pcdata76
Junior Member level 1
Hi,
I have a vintage Compaq Portable 3 computer with broken SMPS. When I turn on the computer, SMPS attempts to start (at the first sight, it can be understood from the power LEDs and the noise coming from the PSU) and then fails but it repeats this sequence over and over.
I need to repair this power supply, because beside the well known +5V, +12V and -12V voltages, it is also producing the -200V required for the display panel, and finding/building a 200V PSU is not very easy. Otherwise, I'd simply change the entire board with a modern and much reliable SMPS instead of spending so much time onto this old unit.
Unfortunately, schematics of this power supply is not available and much worse, it has two hybrids (one for the SMPS controller and one for the feedback circuit). Luckily, hybrids are not covered with epoxy, and I've managed at least to reverse engineer the controller. I've attached the schematic to the post as PDF (also includes the primary circuit- except the 110/220V auto switch part which is not active at 220V operation and not related with the problem, and the AC side components) Some components have no marks, and there is no reliable information on the web for some of the marked ones as well. So the part numbers that I wrote in the schematic may not be all correct). Additionally, TR2 transformer has 6 separate secondary windings (+5V, +12V, -12V, auxiliary winding, -200V, -5V), but now shown in the schematic.
In order to facilitate troubleshooting, I've desoldered hybrids and installed in a way that I can take at least some measurements from the components. (HY1 is soldered back from the bottom side of the PCB)
Following operations were done for troubleshooting so far without success:
- To eliminate the possibility of a short-circuit on the computer (which is actually quite common on that era of computers having tantalum caps), everything is disconnected. Nothing changed except the frequency of restarts. (Btw computer was checked with another PSU and it is working, there are no shorts)
-Tested with dummy loads on the +5V and +12V outputs. No improvement.
-I opened the case of the PSU and checked everything visually. Nothing wrong at visual inspection.
-All caps measured for capacity, ESR and shorts (both in circuit and afterwards also one by one by removing from the board). All are well within spec.
-Secondary circuits are checked with a bench power supply after removing the TR2 SMPS transformer. No shorts, no excessive current draw. Diodes measured fine.
-While PSU is attempting to start, voltages after the diodes were measured and proved that all of them are conducting only one direction.
-All secondary voltages (5V, +12V, -12V, -200V and auxiliary winding voltage) while SMPS is attempting to start are measured with scope. Voltages are coming up in all channels.
Capture of +5V output:
Capture of +12V output:
Capture from emitter of Q3 (auxiliary winding voltage after rectification):
As seen from the graphs, PSU tries to start a couple of times, and during that period all secondary voltages are coming up and then stops. In 0.5 to 1 second, it attempts to start again. Pattern is somewhat similar at each attempt but not exactly same.
Measured across optocoupler U1 (which is responsible for regulation using +5V sense, the other one U2 I couldn't understand the exact purpose. It is very difficult to track the feedback hybrid) using two probes, and GNDs of the probes connected to secondary GND. It looks like there is ~1V difference all the time).
I also measured base of the output transistor (GND on the emitter):
Zoom on the one short attempt:
Zoom of one longer attempt:
Beginning of the longer attempt:
Close to end:
End:
It starts from low frequency PWM and it goes up to 100kHz and then stops. Duty cycle is changing and never exceeds 50% as far as I noticed.
I have experience on repairing many electronic stuff, especially the vintage equipment such as hi-fi stuff, crt televisions, monitors, computers etc. but SMPS troubleshooting is not much my thing except simpler/more obvious faults. Circuits containing oscillators especially if they are made of discrete components are always hard to me to understand. I also couldn't understand how this circuit is oscillating, and changing duty cycle.
As a last solution, I also wonder whether is it possible (and relatively easy) to remove the entire feedback and controller hybrids, and design a modern PWM IC based controller and well-known TL431 based feedback circuits?
I have a vintage Compaq Portable 3 computer with broken SMPS. When I turn on the computer, SMPS attempts to start (at the first sight, it can be understood from the power LEDs and the noise coming from the PSU) and then fails but it repeats this sequence over and over.
I need to repair this power supply, because beside the well known +5V, +12V and -12V voltages, it is also producing the -200V required for the display panel, and finding/building a 200V PSU is not very easy. Otherwise, I'd simply change the entire board with a modern and much reliable SMPS instead of spending so much time onto this old unit.
Unfortunately, schematics of this power supply is not available and much worse, it has two hybrids (one for the SMPS controller and one for the feedback circuit). Luckily, hybrids are not covered with epoxy, and I've managed at least to reverse engineer the controller. I've attached the schematic to the post as PDF (also includes the primary circuit- except the 110/220V auto switch part which is not active at 220V operation and not related with the problem, and the AC side components) Some components have no marks, and there is no reliable information on the web for some of the marked ones as well. So the part numbers that I wrote in the schematic may not be all correct). Additionally, TR2 transformer has 6 separate secondary windings (+5V, +12V, -12V, auxiliary winding, -200V, -5V), but now shown in the schematic.
In order to facilitate troubleshooting, I've desoldered hybrids and installed in a way that I can take at least some measurements from the components. (HY1 is soldered back from the bottom side of the PCB)
Following operations were done for troubleshooting so far without success:
- To eliminate the possibility of a short-circuit on the computer (which is actually quite common on that era of computers having tantalum caps), everything is disconnected. Nothing changed except the frequency of restarts. (Btw computer was checked with another PSU and it is working, there are no shorts)
-Tested with dummy loads on the +5V and +12V outputs. No improvement.
-I opened the case of the PSU and checked everything visually. Nothing wrong at visual inspection.
-All caps measured for capacity, ESR and shorts (both in circuit and afterwards also one by one by removing from the board). All are well within spec.
-Secondary circuits are checked with a bench power supply after removing the TR2 SMPS transformer. No shorts, no excessive current draw. Diodes measured fine.
-While PSU is attempting to start, voltages after the diodes were measured and proved that all of them are conducting only one direction.
-All secondary voltages (5V, +12V, -12V, -200V and auxiliary winding voltage) while SMPS is attempting to start are measured with scope. Voltages are coming up in all channels.
Capture of +5V output:
Capture of +12V output:
Capture from emitter of Q3 (auxiliary winding voltage after rectification):
As seen from the graphs, PSU tries to start a couple of times, and during that period all secondary voltages are coming up and then stops. In 0.5 to 1 second, it attempts to start again. Pattern is somewhat similar at each attempt but not exactly same.
Measured across optocoupler U1 (which is responsible for regulation using +5V sense, the other one U2 I couldn't understand the exact purpose. It is very difficult to track the feedback hybrid) using two probes, and GNDs of the probes connected to secondary GND. It looks like there is ~1V difference all the time).
--- Updated ---
I also measured base of the output transistor (GND on the emitter):
Zoom on the one short attempt:
Zoom of one longer attempt:
Beginning of the longer attempt:
Close to end:
End:
It starts from low frequency PWM and it goes up to 100kHz and then stops. Duty cycle is changing and never exceeds 50% as far as I noticed.
I have experience on repairing many electronic stuff, especially the vintage equipment such as hi-fi stuff, crt televisions, monitors, computers etc. but SMPS troubleshooting is not much my thing except simpler/more obvious faults. Circuits containing oscillators especially if they are made of discrete components are always hard to me to understand. I also couldn't understand how this circuit is oscillating, and changing duty cycle.
As a last solution, I also wonder whether is it possible (and relatively easy) to remove the entire feedback and controller hybrids, and design a modern PWM IC based controller and well-known TL431 based feedback circuits?
Last edited: