This switching power supply came along with me from a flea market. Lost, and lonely and being neglected by owner as it had many burned components and a big label on it ” DO NOT POWER UP”
My curiosity overtook my common sense AGAIN, and I decided to bring it home and give it a new life, very well knowing that it may short circuit my life in that endeavor.
So here it is (after bringing it back to life) , 0-600V DC, switched power supply, 1.7A max output. 
Video of the same –
Entire medical records of the deceased was enclosed with the dead body. The previous owner who did some autopsy on it didn’t put the screws back, so covers were already off.
As always these things needs to be cleaned first, as they have mud and wild animals inside most of them. After exhaustive cleaning, the next step was to do the visual inspection.
Found bunch of burned resistors, capacitors – that too ceramics – totally gone up in flames. Some part of the board was looking similar to the aftermath of a wild fire. PCB traces were burned out.
No schematic was available for this specific model, however I could trace one out for same series (Non-E) with schematics in it. So the adventure starts.
::WARNING:: DANGER ::
THIS IS A DEADLY INSTRUMENT WITH UP TO 600V + DC INSIDE, WHICH IS CAPABLE OF DELIVERING STRONG CURRENT.
This instrument contains mains power rectified and doubled up to 350V DC with capacitor bank holding it. Lowest current limiting is by a 2A Fuse, it takes only 200mA to knock you off your life. You will be long gone before the fuse is burned out. I repeat, do not poke around unless you really know what you are in to. This thing can KILL YOU.
DO NOT ATTEMPT TO REPAIR/OPEN/OPERATE/FOLLOW UNLESS YOU REALLY KNOW WHAT YOU ARE GETTING IN TO. THERE ARE DEADLY VOLTAGES INVOLVED AND IT CAN KILL YOU INSTANTLY
THERE ARE BETTER WAYS TO DIE IF YOU ARE UP TO THAT.
Even if you are familiar with electronics, be very careful when you work on something like this.
Ensure you use professional instruments with proper protection, probes and insulation. Always remember to watch what and where you connect your probes to , specially oscilloscopes, as they are grounded on the shield of the probe. ISOLATION transformer is highly recommended. Also, as old Tektronix manual says do not work on this when you are alone, ensure someone else is at home or around you when you poke around to make the 911 call if needed.
{ Duplicate warning messages are intentional }
Here is the block diagram of the instrument from manual (user). The same manual explains the schematic without the schematic diagram, however you can use the component labels on board to trace the components. However I could refer the old schematic and decode most of it. The part identifiers are different, but they are more or less the same.
The block diagram is explained in the manual, I am not repeating it. Very standard switched power supply. Here is the layout of the same on the board.
NOTE: OVP shutdown MOSFET Q17 is not present in the photo as I removed it from my board, it was bad. Replacement is in order. It is not required for normal operation
This photo was taken after I fixed it, so you will see replacement components sticking out.
I spent the next couple of days decoding the schematic and identifying parts on board, and try to find out the delta between the model in the schematic and one I have. Fortunately, there is not much difference. The extra components I could trace on the board and identify their position and purpose in the new design. Here is the map of the old schematic.
Rule #1 while working on these instruments is to identify what failed, most important – WHY – before you power it up.
So doing the visual investigation around the board, I discovered.
1. R93 – The shunt resistor for over voltage protection is grilled to “rare”. It was not “well done”, so was still working.
2. Natural suspect, the driver MOSFET for the same – Q17 is dead 3N100, 1000V MOSFET.
3. R83,which drives Q2A, which in turns drives Q17 is burned out.
So till now, it looks like over voltage protection kicked in and burned things out in the control circuit, without shutting down PWM. Strange, OR Q17 failed and caused rest of the damage. So decided to pull Q17 out. Replaced R83 of 240Ω. Q2A tested good. Here is the view in the schematic.
4. R141 C38 and C86 are burned. I could not find some of these in the old model’s schematic but I could figure out what they are, to be explained later
5. Traces from the current sense shut resistor (kelvin sense resistor with 4 leads) R91 was burned out. You can see below that I rewired it on the board.
6. Both the fuses and the breaker were fine.
So symptoms 4-5 looks like something is bad in current sense circuit.
Spotted the problem right away, the current sense resistor was open. So this explains rest of the issues. While on load the current sense R91 went kaput sending a high voltage to the sense line, burning things out.
