I wanted to reply with the results of troubleshooting a recent and virtually identical problem with my Howtek SM4500. A little background...
I'm the third owner of this scanner and have had it for about 1.5 years. When I initially bought it, it had a persistent problem with an F719 G error that prevented it from performing any kind of scan (which the previous owner was completely upfront and honest about). After a few weeks of familiarizing myself with the scanner and corresponding with Evan at Aztek, I eventually traced the problem to a bad high voltage power supply (HVPS) on the green channel. To diagnose this, I swapped the suspect HVPS with each of the other channels (R and B) and then tried scanning. When connected to the other color channels, the F719 error would get reported for which every color channel the suspect HVPS was connected to. Evan agreed that it was likely that the HVPS on the green channel was the problem, but cautioned that it might still be something else in the signal chain. I took a slight gamble and purchased a replacement HVPS from Aztek (around $560 in 2018, as I recall), installed it (pretty simple), and was able to start scanning. I run the scanner with DPL 8.1 on a 64-bit Windows 10 machine and have been quite happy with the results (mostly 4x5, but the occasional 6x17, 6x9, and 6x7). The only recurring and slightly annoying problem I've had is that I usually get a few F702 errors in a row when attempting preview scans after initial power up of the scanner (I only scan a couple times a week when not travelling for work). After 3-5 failed attempts at previews, the scanner will eventually successfully perform preview scans, detail scans, aperture checks, and final scans without any trouble. It's almost as if it just needs a little coaxing or warming up to get it past the F702 errors, but when it does, it runs just fine for the rest of the scanning session (which can last several hours). So, that's a nuisance, but hasn't been fatal (knock on wood).
Fast forward to a few weeks ago. My transparency lamp went bad so I ordered a replacement (got an OEM replacement on Amazon for about $16). The scanner started scanning again successfully. Then, after a few days, I started noticing horizontal lines in my 4,000-dpi scans. In other words, lines that are parallel to the direction of drum rotation. Below is an aperture check scan at 4,000 dpi and 19 microns from a small area of a piece of 120 Fuji Pro 400H. (It's not a great sample, but you get the idea.)
The horizontal lines appear one-pixel wide. When I scanned at 2,000 dpi or lower, the lines were not apparent. It seemed to me that the lines indicated either a variation in lamp output, an aberration in the white/black-point calibration (which I understand the scanner does on every rotation of the drum), or a kind of inconsistency to the drum rotation that results in a kind of "pixel offset" error on each rotation. These lines look similar to what Fixcinator reported in his original post. My guess is that the reason the lines didn't show up at 2,000 dpi is because at lower resolutions the pixels either become wide enough that they "conceal" the lines or that the higher drum speed at lower resolutions keeps the drum from "wobbling" as much (or some combination thereof). In any case, I tried the usual stuff: swapped lamps, checked the focus, checked the optical alignment, greased the lead screw, changed drum pads, tried a different UPS, checked the drum for scratches and debris in the calibration area, tried a different piece of film... and to no avail. I did a little reading in the manual and on the Scan Hi End group on Yahoo about what might cause these kinds of lines, and it seemed that the consensus pointed to something having to do with the stability of the drum rotation or the way the scanner reported the drum's position to the scanning software (which, if in error, could cause this sort of line-by-line weirdness).
Long story short: I got the scanner working properly again (no more horizontal lines). The last two things I did were done simultaneously, so I can't say for sure which did the trick, but they were: 1) gently brushed and blew off the carriage code wheel and detector (Pages 5-22 and 5-23 in the SM4500 service manual); and 2) put on a new drive belt. Everything is back to normal now. If I had to guess, the new drive belt is what did the trick as the carriage code wheel looked pretty clean to me (be extremely gentle if/when cleaning it).
My hypothesis is that, because my scanner occasionally sits unused for extended periods of time (sometimes 4-6 weeks), the belt begins to develop an uneven strain along one axis because of the way it's sitting in the scanner on the two pulleys. If you're scanning frequently (e.g., a couple times a week), this is probably enough to ensure that the belt ends up resting in a different position in the two pulleys every few days, such that the strain is more evenly distributed around the belt's circumference. In my case, the uneven strain concentrated along one axis was probably causing the belt to slip a little bit on each rotation (or maybe every few rotations), thereby resulting in the weird, pixel-wide offset. The amount of slippage was presumably not severe enough to show up at the lower resolutions, or maybe the higher rotation speed meant that the drive belt wasn't slipping as bad. Looking at the bad belt and comparing with a new one, I can't see anything obviously wrong with it other than the fact that it's average radius is slightly larger than that of a new belt, but I'd be willing to guess that there is some degree of disproportionate axial strain aligned in one direction. I may adopt the practice of taking the belt out of the scanner if I know it's going to sit for long periods of time.
Hope that helps.
