Saturday, October 3, 2020

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


I went through the power supply, tracing out, beeping and and measuring components to determine what was different between this 2005P supply and its cousin the 2005 for which I had schematics. In fact it was a very minor set of changes. 

I took the schematic for the 2005 and erased the extraneous gear and made the corrections. Now I have a schematic of my unit. I posted this as the prior blog post for anyone who has this unit and is looking for the drawing.

Portion of 2005P schematic covering adjustment

As is shown above, the Programming Constant trim pot is R21 and it is wired to deliver 0 to 200 ohms of resistance from one end of the range to the other. Above it, there is a parallel set of resistors, one fixed at 5200 ohms and the other (R CAL) chosen to make the effective resistance correct for operation.

In my unit, the value of R CAL is 100K such that the equivalent value of the parallel resistors is 4,925K. Since the circuit when adjusted all the way to one end of the Programming Constant trimmer pot, where the resistance is 200 ohms, is insufficient to make the output low enough, the equivalent resistance of this pair is too low. 


I will snip out R CAL and test the resulting output voltage. Based on where the output voltage is with the trimmer pot halfway, I will determine an appropriate parallel or series resistor to make the output of the supply match the external resistor in K ohms by adjusting the Programming Constant pot. 


I set up my resistance board for as close to exactly 20,000 ohms as I could get and wired it into the supply rear terminals as the programming resistance. After cutting out the R CAL resistor, I was able to adjust to 20.000 volts with the Programming Constant trim pot somewhere in the middle of its range.

I did have to recheck and readjust the Zero Trim pot, then finesse the Programming Constant but once that was done, I could set up my resistance board to any value I wanted between 0 and 20K and I would see a voltage equal to 1/1000 of the resistance. This project is complete. 


My 2005P power supply is only adjusted as accurately as the resistance I hooked up and as accurately as my VOM could measure. When I am next in a lab with multiple higher quality instruments, I will recheck the resistance and output voltage but that will be a matter of a minor adjustment of the trim pot.

Power supply with 'programming' resistance board on top

Power Designs 2005P power supply schematic

 This is a schematic I created by correcting the model 2005 version for the differences found in the 2005P, since there seem to be no extant schematics for the latter model.

Friday, October 2, 2020

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

The wine country fires have dumped large amounts of ash and smoke into the air, with that particle laden air wafting down to my area. Air quality is Unhealthy and will likely remain until those fires die down or the wind patterns change. I continued working on the side project for now.

First step was to drill the holes in the top cap, just to the side of the slot where the inner circuit board juts out. Then the wire will bend 90 degrees to exit sideways from the top of the red outer can. As you can see from the pictures below, it has extra room above the top cap to permit the wires to fit.

Top cap on left, outer can on right

I used a small drill bit and a handheld drill on the outer can because of its cylindrical shape, but was able to use the same drill bit in a drill press for the top cap as its bottom would sit flat. With holes drilled and a test made for the wire clearance, I began to think about assembly.

The thermostat wires are too short for the run from inside the oven, out the top cap, along the entire length of the outer can before it is slid down over the oven and still have the wire come out the side holes. Therefore I need to extend the wires by soldering new lengths to the ends and insulating. These new segments should be rated at least the same 105 C as the thermostat existing wires. 

Holes drilled and wire fit tested

The last safety check was to ensure that the shrink wrap tubing that would insulate the solder joints was also capable of safe operation up to 105 C, the same limit as my wiring. I found that the materials used were intended for operation at levels from 125 C up to more than 220 C depending on the particular type, thus making these suitable for my project. 

With the new thermostat installed, I stuffed the insides of the oven with the glass wool and replaced the can covers. I then routed the external wires down the side of the oven can and to the terminals underneath where the thermostat circuit is connected.

Oven back in place ready for rewiring

The next task was to solder all the connections back on the oven terminals. Several of them had 3 or more leads, but most were single wire per terminal. I found for the multi-lead ones I had to completely clean the terminal, clean up the wires and wrap each around the terminal lug ends before I could reliably solder them.

One final check shows connectivity of the thermostat contacts and of the heater, but no shorts, so I tidied up and applied line voltage to verify that this heats up then cycles to maintain target temperature. According to the manual it should take about 10 minutes to reach working temperature after which the thermostat will open and then cycle to maintain the target 70 C.  

I found it was only about 4 1/2 minutes until the oven light extinguished. Since the ambient temperature was about 38 C and the can had been in full sunlight before I put on the cover and started, that is a reasonable warm-up interval. I suspect the manual assumes an air conditioned lab and has some padding in the estimate to ensure the 

Now that the oven is working properly, I have to turn my attention to the process of calibrating the unit. There are meter adjustments, zero voltage adjustments and a 'programming' linearity that ensures that with a perfect 20K ohm resistor attached, the unit will deliver 20V with a variation of no more than 7 millivolts. 

The procedure to adjust the output accurately covers the situation I faced, where the trim pot doesn't get all the way to the desired value when turned to its extreme. There are a set of resistors in a division network on the board, on which you solder jumpers until you get the zero voltage point with the trimmer pot set about midway through its rotation. In other words this is a normal part of adjusting the unit. 

My zero point delivered about 4 mv at its lowest on my VOM, not zero, with the zero trimmer set to its extreme end of the range. To eliminate errors from the alligator clip jumper resistance and the VOM accuracy, I put in a good solid conductor jumper for the resistance and made use of my oscilloscope for the voltage measurement. The results were consistent - I need to change the resistance to achieve a real zero setting with the trimmer pot.

Jumpers on resistors thus only 16.2K (R9) is in circuit now

The four resistors from left to right are R40 (4.32K), R13 (8.25K), R9 (16.2K) and R41 (32.4K) thus they can be set for resistances for 4.32K up to 61.17K in approximately 4.32K increments. These resistors feed one side of the comparator in the oven to match against the target resistance connected to the programming terminal. 

I will cut the existing jumpers and use alligator clip jumpers to experiment, then solder in new jumpers to the desired resistor value I select based on the outcome. The correct combination in circuit were R40 and R13 for a combined resistance of 12.58K ohms. I was able to set the zero point with the trim pot and the unit was very stable within the precision limits of my VOM. 

Setting the zero point with short on programming input terminals

With the zero point properly set, I attached my resistor board and set it up for 10K ohms. My VOM shows the resistance at 9.9K within the precision of my unit. Then I attempted to set the linearity to achieve 9.9 volts where it is between 9.893 and 9.907 on the voltage measurement, again subject to the precision and accuracy of my VOM, in order to meet the specifications for this unit of +/- a millivolt. I could not. At one extreme of the programming constant trim pot, it was still over 10.3V output. 

Unfortunately, the schematics I own are all for the regular 2005 unit, not the 2005P that is programmed by external resistance. In the regular units, high precision resistors are wired onto the setting switches and necessary adjustments are made internally, but on my unit there is a trimmer pot to make sure that 1000 ohms produces exactly 1V +/- 7 mv. 

To finalize the restoration of this unit, I will need to reverse engineer the circuit portion that includes the trim pot to see where I might be able to adjust other values so that the trim pot can set the target somewhere near the middle of its adjustment range. Stay tuned for part III of this project.