help me reuse this receiver frontpanel

[gcb]

i got this old busted receiver (marantz SR7400. the whole issue was just one corner of the power board broken, if anyone needs parts from it you just pay postage) to use as the case of my HTPC... mostly because looks was high on the list because of the wife restrictionsTM (and we don't have a basement)

it will probably fit the contents just fine, but now i'm dying to use the front panel!

it has a cold-something display (will open another thread to try to reuse that as well) with a few fixed info (such as DISC, FM, etc) and some 8 general purpose character blocks (the interesting part).

for now, i'm woring about the miriad of buttons that i thought i could simply hook up to a keyboard board and use as is... but i fear there is something more complex going on. I'm pretty sure it's a completely uninteresting pattern for anyone that is familiar with eletronics and panels, but for me it's weird.

it's basically this:
a +5V rail that attaches to one side of all the buttons.
another rail that is interconnected by random value of resistors and then connected to the other side of the button by yet another random value resistor.

here are some pics (zoom in!)
https://picasaweb.google.com/1137908232 ... directlink

here's some random 3 buttons schematics from the bottom/middle of the board. have no clue where the bottom rail is going to, the top one is the uninterrupted +5V.

if I touch the multimeter probes around any of those resistors on the bottom rail i get no connectivity at all. it says OL for every resistor pair of legs i choose to probe there. which is very weird for me.

[UAirLtd]

could be some kind of analog thing, each button pushed causes the analog voltage at that rail to be different, there should be another resistor somewhere on that rail tied to GND (perhaps R814?). You just measure the analog voltage on the rail, and each button press results in a different voltage, takes up one ADC input pin on a microcontroller, saves some IO pins.

[gcb]

could be some kind of analog thing, each button pushed causes the analog voltage at that rail to be different, there should be another resistor somewhere on that rail tied to GND (perhaps R814?). You just measure the analog voltage on the rail, and each button press results in a different voltage, takes up one ADC input pin on a microcontroller, saves some IO pins.

That makes sense... i still have to finish tracing those circuits to the 'output' connectors and test it with live power... but yesterday my eyes were already tired of following traces on a keyboard that i will use as the bridge from those to the computer.

In the end, i think I will simply remove all the resistors, so the board becames just a bunch of loose buttons (not part of anything that can mess up my individual button signals), and then i will solder the buttons to the matrix of the keyboard board.

but on the other hand, i will still need an arduino->usbHID to get input from the volume and input source knobs.... So i think i'm trying to understand how those panels works so that i can use only the arduino, and ditch the keyboard board.

I think you are spot on on the voltage drop. just have to move my lazy ass and test those. ... or maybe i can make something simpler than an arduino to interface the rotary knobs to the keyboard board... hum... options, options...

[gcb]

You were spot on. Seems like it reads the final drop down voltage on those button tracks. i got 3 'outputs' from those buttons, 2 goes to pins on a ribbon cable that goes to a logic board... another one is giving me some more trouble.

KEY0 and KEY1 output pins each handle 8 buttons and vary from
2 Ohm,
680,
1.2K
3K
4.8K
7K
9K
15K

I measured all those going from the +5VL pin (from the power board) to the respective KEY0 and KEY1 that goes to a logic board, (btw what is VL? ...it sits right next to one called +5VD), [o]but KEY2 do not seem to go to any logic board, but instead goes back to the power board ...

ah, ok, gotcha. the other question was why only this rail has the resistence ranges all in the >1K Ohm... it's because the last button on this chain is the power button. with 0Ohms. So i think i solved the mistery... just let me know if i'm way off here:
the KEY2 is like the ATX power button in computers... it's "always live" and anything <1Kohm will trigger a power on.

So KEY2 last button is power, which has 0ohm, pressing it powers on the board. after it's on, it probably forwards the pulldown information from the remaining 5 buttons on the track (1k, 2.5k, 4.5k, 7k, 12k) to the logic board somewhere on the power board... probably including the power button so the logic board decides how/when to put it back into stand by or something.

Ok, so if that's correct, all that's missing is VL vs VD? is this something standard? or is it something particular to this setup (i'd imagine Voltage Live, Voltage Dead or something [/o]

edit:
that's what i get for doing stuff in the interval of a game... that's all wrong. as soon as i tested with the daugher board attached i was able to get a KEY2 output matching exactly the same impendances as KEY0 and KE1 for the key2 buttons. it messed me up a little because for the daugther board signals there's a key2+ and key2- pins, but of course, when you attach the damn thing it closes the loop and now it's all the KEY2 rail. so it's pretty much powering the logic board at all times and it decides when the power button was pressed or not. nothing like ATX here... also there's two buttons i missed on the other side, so it's all 8 buttons with the very same values distributed.