#42 Presonus Audiobox 1818VSL - revisited and revived!

[Part one can be found right over here]

Through a mix of patience, persistence and good luck, i managed to get my grubby mits on another, fully functional Audiobox 1818VSL, for considerably less than the 1-2 pricey ones available on eBay in Europe.

After making sure this second one was indeed working and everything, i proceeded straight to removing the firmware chip and dumping its contents. It came with what seemed like the latest version available, 1.49.

Speaking of which, the v1.49 dump (binary format) can be found right over here.

Here's the point where some clearing-up of details should be made. This 1818VSL has two versions of drivers available:

1.  Audiobox ("AB", latest version is 1.3, from March 2016)

• pro: offers access to the built-in mixer & DSP effects
• con: only(?) works with firmware v1.18

1. Universal Control ("UC", latest version is 3.0, from March 2019)

• pro: "technically" newer
• pro(?): updates the firmware to v1.49
• con: is a barebones driver (no more access to hardware mixer & DSP effects)

So, after "cloning" the firmware into my original unit, it booted right up, and the AB i had installed at the time detected it, but claimed it needed a firmware update. Fair enough, i let it have its way. After allegedly successfully completing that and the unit rebooting, though, it then greeted me with the same message again. That can't be good...

Uninstalled AB, installed UC. As one might expect, that also demanded a firmware update, but interestingly enough, it indicated the existing version as 1.00. I chose not to go ahead with that, since i wanted access to the hardware mixer and effects (routing signals in and out for testing other gear without opening up a DAW every single time).

So i took to doing some digging. I "copied" the layout of the board in the CPU area along with its surrounding resistors, and tried to identify (on the second, working unit) what signals went where. At this point, the I2S clocks to the converters (MCLK, BCLK, LRCK) in the first unit were still absent, just as before.

After a couple of hours of on-and-off probing (multimeter continuity test, as well as oscilloscope), along with consulting of datasheets, i managed to figure out most of the connections.

The "north" and "east" sets of resistors connected the Xmos chip to the USB3318 interface, apart from R93 and R101, which were in series with the SCLK and MOSI lines of the SPI bus going to the flash memory chip.

R132 was a 300Hz clock line, going to the clock input of the CS2300 PLL chip. On the first (bad) unit, the clock was either absent, or working at "only" 50Hz (not 100% sure if my multimeter was actually measuring 50Hz there, or just picking up some mains interference; i didn't think of probing with the scope just then). That was an obvious but interesting sign. Sure enough, nothing was coming out of the PLL chip.

In the mean time, i managed to identify the tiny 8-legged component near it and connected to it - an NC7NZ17 triple buffer. Sure enough, the commoned inputs were connected to the CS2300 clock output (through R115), one of its outputs went to the four codec chips (through R145), a second one to a small component near the S/PDIF connectors (through R125), and the third probably / likely back to the Xmos (through some vias and R116).

I was preparing to try to reflow the Xmos, thinking that who knows, maybe some of the solder balls might've cracked or something. But in all honesty, i quite doubted that was the case, since after temporarily removing R137 and R138 (LRCK and BCLK respectively), i could still measure around a diode-drop from ground (450-odd mV), so they most certainly were not disconnected.

In a bit of a hail-Mary attempt, i reinstalled AB 1.3, reconnected the faulty unit (which i had re-updated to v1.49 in the mean time), let it "update" the firmware to whatever version it wanted the first time, but also allowed it to do that the second time in a row it asked. Aaaaaaaand whaddya know - the thing came to life! I guess the first "update" reset it to v1.0, and the second stage updated that to v1.18.

Probing revealed the expected 300Hz being sent to the PLL input, as well as the expected clock on its output, as well as LRCK and BCLK on the Xmos side of R137 and R138. At this point, the two resistors were still disconnected, but after putting them back, i got some meter activity when turning the input gain controls!

Yes, i very likely "took the scenic route" on this, due to one (or several) brainfarts along the way, but hey - the end justifies the means... i guess(?). Still, no harm done, and a very capable little unit brought back from... well, more like a "coma", than death. From the brink of death, perhaps?