Well I opened up the Power Designs 5005R in an effort to repair it. Let me start by giving you some background:
This model is fairly old like most Power Designs supplies and has a low granularity (coarse) single turn voltage knob (0-50V) with a higher precision single turn 0-1 V control knob inside of it. Inside the unit, there are two pots piggy back mounted with the fine on top and the coarse on the bottom. It was a real bitch trying to separate them to clean them. The two pots are wired in series and the sum of their resistance is presented to the voltage regulation circuit.
Now, the problem presented itself when I tested the unit. Because all of these analog pots are older than dirt, tiny crap inevitably got inside of them. The result of this is a very VERY serious problem. To make matters worse, the main voltage control pot is not wirewound. It has a solid circular (ring) resistive metallic (or more likely ceramic because it's a Cermet pot) element inside and offered (notice the past tense here) seemingly true contiguous adjustment. Unfortunately, because the thing is older than dirt, crap managed to get inside of it, whether it's the lube used to lubricate it (and there's quite a bit in there) or metal/ceramic dust from too many turns or the environment (or whatever). So, when the wiper (which is tiny and barely makes contact with the resistive element) gets dirt between it and the smooth track, guess what happens? Basically, you get a "break before make" open circuit scenario telling the supply to shoot a full fifty volts to its output terminals. So, any reasonable circuit you have connected, save for your current limit setting, gets instantly fried. This made me appreciate modern digital circuits with rotary encoders providing inputs to microcontrollers (i.e., solving this entire problem).
I first noticed the spikes when I tried to test the supply through its entire 50 volt range. As I turned the coarse voltage control pot from 0 to 50, I saw occasional brief spikes to 50 V along the way. More surprisingly, the fine tune control (which has a wirewound pot backing it) also caused spikes. This behavior makes the supply not only useless, as is, but also dangerous, both to DUTs and small furry animals. One can argue that you can set the voltage first and then connect a circuit or device, but I feel that this is too much to ask and a mere touch of the voltage control pot can shoot a 50+ V spike (i.e., infinite resistance in the regulation circuit) through the device connected to it, because a single grain of crap got under the wiper or it (i.e., the wiper) left the resistive element for ten milliseconds due to vibration and tight tolerance (combined with a crap speed-bump it was balancing on partially).
I tried to take the pots apart and clean them, but this was futile. The things are just way too old and the tolerances too tight inside. They were never meant to be taken apart and cleaned. There's lube inside and the wiper is at such tight tolerance with the cermet track that any shift when it is taken apart will result in more of that "break before make" action when put back together. It's just not worth the effort and danger. I certainly would not trust any sort of "drill holes, blow and clean solution," because even if it works perfectly on day one, all it takes is for a single particle to get in between the wiper and track and your circuit under test is literally toast.
So, the solution to the dilemma was to take a single 10 kOhm Chinese Bourns imitation 10-turn pot and replace the two (coarse and fine) with the single multi-turn pot. Sadly, the hole in the metal front panel was too small and I had to drill it slightly larger to fit the fake Bourns pot. The whole endeavor including removing the old pots, examining them and mounting the fake Bourns took about two hours.
In the end, this seems to have worked fine and allows, with some manual dexterity, adjustment on the order of about 5-10 mV. Also, there is no longer a "break before make issue," because with a new non-corroded, non-polluted wirewound pot, it's always "make before break." My only concern is not knowing how many watts of power are being placed through the pot itself to make sure that this is within the 0.6 W or so that it is rated for. In a conventional voltage regulator circuit, the current flowing between the Adj and Out pins is tiny. In this particular supply's circuit, who knows...
I have two TW4005 dual-supplies I got recently that need similar treatment due to the same voltage spike issue when the coarse control is turned. The bottom line is that unless you are willing to replace the pots on Power Designs analog supplies that use pots for regulation (i.e., the non-precision supplies), they are a bad idea (to buy). Even on the precision units, you absolutely must properly clean the switch contacts for them to work properly and without danger. All it takes is one open circuit caused by "crap interference," even in a switch designed to make before break, and you can kiss your DUT goodbye.