Simard PD-250 Notes ----------------------- Initial Rev. 14-Feb-2022 Current Rev. 2-Aug-2022 This file is notes about the Simard PD-250 power supplies for the sputtering system in Rm B148. I assume that PD stand for Plasma Discharge. I don't think that Simard has existed as a company since perhaps 1990. I think that most of this sputtering system is from the mid 1980s. We have 3 versions of this Plasma Discharge Power Supply: 20, 200, and 250. It is believed that the model 200 and 250 supplies are much like each other and that the older model 20 supplies have major differences. The Plasma Discharge supplies contain the Filament supply ( 0-10 VAC at up to 40 Amps) and the Anode supply ( 0-100 VDC at up to 10 Amps). I believe that we have a complete print set for the model 200 supplies, that is a print of the main chassis power wiring, and prints for the other internal circuit boards, e.g. Control Brd, Interconnect "F" Brd, and the Front Panel Brd. For the model 20 we have only the main chassis power wiring print. For the model TS-2 Target supplies used on all 4 of the big triode guns we also have only the main chassis power wiring print and have no information about its internal circuit boards. The layout and construction of the PD-20 supplies is quite different from the PD-200 / PD-250 supplies. This may fit with the PD-20 supplies actually being labeled, "Eratron" from Campbell, CA where as the PD-200 / PD-250s are labeled, "Simard" from Santa Barbara, CA. The TS-2 supplies are also labeled, "Simard" from Santa Barbara, CA. I have found no instruction book at all for these supplies. I can find nothing useful about Simard sputtering systems on the web. Reza wrote a description of how these supplies connect to the guns - there was no documentation about this before he wrote a note about it. Operating Modes of the PD-200 / PD-250 Filament & Anode Supplies: ------------------------------------------- As built the PD-200 / PD-250 supplies had 4 different operating modes. These modes differ by Local Control vs Remote Control and by how the Anode Voltage is regulated or not. These modes are described below. It's also clear from some 1986 notes that these supplies have been modified to provide a special operating mode that I believe picks just a subset of the parameters for being Remote Controlled the other parameters remain under Local Control. The normal stock operating modes of the PD supplies are: 1. Local Control of both the Filament Current and the Anode Current, the Anode Voltage is not directly controlled or held at a set value. Both small toggle switches in the middle of the back panel are in their Up position and the back panel Auto Emission 10 turn pot has no effect. The front panel Filament Current 10t pot has control of the Filament Current and the Filament Current will regulate at its set value, i.e. the output voltage of the Filament supply will automatically go up or down to hold the set value of Filament Current. The front panel Anode Current 10t pot has control of the Anode Current and the Anode Current will regulate at its set value, i.e. the output voltage of the Anode supply will automatically go up or down to hold the set value of Anode Current. Note that the Anode Voltage is not directly controlled, i.e. it is not held constant. That is, the kinetic energy of the electrons that are trying to ionize the Argon atoms is not automatically regulated at a set value by the power supply itself. 2. Local Control of the Anode Current and the Anode Voltage is held at a set value by the supply automatically adjusting the temperature of the Filament to hold the voltage drop across the Filament-Anode Diode at this set value. The back panel's small left-hand toggle switch, confusingly labeled, "Off or Remote ON / Internal On", is in the Down position. The back panel's small right-hand toggle switch labeled, "Local / Remote" is in the Up "Local" position. The front panel Anode Current 10 turn pot has control of the Anode Current and the Anode Current will regulate at its set value. The rear panel Auto Emission 10 turn pot sets the value of the Anode Voltage. The Anode Voltage will regulate at its set value. The front panel Filament Current 10t pot does not have moment to moment control of the Filament Current but it does set the upper limit to which the Filament Current is allowed to rise. How does the Anode Supply magically control both its current and voltage outputs ? It does this by effectively controlling the value of its load resistor, i.e. the effective resistance of the Filament-Anode Diode load on the Anode supply. The effective resistance of the Filament-Anode Diode is controlled by controlling the temperature of the Diode's Filament. The Filament's temperature is controlled by adjusting the heating current flowing through the filament. 