Becker Grand Prix 780 Headunit Auxiliary Input DIY - Page 4 - PeachParts Mercedes-Benz Forum

bmwpowere36m3 · #46 12-16-2015, 10:38 AM

Quote:

Originally Posted by ebasta

@bmwpowere36m3 can you provide pictures of the alarm bypass mod that you did and how did you do the mod where the "CR" button controls the jack audio on and the eject turns the jack audio off?? Thanks.

The alarm bypass is easy. Just connect the five wires on the flat five-pin connector to the three-pronged spade connector on the back-bottom of the head unit. The three-pronged connecter is grounded (i.e. negative, chassis ground) and all your trying to ground all the wire in the five-pin connector.

You can connect them to any ground you like, just the three-prong connector is right there and convenient.

The switch... I don't recall the specifics. I think its a momentary ground to begin with, you'd need to verify with a DMM. I cut/scratched one side of the PCB trace away from the switch (NOT the grounded-side). Soldered a wire from the switch (side that is grounded when switch is depressed) to the copper pad/trace on the PCB where the copper strip contacts when a tape is inserted.

Basically your grounding the copper PCB pad, tricking the head unit into thinking a tape was inserted.

I'll see if I can get some pics.

bmwpowere36m3 · #47 12-16-2015, 10:41 AM

Lastly

I'll put a thread in the for sale section... but I have no real use for this head unit anymore (like I said I sold my W123 and have no use for it on my W124).

So if anyone is interested in it, let me know. Otherwise I'll toss it.

bmwpowere36m3 · #48 12-16-2015, 11:14 PM

ADDED THE PICS BACK AGAIN AND VIDEO [HOSTED BY FORUM, should they shouldn't disappear]

I'll try piecing together a DIY on how I modified my Becker Grand Prix 780 headunit (HU) to accept an auxiliary input (ipod for me). I started with a Becker HU I grabbed from a '89 420SEL at our shop. The first thing I did was verify it was working before proceeding. The HU does not require a CODE, but it features an 'anti-theft' wiring scheme.

I grabbed the above picture off another site, the 'anti-theft' feature is the wiring connected to the flat five-pin connector (#9, centered) and the three-prong copper connector (#10, center-bottom). To bypass the 'anti-theft' feature, all that needs to be done is connect ALL the wires out of the flat five-pin connector TO the three-prong copper connector (which is a ground). In essence, you are grounding all the wires in the flat five-pin connector.

There a many ways to do it, such as: use the existing anti-theft harness, tie all the leads together and connect them to the spade connector (w/spring clip) that goes on the three-prong connector.

However, I simply desoldered the wires from the inside of the anti-theft connector (blue arrow), tied them together and soldered them to the wire coming from the three-prong connector (red arrow) all inside the HU. You can see the thin/thick brown wires leading down towards the three-prong connector, that's what I soldered together.

Now this will prevent the HU from locking when used in another vehicle w/o the 'anti-theft' feature or it allows one to bypass it. Just note that if your HU is already locked, then I believe you need to go to the dealer to get it unlocked. As long as the HU is removed from the car w/o the alarm activated, the HU won't be locked. *IMPORTANT*, the 'anti-theft' feature needs to be bypassed or connected (to a working OE alarm) before powering the HU, or it'll lock.

Here is my "testing" setup:

^ Becker HU, iPod Mini (auxiliary source), and HK (BMW) speakers

bmwpowere36m3 · #49 12-16-2015, 11:15 PM

Now onto modifying the cassette section in the HU, thus rendering it unable to play tapes in the future (no loss here

).

^ Bottom cover removed

First, to allow the HU to "play" your auxiliary source it needs to "think" that it's playing a tape. In the above photo, the red arrow points to a copper strip. The strip itself is connected to ground, and when a tape is inserted, the strip's tip gets pushed onto a "pad" (directly below it) on the printed circuit board (PCB). Thus grounding the pad on the PCB and activating some sequence in the HU's circuitry.

So there are two options: Insert a tape whenever you want to use the auxiliary input or Wire a switch to ground the PCB pad (tricking the HU into "tape mode"). I did the latter and wired the "Tape Selection Button w/Indicator" ("CR" button, seen below) to ground the PCB pad.

Now to actually add the input, I bypassed the original signal that leaves the tape amplifier chip (seen below).

The tape amplifier is the small black chip (centered). I traced the wires back to the HU's circuitry (from the tape amp chip). There are 6 wires that connect the PCB (where the tape amp chip is) to another "vertical" PCB (where the three gold circular trimpots are). Those are the wires we will tie into...

The 6 ORIGINAL wires from left to right (looking at the HU like the photo above)are: GREEN, RED, BLUE, BROWN, YELLOW, and GREEN. As labeled in the above photo, the left-most green wire is the "left-channel," the center brown wire is "ground," and the right-most green wire is the "right-channel".

Now proceed to cut the three wires (green, brown, green) as close to the tape amp PCB as possible. On their other end (vertical PCB), you'll need to desolder them. Once desoldered, solder in a new set of wires. I'll leave it at that, since you can run a long cable (containing those three wires) out of the HU to a connector of your choice (female 3.5 mm jack, like Becker does) or as I did, run them to the HU's back panel (metal) and terminate them to a panel-mount DIN connector (seen below, green arrow).

Finally here's the headunit in operation: https://www.youtube.com/watch?v=3dB8gZaPe4E

I'm purposely leaving this DIY somewhat general because there are many ways to accomplish this modification. Also the assumption is that any takers will be somewhat adept at figuring things out themselves. I'm more than happy to answer specific questions.