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 this picture off another site, the 'anti-theft' feature is the wiring connected to the flat five-pin connector (centered) and the three-prong copper connector (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.

[SPACE] I'll add a picture later of my modified connector.

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.

I proceeded to disassemble the unit and clean it throughly. Now onto modifying the HU to accept an auxiliary (line-level) input. Here is my "testing" setup:


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



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" (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 for $100) or like me, run them to the HU's back panel (metal) and terminate them to a panel-mount connector.

I'll update the connector termination once I get that far.

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.

Try uploading the photos to the forum software. We may want to add this as a Wiki.

I bought a 780 for my 83 years ago and never had a problem with the anti-theft feature. I don't believe it locks until it is connected to the factory alarm system. I have no bypass and it works fine.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

Are you aware the tape head preamp has a 6db/octave EQ curve?

The freq response will be far from flat if you're going thru the head preamp.

I'm not sure on the specifics but, I'm pretty sure I bypassed the little IC shown in this pic:



which is a Toshiba TA7705F (Low Noise Dual Amplifier For Autoreverse Car Stereo). So unless that's further downstream... any insight would be great. I tested them with some HK speakers I had any it sounded great for what it is .

On my later 1432 head unit the 5 pin connector has nothing to do with the alarm... one pin is the speed sensor input (for speed sensitive volume) and another is a telephone mute. The other pins are not connected. There is an alarm wire that hooks to a single terminal on the back, like the one for the power antenna trigger. It has a big spring on it so it gets knocked off every time you remove the head unit. My guess is if this is powered up (alarm set) and gets knocked off it kills the radio. I just leave it unplugged.

That's for this information, I'll probably use this info to mod the spare 1432 head I have for an input

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I'm not seeing the photos. Can you send them to me or try to reload them on this page?

When I get a chance I will, but you should be able to see them unless your work blocks certain sites...

Not sure why I couldn't see them the first time but they are there now.

Question: Are you playing your iPod through the headphone out or using a plug to get the line out? I'm wondering on a tape head amp what it is expecting. I'm assuming that it should be line-out for best output and quality.

I would assume its expecting a "line-out" signal. Though you could feed it almost anything, since you can still control the volume with the headunit (as long as the input signal isn't too low or high). *Best sound quality would come from a "line-out" since it bypasses all the internal amplification and circuitry of the portable device. *I say best, but given the input method, the age of the radio, and other factors.... I wouldn't be putting my hopes on achieving some grand sound out of this headunit. I did it because I wanted to keep things simple, low cost (this is for my secondary car, I already have a bank drainer = M3), and original looking.

what happens to the tape mechanism when you power off the HU?

z168, that's a great question. Part of the "convenience" package on my 500E allows the player to turn on when car is off...

bmwpowere36m3: I'm doing this on a 1432, which guts are almost identical to the 780. The Bose system in my 500e is solid...pretty nice quality especially since the amp is new and all speaker are new OE as well (except rear deck). I prefer the bose amp to power everything rather than have the iPod headphone amp pushing into the Bose amp...

I was just thinking through this some... because the 1432 allows you to switch between different modes you can alter the tape sensor to make it think a tape is present all the time. Then you can simply use the mode button to select the tape deck. You'd need to do some other trickery though, because the head does check that the tape is actually rolling.

-Jason

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

That's probably why the copper strip in the 780 only has one arm and the one in the 1432 is the same size arm but splits off into 3 different "fingers". I think I'm just going to wire it so that a tape has to be inserted. Hopefully it will work. I'll just gut out a tape that I have so that it doesn't make noise, etc.

edit: luckily I have an extra 1432 to test on before going to the one in the car...

It automatically "ejects," thus disconnecting the input. I wired mine up so that pressing the "CR" button enables the tape-mode (simulates the grounding of the copper finger inside when a tape is inserted). Then to return to the radio, I press the "Eject" button.

you think this DIY would work for a becker 1492, going into a w123 car that originally had a becker 612 head unit?

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