Roll another one…modifying the Durst RCP20 for RA4 processing

Sometimes I’m just incredibly lucky. Tabletop RA4 roller transport processors are unobtanium these days and if you find one, it is likely to be insanely expensive. I happened across a Durst RCP20 (which in fact is a Thermaphot machine, but Durst sold them under their own brand). These machines have a few drawbacks though, which boil down to them being darn old pieces of equipment. First and foremost, to be able to use mine, I had to convert it to run at the right speed for the current RA4 color paper process. Here’s how I did this.

First of all: don’t tell me how lucky I am to have found one of these machines. I wasn’t really looking for one, and before I know it, I had…two of them! One I had to buy, the other was a gift (!!!) And no, don’t ask me to sell or donate you one. I already did, so I’ve only got one left and that one I intend to put to good use.

Brand spanking new….actually, no. A very old RCP20. But still. It’s an RCP20!

Now, let me start by whining and complaining a bit, because that’s what I like to do the most. These machines are really old – the ones I got were both around 40 years old. This means that the rubber rollers on the racks are generally toast, to a greater or a lesser extent. I suspect someone already did some refurbishing work on the one I kept, because some of the rollers look like they’re coated with a different material than the original black rubber. The problem is that these rollers start to bloat, which causes friction in the rack, and ultimately this will result in paper jams and the motor burning out if the racks get too heavy to turn. Essentially, these machines are ticking time bombs, in a way. I got a quote from a company to refurbish the rollers, but this came out at around €55 per roller. There are something like 16 rollers in a machine like this, so errm…well, let’s just…not. I just hope that mine will remain operational for some time until I figure out how to refurbish these rollers myself.

Roller racks from the RCP20. The developer and blix racks (left and right) are identical but for the marking on the center plate. The stop bath rack (middle) has a foam roller that rubs the print with stop bath solution.

Then, there are numerous other things that may go wrong, get lost in time etc. One of the machines I got lacked the little couplers that link the drive shafts of the individual racks to the machine as well as the white bar that keeps the shafts in place. This is the bit I’m talking about:

The red bits are the couplers that link the rack drive shafts to the machine itself. The white horizontal bar keeps the couplers in place.

They’re simple enough components, so a replacement could be fashioned from junkbox parts or (partly) 3D printed if they are missing.

Another issue is the thermostat. One of the machines I got had a faulty thermostat. In fact, it did work, but the temperature setting was way off. I had to set it to around 20C for the developer bath to remain at 35C. Still, it kept the temperature stable, so it wasn’t much of a problem. But I’m not sure if a replacement thermostat is easy to find if it really goes pushing up daisies. Interestingly, the other day I replaced the thermostat on our refrigerator (my sweet girlfriend had almost ordered a €300 fridge as I was just finishing the order process on a €15 thermostat for the otherwise perfectly fine machine…) and the thermostat I exchanged looked pretty much exactly like the one in the Durst RCP20. Now, the temperature range is obviously different, but it makes me hopeful that perhaps a replacement could in fact be found in the marketplace, still. I haven’t looked for it, yet.

Thermostat on the RCP20. Yes, it’s basically just like a refrigerator thermostat.

But now for the really major issue. These machines are originally geared for the EP2 color process, which was far slower than today’s RA4 process. In addition to the slow speed being unnecessarily annoying, your RA4 paper will in fact hopelessly overdevelop (and throw nasty color casts, fog etc.) when developed for as long as EP2 paper took. So forget about just dealing with the low throughput, pretend it’s still 1978 and run your RA4 paper through an unmodified RCP20. Something needs to be done.

Now, fortunately, again I simply have to steal the good idea that has been published long before I ever thought about any of this. In fact, there’s still a very nice web page online by a German fellow who published the essential details on modifying an RCP20 to run RA4 process: Not only is the page still online, the exact parts he used are also still available from the same online store! Here’s that parts list:

Alright, prices are not the same anymore, but still, for less than €50 at the time of writing you can get a (nearly) complete Durst RCP20 modification kit. Nice! So that’s what I did. Now…where do those things go, exactly? Let’s have a look.