3. Remote Control of both the Filament Current and the Anode Current, the Anode Voltage is not directly controlled or held at a set value. The back panel's small left-hand toggle switch, confusingly labeled, "Off or Remote ON / Internal On", is in its Up position. The back panel's small right-hand toggle switch labeled, "Local / Remote" is in the Down "Remote" position. This mode is like the mode described in #1 above, except that two remotely supplied signals are used to set the operating points of the Filament Current and of the Anode Current. The remote signal supplied to J5-16 sets the regulation point of the Filament Current. The remote signal supplied to J5-18 sets the regulation point for the Anode Current. Both of these remote control voltages are referenced to J5 pin 19. The value of the Anode Voltage is neither directly controlled or regulated at a set point. 4. Remote Control of the Anode Current and of the Anode Voltage. This mode is just like the mode described in #2 above except that remote signals are used for control inputs. The remote signal supplied to J5-18 sets the regulation point for the Anode Current. The remote signal supplied to J5-14 sets the regulation point for for the Anode Voltage. The remote signal supplied to J5-16 sets the maximum value to which the Filament Current is allowed to rise. All of these control voltages are wrt J5-19. The Anode Voltage is held at its set point by the supply automatically adjusting the temperature of the Filament to hold the voltage drop across the Filament-Anode Diode to this value. To operate in this 4th mode set the rear panel right-hand toggle switch in its Down, i.e. Remote position and then either set the rear panel left-hand toggle switch in its Down "Internal On" position or else jumper together pins 1 and 19 in connector J5. Why is this all so confusing - it's because: - They use different names for the same thing, i.e. Emitter Current = Filament Current Plasma Current = Anode Current Plasma Voltage = Anode Voltage - The switch with the confusing name, "OFF (or Remote On) Internal On", should have the label, "Automatic or Manual Control of the Filament Emission". - All controls should have been on the front panel. - The rear panel 10 turn pot labeled "Auto Emission" should have been labeled "Anode Voltage". - They provided two ways to initiate Automatic Control of the Filament Emission, either by putting the rear panel switch, "OFF (or Remote On) Internal On", in its Down "Internal On" position or by jumpering pins 1 and 19 in connector J5. Special MSU Operating Mode: This special MSU mode is described in a note from Paul Dargan at Simard to Bill Pratt on 7-July-1986. It's quite simple. In a modified supply, putting the right-hand rear panel toggle switch in its Down "Remote" position only switches Anode Current to Remote Control. Anode Voltage and the Maximum Filament Current are still under the control of the Local 10 turn pots, i.e. the rear panel Auto Emission pot and the front panel Filament Current pot. The modification to the supply to implement this special MSU mode is accomplished by adding two jumpers to the rear panel right-hand "Local/Remote" switch. S401 pins 1 and 3 are jumpered together so that the front panel Filament Current pot remains in control when the switch is in the Remote position. S401 pins 7 and 9 are jumpered together so that the rear panel "Auto Emission" pot (aka Anode Voltage) remains in control when the switch is in the Remote position. Note that connector J5 pins 14 (Auto Emission) and 16 (Filament Current) must not have any connection to allow this special MSU mode to work correctly. Contemporary Mode of Operation in Rm B148 Sputtering Lab: - Both rear panel switches are Down, i.e. "Internally" enable automatic control of the Filament Heating Current to provide control of the Anode Voltage at a fixed value, and enable "Remote" control which in our case means remote control of only the Anode Current. - The remote control panel "Current" pots sets the regulation point for the Anode Current. - The rear panel "Auto Emission" pot set the regulation point for the Anode Voltage. Setting this pot to 5.8 will cause the supply to regulate at about 60 Volts of Anode Voltage. - The front panel "Emitter Current" pots sets the maximum value to which the Filament Current is allowed to rise to hold a constant Anode Voltage. Setting this pot to 8.5 will allow a maximum Filament Current of about 40 Amps. Remote Control Panel: --------------------- At some point someone built a remote control panel for the sputtering system so that an operator could run all 4 guns from a single control panel. I have no prints or any description of this remote control setup. It may be just 10-turn pots and digital panel meters - I do not know who made it - but I assume it was Barry Tigner and Bob Rains. This remote control panel is poorly labeled. What it controls is the: Anode Current and the Target Voltage. With the back panel switches on the Filament & Anode supplies in the positions that I've found them these supplies will try to keep the Anode