I got my V5.1 kit today, assembly went smoothly, except the control board FFC, which didn’t feel quite right when plugging in (at both ends).
After assembly, I got an error message on the screen. Tore it back down and took a closer look at the FFC.
This is how the FFC cable looks after plugging it in just once:
The issue is that the connector part number used is the locking type, while the FFC cable is not.
According to the BOM available at the github repo, the part number of the connector is SFV20R-4STE1LF.
Because the shipped FFC is the regular type, those two locking features push against the FFC and even bend the connector when fully pushed in:
Either the FFC cable or the FFC connector should be changed so that they’re more compatible.
I’m not criticizing the project in any way, just thought I should bring this up, as it might fix various issues.
Also, sorry for the multiple posts. As a new user I can’t attach more than two photos per post.
Your criticism is valid and constructive and i really appreciate it. You are right and the cable type we are using is not 100% OK. Probably this is causing trouble to more than one user, although i personally never had issues with it.
FYI, we are looking for a replacement, but it’s not a common cable type and not so easy to find.
Anyway, the current flat cable normally works if it’s well inserted and you don’t plug/unplug many times. Of course, we are going to improve this ASAP by changing the cable or the connector (in next hardware revision).
All the best and big thanks for your feedback!
I guess some variation is to be expected, but in my case, the FFC was fully plugged in at both ends (checked again after tearing it down) and did not work. I did not take a picture, but on the control board, it looked very similar to this, but with a more bent connector:
from Zynthian 5.1 Buttons Not Working [Solved]
I suspect that one or more of the middle pins were not making contact due to the bent connector.
Yes, those FFCs cables are less common. An easy fix would be to just replace the connector with the non locking part number. The PCB footprint is the same. For existing stock, those locking features can be removed in a couple of seconds with a scalpel (that’s what I did on mine today).
Currently tinkering with it, and it seems to be an awesome device 
All the best!
Hi @simmeah !
I just tested the solution you proposed. it works and it’s easy enough, so we are now re-working all the boards we have stock-pilled.
I hope this helps to reduce still more the number of “building issues” with the Kit V5.1 (i must say is quite low 
).
Indeed, it could be that many SD-card extender issues are related. I will post a fix with detailed pictures ASAP, so people having SD-extender issues can test this solution.
Of course, next batch of boards will replace the SFV20R-4STE1HLF by the SFV20R-1STE1HLF and similarly for the rest of FPC connectors.
I hope it compensates the issues 
Thanks a lot for your advice!
These perhaps?
15 cm, reverse direction, pitch 0.5mm, 20 pin
Yes, this is the FPC control cable used in zynthian kits.
As @simmeah has noted, it doesn’t fit 100% the current on-board connectors. To be clear, they mostly work, but i suspect that a number of wiring issues could be caused by this. We are re-working the boards, cutting the tiny plastic bumps that cause the cable to bend when closing the connector.
I will post some pictures ASAP to illustrate the process so people having wiring issues can rework the connectors as needed. It’s really easy to do with a cutter tool (and magnifying glasses!) but also quite delicate because of the tiny size of the elements to cut.
Regards,
I’m glad I could help
It more than does! I got the Zynthian only as a lazy way to quickly test FluidSynth (the plan is to design custom HW capable of emulating an SC-55, for a one off project, where a PI doesn’t physically fit) and didn’t know much else about this project. Well, I spent the whole last day tinkering with it, and it is an awesome device! For a long time I considered buying a MIDI keyboard, and now I have the perfect excuse!
Does this only affect V5.1, or the earlier RPi4 V5 as well ? I’ve been having minor issues with my (RPi4) V5 occasionally starting up with a blank screen (the rest of the system, like the buttons, seems to work).
This affects all V5 main boards, older Pi4 and recent Pi5 ones.
I’m adding this to the wiki tutorial, so users with related issues could try this fix.
To resume, and as @simmeah as already explained, the FPC connectors doesn’t fit perfectly the standard ribbon cables we include in the V5 kit, causing the cable to bent when closing the tonge and potentially causing bad contact or, at worst case, damaging the flat cable.
The reason is we accidentally choose a wrong part number and didn’t realize until now because the difference is really small it works most of times. Indeed i never suffered the issue in any build i did here, in zynthian labs. And i did a few ones!
We have already fixed the BOMs so next boards will be manufactured with the right part number. Also, we are fixing all boards we have stocked so:
ALL V5 KITS SOLD AFTER 2025-06-01 ARE FREE FROM THIS DEFECT and you don’t need to fix this because we already did. If you have issues, the cause is not this.
Anyway i would like to offer a solution for those having issues with this. Indeed, the solution is simple and it has been already explained by @simmeah:
Use a cutter or scalpel to cut the 2 tiny plastic bumps that cause the issue.
The bumps are really tiny and you probably need good eyes or magnifying glasses. The tool must be well sharpened, so the plastic bumps can be cut will minimal force. Be extremely careful to not worsen things by breaking the FPC’s tonge.
You must repeat the operation for all FPC connectors used by V5. They are:
Main Board
Control Board
I only recommend to perform this action if you have any of these problems:
Control board connection issues, like:
SD-card extender doesn’t work or it’s not stable enough. You should be sure of using good quality SD-cards of the right sizes (32 or 64GB). Also, check the SD-card extender is not broken nor damaged.
and YOU HAVE DOUBLE CHECKED the build tutorial and you are sure you followed the instructions carefully and everything is according to them. This should be a last-resource action before considering to replace the board.
ALL V5 KITS SOLD AFTER 2025-06-01 ARE FREE FROM THIS DEFECT, so you don’t need to fix it because we already did. If you have issues, the cause is not this.
May the force be with you 