If you open up the RCP 20, which works best if you turn it upside down and take off the bottom, you find this area around the motor assembly where the original gears and drivebelt are:

Original motor and drive assembly inside the RCP20.

The motor is at the top of the picture; note the inspection date of 9 Nov. 1976 – did I mention these things are old? The motor drives a transmission consisting of some metal gears, and that in turn drives a small plastic cogwheel. This is connected through a drivebelt with a bigger cogwheel (bottom of the photo), which is attached to the actual driveshaft. The driveshaft extends outside the bottom right of the photo to three worm wheels that drive the racks. Note also the plastic fan mounted directly onto the rotor of the motor in the top of the picture. Our new parts will fit right in place of the old parts, like so:

New components roughly where they should be.

Note that the bigger wheel will go at the motor/gearbox side, and the smaller one goes onto the driveshaft. This creates a 30:10 transmission and that in itself is something we should take into consideration. You can see that the cogwheel order is pretty much the opposite of what it was in the original situation. In the original machine, the gearbox drives a small cogwheel that in turn drives (through the belt) a larger wheel. This means the gearbox and therefore the motor experiences a low torque situation. If we then flip things around in the new situation, the motor actually ‘sees’ a higher torque – which stands to reason, because the actual racks run much faster in the new setup, so it’s logical that more force is needed from the motor.

The consequence of this is that we need to consider cooling. The motor already tends to run warm in the original setup, and having it work harder won’t make things much better. The original fan is nice, but more for decoration purposes. Its blades are at a straight angle on the axis, which means they flap about a bit without creating a well-directed airflow across the motor at all. Probably this has to do with the more rudimentary injection molding possibilities in the mid-1970s. Now, we’re going to lose that fan anyway, because it’s in the way of our new cogwheel. Out it goes! The fan can simply be pulled (with some force) from the shaft as it’s a simple friction fit. But make a mental note that we fix the cooling issue later on. I know that the German bloke chose to just make the fan smaller by cutting the blades, but I don’t think this is a good idea at all. The fan isn’t there just to look pretty (they would have made it prettier if it were).

The new, big cogwheel next to the old one it should replace. Note how the bigger wheel conflicts with the fan.

Sadly, it’s not a plug & play affair, this one. The 8mm bore hole on the bigger cogwheel is a little too small for the shaft that extends from the metal gearbox, so it needs to be machined to a larger diameter. I forgot, but I think it needs to be 9mm instead of 8. Just measure the shaft and machine accordingly. It’s worth mentioning I got help from the great friend who gifted me one of these machines – not only is he a really swell guy (and he really is!!), he is also mighty experienced with machining stuff. I’m more of an electronics person, so I’m over the moon if someone not only has a machine tool, but also knows exactly how to tweak its knobs!

Then there’s the drive shaft of the racks, the long one with the worm wheels on it. This one needs to be machined down a little to accept the small cogwheel, which has a smaller bore. Again, if you know how to work a machine tool, this is a simple affair.

Once the machining and drilling are done, it’s time to mount everything together. Now it gets a little tricky, because you want to tension the belt as well. Especially in this new, higher torque situation, you don’t want that belt to be all loose and flappy. Besides, the new belt is longer than the old one, so something had to be done anyway. The motor assembly is bolted to the chassis, and it turned out to be the easiest solution to just lengthen the mounting slots a little so we could slide the motor along the same axis as the drivebelt. This way we could easily fit the belt, and then slide the motor backwards until the belt was tensioned, and then fasten the motor firmly.

Motor assembly with the old cogwheel still in place, but the fan and belt have been removed. Note the slotted hole (approx. center of the image) that needs to be extended to the left to accommodate the new drive belt.