Voltage at a constant value (set via the back panel 10 turn Auto Emission pot) by adjusting the heating current flowing through the Filament. The maximum value to which the heating current is allowed to rise is controlled by the front panel Filament 10 turn pot. A major question that remains about this Remote Control panel is to what extent are the "grounds" from the various power supplies kept isolated from each other. This is important because the "System Ground" from the PD-250 Filament & Anode supplies appears to float wrt the mechanical chassis ground by a value that is equal to about 1/2 the Anode voltage. This float comes from the way that the Control Board senses the Anode Voltage, i.e. with a divider that taps the Anode Voltage in the middle between its positive and negative output terminals. If Simard was expecting a generic remote control panel to consist of just 10 turn pots and analog meters, that both float wrt ground, then this scheme would have been OK. But Barry used digital panel meters that run from a common supply. In a test of the A and B Filament & Anode supply connections to this Remote Control panel the pin #19 "System Grounds" from the two supplies appear to be isolated in the Remote Control panel - but by how much ? Are these digital meters true differential input with a > 100 Volt common mode range ? Filament & Anode Supply Grounds: -------------------------------- Two big topics that are not currently understood about the Filament & Anode Supply grounds are: - which if any of the output terminals of the Filament & Anode supplies is connected to Chassis Ground, e.g. connected to the racks and vacuum chamber and general lab safety ground ? - In the PD-250 supplies, that do not have optical isolation of their Anode supple Voltage and Current Sense Signals, what is the potential of the Controller Board J3-3 "System Ground" with respect to the Chassis Ground ? Three points besides the human safety of this system that make the above questions important are: - There is a 5k Ohm resistor from the Anode positive output terminal to Chassis Ground. What is the full path of the current that flows through this resistor ? - There is a symmetric voltage divider on perf-board to sense the Voltage output of the Anode supply. This voltage divider is approximately: 70k Ohm to the positive Anode output terminal, 970 Ohm, 70k Ohm to the negative Anode output terminal. The upper connection to the 970 Ohm is the "Hi" input to the Anode Voltage Sense circuit. The lower connection to the 970 Ohm is the "Low input to the Anode Voltage Sense circuit and this point connects to this supplies "System Ground". - The above point means that the "System Ground" is floating somewhere between the Anode positive and negative terminal output potentials. For this to work everything must be floating well wrt the 70k Ohm divider resistance. For this to work the "System Grounds" from the 4 PD-x supplies must not be connected together at any point, e.g. in the Remote Control Box. Check for Grounds at the Gun Terminals: --------------------------------------- I checked the Filament, Anode, and Housings terminals on all 4 guns to measure the resistance from these terminals to ground and the resistance between the various terminals on a given gun. Here "ground" refers to the large metal vacuum chamber of the sputtering system. For now I did not look at the Target terminal. I did not want to disturb anything so these resistance measurements were done without unplugging the guns from their Filament & Anode "PD" supplies. Gun A has a PD-250 supply Guns B and C have a PD-200 supply each Gun D has a PD-20 supply with a separate igniter box Results: As expected, on all 4 guns their two Filament terminals are tied to each other with a low resistance. On all 4 guns the Filament terminals appear to be an open circuit wrt Anode, Housing, and ground. On all 4 guns the Anode has about a 5k Ohm resistance to ground. This 5k Ohms connection to ground is from a resistor inside the Filament & Anode power supply. On all 4 guns the Anode terminal appears to be an open circuit wrt Filament and Housing. On the A, C, and D guns the Housing terminal appears to have an open circuit wrt ground. On the B gun the Housing terminal appears to be directly connected to ground. Further investigation shows that this Housing to ground connection is inside the B gun and not in the Filament & Anode power supply that runs the B gun. On all 4 guns the Housing terminal appears to be an open circuit wrt Filament and Anode. Relationship of the Various Grounds and the Anode Voltage Sense Circuit: ------------------------------------ Filament Current Ramp Circuit: ------------------------------ The PD-250 supply contains a "Ramp Circuit" that at turn On time forces the Filament Current to start out at zero independent of the other inputs to the Filament Current control loop. This ramp is generated by a 100 uFd capacitor and a 470k Ohm pullup