Hi Andy,MoonUnit drew my attention to IC906 where power is on pin eight and is maintained by a 3.4V battery.
This is a PCD8571. This is an eight-pin serial RAM chip containing 128 bytes. If the battery dies the contents of this RAM chip also die.
My guess is that when new from the factory this part is programmed with some magic value that tells the radio to work. When the battery dies the contents are lost and the microcontroller shows dashes. Then when the workshop tool is connected it gives the microcontroller a specific sequence of some kind of signal or signals which instruct it to restore the contents of the RAM chip and it works again.
In theory it would be possible to take a working radio and connect some test hardware into the circuit then read out the contents of the RAM chip. It might then be possible to restore a locked radio by again connecting some test hardware and reprogramming the RAM chip. Tedious work, time consuming and not easy for anyone to just DIY.
Yes - I think they are. I got them from various sites on the net. I tried without success to program the IC906 and put the failure down to it being still on the circuit board and thus not being powered properly by the programmer (clip on lead). But more fundamentally, if IC906 isn't an EPROM in the first place, it wouldn't have worked [slap forehead].Looks like those hex dumps are from a 24C01 which is an EEPROM. Didn't see that on the schematic, but if it's on there somewhere then it could also hold the theft status.
Hmm, well that's not good, I'll have to go back to see exactly what I did try to do. I've been ferreting around in the dark on this and had come to believe the tool was an urban legend, so to see it here is exciting ...There is a good chance you have damaged IC906 by doing that. IC906 uses I2C and has a different pinout to 24C01.
I don't believe there is a 24C01 - I think it's being confused with the PCD8571 chip. Certainly, when I was looking into all this a while back, I found a Polish site, which I thought implied the RAM chip was / could be replaced by an EPROM. See https://www.elektroda.pl/rtvforum/topic129638.html#16980526 although you have to put the posts through google translate. Some tantalising hints.I looked again at the schematic - can't see a 24C01 on there... if you know where it is please post an image of that part of the schematic.
That button is actually a depiction of an aerial. On the 754 model, it does not exist, but on the 755 model (which I think is a US version) it does, and successive presses increases the scanning sensitivity when scanning for channels.I noticed on the schematic there is a key that has a symbol that looks like a keyhole. Does this button actually exist?
Thanks for posting this. I'm getting a new (old) Becker this week from a parts car.I had an extra Becker 754 radio (previously from a 300E) that I wanted to install in my '89 560 R107. As many know, the 754 model utilizes a lithium battery (cutting edge technology at the time) and when the radio is removed from a power source (e.g., stolen and or pulled out of the car) the lithium battery will eventually go dead, putting the radio into a locked mode where once powered up again, the display will show dashes. Unfortunately, these radios do not have a security code like many of the Becker units. The Becker 754 requires a "reset" by the dealer. I wanted to provide what I did to bring this radio back to life, as there is little information on this subject.
1. I took the radio to a local electronic repair shop and we replaced the lithium ion batter (3.6 volts / half AA size) and soldered the new battery to the board. (This cost $20)
2. I then approached a contact at my local Mercedes dealer (Hendrick Mercedes - Charlotte, NC) and asked if they had any "old timers" that may remember how to reset one of these radios. Luckily, one mechanic knew how to do this, and still had the tool to "reset" the radio. He said he would do this at no charge!
3. Remove the face plate of the radio (easiest when removed from the car). With face plate removed, install the radio in the car / connect all wiring to the rear of the radio.
4. With the ignition off, insert the reset tool (see photos) into the face (where to connect should be obvious). Hold down green button on the reset tool, then turn on the ignition to power the radio. Release the green button. Remove the reset tool and install the face plate back on the radio. The radio should now work and it should be "unlocked" / eliminate the dashes that previously appeared on the screen. The instructions for this process are also located inside the case that houses the dealer's reset tool.
Hopefully this helps others who may want to restore / revive a Becker 754 radio. Also, many thanks to the folks at Hendrick Mercedes in Charlotte, NC.