So now for the cooling stuff. Well, I got this 3D printer not too long ago, so I did it the fancy way and started by replacing the original fan with a brand new one. I started up Autodesk Fusion 360, googled a bit on how to design a fan and engineered one to the exact measurements I needed. Yes, it’s a friction fit, just like the old one, so it slid right onto the motor’s shaft. Moreover, it turns out to run really well and make a lot more airflow than the other one. Only problem is I designed it the wrong way around, so it sucks air from the motor towards the developer bath, while I intended for it to be the other way around. Ah. Well. Doesn’t matter too much, because it still works this way. Besides, I also added a separate fan that blows across the motor’s coil, which is the part that actually heats up. The original fan, even if I had done the replacement job right, would never really reach that area very well anyway. I added a small 12V power supply and fuse for the Intel CPU fan (I think it was the stock fan from a Pentium 7?) Here’s what the final result looks like:

Modified RCP20 drive mechanism: new cogs and driveshaft, 3D printed fan and CPU fan to cool the motor coil.

Note the new cogwheels on the motor shaft and the rack drive shaft (bottom of the photo), and the 3D printed fan in transparent PET-G. Yes, there’s ample clearance between the new fan, the developer bath and the drive mechanism! In the center of the image is the little power supply for the CPU fan right above it; there was a convenient empty space it could be strapped into with a couple of tie-wraps. For the power supply I used a Chinese off-the-shelf low-power SMPS comparable to what you’d find in a typical phone charger, but 12V instead of 5V of course.

And that’s basically it! The rest was just a matter of remounting the base plate, flipping the machine over, fill’er up and go! I haven’t done all that much RA4 printing lately, only a couple of sessions, but the machine has worked like a breeze so far. I used to do RA4 in trays at room temperature and while that worked quite alright, this is much faster. Yes, I’m limited to 20cm print width, but for me, that’s OK. I generally print small anyway.

21 thoughts on “Roll another one…modifying the Durst RCP20 for RA4 processing”

  1. Hi!
    I found your site through Photrio, and I just got my hands on a modified RCP 20.
    I was testing it yesterday with water and everything looks good but I noticed that the temperature in the blix bath is lower than the developer bath by around 5 degrees celesius or more. I tested after having it running for more than half an hour. Do you think that would be a problem?
    Looking forward to your reply.
    Thank you,

    1. Hello Abdela, congratulations on finding an RCP20! It’s strange that the blix bath is so much cooler than the developing bath, because they’re virtually identical electrically and mechanically. They use the same heating element, bath capacity is the same and the roller rack is also the same. So by all means, the temperature should end up being more or less the same as well. Now, in principle, I don’t think it will be a problem because in my experience, blix is super fast. Even at a lower temperature, I’m confident that your prints will blix out completely. If you have doubts about it, you can always re-blix them manually in an additional tray; this can be done in room light and at room temperature. But I don’t think it will be necessary.
      However, I would recommend looking into the cause of the temperature difference. For instance, if it turns out that your racks have more friction than they should have and the motor runs hot, this may heat up the development bath (as it’s close to the motor) causing a temperature difference. In this (fairly randomly chosen) instance, the underlying problem would ultimately cause malfunction of the motor and should really be resolved. So please take the time for a thorough inspection of your machine, or have someone else with a good working knowledge of electrical equipment do this for you. Our machines are very old by now, and although they are low-maintenance, we can’t expect them to run forever without any care.
      Let me know how things pan out!

      1. Hi Koraks,

        Thank you for replying so fast, I know where the problem was – I had the machine uncovered the whole time and I was checking the temperature and checking if everything is working correctly, and now it’s working normally, although I have another issue unrelated to the temperature.
        Now there’s some noise when the machine is working, I’m guessing it’s something to do with the rollers maybe. I don’t know if I’m allowed to post a link here to illustrate what’s happening, but here it is: (it’s a 16 MB mp4 file)
        Let me know if you can see it and maybe you know what might be the reason, thank you so much!
        Prior to commenting here I wanted to email you so I don’t make a mess in your comments with something unrelated, I hope you don’t mind. Thanks a lot. Looking froward to hear from you.