to +15 Volts. This is a 47 second time constant. The scale here is about 1 Volt ---> 10 Amps of Filament Current and this ramp will clear the 4 Volt --> 40 Amp point in about 20 seconds. So this ramp limits the Filament Current output of the supply for only about the first 20 seconds after turn On. Before turn On the 100 uFd ramp capacitor is clamped to zero volts by an N junction FET. When the supply is turned OFF, by its On/Off push button, the gate of this FET is grounded by a second pole in this front panel On/Off push button. When the supply is turned On by this On/Off push button the gate of this FET is pulled to -5V and the ramp capacitor can begin to charge up. The circuit path of this FET gate signal is very convoluted: Pin #1 of the front panel Power On/Off push button goes via a violet wire to an in-line 1k Ohm resistor and then via pcb trace to pin #10 of front panel connector J8 and then via a white wire in cable CA4 to the back of the power supply where this white wire connects to pin #10 of connector J4 on the Interconnect Brd "F" and then via trace to pin #1 in header J7 and then via a pink wire in cable CA3 back to the front of the power supply where this pink wire is spliced to a white wire which runs vertically up the right-hand side of the Controller Board to a single isolated pin that is near the top right-hand corner of that board and then via trace on the Controller Board to the gate of the clamp FET. Pin #2 of the front panel Power On/Off push button goes to "System Ground" on the Controller Board via: a green wire that makes a connection to another green wire on the back of a front panel meter and then via that green wire to a trace on the Front Panel Board that connects to pin #9 of Front Panel Brd connector J8 and then via a gray wire in cable CA4 to the back of the power supply where it reaches pin #9 of connector J4 on the Interconnect Brd and then via trace on that board to red wire in cable CA3 that runs back to the front of the power supply where it reaches pin #3 of connector J3 on the Controller Board which connects to the System Ground net on that board. Note that the System Ground pin #3 in connector J3 of the Controller Board is immediately adjacent to the 120 VAC connections to pins #1 and #2 in that connector. Controller Board Connector Pinouts: ----------------------------------- J1 8 pins Isolated Transformer Coupled Outputs: J1-1 Fire the Igniter Emitter Q403 Q404 Black J1-2 Fire the Igniter Base Drive Q403 Q404 Brown J1-4 Anode Supply SCR SM302 Cathode Red J1-5 Anode Supply SCR SM302 Gate Orange J1-7 Anode Supply SCR SM303 Cathode Yellow J1-8 Anode Supply SCR SM303 Gate Green J1-10 Filament Supply SCR SM301 Cathode 2 Blue J1-11 Filament Supply SCR SM301 Gate 2 Violet J1-13 Filament Supply SCR SM301 Cathode 1 Gray J1-14 Filament Supply SCR SM301 Gate 1 White J2 Connector now 6 pins original design was 8 pins voltage and current sensor inputs: J2-1 Red Input from the Filament Current Sense Transformer J2-2 Blk 60:5 Transformer parallel with 0.1 Ohm 417 mVAC --> 50 Amps Filament Current J2-3 Red Input from Anode Voltage Sense / Divider J2-5 Blk board Input is about 0.00694 of the actual Anode Voltage J2-4 Red Input from the Anode Current Sense Transformer J2-6 Blk 60:5 Transformer parallel with 0.51 Ohms J3 Connector 14 pins mixed inputs and outputs: J3-1 One side of 120 VAC line power J3-2 Other side of 120 VAC line power J3-3 System Ground J3-4 Auto/Manual signal Input from the Rear Panel Switch Ground --> Auto Open --> Manual J3-5 Auto Voltage Centering Output +-5 Volts output from U18 no documented connection J3-6 Filament Current Control Input from Local or Remote 10 turn pot 0 to -5 Volts for 50 Amp Fil Current J3-7 Line Lock Output Signal High when PLL is locked to an LED driver on the Interconnect F Board J3-8 Filament Current Sense Output direct load on U7 signal is wrt System Ground 1 V --> 10 Amps Fil Current J3-9 Anode Current Control Input from Local or Remote 10 turn pot 0 to -6.33 Volts for 12.66 Amp Anode Current J3-10 Auto Emission Control Input from Local or Remote 10 turn pot 0 to -5.47 Volts for 109.5 Volts Anode J3-11 Anode Current Sense Output direct load on U10 signal is wrt System Ground 1 V --> 2 Amps Anode Current J3-12 -15 VDC Output reference voltage Output for the Remote Control 10 turn pots J3-13 Anode Voltage Sense Output direct load on U10 signal is wrt System Ground 1 V --> 20 Volts Anode J3-14 Fire Igniter LED Driver Output High when Anode is > 120 Volts to an LED driver on the Interconnect F Board J5 Remote Connector on the Interconnect Board "F": -------------------------------------------------- Pin Function --- ------------------------------------------------------ 1 Input - when pulled Low, i.e. when jumpered to pin 19 this will cause relay K1 on the Controller Brd to pull in and will put the supply in the mode for Automatic control of the Filament Current to hold a constant Anode Voltage. Rear panel left-hand toggle switch in the Down "Internal On" position does the same things. 