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HP 1652B Scope/Logic Analyzer
Power Supply Repair
My HP1652B logic analyzer in use
I 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 smelled a burning smell. I did some searching and found only this account of someone repairing theirs. In that case, it appears the capacitors had gone bad. I hoped mine would be the same issue and 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 one out).
- 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.
Photo of the component side of
the board.
Starting 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 EMI filter.
- The small black block below the bottom grey is the full wave bridge on the input power.
- The 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 be about 165V. To measure this, use the 'top' lead of R1 (between the two black capacitors) as the return. The bottom lead of R1 is -V. R2 (visible here) is above the right most cap, and its left lead is the +V terminal.
- The big transistor (TIPL755) with the heat sink in the middle is the main chopper of the DC bus.
- The 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 voltage.
- The board on the bottom is the main controller. The five pots allow you to trim the voltages. I say that because these five pots are accessible through a slot machined into the top cover.
- Above 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, and diodes soldered to them. These are the rectified and filtered output buses. 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.
The damage/short area on the
board is right by C8.
I 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 be ~0.05 ohms, and their insulation resistance was in the MegOhm range. As a result, I judged them all undamaged and installed them back into the board.
On power up, the unloaded power supply board still produced a soft ticking noise, but no burning smell. I reasoned 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 is on the left most terminal:
- 4.8
- 4.8
- 4.8
- 4.8
- Return / ground
- Return
- Return
- Return
- 3.5V
- Return
- -5
- Return
- 12V
- Return
- -12V
- Return
- 10.7V
- -5.2V
- 11.2V
- Return
Transferring Data
The 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) |
| 2 | 2 |
| 3 | 3 |
| 7 | 5 |
| 6 looped back to 20 |
Adapter to connect the serial
output to a PC
My notes on setting this up (on Logic Analyzer):
- 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
- 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:
PIA
- STATE
LISTING
Label > R/_W RS DATABS SPARE CS
Base > Symbol Bin Hex Bin Hex
+0000 READ 01 00 1 5
+0001 WRITE 01 00 1 5
+0002 WRITE 00 7F 1 5
+0003 WRITE 01 34 0 5
+0004 READ 00 00 0 5
+0005 WRITE 00 00 0 5
+0006 READ 11 00 0 5
+0007 WRITE 11 00 0 5
+0008 WRITE 10 FF 0 5
+0009 WRITE 11 34 0 5
+0010 READ 10 00 0 5
+0011 WRITE 10 00 0 5
Label > R/_W RS DATABS SPARE CS
Base > Symbol Bin Hex Bin Hex
+0000 READ 01 00 1 5
+0001 WRITE 01 00 1 5
+0002 WRITE 00 7F 1 5
+0003 WRITE 01 34 0 5
+0004 READ 00 00 0 5
+0005 WRITE 00 00 0 5
+0006 READ 11 00 0 5
+0007 WRITE 11 00 0 5
+0008 WRITE 10 FF 0 5
+0009 WRITE 11 34 0 5
+0010 READ 10 00 0 5
+0011 WRITE 10 00 0 5
The above is a capture of the pins on a PIA on a System 9 board upon startup.
Floppy Drive Emulation
Being mechanical, the floppy drive has always
been a concern for me. In July 2024, I started having
problems with reading a few of the disks that have always worked
fine. The original floppy is the Sony 3.5" MP-F52W-30 that
spins at 600 rpm and is Double Density (DD). I found out
that there are a few floppy drive emulators and the one most
often used and available for a low price on Amazon was the Gotek
SFR1M44-U100.
One objective I had was to figure out a way to have _both_ the floppy and the Gotek connected, and switch manually from one to the other. In this manner, I could copy over all the configuration and setup files that I had on my floppies. Fortunately, these drives come with enable lines (called Drive Select in the manual), and my intent was to use these and a manual toggle switch to go from one to the other.
I worked on my own at first and tried the factory firmware that comes on the Gotek. That did not succeed and the Analyzer did not recognize the drive. It would beep after power up with a Disk Error. In my research I found that there are some firmware alternatives such as "HxC" that are developed by private individuals. And although there were no details, someone had interfaced an HxC to a 1652B in the past.
HxC has a powerful firmware customizer that allows you to put in all kinds of options to modify the drive behavior, which is useful as the Sony drive is not exactly the common PC format. Despite my best efforts, the closest I got was to be able to boot off the floppy, switch to the Gotek, and format the Gotek. But copying files to it still did not work. In desperation, I reached out on the HxC user forum, and Jeff the administrator was able to help me solve the problem. The exchange is documented in this thread. In the end, the problem was found to be specific to the MCU chip on my Gotek (QFN32 version of the AT32F415). Jeff had to move the Disk Change line to an unused pin, and I was finally able to use the Gotek to boot from and store my user files.