        1. No problem! To be frank, the temperature of the blix bath should still end up pretty much the same as the developer without the top on the machine. So I’d still recommend checking inside the machine, measure resistance of both heating elements (dev + blix) and in short do a thorough inspection.
          The boink-noise in the video sounds like the result of excessive friction in one of the racks, probably due to one or more rollers being badly warped/expanded. The ‘boink’ noise happens when the friction is overcome by sheer force of the motor and the affected rack’s drive mechanism jerks back into step. Inspect the racks closely and in particular each of the individual rollers. My bet is that you’ll find at least one that’s so warped that the rack creates excessive friction at one particular point of its cycle. This roller, and likely other affected ones will have to be replaced or refurbished. You should also check all of the plastic/nylon gears on each rack as it’s quite likely that as a result of the friction problem, one or more gears on one or more of the wheels have been stripped, entirely or partly. If a gear is damaged, you might be able to print a new one; a material like PET-g might be tough enough to make a reasonably durable gear for a rack that otherwise runs smoothly.
          Note that if you remove the racks, you should be able to turn them fairly easily by hand by applying torque to one of the gears or rollers. You’ll notice when you hit a ‘tough spot’, which may help nail down the problem.
          Sadly, this is the reality of these machines today. It’s rare to find one with all rollers in good condition. My machine too has some warped rollers, but not as badly as yours by the sound of it. It’s just a matter of time, though.

  2. Thank you again for your reply Koraks.
    I was thinking the same about the noise, but weirdly enough, yesterday when I tested it didn’t have this problem, and the whole RCP20 seems to be in a new condition, the rollers as well, I will go check again and see where that might be exactly.
    Another point is, the seller told me that he hasn’t put a fan inside, as he recently modified it and told no need to add it, and in worst case scenario, have a fan at the bottom facing the opening towards the motor, hmm, but I don’t think that’ll work. What do you think?
    Thank you!

    By the way, on many occasions, I can’t post a comment and it takes me sometime. I get this error “Error: You have entered an incorrect reCAPTCHA value.
    Click the BACK button on your browser and try again.”
    But I actually don’t see where I can enter the captcha.

    1. I would certainly add the fan. Even in unmodified state, the motor runs quite warm. Originally it also has a (fairly ineffective) fan mounted onto it. Modifying it and not adding a fan does not sound like a good idea. I’d certainly install a fan; there are any number of ways to do this. The important thing is that the heat is somehow removed from the motor.

      About the captcha: it’s invisible and indeed many people complain about it. I need to look into it, because it doesn’t seem to work very well at all, my apologies!

  3. Hi Koraks,

    I managed to print without the noise happening for a whole session, cleaned everything and all, and in the next session, the noise came back, so I tried to find the origin of why it’s happening because there’s no bump in the rollers, and still look new.
    I tried placing the tanks one by one to see when the noise starts, and it’s only with the developer tank. Upon close observation, I found that it’s at the level of the axis that connect the racks’ rollers to the machine’s, along the red coupling sleeves that serves to lock the two up. The 2 axes (coupling surfaces) seem not to be in sync and maybe one might have been deformed a bit?

    Here’s a photo of where I think the issue might be coming from:
    I removed the red coupling sleeve to see how that acts when I turn on the machine.

    What do you think?

    Thank you very much for you time Koraks!

    1. Ah, it’s unfortunate you have not yet tracked down the problem, but at least you’re closer now! I’ve had a look at the photo of the coupling surfaces. They look fine; in fact, they look like absolutely brand new. Indeed, there is some space between them, but this appears to be normal. It generally only creates a jerky movement when the machine is first turned on when the surfaces contact; after that, they should remain in contact as the machine drives the developer rack. In any case, this is not likely to be the actual cause of the problem; it’s just where it shows up, because as soon as there is some friction, the drive mechanism will have to force its way through that point and the axis coupling is the bit where you can actually see that happening.
      I’d suggest taking out the developer rack, putting it on one end and then turning the cogs so that you can feel at what point exactly the friction emerges. Then see if you can link that to deformation of something in the rack.

    1. I think that lever is there to adjust the exit roller pair of the developer bath for different paper weights. I’d run a sheet of scrap paper through it on both settings and see at which setting the paper goes through properly without getting stuck in the path. Btw, I don’t think the position of that lever should be the cause of the problem. If it makes a difference, inspect both of those rollers and their axes + cogs for damage or deformations.

