Radio/Stereo: CR-4024/CR-4123: The Definitive Thread

[Juular]

You absolute legend.

I have this unit in my 240 GLT, yet I have found almost no information about it online.

The pinout is especially useful.

[TheHungriestBadger]

I did manage to get the part number of the main control IC (located near the front of the unit on the bottom PCB, easily accessible by simply removing the bottom plate of the unit). It is a D1707G chip by NEC.

If this is the source of your issues, then give up now. It is a highly integrated, and application-specific chip. It contains Phase-Locked Loops (PLLs) to aid with the tuning, as well as the drive circuitry for the Vacuum Fluorescent Display (VFD), handling the buttons and tuning knob, storing saved stations, etc.

Whilst some parts of the IC could theoretically be replicated on a custom retrofit daughter board (i.e. the PLLs, VFD driver, etc.), this would be incredibly difficult to achieve, especially given some of the VFD segment outputs are multiplexed with control inputs as well (god knows how). The datasheet does contain a nice pinout though which may aid in basic debugging (IC enable signals, voltage rails, etc.).

The IC itself is no longer available for purchase to my knowledge, and even if it is, you would have to reverse engineer the microcontroller code yourself, and even if you could do that, there would be no way to assemble the code or flash it onto the IC since there are no programming specifications, nor the toolchain available to do so.

As a heads up, modern flash-based microcontrollers typically specify program code (ROM) retention for a typical span of 40 years, thus these stereos from the mid '80s are approaching this date (at the time of writing - 2022). I do not know if older One-Time Programmable (OTP) microcontrollers had different architectures for storing code which may increase this span. I also don't know whether this microcontroller is OTP or flash-based. Either way, it is possible that code could start to become corrupted due to age, if not now then in the future.

Alternative Links:
https://drive.google.com/file/d/19O2...ew?usp=sharing
https://drive.google.com/file/d/1cH7...ew?usp=sharing

[TheHungriestBadger]

It appears that even these stereos follow the same nomenclature as other Volvo units: The CR-4123 is a popular unit in North America; however, a friend of mine recently came across a CR-4023 for sale in mainland Europe. This is similar to, say, the CR-7x2 where the 'x' designates the region code - 0 for Europe, 1 for North America, 2 for Australia, and is it 3 for Japan?

In the case of these units, they will take the form in the title: CR-4x2y where x appears to be the region code, and y may refer to some firmware/hardware version for the unit. So far I have come across a CR-4021, CR-4023, CR-4024, and a CR-4025.

[Suterman]

Hello.

I am trying to wire up my original cr4024 stereo to an aftermarket amplifier, the old amplifer went ages ago and I had an old cd/radio installed but now that has given up too.

I have recently started using my mobile telephone to play media to my car stereo via a bluetooth/fm converter plugged into the cigar lighter so I thought that I would like to refit the old cr4024 and do the same with that. However, there is a problem in that the cr4024 has the 6 din socket audio output.

I still have the old loom for the volvo amp with the 6pin din plugs on it so would like to chop this up and make a conversion loom using a audio output/rca converter.

My queston is could anyone tell me which pins do what on the 6 din socket on the cr4024 unit? would this work anyway? would it be better to just splice the 6din wires directly to the rca lead and not use the audio/rca converter? (the converter turns the signal down somewhat I believe)


Many thanks

[TheHungriestBadger]

The pinout for the DIN connector going to the external amplifier is given below. I believe it to be consistent for all Volvo CR units (and possibly TD units too?). Certainly the external amp DIN pinout between a CR-702, CR-905, CR-906, and CR-4024 are all verified as being cross-compatible.

Please note that I have not personally verified the displayed pinout to be correct - the only way to do so is to play some audio and use an oscilloscope on the audio signals. I have no reason, however, to doubt that the pinout given is correct.

Also note that the CR-4024 9-pin connector (the one providing permanent & switched live, as well as ground) only 'breaks out' the two front speaker outputs (FL+/- and FR+/-).

Finally, you may need to rotate the image to match the orientation of the connector on your unit - pay attention to where the notch is for the DIN cable 'latch' and use that as a reference for the connector's orientation.

Alternative Link:
https://drive.google.com/file/d/1CMy...ew?usp=sharing

[TheHungriestBadger]

As shown in the post regarding the pinout of the external amp DIN connector, there are six signals:

1) Ground - also connected to the chassis of the unit.
2) Remote Turn-On Signal - 12V or 0V, I am not certain which voltage correlates to an 'on' signal, but I would assume that when 12V is present, the amp is signaled to turn on.
3) Four audio signals (FR, FL, RR, RL). I believe these are tapped directly off of the preamp board (wherever that is inside the unit), and do NOT have +/- signals. These are line-level audio signals.

[Suterman]

Thanks,

I just powered up the cr4024 in order to test the din outputs, it worked, then did not work and appears to have died.

Back to the drawing board.

[TheHungriestBadger]

Define died? What are the failure symptoms? I wouldn't bank on it being repairable if it is genuinely dead but would be interesting to see what is/isn't working on yours and compare it to mine

[Suterman]

I wired up the permanent and switched power and ground, turned on car ignition and switched on head unit, the display lit up for a moment then went off and now it wont work at all.

[TheHungriestBadger]

That sounds exactly like what happened to mine when I went on the test drive before buying my car. I would bank on it coming back, but possibly remaining intermittent? I don't know of any way to persuade it to come back to life when one wants though.

There was only one pattern I noticed consistently:

Looking at the yellow flexible connector back in post #12, one of the pins towards the left (as shown in the photo) measured at roughly -17.4V with respect to ground (i.e. the chassis of the radio) when the display was working. When the display was off, the same pin would always measure about -16.7V.

This might not seem significant; however, this difference is almost exactly 0.7V, which is the typical voltage drop across a silicon diode.

I know how Vacuum Fluorescent Displays (VFDs) work, and I also know that the negative bias on the segment/grids is approx. -20V. In my mind, putting these two bits of information together, I would expect that something to do with the filament of the display has gone open circuit (or near to it). I also suspect that it's on the 'front panel' PCB containing the display and memory buttons, as I reflowed literally every joint on the main board of mine, and quite a few on the smaller boards accessible from the top with no change in symptoms.

Of course, the issue could be different in your case, Suterman. Double-check the inline fuses as an easy starting point, but I would probably advise against trying to go inside too much - these units are not designed to be serviceable from my experience so far.