DIY PS503A Rebuild Prototype

 This is the start of a DIY rebuild project for the Tektronix PS503A dual tracking power supply.

I once build this unit a long time ago, in the late 70's, using several original parts.

This version of the PS503A is a little strange because I can't find any information on the net about this particular unit, that uses 14-pin Fairchild 741 amplifiers, and I don't have the manual anymore. All the pictures that I could find use the more common 8-pin versions. In any case, I sold it to a collector together with all my Tektronix gear quite some time ago. Overview here.

An attempt to create my own replacement many years later is described on my Blog here.

I'm showing this instrument because my plan is to use the same enclosure and the controls, but with a new PCB front panel. The PS503A does not have a display, but I'm going to use this one. After I build it, I made several changes to the way the display is used, hence the handwriting on the panel.  It is actually an accurate voltmeter with an LCD. I use a switch to show either the positive or the negative voltage. The switch above with the +/-V does that. The switch labeled Tracking will tie the two supplies together such that the voltage set for the positive supply will be tracked by the negative supply. This Master-Slave method is quite common, but the Tek engineers figured out a way to select separate rail voltages and use the tracking to change the set voltages by a percentage. So you can have a rail for +15V and one for -8V and change both voltages together. That's what I will use too.

Unfortunately, this supply, that started as a study on using the standard LM317 and LM337 regulators works OK, but not great. The main drawback is that I used a transformer that only allows +/- 100mA and that's not enough for many of my projects.

The goals for this DIY PS503A project

The goal is to build a standalone unit that can be placed close to a prototype to supply the required rail voltages. My intention is to use this supply when I'm working with Opamps that need a positive and a negative rail. 

The rather special feature of the PS503A is that it not only had dual tracking, but also with two different voltages. The dual tracking in this case will supply a percentage change on both rails. More information can be found on the Tek Wiki pages here. 

The SG503A goes up to +/-20V and 1A, and I'm planning to see if I can increase the voltage to +/-30V, probably not with a 1A output, but maybe half of that above 20V.

Another slight draw-back of this design is that the Current limiter only works on one rail. I would like to see if I can add circuitry to limit both rails if one of them gets triggered.

The next item I would like to address is the over-voltage protection. The original over-voltage protection for the PS503A uses a Zener with an SCR to cut the output if it is above about 27V.  In earlier versions, like the one I had, the SCR would actually short the raw supply rails with a small resistor to the common such that it would blow a fuse. That's pretty dramatic, and that method is called a crow-bar circuit. In later versions of the supply however, the SCR now cuts the drive to the output transistors, and will discharge the output capacitor. A clever trick also protects the DUT in case the power transistors fail. And again, also this circuit works on only one rail.

There are several components that do not like a dramatic disappearance of one of the rails, it could actually damage the chip. So my plan is to see if I can couple the fault mechanism to both rails at the same time.

I hope I'm smart enough to figure this out, or I may try to get some help, because this old tinkerer has it's limits.

Building a simple prototype

To better understand the circuitry and try some of the enhancements or changes, I built the positive supply section and the voltage reference section on protoboard. I did that in several steps to test it out before adding more complexity.

Here is the complete positive supply section, working and functional with some improvements already. It does not have the over voltage protection. That was added later.

What you see here on the left is the original voltage reference section with an alternative. Then the dual 10V references (for the +/- supplies) followed by the voltage controller and the output section. The red test lead in the middle is connected to the output terminal, which is the right side of the green 0.6 Ohm current sense resistor. To the left of that are the two output transistors. On the top right edge of the first board you can also see a device on a carrier which is the new current source.

The circuit on the protoboard on the right is the current controller. The red LED is lit, because I adjusted the output current with my Dynamic load and dialed in the tripping point of the Current Limiter, such that the supply is now in the Constant Current mode. The original PS503A uses a little incandescent light bulb (18V 26mA) to show an indication of the output level voltage. I changed that circuit slightly to use a yellow LED, and you see that lit on the right hand side. More about that below.

Improvements

As a reference to what I did and will be doing, you can use the original schematic, published by Tektronix of the latest revision.

This can also be found here, when you look at the manual on the right hand side of the page. 

The quality is not great, because it is a scanned copy of the manual.
Here is the positive supply section I prototyped and will be addressing below:

The reference supply

While I was building and testing the prototype, I noticed a few items that I wanted to address. First of all, I found that the reference supply is not as stable or resistant to sudden temperature changes, although it has a form of compensation built in. The engineers used a common trick by using a diode (CR24) in combination with a Zener (VR24) to counter the voltage change due to temperature. (one has a positive coefficient, and the other has a negative effect) However, that does not work well with sudden changes, like a draft. It also takes quite some time to warm-up to a stable reference voltage. When I used my DMM on the reference voltage and briefly blew some air over it, the voltage jumped before it slowly normalized again.

Granted, this will not happen this drastic when everything is warmed-up and inside an enclosure.

But, it's also not as stable as I would like to have it (see below), so I tried a voltage reference chip that the engineers at the time most likely didn't have access to, or it was deemed too expensive, or deemed to be too good for the intended applications for this supply. I will most likely make both options available on the PCB, if there is enough room.