  4. Hey everyone!

    Thank you for the discussion, it’s pretty difficult to find information about those RCP. Since i found you i have a question to submit you. I just “bought” a PCR 50 wich is the bigger brother of the RCP 20 i guess, it’s been working quiet nicely the only problem I have is the temperature of the developing bath.
    That bath doesn’t got above 18-19°c and the fix is at 38°…

    I’ve red in your coment that it is supposed to be the same heating system (at least for the 20) do you have any information to provide me? Any suggestion?

    Thank you all!

    1. Hi Antoine, interestingly, I’ve come across one RCP20 with a very similar defect to yours: the temperature would remain constant, but it would be completely different from the temperature set with the dial. I assume it’s a simple matter of a defect in the thermostat. If you open up the processor, you’ll find the same type of thermostat that is present in many refrigerators – only this one is obviously made for the temperature range that’s relevant for this kind of machine. I have not (yet) looked for a replacement thermostat, but I bet those are still available somewhere, as it seems a pretty generic thermostat that’s probably used in many pieces of equipment.

      As a workaround, you could see if there’s a temperature setting on the dial that does yield a real temperature of 38C. So feel free to dial in a much higher temperature than 38C while observing the actual temperature of the developer bath. With a little bit of luck, there’s a setting where you get 38C. Mark it on your dial and use that!

      If the bath doesn’t heat up at all at any temperature, it’s likely still a thermostat problem and it will need to be replaced. I don’t have good experiences trying to fix a thermostat like these; once they’re broken, they should simply be replaced. The same is true of the bath temperature starts to fluctuate erratically or overshoots.

      So long story short: the thermostat likely will need to be replaced, but if you’re lucky, you can work with the ‘broken’ one provided it will still regulate properly.

  5. These gears can be customized on China’s Taobao site for about $12, including the drive belt. Store Name 玖阳传动配件. The motor part can be moved to adjust the distance of the conveyor belt.

      1. Yes, they can customize the gears with any transmission ratio according to their needs. I had it custom made in their shop and it was very good. They will help you calculate all the parameters you need, and you can use Google Translate for communication.

    1. Hey Nicole, yes, you can remove the old fan if you ensure that the motor is cooled in another way. For removing the cogwheel from the axel, I used a tool called a ‘pulley puller‘ in English. It’s essentially a clamp that grasps the cogwheel and a threaded pin in the center that pushes against the axel. By turning the screw, the clamp pulls the cogwheel off the axel. It’s just a friction fit, so with a little care, you can pull it right off. You do have to unmount the motor and gearbox (if memory serves…) so you can make space for the pulley puller. You need to unscrew the motor anyway, since you have to move it a little later on to tension the new drive belt.

  6. Dear Koraks, Do you have the 3D file to share for the ‘new’ fan blade ? Also with the new gears how do you make sure they fit without spinning on the axle – are they a push fit or is epoxy glue the answer (with the additional torque I think they’ll need to be attached 100%) ?

    1. Michael, if you drop me a note through the contact page, I can send you what I have on the fan. As to the gears: they slide onto the axles and then lock into place with a tiny locking screw. Some kind of cement or friction fitting might work too, but given the torque on these things, I’d be hesitant to think along those lines. I recall that on the small gear that went on the longitudinal drive axle for the racks, I flattened the axle so that the locking screw would stay put. This seems to work very well in practice.

  7. Hello Konraks!
    I just bought a RCP20, another one in the club!
    I don’t know why the second time I set the machine on, always with water, to check the heat and the developing time, the pump stop working.
    Do you know what could it be?


    1. Welcome to the club, Julián!
      My guess is that the pump is on its last legs. I’ve never inspected it closely, but a wild guess is that if there’s a capacitor on the pump motor, it may need to be replaced. You’ll have to take the machine apart and inspect the pump closely, and see if it indeed has a capacitor. I expect it does. Look for one with an identical value and replace it; see if that makes a difference. It’s usually a cheap fix, but you need to be comfortable wielding a soldering iron.

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