2 Output - "Auto Voltage Centering Output" this comes from the Controller Board opamp U17 output and indicates how far the actual Anode Voltage is from its set point. 3 Output - Goes Hi when the "Firing Circuit" is activated. It actually goes Hi when the Anode Voltage is above 120 volts. The "Firing Circuit" is the big relay that when activated ties the "Housing" to the Anode and disconnects the "Target" from its power supply. Open collector output with no pullup. Normally Low. 4. Output - Goes Low when the PLL on the Control Board is locked to the 60 Hz line power, i.e. the SCR firing circuit is ready to run. High otherwise. Open collector output. 5. Output -15 Volt DC source wrt pint 9 for use as a reference for the remote 10 turn control pots. 6 Output - Anode Voltage scale 1 Volt --> 20 Volts of Anode Voltage reference to pin 19 1k Ohm internal impedance 7 Output - Anode Current scale 1 Volt --> 2 Amps of Anode Current reference to pin 19 1k Ohm internal impedance 8 Output - Filament Current scale 1 Volt --> 10 Amps average of Filament current reference to pin 19 1k Ohm internal impedance 9:13 to J7 wire wrap pins the mapping is to be determined 14 Input - remote control of the Anode Voltage set point 1 Volt on pin 14 --> 20 Volts on the Anode reference to pin 19 15,17 Output - contact closure closed when the rear panel right-hand switch is in the down "Remote" position. 16 Input - remote control of the Filament Current set point 1 Volt on pin 16 --> 10 Amps of Filament Current reference to pin 19 18 Input - remote control of the Anode Current set point 1 Volt on pin 18 --> 2 Amps of Anode Current reference to pin 19 19 Signal Ground for analog inputs and outputs Has but a very weak connection back to the Main Controller circuit board signal ground. 20 Signal Ground for digital inputs and outputs Pin 20 is actually the exact same thing as pin 19. 21:24 to J6 wire wrap pins the mapping is to be determined 25 to J7 wire wrap pin the mapping is to be determined Aluminum Electrolytic Capacitors in the PD-250 Supplies: -------------------------------------------------------- The various low level control boards in these supplies use a significant number of low quality 85 deg C aluminum electrolytic capacitors now all 36 years old. The following table just tries to enumerate all of these capacitors. Main Controller Card: 1 uFd 50 V C36 * 10 uFd 35 V C25, C28, C45 * 22 uFd 25 V C26, C29 100 uFd 25 V C39, C40, C42 * 220 uFd 35 V C22, C24, C27, C43 * 470 uFd 16 V C23 * ---> The value of this capacitor controls a system timing function Both 22 uFd capacitors could also be 10 uFd with no change to the system operation. All of these caps could/should be moved up to 50 V especially: C23, C39, C40. Note that the main controller boards on our PD-250 supplies are quite different from the print set for the PD-200 supplies, i.e. the only main controller board print set that we have, e.g. the whole isolated input section for the Anode Voltage and Current Sense function has gone away. Front Panel Circuit Board: 10 uFd 35 V C201 this filters the -15 V reference for the 10 turn pots and could be a larger value if needed to minimize part types Interconnect Board "F": 10 uFd 16 V C405 100 uFd 35 V C401, C402, C404 *, 220 uFd 35 V C406 * 250 uFd 50 V C403 * ---> The value of this capacitor controls a system timing function All capacitors could be moved up to 50 V to minimize the number of part types. Filtering of the Anode supply uses a large inductor. There are no dangerous large energy storage capacitors in either the Filament or Anode supplies. Replacement Parts Used in the PD-250 Supplies: ---------------------------------------------- A number of replacement parts were needed to get all of the Simard PD-250 supplies running again. In some cases the original part from many years ago was not still available. This is a list of some of the replacement parts that were used. Anode Volt Meter on the front panel: Newark Part Num: 55F5373 Simpson Part Num: 17533 Description: Analog Panel Meter, Black Knife-Edge Pointer, DC Voltage, 0V to 150V Approximate Price: $ 110.16 each This meter has the appropriate 0-150 Volt scale and the "basic" meter movement is correct, i.e. 0-1 mA FS but as sold this meter contains an internal "multiplier" resistor (about 150k Ohm) and this resistor must be removed for this meter to work correctly in the Simard circuit (which needs just the basic 0-1 mA movement). Relay K1 on the main Controller Circuit Board: Digi-Key Part Num: 306-1287-ND Coto Part Num: 7141-12-1011 Description: RELAY REED SPDT 250MA 12V Approximate Price: $ 17.80 each The original part was made by Gardian, their part number A410-366183-12. Gardian no longer makes anything even close to this and I could not find anything to match the pcb layout pattern. This Gardian part was also a very poor choice for Simard to have used - it has