select "Alternate"
- Setup LA for "State" and "State Listing" display.
- I/O -> Print All. Then "Select"
- You can hit "Stop" to Halt
Example capture stream with Hyperterm:
Base > Symbol
The above is a capture of the pins on a PIA on a System 9 board upon startup.
- 8/30/2015 - successful repair of the power supply.
- 12/12/2020 - Added note for transferring data to PC.
- Info on the one mechanical element: the disk drive. It is 4"
wide and has a single 34 pin ribbon connector. A label on the board
shows the model number is MP-F52W-30 manufactured by Sony. There is also the number "4-621-720-41" on the label.
- 3/23/2021 - I set up an Ebay 'watch' that flagged an available used drive and bought it for $20.
(c) 2020 Edward Cheung, all rights reserved.