So as you can see above I rewired the burned traces and replaced burned caps. The new resistor (in this model) R141 and C86 are in parallel and is wired like frequency compensation for the current sense input and main output of the current sense resistor R91.
Here is the view of these new components in the schematic, as well as the path of damage.
Now to fixing thigns up. I have replaced all the burned components. The main current sense resistor, I opened it up and re soldered the broken leg. It is a .1Ω/7W Kelvin sense resistor.
Now to start testing things out.
First step is to isolate and remove the DC to main switching transistors. Plan is to check the control circuit and confirm its working before putting switching voltage. You dont wanna blow things up with full voltage. So removed F3 from the board. This disconnects main DC to switching transistors.
{ Remember only one leg is disconnected, -ve is still active and connected}
Since parts of the board was burned out due to over voltage, I wanted to check all op-amps to avoid issues later. So I made the simplest possible inverting amplifier circuit to check every op-amp in the board. Here it is for your reference.
This is an amp with gain of 10, typical circuit you learn in high school. Apply say 50mV input AC from a signal generator and you should see 500mV on the output. Ensure you use dual rail to power the op-Amp (+ 12 and -12 for example) and signal generator, power supplies and oscilloscope share the ground.
So for me all op-amps (surprisingly) tested good.
Now powering this unit up with variac, and isolation transformer should give you a click after a second or so @ around 80V input. The click is from main relay K1. The delay is to avoid inrush during power on where the main caps charge through a resistor till the relay clicks. You can measure the rectified DC voltage at F3 Terminal. This will confirm the main filter, Inrush ckt, LV (aux) power supply and associated components are good.
Also verified the auxiliary power supply by measuring the voltage across the op-amps power pins and I found all required low voltages were up to spec. Rectified DC was at 340V on the Fuse 3 terminal. All looks good till now.
The main PWN controller for the SMPS is IC 3846, you can check if it is alive by checking the sync and saw tooth signals on the chip pin 8 and 10. Example here directly from datasheet. 
I verified the same and it was alive.
Now I noticed the OVP indicator – aka Over voltage Protection indicator – is lit in the main panel. This means the SMPS or PWM controller U3846 is shut. If you refer the schematic they drive the over voltage protection signal to pin 16 of 3846 to shut it down. I verified that is the case here. No PWM drive output as chip was shutdown.
Basic op-amp troubleshooting, check both the input and verify the output. LM358 is in use here. 2nd op-amp in the chip generates OVP signal. Pin 5 is the sense line from output and pin 6 is reference from front panel.
Here the issue was very simple, someone got frustrated and turned the OVP control all the way down, that it wont let the unit power up by flagging OVP. I turned the front panel control the other way around and OVP indicator went away.
Now to the next step, to see if its alive, if its heart is beating. Pins 11 and 14 are the pulse output from the chip. So simple probing with oscilloscope is all needed. Remember to ground the scope to the auxiliary power supply ground reference.
Turn the voltage output knob clock wise by a turn or two, so do current control knob and I could see the chip was driving PWM signals to the driver transistors.
So the unit all the way till the drivers are working. Fuse F3 is still out,
Now to next step. Checked all main MOSFETs with multi-meter and confirm none are short. It does not appear to be as the fuses were intact.
There is a trick to check the entire system without applying full voltage. You need to ensure that you are on isolation transformer and you have DC power supply which has got floating outputs.
Here is how it works, since Fuse F3 is out, you could insert a small DC voltage, say like 24 volts or so in to the system and see if you can record any voltage in the main output. Remember this is dangerous if you dont have isolation transformer and floating power supply.
I did notice a small output voltage in the main output and varying width of PWM signal as I turned the voltage POT on.
So all looks good till now, so time to reinstall the fuse F3 and go full throttle. Power off the unit and wait a minute or so for the main caps to discharge. Then I reinstalled the F3 fuse and powered up, with one hand still on power switch and here is the result.
It was wait and watch for next few minutes to see if anything is heating up, or smoking. All looked good for several minutes. so it was time to try powering up and cranking the voltage all the way to the top
Since I am still alive and my house is not burning down, I decided to go ahead and test (burn) few resistors to check loading, and it all turned out to be good. It does deliver the juice 🙂
Its all done for this little puppy, and is gonna join my Heathkit 2717, Fluke 407 HV power supplies, making this the 3rd HV PS I restored.
Back to Home page for more restoration/Repair Projects
