This afternoon I got a *istDS from eBay for (converted) USD 77.
It turned out there are only very view signs of use, and only about 6500 actuations.

But, after 3 hours of troughly testing, I found 2 problems, where the second one may be "a feature, not a bug" (an original Microsoft saying).

1) The diopter compensation has low resistance, and does not change anything. Unfortunately, it is set to a position which doesn't provide a very sharp picture for my eyes. Is there anyone whith experience what kind of defect could it be, and how to repair it for cheap?

2) The TTL mode needs a flash FStop correction of -1, but this is not a problem, as it is consistant for all my 4 Metz and 1 Regula TTL flashes. Even the AF assist light works with the two of the flashes which offer it.
BUT!
I own only one pTTL flash (a Soligor DG-340DZ), which works perfectly with my K200D, including the AF assist.
With the *istDS it does not pre-flash, always uses full power, and no AF assist light. Has the communiction protocol of pTTL changed between the *istDS and the K200D?

EDIT:
To make sure, I now used the Pentax DA 18-55 WR instead the Cosina MC 3.5-4.5/19-35.
With this lens now pTTL works, but it is not very reliable. If I set flash compensation to -1 FStops, two of three exposures are correct, the third is randomly either over or under exposed (same subject, camera not moved). So I may stay with TTL or flash AUTO.
But it is strange, as the Cosina should be FA compatible, and even the Pentax F series lenses do support pTTL.

I have posted the same question in the General Technical Troubleshooting section.

EDIT 2:
I forgot to say, before starting serious testing I updated the firmware from 1.00 to 2.02.

I think your item 1 means that the camera has been disassembled for service, and re-assembled in such a way that the claw on the diopter adjuster did not engage with the mechanism.
( that happened to me) To correct that will require disassembly. That is a difficult job for me, also it is likely that the one or more of the fine wires connecting to solder pads in the top hat will break
(that also happened to me).

@wombat2go

Thank you, that's what I thought, too. Also for me it will be difficult. I used to be quite good in such jobs, but then I was 30 years younger . . .

I'll look for a service manual or an instruction, as I wanted to put a cheap split screen in it like I did with my K200D; this wouldn't make much sense if the viewfinder picture isn't sharp.

I hope there is not anything broken, as I am not sure how to get spares 10 years after the introduction of the *ist DS.

https://app.box.com/s/b5b117f4f064fa88a965
https://app.box.com/s/cc0deffe99cc5e842106
https://app.box.com/s/fcd623262b4dda0d947e

In case you attempt to fix it, here are some notes I took. (and find a 14 year old to do it)!

I don't think you will need parts to align the dioptor claw
If you do, I have some donor ist ds in the parts box.

Before dis-assembly, photo some blue sky and examine for defects at iso 3200.
On this camera, the main mother board had the sensor defects mapped to it during manufacture, so the camera is a throw- away if the sensor goes bad.
On dis-assembly, open the flash before you pull the batteries out.
Be really careful not to tension the wires when the top hat lifts up

On re-assembly, tricks, apart from the diopter claw, as i recall:
Firstly align the eyepiece as you lower the top hat.
Make sure the flash latch and the diopter are both correct as you hold the the top in position before adding screws.
Also make sure the main switch rotor (in the top hat) does not get "cross fingered"
Fully tighten the screws in the battery compartment last, because they control the main switch contact pressure.

Unfortunately I think you have to go through all this drama just to re-engage that diopter.

Good Luck!

Edit: Oh have to add the bit about the flash capacitors in a frame on the top left side of the camera looking down.
You will remember them forever if your finger gets in there !

I tried to repair an iST DL and found the wires to be very fragile so I suspect the DS is the same way, they kept breaking and I kept getting shocked. You would think once the capacitor discharged with no battery in the camera it would be safe. I never did get back together. And yes the flash capacitor really bites! I ended up buying a used iST DS and like better than the DL.

Thank you all for your help and suggestions, and specially wombat2go for offering his notes.

Today I checked outside in better light, and found the diopter setting is only slightly wrong for my eyes. It may be good enough for using a split screen.

In any case, I will wait for a couple of weeks to see whether it will develop other problems.
If not, I will decide whether to start this enterprise. And order a split screen only if it is successful.

EDIT:
I really start to like this camera. Handling is nearly identically to the K200D (less options, of course).
What I am missing most is the program shift in manual mode of the K200D, and the possibility to save PEF and JPG at the same time, or alternatively "in camera" development. As the auto WB is really poor even compared to the K200D, this means to save everything as PEF, and process ALL pictures on the computer.

Congrats on the DS! I have the original D & wouldn't mind finding a DS in good shape. I love the CCD sensor & simplicity of these older models. Odd that you're having auto WB problems - I find it works very well on my D and K100D for outdoor shots (but tends to underexpose a bit) & can shoot jpg just fine. It's the indoor lighting that really trips it up in my experience.

@paulh
You are right, WB seems to be ok for daylight.

It is any kind of artificial light, which it does not get right. In this respect, the K200D is not so much better.

But there I can save RAW+JPG, and only need to develop pictures which need improvement.

The only light I find it doesn't do well in is fluorescent light, but that's very typical of most digital cameras really. And as Paul said, congrats - it's a great camera - wonderful CCD sensor, and so small compared to most other DSLR bodies. Also, if you happen to have Photoshop (a version of it that supports editing raw images), you can use camera raw to edit JPEGs, which makes it very easy to adjust white balance. A lot of people don't know that, but in Bridge, you can choose "open in camera raw" from the file menu (command-r on the Mac). It actually works on JPEG and TIFF as well. It doesn't give you all the benefits of shooting raw, but all the image sliders still work and changing white balance there is very convenient.

