In the previous part we have disassembled VS-1880 to get access to faders located on the Panel Board. Looking at the faders close-up below can see that most faders are not in the best shape (spill damage ?).
Decided to replace all faders, but finding replacements turned out to be a bit of a challenge, Here is original fader pot removed from Panel Board:
Approximate measurements gave the following specification:
Slide Potentiometer 50k linear
lever height ~ 10mm
side A pin distance 64.5 mm
side B pin distance 67.5 mm
It took a while to find replacement parts without paying exorbitant price, but the deal was – minimum quantity 50 pcs. Fine. Now I have enough spare faders for more than 100 years of moderate use. Lucky me.
New faders look real good, have also dust protection layer, and are the exact size of the old one.
Having new fader pots on hand, I have replaced all thirteen. Also from reading the schematic of VS-1680 (could not get a hold of VS-1880 service manual) that there are only two ICs 0- IC7 and IC8 responsible for reading the state of all faders. You can see them on Panel Board faders close-up in lower left corner. Both are 74HC4051AP, mux/demux chips. Since access to components on the Panel Board is not easy – I decided to replace them as well. So far so good.
I have assembled back VS-1880, Run diagnostic mode, test faders and… Nothing works. At all. Moving lever on any fader does not cause even slightest change of the output values. Hmm…
After some head scratching, when I was going to give up – a question popped out – did you compare pin mapping on old and new pot ? Nope. I didn’t, Apparently I have assume that it is some sort of a standard. Well, assume nothing – it wasn’t, Quick compare test gave the following table:
Once mapped, solution became quite obvious. All that was needed was to ‘jump’ pin nr 5 to pin nr 2 on every fader, as in the following picture:
This time after assembly all fader tests passed perfect.
Conclusion: if faders on VS-1880 (and most likely on VS-1680) start acting up and fail diagnostic test – almost 100% chance is that fader sliding pot need to be replaced. I do not think that there was any problem with 74HC4051AP.