LED mods for Arp 1601 clone

Still time for a quick one before it’s ‘BYE BYE 2016’! :)

While building my TTSH I also got myself a DIY kit of a Arp 1601 sequencer clone. The build itself was so straightforward that I didn’t see the point in blogging the details. However I made some LED mods to it and thought maybe this would be of interest to owners of this device. So if you ended up here looking to change the color of LEDs on your Arp 1601 clone, look no further! If you’re in a hurry, check the summary section at the end of this post.

While you can also find my posts related to the mod at Muffwiggler (Muff) forum, I thought my blog here is a more compact way to present the information. For one, since forum threads at Muff are created by product / project, there’s a number of discussions going on at the same time (esp. if a DIY build). So a thread can become convoluted. Second, threads which span years (or multiple production runs of a kit) can become l-o-n-g, adding to the ‘needle in a haystack’ effect. Third, if you’re not logged in the board system won’t show post attachments or the like. So it can be quite the dig looking for information you need.

But don’t get me wrong, I love the Muffwiggler board and a lot of highly skilled people dwell there. It’s just that often the format isn’t good for finding information that you need instantly.

Well maybe my ramblings aren’t either, but let’s not follow that tangent! :D

My finished 1601 build

My finished 1601 build

Ok so the Bourns PTL60 and PTL45 sliders used in the 1601 clone have regular 2*3*4mm rectangular LEDs, and it’s easy to find these in bulk eg. on Ebay. I wanted to go with the slider pip colors of the original 1601, so on my build step slider LEDs are two blue followed by two white etc. whereas the clock, FM and PW slider LEDs are red. White and blue turned out to be nice in the sense that their current draw is a pretty close match, so they’re about equal brightness too.

Which brings us to the small catch with the slider LEDs: with the instructions here you won’t be able to change each of the step slider LEDs to different color and keep their brightness constant. You can of course have the most super ossom rainbow showcase (<3) if you like, but it just won’t be equal brightness. Why so?

It’s because the original design has these sliders grouped to two banks of 8, both of which have one resistor that both sets the LED brightness and drives the step out transistor (to chosen gate bus). So modifying each step LED for different color and consistent brightness would require a lot more PCB modding. So here it’s about balancing between the original circuit and modification complexity. At first I was planning to take this route, but luckily didn’t have to.

Mod design

For my mod I was fortunate enough to get pointers from Kipling, who both designed the electronics and handled first few parts production runs of the 1601. Personally I often have the habit of overcomplicating designs, and such was the case here too to boot with. Here’s what Kipling had to say about the design related to step slider LEDs:

There’s no LED brightness trimmer on the 1601. Although I have not tried any other LED colours this question has come up before and the answer is be careful, as reducing the current to dim the LEDs will impact on the operation of the gate output circuitry which is part of the same current path.

Z4C, D, and E have to be able to turn on when current is passed through the slider LEDs. This could be compensated for if using a single colour of LEDs requiring a lower current by raising the value of R81 and R106 for banks A and B (both normally 680R), and that of R133, R142 and R151 (all normally 100R) to achieve the same base-emitter voltage on Z4C, D and E respectively.

If you need a different current for two different colour LEDs in the same bank you could be into track mods to add resistance to only the lowest current LEDs and possibly bypass resistors to maintain a similar current through to Z4C, D and E.

So yeah like said, going all rainbow-y here would complicate the design, requiring traces to be cut on the PCB (“track mods”) and more resistors to be installed. If sticking by the original design then it’s enough to alter the values of R81 and R106 and provide alternate current path (resistors) to each base of each step transistor.

Clock, FM and PW slider LEDs, on the other hand, are much easier to mod. Clock being a blinking LED, it has a resistor of its own (R130, 750 ohms stock) while FM and PW LEDs are connected in series and share the same resistor (R242, 650 ohms stock).

Step Slider LEDs

Ok so blue and white LEDs for me here. Using part designators of step slider 1 as an example, here’s how the schematic looks original vs. modified:

Step 1 slider in stock config

Step slider 1 circuit in stock configuration

Modified step slider 1 circuit

Modified step slider 1 circuit.

So while R81 and R_TRIM are used to limit current to bank LEDs, there’s a new resistor R_EXTRA installed that supplies Q11 with current close to stock configuration to properly drive the gate output stage (via gate bus switch). R_TRIM is added in series to each bank (R106 being equivalent of R81 in bank B) whereas R_EXTRA needs to be installed for each step.

I used 825R for R_EXTRA because my local electronics shop didn’t have 820R in stock. The difference isn’t big and both work, but 820R is the closer match to original circuit by values calculated from voltages measured off my 1601. For R_TRIM I chose to go with 47k; I did try 10k and 22k too but the blues and whites were just too bright. I ended up installing step slider resistors like this:


R_EXTRA (825R) resistors for slider bank A..


..and the same for slider bank B.


R81 & R106 with R_TRIM (47k) to adjust brightness for both slider banks.

Clock, FM and PW LEDs

Here it was 2k2 for R130 and 10k for R242 for me. Also it seems I forgot a small catch here too: There really isn’t a single good choice for clock LED as its brightness depends a bit on the Pulse Width slider setting. After testing a multitude of values I went with 2k2 because that seemed to give about equal brightness with the other sliders, when Pulse Width slider is set to about 50%. So on the lowest PW setting the clock LED is more dim, and at highest setting more bright.

I soldered R242 and R130 slightly off the PCB so that their ref. designators remain readable.

I soldered R242 and R130 slightly off the PCB so that their ref. designators remain readable.


In brief what got changed was:

  • R81, 680R stock -> Add 47k trimmer in series with R81 (step sliders 1-8, Bank A).
  • R106, 680R -> Add 47k trimmer in series with R106 (step sliders 9-16, Bank B).
  • R130, 750R, clock LED -> Changed to 2k2.
  • R242, 680R, Pulse Width & FM LEDs -> Changed to 10k.
  • Install a 820R resistor per step to provide separate current path to base of gate out transistors.

And that’s all. It’s about 3 hours left of this 2016 where I’m at, so I hope I got everything down here :).. Now I’m on to some (non-blog) festive stuff! Happy New Year 2017!


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