Okay, thesteve has pretty much un-idioted me, by which I mean to say that he has taught me very quickly how to use my oscilloscope, which is the greatest tool in the history of ever, and which I consider to be invaluable for anyone attempting to tweak their PCE CD system (whatever it may be) back to life.
Seriously, I can't overestimate how immensely powerful even a cheap scope like mine is.
As far as the white lens pot is concerned, most of them should come with a hard epoxy on the pot, locking it in the factory-determined ideal impedance. That being said, it's okay to have it 'too strong'. Sure, it'll shorten the lifespan of the laser itself, but better overpowered than underpowered.
VR105, as I've said before, should be clocked at a fairly precise 4.3MHz frequency. Because the very nature of a waveform is its undulation, you're going to get readings at both the top and the bottom of the waveform, meaning you'll see the frequency go up and down on your meter. As a rule of thumb, try to get the frequency of VR105 to go NO LOWER than 4.321MHz, which should mean your peak should be at 4.400MHz, or more likely just under. Again, better a little strong than a little underpowered. A decent frequency counter (at least 10MHz) shouldn't cost you more than $15 USD on eBay, and is well worth the investment.
VR103 is your go-to pot if you have scratchy sound or sound that drops out. On all the Duos I've played with, it seems to have a very, very small window of perfect operation - maybe ten degrees or so, but I'm not sure if this is typical, as my sample group is comprised of the four Duos (all PCE) I have ever seen.
If you're starting from scratch on adjusting the pots, thesteve's directions are perfect:
1) Pull the RF signal from pin 2 on the P5 connector, and ground it on pin 1 of P5.
2) Set your scope to 2.00V/Div and 1.000us/Div. Get your readings while your CD system is trying to load the disc.
3) While the CD attempts to load, use a plastic or other non-metal small Phillips-head screwdriver to turn VR102. You're trying to get the greatest amplitude (biggest wave) you can. Once you've found the spot that makes the wave as large as it can be, move on to VR101 and do the exact same thing.
4) After you've made the biggest wave you can, set your scope trigger to be at the far left of your screen, and about 3/4 of the way up the waveform you're pulling. Turn VR104 clockwise until the disc stops spinning. Once this happens, make the smallest turns you can COUNTER-clockwise and wait a few seconds between each turn to see if the disc starts spinning. Once it does, move on to VR103.
5) VR103 is kind of the odd pot out - it's incredibly touchy, and it seems to vary on a per-Duo basis how to set it. Ideally, you want to turn it until your drive is making no real noise - no squeaks, no whines, no chugs or repetitive clicking noises. It should run almost silently. You can find your starting point by turning it counter-clockwise until it starts making a faint whine-like noise. When this happens, turn it CLOCKWISE by a hair, and I do mean 'just a hair'. I'm not exaggerating when I say that the VR103 pot has a very small window of operation. Use a game with a soundtest available right off the bat, like Rondo of Blood. Playtest the first and last tracks while making minute adjustments. There should be no crackling or audio dropouts at all. This can take some time, so be patient, and you'll pull through.