To replicate this setup, you will need to modify three areas: 1) Ribbon Cable, 2) Gotek, and 3) the USB stick.
1. Preparing the ribbon cable
Per page 12 of the datasheet (link below), we can see that the power feed is integrated onto the 34-pin drive connector. The first thing I had to do was modify the ribbon cable to interrupt and intercept the power. I did this by purchasing a 34-pin male-female extender, and then pressed on a new 34-pin IDC female connector into the middle of it, which will be the tap for the Gotek.
The "Y" cable made from a male-female 34-pin extender.
Two drives connected to the Logic Analyzer.
In the image above, you can see the two drives plugged in.
2. Preparing the drive
I followed the instructions on the HxC site to program a bootloader onto the chip, and programmed custom firmware from the HxC site for the drive. This writes over the factory firmware. I then followed Jeff instructions and wired Pin 4 on the ribbon to an unused MCU input. This line does not exist in a normal PC disk drive but is used by the Analyzer to know when a new floppy has been inserted. The only other mod is to insert the S0 jumper to connect the above signal through. There is a file package in the links below with more details.
3. Preparing the USB stick
I formatted a USB stick with FAT32 and copied over the configuration file (.CFG) and the .HFE file that Jeff sent me to it. The bootable HFE file has the same revision of System file that I have (rev 2.11), but this also includes the "Inverse Assembler". I have not encountered this before and do not know what that is. But I was able to use this system image just fine after deleting the Assembler files. Each HFE file forms an image of a floppy, and you can have many in one USB stick. The images are selected by pushing the black buttons on the Gotek and the numerical display will show which images the Analyzer will see. This is super convenient compared to swapping out floppies. The HFE files can also be backed up on the PC. In my situation, disks 0 and 1 are boot images, disk 2 and 3 has my Logic Analyzer configuration files. Disk 4 boots the system up in scope mode, and Disk 5 has scope mode configuration files.
In use
The image below shows the finished single drive configuration after the development and the copying over of the configuration files. I also shot a video but of the dual drive configuration to show the disk switching in action.
HP1652B Logic Analyzer with Gotek drive and HxC firmware installed.
The result looks very clean.
YouTube video of the Gotek in a dual-drive setup
One objective I had was to figure out a way to have _both_ the floppy and the Gotek connected, and switch manually from one to the other. In this manner, I could copy over all the configuration and setup files that I had on my floppies. Fortunately, these drives come with enable lines (called Drive Select in the manual), and my intent was to use these and a manual toggle switch to go from one to the other.
I worked on my own at first and tried the factory firmware that comes on the Gotek. That did not succeed and the Analyzer did not recognize the drive. It would beep after power up with a Disk Error. In my research I found that there are some firmware alternatives such as "HxC" that are developed by private individuals. And although there were no details, someone had interfaced an HxC to a 1652B in the past.
HxC has a powerful firmware customizer that allows you to put in all kinds of options to modify the drive behavior, which is useful as the Sony drive is not exactly the common PC format. Despite my best efforts, the closest I got was to be able to boot off the floppy, switch to the Gotek, and format the Gotek. But copying files to it still did not work. In desperation, I reached out on the HxC user forum, and Jeff the administrator was able to help me solve the problem. The exchange is documented in this thread. In the end, the problem was found to be specific to the MCU chip on my Gotek (QFN32 version of the AT32F415). Jeff had to move the Disk Change line to an unused pin, and I was finally able to use the Gotek to boot from and store my user files.
To replicate this setup, you will need to modify three areas: 1) Ribbon Cable, 2) Gotek, and 3) the USB stick.
1. Preparing the ribbon cable
Per page 12 of the datasheet (link below), we can see that the power feed is integrated onto the 34-pin drive connector. The first thing I had to do was modify the ribbon cable to interrupt and intercept the power. I did this by purchasing a 34-pin male-female extender, and then pressed on a new 34-pin IDC female connector into the middle of it, which will be the tap for the Gotek.
The "Y" cable made from a male-female 34-pin extender.
The following mods were performed:
The Monitor connector above was very useful in allowing me access to figure out why the initial code loads were not working. I connected a second logic analyzer to this location to monitor the control signals.
Example waveforms showing signals on the ribbon cable. It provided Jeff from
HxC insight into what the interaction was with his code and the Analyzer.
More in the HxC forum thread.
- 5V was interrupted and wired past the middle
connector. Power was split off for the Gotek
(red).
- 12V was interrupted and wired across to supply the
floppy drive (yellow wire).
