What CC knows about stock Jeep radios

[NickyV]

I will be paying attention!

[ghetdjc320]

Just ordered up an RX-173 that seemed to be in good shape from eBay. will be tearing that one apart as well. There is one glaringly obvious solution that occurred to me for the am/fm radio bleed through when adding an aux input, a switched 3.5mm socket. It's a super simple device that passes through signal on normally closed contacts when there is no plug inserted in the jack and automatically switches inputs when plugging in a jack. Instead of using those two pin connection on the pre-amp board, why not just intercept the am-fm radio signal to the board? 

 

I do like bluetooth options as well but unfortunately apple still does not support any real HD bluetooth options such as Aptx HD. Therefore the best solutions tends to be a USB C DAC. Installing the DAC close to the head unit 3.5mm port would shorten the signal path. The connection would be USB to the DAC then switch to analog and go to the RX series radio pre-amp board where the am/fm radio would normally plug in. It's just a 3 pin JST connection for the stereo signal. If done properly with quality components, that would be the best audio signal one could get to the stock RX series internals. The DAC analog output may have to be attenuated down to the appropriate line level voltage to match the am/fm radio levels when switching.

 

On the output side, a nice DSP would work great to not just make up for the stock amplifier limitations, but also to tune the cab acoustics. I'll post up how this works out. I think the current RX-172 I have I'll just gut entirely for a future raspberry pi setup with an OLED screen but the RX-173 I'll use to optimize a factory component system. 

[fiatslug87]

That sounds like the Scosche IPFMRDS I have in my three MJs. Made for iPods but a lightning adapter lets me use my iPhone.

 

 

[ghetdjc320]

5 minutes ago, fiatslug87 said:

That sounds like the Scosche IPFMRDS I have in my three MJs. Made for iPods but a lightning adapter lets me use my iPhone.

 

 

Ive used those in the past. But those still use the FM radio tuner to carry the signal. I'm wanting to run the DAC signal direct to the pre-amp board and disconnect the tuner output altogether. 

[ghetdjc320]

I was staring at the disassembled radio for a bit… Really wish I had the diagram or schematic for the RX series of radios. However, I’m wondering if, when the cassette is inserted into the deck it mutes or shuts down the radio tuner entirely, thus preventing the bleed through. I wonder if there is a way to emulate the tape being present when an aux input is applied. I’d assume there is some simple 12v or 5v logic commands that trigger the display to show tape commands. And since there isn’t really any input change command, I’m assuming this is just a logic curt unit that mutes the tuner or puts it in standby. I’ll either have to dig up a schematic somewhere or just get the multimeter out and test away.  
 

In a nutshell, here is what I’d like to accomplish on a mostly stock radio:

1. Eliminate the tuner bleed through without disabling the radio or attenuating the gain. For this I’m thinking simply intercepting the radio input to the preamp board in some fashion. A switched input jack would do the trick nicely. But an automatic method would also be nice. 
 

2. Get the display to show different information other than radio station while playing aux input. Emulating tape deck display functions seems like the go to option here. 
 

3. Get the stock play, rr and ff buttons to control the aux source. This one is a bit tougher with stock components. 
 

 

[ghetdjc320]

Did a bit of signal tracing on the preamp board. Also desoldered the mounting plate for the volume and tone pots. I hadn’t noticed before that there are several adjustable potentiometers on the board. Two of them attenuate the tape deck input. How I would love a good diagram of the board lol

 

 

[NickyV]

14 hours ago, ghetdjc320 said:

How I would love a good diagram of the board lol

Check your DMs!

[ghetdjc320]

4 hours ago, NickyV said:

Check your DMs!

Awesome! That was the missing link. So it looks to me like those pins everyone uses for aux are just test points on the PCB. My current goal is to intercept the am/fm signal pre tone controls, and add a switching input that will pass through am/fm but switch to aux or Bluetooth when signal is present. I want to have this trick the radio into thinking a tape is playing so the radio station display goes away. Then have the play, rw and ff buttons trigger a DSP to skip songs and play/pause. That logic looks simple enough. I’ll add a transistor to transfer that logic to the DSP to trigger this functions. I’m liking the tinysine DSP boards with Bluetooth. I can have a clean aux input for my DAC and a Bluetooth for regular wireless pairing 

[ghetdjc320]

As I’ve been compiling data and studying these radios in some detail, I need to jot down some of the info I’ve found so far. Some of this is my operating theory and I’ll update it with further confirmation. To preface, I have both an RX-172 and RX-173 in hand and a schematic from an earlier RX. Here are some of my findings:

 

1. TP pins such as TP371 and 471 are “test points”. There are many test points on the board, likely to validate the circuits upon or before assembly. 
 

2. TP371 and 471 seem to just sit on the line level input to the tone controls (starting with treble). They are not specifically the tape or tuner signals but rather  combined signal. 
 

3. I do not see that there is necessarily any discreet “input selection” in these units. Currently, I’m thinking it is the presence of the tape that sends a “mute” signal to the radio. Some have reported long ago here in CC that removing the tape deck, inserting a blank tape or disabling the am radio will eliminate the audio bleed through issue. There seems to be some logic here but I’ll have to do some testing to confirm. 
 

4. The preamp is labeled as PCB-VR on the drawings. It looks like there have been a few variations of this over the years but the final output to the amplifier portion of the board is the 3 pin connector with red white and black wires. On my RX-172 that is P304 on the pre-amp board (the one with the tone and volume controls). You should see a cable coming from there going to the main board input P304 near the amplification stage. NOTE: this is where one should be able to get line level aka RCA pre-out signals for an external amp. This is before any amplification takes place. The amplifier in these radios seems to just be a simple 2 channel design, which is further substantiated by the lack of a fader control (front and back biasing) and only a balance control (left and right biasing).

 

5. The AM tuner is the longer vertically mounted pcb that is against the left side of the radio viewed from the top. 
 

6. the FM tuner is smaller but parallel to the AM tuner and also sits near the left side for eh radio as viewed from the top. 
 

7. The amplifier itself sits on the main PCB board near the antenna input. It is not the vertical board mounted behind the main radio harness connector. 
 

8. The AM/FM radio inputs appear to be coming from the 7 pin connector from the main board near the FM tuner to the pre-amp board. The tape input is separate and comes into the preamp board on a 3 pin ribbon wire. 
 

9. There are adjustable potentiometers all over this board. I’ve heard the rumor about “lowering the AM radio volume” as a work around for the signal bleed through issue. I can see how that could be accomplished. I’ll have to determine which pot that is, but a simple adjustment with a small flat head screwdriver would take care of it. 
Edit: I think it’s pot 71. You can see it circled in the picture just below the volume and tone controls. Turning it counterclockwise should lower the output. 

 

More to come as I delve deeper into these head units. 

[fiatslug87]

They do have fade, front/rear. Can you also pdf the wiring diagrams and add them to this post? Thanks

[ghetdjc320]

1 hour ago, fiatslug87 said:

They do have fade, front/rear. Can you also pdf the wiring diagrams and add them to this post? Thanks

Interesting, I’m only finding a balance pot on the RX-172 I’m working with. Since the diagram I have is from another member, I’ll leave it to his discretion to post. There are many aspects that are the same as the later models but many that aren’t. The manual I have is of an RX-170. I’ve been trying to dig up the manual for the later 172 or 173 but haven’t found any yet. If I am able to get one, I’ll post up whatever I can. If anyone else has one, please let me know. The signal traces from the AM/FM tuners to the PCB-VF are quite different on the 172/173 than on the 170.