large 5 Amp silver contacts and is being used in a dry circuit and Simard did not include a flyback diode. The replacement reed relay is mounted on a small piece of perf board a short direct wires are used to connect it to the pcb. Relay K405 the Igniter Relay at the back of the supply: Newark Part Num: 24F1433 Struthers Part Num: PM-17AY-120 Description: Power Relay, 4PDT, 120 VAC, 35 A, PM Series, Socket, AC Approximate Price: $ 115.45 each This was originally a standard Potter & Brumfield relay a 4 pole with part number PM-17AY-120. I do not know if Potter & Brumfield still exists but this part from China by a company called Strthers is the only available replacement. Be careful of the flying flex leads that they do not get hung up on other parts in the supply. Simard uses 24V DC to power the coil on this 120 VAC relay. This appears to work OK and they may do this to help this relay drop out as fast as possible. The TIP transistors that switch this relay contain the required flyback diodes. Switching an amp or two of DC at up to 1000 volts gives the contacts on this relay zero chance of a long life. Note that Simard originally used a 2 pole version of this relay: Potter & Brumfield part number PRD-11AG0-120 but seems to have switched to the 4 pole version using 2 poles in series for each circuit I assume to try to control the arcing. SCR Diode Combinations in a "power pack" TO-240AA Crydom made the original parts used by Simard. Crydom seems to be out of this business now so their parts like F1857DK1200 are not available. Companies like: IXYS MCC56-12101B, MCC56-12108B, MCD56-12101B, MCD56-12108B appear to be a replacement. Note that the "10" in these part numbers may be an "io". Also Infenion must make a replacement. Transformer T303 240 VAC to 24 VAC with a Split Primary: Digi-Key Part Num: 595-1894-ND Signal Trans Part Num: CL2-80-24 Description: PWR XFMR LAMINATED 80VA CHAS MT Approximate Price: $ 30.54 each Note that the Split Primary of this transformer is necessary to supply the 110 VAC power that is used by things like the fan and the supply to the main control board. This replacement from Signal Transformer works OK both physically and electrically for the custom part that Simard used. Gun Voltage Measurement During Normal Operation (1-Aug-2022): ------------------------------------------------------------- When Bob finished his work today putting top leads onto a sample he restarted the Niobium gun, gun C, so that I could make some measurements right at the gun terminals during normal gun operating conditions. Gun C uses a PD-200 Filament-Anode Power Supply, i.e. the type for which we have an official Simard print set and the type that has a real optically isolated measurement of the Anode Voltage and Current. I recorded the following readings: Remote Control Box Meters: 601 Volts Target Voltage 0.60 Amps Target Current TS-2 Target Supply Meters: 600 Volts Target Voltage 0.60 Amps Target Current PD-200 Fil-Anode Supply: 26 Amps Filament Heating Current 45 Volts Anode aka Plasma Voltage 3.2 Amps Anode aka Plasma Current Filament wrt Ground: -32.7 VDC (and there must be an AC component) Housing wrt Ground: -29.3 VDC Anode wrt Ground: +11.5 VDC Target wrt Ground: -598 VDC So as the cable tracing indicated, the digital meters on the Remote Control Box indicate the Target Voltage and Current. There is still the big question - do the connections within the Remote Control box (e.g. one power supply for all 8 digital meters) force the "signal commons" of the 4 controller boards in the 4 PD-200/250 power supplies to be all tied together and if so how does that effect the not truly isolated measurement of Anode Voltage in the PD-250 supplies. It's seen that the Filament is negative wrt ground and that the Anode is positive wrt ground. What determines this balance point when operating with the fully isolated PD-200 power supply ? The positive wrt ground Anode output of the PD-200/250 power supply is tied to ground via a 5k Ohm resistor inside of the supply. With the Anode operating at about +11.5 Volts wrt ground this resistor must be carrying about 2.3 mA of current. This resistor will tend to force the Filament terminals to be negative wrt ground by the full value of the Filament to Anode voltage. But the Filament being negative wrt ground will allow a space charge current to flow from the Filament to grounded parts of the gun in the vicinity of the Filament. This current from the Filament to grounded parts of the gun will tent to resist allowing the Filament to be negative wrt ground. A balance must be reached with a few mAmps of current through the 5k Ohm resistor and a few mAmps of current from the Filament to grounded parts of the gun's structure. Perhaps Simard wanted this balance, i.e. Filament somewhat negative wrt ground and Anode somewhat positive wrt ground, so that the electronics were more or less neutral wrt ground in the vicinity where they ionize the Argon atoms.