- Pin 1 (Disk Reset) was interrupted and wired across the Gotek. Since the Gotek grounds pin 1, it disrupted eject detection.
- The bypass line for Pin 1 is connected to Gotek Pin
4. On the board, this line goes to J8-JK terminal.
- Pin 10 (Drive Select 0) was interrupted and used to
select between the two drives. Pin 10 of the
desired drive is grounded by a toggle switch to select
it. The Logic Analyzer does not actively toggle
the Drive Selects and always has pin 10 low.
- Ground/Return (three lines) were tapped for the Gotek power connector (black).
- I also added a second female 34-pin IDC connector so I
could monitor the signals with another Logic Analyzer
(Monitor).
The Monitor connector above was very useful in allowing me access to figure out why the initial code loads were not working. I connected a second logic analyzer to this location to monitor the control signals.
Example waveforms showing signals on the ribbon cable. It provided Jeff from
HxC insight into what the interaction was with his code and the Analyzer.
More in the HxC forum thread.
Two drives connected to the Logic Analyzer.
2. Preparing the drive
I followed the instructions on the HxC site to program a bootloader onto the chip, and programmed custom firmware from the HxC site for the drive. This writes over the factory firmware. I then followed Jeff instructions and wired Pin 4 on the ribbon to an unused MCU input. This line does not exist in a normal PC disk drive but is used by the Analyzer to know when a new floppy has been inserted. The only other mod is to insert the S0 jumper to connect the above signal through. There is a file package in the links below with more details.
3. Preparing the USB stick
I formatted a USB stick with FAT32 and copied over the configuration file (.CFG) and the .HFE file that Jeff sent me to it. The bootable HFE file has the same revision of System file that I have (rev 2.11), but this also includes the "Inverse Assembler". I have not encountered this before and do not know what that is. But I was able to use this system image just fine after deleting the Assembler files. Each HFE file forms an image of a floppy, and you can have many in one USB stick. The images are selected by pushing the black buttons on the Gotek and the numerical display will show which images the Analyzer will see. This is super convenient compared to swapping out floppies. The HFE files can also be backed up on the PC. In my situation, disks 0 and 1 are boot images, disk 2 and 3 has my Logic Analyzer configuration files. Disk 4 boots the system up in scope mode, and Disk 5 has scope mode configuration files.
In use
The image below shows the finished single drive configuration after the development and the copying over of the configuration files. I also shot a video but of the dual drive configuration to show the disk switching in action.
HP1652B Logic Analyzer with Gotek drive and HxC firmware installed.
The result looks very clean.
YouTube video of the Gotek in a dual-drive setup
Software Capture of Screen Print
The other mechanical input/output of this old
device is screen captures. It supports printing via serial
(or HI-IB) to a Thinkjet printer. I decided to try and see
if I could convert this stream to an image.
I first started with the capture of the serial stream and pulled the result up on my Neo Hex Viewer. From what I could see on page A-2 of the Thinkjet Owner's Manual, the graphics commands are preceded by 6 byte header consisting of "<ESC>*b74W" (hex 1b,2a,62,37,47,57). I could see these packets in the file, but the packet is a lot longer than expected and interspersed with additional random bytes. See below, with the benefit of hindsight, I know the first (top) line of the image is a line of black pixels. So the packet should only contain 0xff bytes. But there are many characters interspersed.
I first started with the capture of the serial stream and pulled the result up on my Neo Hex Viewer. From what I could see on page A-2 of the Thinkjet Owner's Manual, the graphics commands are preceded by 6 byte header consisting of "<ESC>*b74W" (hex 1b,2a,62,37,47,57). I could see these packets in the file, but the packet is a lot longer than expected and interspersed with additional random bytes. See below, with the benefit of hindsight, I know the first (top) line of the image is a line of black pixels. So the packet should only contain 0xff bytes. But there are many characters interspersed.
Hex view of one packet in the captured stream.
Looking at the viewer above, you can see
the packet header in the top-left. After the 'W' is the
start of the pixels (top row of image), and they should all be
0xff. Counting the number of good 0xff bytes, we have 74
(decimal). That matches the expected quantity in the
packet header. Total length of the packet is 192
(decimal), or 118 extra bytes.
The first rendering of this stream produces the image below. You can see these columns of garbage data breaking up the image. What is interesting is that these are perfect columns. So they must be produced by the Logic Analyzer and not the PC or cable doing the capture.
The first rendering of this stream produces the image below. You can see these columns of garbage data breaking up the image. What is interesting is that these are perfect columns. So they must be produced by the Logic Analyzer and not the PC or cable doing the capture.