Side project while waiting for outdoor air to become fit for human life - restore a Power Designs 2005P Power Supply - Part I

 CALIFORNIA FIRES AND AIR QUALITY

The many wildfires, most started by lightning during one day of thunderstorms, have been joined by fires up north of California and a few new blazes started later, to produce staggering amounts of smoke and ash. While I am safely away from any fire danger, the air quality has had very large quantities of 2.5 micron or finer particles that are quite unhealthy. 

TERMINAL PROJECT WORK REQUIRES OUTSIDE EXPOSURE

I have set up tables with the various 317x terminals, the 3174 controller and the other components for my main project driving the green and color screens from MVS running on PC based Hercules and a P390 mainframe. The bad air has been an impediment to further testing, thus I have been spending my time in alternate projects and reading.

RESTORATION OF POWER DESIGNS 2005P POWER SUPPLY

I had bought a 2005P power supply from eBay because it is a highly accurate supply with an internal oven to ensure excellent regulation and accuracy of the voltage produced. The model I bought is remotely programmed, which simply means that an external resistance is used to set the voltage. It produces 1V for every 1000 ohms of resistance across its 'programming' terminals.

I have a resistance substitution board that produces essentially any resistance I want from 1 ohm to 11.1 MOhm by setting switches, which when connected to the power supply programming terminals will let me produce voltages from .001 to the full 20V capability of the supply. It also supports current regulation, thus will be a good lab tool when used for experiments where controlled targeted voltages are important. 

There are some adjustments that I can't make with the extreme range of the calibrating pots, but that I something I can deal with by adjusting some internal resistances and pots. The more serious issue I detected is that the oven is not heating up. Any time the supply is plugged in, even if the switch is off, it should warm the oven and maintain its  temperature. 

The amplification and regulation components are set inside an oven can where a heater is thermostatically operated to establish a narrow range of fixed temperatures for the remaining parts. Operation of the heater is shown with a neon indicator bulb on the supply faceplate, but it remained dark.

Further, I checked the terminals on the oven that feed the heater inside and found zero voltage on the pins. Checking the pins for the thermostat, I found it stuck open when it should be closed at room temperature. I therefore have to disassemble the oven, find the thermostat and attempt to fix it. If I can't fix it I will need to replace it with a comparable unit. 

Oven with components inside

Oven desoldered and removed from the main turret board

Following the instructions got me to the point where I can see the board with all the amplifier components mounted on it but when I grab it and try to pull it out it isn't moving. Moreover, the diagrams of the board don't have the thermostat (or heater) on them so they may be embedded inside the base of this can. I did have to pick out quite a bit of glass wool that is the insulation inside. 

Amplifier components on small turret board inside overn

I suspect the thermostat is inaccessibly embedded in this nylon base

While there is no definitive spec for the temperature inside the can, several others who have restored these units report that it activates at 70C. Some have chosen a lower set point of 50C, claiming the regulation is just as good and they believe the lifetime of the components would be enhanced. 

I shopped for a 70C thermostat. There must be room inside to mount it and a way to connect the wires to the pin on the base, or a way to route the wires out without compromising the heat seal of the oven. Putting it inside the can won't be a problem, I believe, but the wiring is challenging. 

70C Thermostat

My plan was to drill two small holes in the base for a snug fit and epoxy the thermostat wires in place. The body of my new thermostat is metal thus I had to ensure that it was fully insulated from the components on the small turret board. Fortunately it comes with a plastic cover.

Drilling from the bottom was too risky, since I can't see inside the nylon base. My plan B is to drill holes in the top metal cap to allow the thermostat wires to protrude, then bend them 90 degrees and route them out a hole in the top of the outer metal can. This won't be ideal cosmetically but should minimize additional heat loss and most importantly, work properly.