Not surprised there - I'm sure the DL is very capable, but it was a downgraded DS. The most obvious difference was the DS has a true pentaprism viewfinder while the DL was given a cheaper penta-mirror viewfinder. In that regard the DS (and DS2 which I own now) are rather like a digital ME Super (if you're familiar with that film body) - very small and compact with a large bright pentaprism viewfinder.

I'm living in the EU, and here artificial light which is not fluorescent light ceases to exist.
I don't use Adobe programs, but I do own and use a bunch of programs which can shift colors. But this is not the same as setting WB of an original RAW file, as the information of what the sensor received is gone. The JPG process has already done a WB, and has done it with a reduced color resolution. I agree only that in many cases it may be "good enough".

I agree! I shoot raw 100% of the time myself anyway. I also shoot film and I use camera raw in Adobe Bridge on the TIFF files after I scan negs. But anyway, the thought was if one chooses to shoot JPEG over raw for any reason, one could at least adjust the white balance on just the indoor shots where the white balance didn't come out right. All the outdoor shots would likely be fine. One could also change WB settings in the camera when they're going to shoot indoors too - might give better results than AWB in fluorescent lights. But then you have to remember to change it and change it back later - I used to do that all the time, but don't have to worry about it when shooting raw.

Another question regarding flash usage:

The *istDS is capable of TTL as well as pTTL.
But there is no hint in the menu (nor in the manual) how the user could influence what it will use.

I may get a newer Metz flash which can do TTL and pTTL, but choosing which to use is done automatically. Does anyone know what will happen if both camera and flash decide automatically without a possibility for user intervention?

With my present collection it works as one would expect it:

My older Metz flashes work in Auto mode and in TTL mode.

My only pTTL flash (which has no TTL or Auto) works alright with KA, KAF and KAF2 lenses.
However, with a Cosina AF zoom lens (KAF mount) it works always with full power and no pre flash. On my K200D the same lens will use pTTL.

EDIT:
To add some more information: lens and pTTL flash are the ones mentioned in my original post, lens Cosina MC AF 3.5-4.5/19-35, flash Soligor DG-340DZ.
And I may get a Metz 24 AF-1 for cheap.

Not really related to this thread, but maybe interesting for some users:
The Soligor flash is specified as Guide Number 34, the Metz GN 24.
But if you look carefully at the tables and do some maths, you will find that there is no difference in flash power. Soligor (like, sadly, many companies) are using an advanced interpretation of GN to inflate the numbers.
The Metz uses (honestly) the traditional specification:
GN 24 for ISO 100 in not too big rooms and white ceiling. As it has no zoom reflector, this GN is independent from film/sensor formats and focal length, and valid for the maximum angle of view, which is as for a 35mm lens on FF.
The Soligor is a zooming flash, so the actual guide number is different for each setting of the reflector. In 35mm (FF) position (which is the fixed one of the Metz because of the built), it also has just GN 24.
And adjusting the reflector indoors to the used longer focal length is not always a good idea, as it may increase contrast just too much.

Pentax ist ds works well with the AF360 in TTL mode.
The camera jpgs , color and exposure, track accurately with f/ .
Below are 2 photos with the SMC Pentax -M 1:2.8 35mm, with the camera quenching the flash.
That lens is a very sharp one, close in , I think.

(~3MB)
# at f/4
https://app.box.com/s/4b3rmumhw5627m2n803v

# at f/11
https://app.box.com/s/gw2gg29zsbnx44ht3n5l

Thank you.
Is the flash the FTZ or the FGZ version?
I think with the FGZ you can manually select between TTL an pTTL (not sure, you will know), and the FTZ has TTL only, so no problem.

My question was what will happen if camera and flash both are capable of TTL and also pTTL, and you can't select manually.
But I now decided to buy the very small Metz, so I may know end of this week.

I do own the same lens (SMC-M 2.8/35), but it suffers from slow aperture, and I haven't found the time to try to fix it (yet). Meanwhile I use my Tokina RMC 2.8/35 or the Revuenon special 2.8/35 (M42), which both are not bad, but the SMC coatings are definitely better. And whereas the Tokina is as small and lightweight as the Pentax, the Revuenon is a brick (and I am badly missing an aperture preset).

By the way, I wonder about one item in your EXIF data. They state you have not allowed to use the aperture ring. Is this an error in PhotoMe? As far as I remember, my K200D would not fire at all with this setting if a M lens was attached.

Good Morning RKK
Flash is AF360FGZ
Ist ds is set with "Using Aperture Ring = 2 Permitted"
The flash shows "P-TTL" until the shutter button is half pressed on the ist ds whereupon the menu on the flash changes to "TTL"
That is confusing because it reverts to "p-TTL" after a few seconds of inactivity.

In TTL mode, the zoom settings selectable on the AF360FGZ are 16-19-24-34-48-58 mm
In p-TTL mode, the zoom settings selectable on the AF360FGZ are 24-28-35-50-70-85 mm

I have some -M lenses that in cold weather, are becoming slow to close aperture ( the 24mm, 28mm) but so far the 35mm remains snappy.

I have 2 Pentax flash units that also work reliably as off camera optically triggered slaves, the AF360 FGZ and the AF 500 FTZ

The AF500FTZ is TTL > Manual > Manual burst only, so it is not much good for Pentax cameras later than the ist d series.

I hope your Metz works OK.