Here is the schematic (in KiCad) for the VRef and Tracking circuit. On the left in the box the original voltage reference circuit and in the smaller box the new one.

The Vref of 9V, or in my case 10V is fed to the two Opamps, in a configuration where one is a voltage follower with the same output as the input, U1, to provide +10V for the posive supply, and U2 that changes the polarity to a -10V reference voltage for the negative supply. By flipping switch SW1, the reference voltage is now made variable and that sets up the dual tracking feature. The two trimmers are used to calibrate the output of the supplies to the maximum output voltage, plus a bit.

Here is the measurement that I took from the original reference voltage output.

This is after a warm-up period of about 15 minutes. It takes a very long time to settle. Note the span of 14.3mV over this 1hr period. This is not all together bad, but it also means that the output will swing with it, amplified by a factor of two. (10V reference amplified to get a 20V output)

The real test is to measure the output of the supply and include all components that could contribute.

Here is how that looks for a small snap-shot:

I used a REF01 10V reference chip to replace the original 9V reference, and this is the result measuring the output again:

The swings look much more dramatic, but the span is only 3.8mV, and this was after a rather short warm-up with a reference that did not have a burn-in period yet.

This needs to be repeated with the circuits on a proper PCB and within an enclosure, but the result is good enough to include both as an option.

Lets have a look at the various building blocks of this instrument.

The positive supply

This schematic already shows my latest changes in it, explained next.

Please note that this is still under development so can easily change. I will do my best to update the schematics as I'm progressing, but don't use them yet to build your own, give me some time to complete it first.

The improvements I already made

The Opamps

Instead of the 741 Opamps that were used in the earlier generations, I selected the TLE2141 Opamps. Mainly because they can stand a 44V supply maximum, which makes it more reliable than the originals that went kaduk quite often. Even so much so that lower (+/-27V) rails were added to the design, and even that did not stop all the failures. This could also be contributed to the fact that the early 741's were not that reliable stemming from earlier manufacturing processes. I could have use the TL071, but that also has a rather limited maximum supply voltage specification. As a minimum, during the prototyping phase, I will be using the more rail tolerant TLE2141. It also allows me to raise the +/-6V2 voltages to +/-8V by using regulators.

The current source/bias for the diode OR and output transistor

This step is a pre-cursor requirement for increasing the supply voltage.

If you look at the original schematic, I replaced the bias setting resistor (R87) to the Base of the driving transistor Q14 from a 3K resistor to a 5mA Current Sink. This will make the diode OR circuit and Base current independent of the raw supply and the variable current flow when the output voltage is changed from the minimum to the maximum. In the 20V version, that point moves from 9V to about 25V, and hence the Base current changes for Q14.

The just about standard method is to use a current source and not a resistor. In my previous design for a power supply, I used a typical circuit to create a stable current source. 

Below is that circuit. It used a red LED as a rather stable voltage source to bias the PNP transistor Q2. The Emitter resistor R4 sets the current, R15 sets the LED current. The capacitor of 220uF was added as a startup glitch protection, because it delayed the bias setting until the other supplies where stable.

In that supply design, I used LED's as the diode OR because they were used on the front panel showing the CV or CC mode activity.

There are many other possibilities to create a current sink/source, like a J-FET but this time I opted to use a dedicated LTC chip (LT3092) and two resistors to precisely set the current. I already have two of them in my stash, waiting to be used. I tried it out and it works really well. By using the formula in the datasheet, I could easily select the resistor value for the 5 mA current I wanted. This is a little higher than the original current that varied between 1.7mA and 4.9mA using the resistor, but that is not very critical.

The Vref for the current limiter

Because I'm now using a different VRef circuit, I needed to change the Voltage reference for the current limit circuit. In the original, that voltage was tapped from the VRef circuit (connection between R28 and R29), and then fed through a diode (CR78) to the base of Q80. I simply adopted the circuit from the negative supply and used that for the positive supply as well. This adds R41, Q11 and R40 in my schematic.

The Voltage output indicator

Because I don't want to use a light bulb for the Voltage indicator (old school), I'm using an LED that many users of the PS503A had to change to as well because these bulbs are hard to get. I opted for a  yellow LED, and that required a value change for two resistors, R14 from 5K6 to 15K and R15 from 510R to 150K in my schematic. Both values depend on the LED current, so may need some tweaking. I'm simply using one that I already have, but is without specifications. In my case, the LED is very dim but still visible with a zero Volt output and will be bright at 20V. When the output is switched off with SW2, the current from R15 is disconnected and the LED is off to show you that there is no output voltage present. Just like the original.

Decoupling the Opamps

I also added decoupling capacitors for the two Opamps.

Sziklai output transistor pair

The last change, for the time being, is the change of the Sziklai output transistor pair. In the original, the power transistor was in the TMXXX mainframe and is not so easy to find. I selected a combination of the BD139 and the D45H11. I had a few left from my Walt Jung voltage regulator design, and if Walt found this transistor good enough, who am I to disagree? This selection may change however when I have a real setup that will allow me to do some power stress tests later on. 

