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.
Exploded view of the HP1652B from the Service Manual.
You should then see the image below.
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 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.
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:
- Return / ground
- 8/30/2015 - successful repair.
- HP1652B manuals.
- Suzy's blog on this Logic Analyzer. She found me due to needing to repair her power supply (5/2016).
(c) 2015 Edward Cheung, all rights reserved.