If you want to know more about this Sziklai configuration and the difference or pro & cons compared with a Darlington pair, use Google.

There are two main differences that I care about. First of all the input voltage for the first transistor is 0.6V for the Sziklai and double that or 1.2V for the Darlington combo. The other difference is the thermal stability that is reportedly better with the Sziklai, and the two transistors do not need to be on the same heat sink, which was important for the original PS503A with the output transistor located in the TM5XXX mainframe.

Resistor value changes

Although I purchased all the required (odd) resistor values, I will probably change them to the E96 range. The reason Tek used these sometimes odd ball values, like 1K05 or 1K62 and 976 Ohm is that in those days, through-hole metal film 1% or better resistors only came in these values (not even in E192). 

I started to make some changes already, but want to be careful not to make mistakes.

The over voltage protection

I waited to describe this circuit because I wanted to test and understand it first and also see if I could improve on it.

After adding the circuit to my prototype I finally figured it out. It turns out that I totally overlooked the presence and functionality of CR66 in that original circuit. (D29 below)

Let me explain it with this circuit section in the box.

The output voltage is tapped by R13 and the 24V Zener D28 that go to the Gate of the SCR. That voltage is filtered by C47, and R42 pulls the Gate low to avoid false triggers. 

So, if there is a voltage on the output that exceeds the Zener voltage, the Gate of the SCR gets pulled-up and is triggered. When that happens, the Anode of the SCR will be shorted to the Cathode at GND level, pulling the Bias of the diode OR also to GND and thereby removing the drive to the Base of Q14 and that in turn turns off the output transistor Q13. So far so good.

However there is also the 50uF output capacitor C2 that still dumps it's capacity into the DUT. The Cathode of D29, which function I initially overlooked, gets pulled down and conducts and that shortens the output through D29 and the SCR to GND. Great solution, however, there is even more to it. 

In case the output transistors malfunction, like an E-C short of Q13 which will be the cause of the over voltage, they will no longer react to the removal of the Bias to the Base of Q14. D19 will come to the rescue again and will also short the unregulated 33V rail to GND and that will blow the main fuse. 

 

So this clever circuit takes care of all possible over voltage causes and protects the DUT.

Hats off to the Tek engineers, they are much more clever than I first anticipated, and I just didn't get it at first. At a later stage I'm going to try to couple both protections together, so if one fails, both outputs will be removed.

The negative supply

This supply is almost a mirror image of the positive supply.

Note that I copied the current limiter reference voltage circuit around Q10, D21 and Q4 and also use that on the positive supply, because I use a different main voltage reference. C18, on the Base of Q9 is not in the original circuit, but I've added it here too, just in case it is needed.

The Power rails

There are actually three different dual rail voltages, the unregulated +/-33V, the regulated +/-27V and the regulated +/- 6V2. Regulated means simply set by a Zener diode. The original for the +/- 6V2 rails used large 750 Ohm 2W resistors, but I don't like that at all so I'm simply using 8V regulators. 

Depending on how I'm going to implement the over-voltage protection, I may have to change the PTC fuses back to glass fast blow fuses.

I have also not selected the main transformer yet. I have two versions that I will try, but they also have to fit in the enclosure, together with heat sinks for the power transistors. That decision will be made soon, because it will dictate the PCB I'm going to design next.

I added a second main filter capacitor of 1,000uF separated by a small 2 Ohm series resistor to better filter the unregulated raw rail voltages. This will become more important when I increase the output voltages with as high a current as possible at the maximum output of hopefully +/-30V. Both the resistor value and the 1,000uF size can change based on actual stress tests.

The circuit around Q5 and Q6 is a startup protection circuit to make sure everything powers-up correctly and at the same time. It also functions as further protection if one of unregulated rails have a fault.

The resistor values for the Sobel networks on the secondaries of the transformer will be determined when I have selected the main transformer.

The transistors used in the +/- 27V rails may change once I know the total current consumption, which is unknown at the moment.

Note that there are several different GND symbols used in the circuits indicated by a number. Eventually, they all come together at the Common output connection, but are kept separate from each other to create a "Star" GND. You can easily see that when you study the PCB pictures on the Wiki site.

The Voltmeter

This circuit is quite simple but in order to switch the display voltmeter inputs around so we can measure the positive and the negative supplies, the power rails for the meter have to be galvanically isolated from the rest of the circuits, hence the small additional transformer. The minus input from the voltmeter is internally connected to the minus supply voltage of the meter/display, so that's why this is needed.

Next steps

Before I can start to work on a PCB layout, I want to invesigate the coupling of the protection circuit, and also see if I can make it "tracking" so it will not just watch out for a +/-27V overvoltage, but a certain voltage delta above the actual voltage setting.

The other improvement I want to investigate is if I can couple the current limiters together if there is a fault, like a short.

More later....

Github project repository

There will be a dedicated Github project when I have finished the design, that will have all the information you need to build one yourself and I will also enter the project in the Shared Project section of my sponsor PCBWay.