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by Jeremy Schrag
As we continue to move into the future and ever more complex automotive entertainment, some of us desire more and more of a connection to the past. There are a lot of people out there restoring Mopar vehicles from the eighties with the intention of keeping them as stock as possible. But what to do for a sound system? What can we use that presents a factory look, yet still sounds good?
I'm going to try to answer that question today by looking at one of Chrysler's most advanced head units of the mid eighties, the Ultimate Sound System.
In the above picture, an initial glance might tell you that you're looking at the same unit, only in different colors. In fact, these are two different models, though they are very similar in both construction and operation.
Let's start with the bottom unit. Available from 1985-1987 or so, the Ultimate Sound deck was one of Chrysler's best of the decade. It features a defeatable five band equalizer, selectable tape equalization for metal or chromium dioxide cassettes, and dynamic noise reduction for both the cassette and FM tuner. It was also the first unit to use the joystick style fader and balance control.
The top unit came into the picture at the end of the Ultimate Sound era. Available in 1987 only, it changed a few features around. This was the stopgap deck of choice for Chrysler's first 1987 Infinity sound systems, thus the Infinity logo you see in the picture above. The Infinity I and Infinity II weren't ready yet. What was Chrysler to do? Why, take their best sounding deck of the day and put the Infinity logo on the front, that's what. These decks are quite rare, but are out there in both black and silver trim.
Now, you're probably wondering what the difference is between the two. They look so similar, it's a little hard to tell. In fact, there are only two minor functional differences. The Infinity model replaces the more generic DNR from National Semiconductor with the more familiar Dolby Noise Reduction. The second difference is in the tuner. As you see on the bottom unit, there is an "SRC" control around the missing tuning knob on the right. This stands for Stereo Reception Control. What this does is force the FM to go into monophonic sound to compensate for far away stations. It has the tendency to meddle with strong stations, thus the ability to defeat it. We are so used to modern decks doing this for us, we forget that it was not always so.
And indeed, the top deck does do that automatically, so that control is no longer there. Instead, we get an AM stereo defeat control. This is perhaps the only deck Chrysler has ever produced where you could manually shut off the AM stereo decoder.
I hear some of you asking about that "AMB" control. Well, that functions the same way on both units. Only functional in the FM band, it's a stereo separation enhancer. The idea is to make the sound more like a concert hall. The rest of the controls are pretty self explanatory. Set the clock by pressing "set" and then the volume knob. Change the cassette EQ by pushing the “70µs” button. Eject the tape by pushing the tuning knob.
Before we move on, there is a not-well-documented feature on both of these decks that make them absolutely perfect for restoration projects on cars that have a dashboard clock: you can defeat the clock. Press "set" the volume knob, and then the "70µs" button twice. Repeat the procedure to bring it back. This will enable the deck's display to only show tuner and cassette operation, leaving the clocking stuff for the other clock.
Let's take a look at the labels on each of these units.
This is the label for the Infinity deck. There is a model number present: RX-671S. Supplier code of 28046 indicates Mitsubishi as the manufacturer.
The bottom deck has this label, and is identified as model RX-650B. Again, it is made by Mitsubishi.
While these are some of my favorite decks in my collection, it is entirely likely that you are reading this having found one of these cool decks and discovered it to be flaky. Unfortunately, these are probably the most trouble prone decks you can get for your Mopar. The reason why can be summed up by three words: “cold solder joints.” These decks are absolutely notorious for them.
I have not found one single deck of this ilk to be fully working at the yards. Indeed, my rare Infinity model was suffering from channels randomly cutting out and display issues. That unit actually made things worse because it had a bad amplifier chip on one channel, which prompted me to use the black deck in this article as a source of parts.
So, to save myself some trouble, I am going to use my parts deck again to show you how to get these wonderful sounding head units back to full working order.
First, I'll show you the back panel, with a multitude of arrows pointing out the important screws. Each pin on both connectors is labeled beneath, a feature I very much appreciate.
Blue arrows indicate the screws that hold the top and bottom on. Some are missing here. The green arrows are important if you need to replace an amplifier chip - they hold the inner sleeve to the outer case and hold the amplifier chips themselves to the case (thus the indentations). The red arrows indicate two of the six screws holding the upper printed circuit board (PCB) tray in place. Finally, the purple arrow indicates one of the two screws holding in the power and filtering module.
Don't worry about the screws by the antenna jack just yet. We'll get to those later.
This is the bottom of the deck, and the only screw we need to be concerned with here is the missing one arrowed in red. This is where the fifth faceplate screw goes. Yes, these decks have five screws holding the face on.
The left side of the unit. Orange arrows indicate the missing faceplate screws. Green indicates one of the two screws that hold the front panel and mainboard assembly to the rest of the unit.
Of the two red arrows, the top one points out another screw that secures the inner sleeve to the main casing. The bottom one is the other screw that holds in the power and filtering module.
Blue arrows indicate where another amplifier chip is located. As is the case on the back panel, these also secure the inner sleeve. More on that later.
There is one screw here without an arrow in the top right. This is one of the six screws that holds in the upper PCB tray.
The other side. All screws here serve the same functions they did in the last picture. Is there one missing? Nope, that bottom red arrowed screw in the above picture has no counterpart on this side. This is because there is no power and filtering module on this side.
Let's remove the faceplate.
This is what you'll be faced with should you remove the front panel. The button switches on these decks are quite hardy, and should rarely need replacement. The joystick assembly is self contained, requiring the near total disassembly of the deck to work on. The illumination lamps used are standard "grain of wheat" style 12 volt 3mm bulbs with leads wrapped around twist lock bases, but these too require a fair amount of disassembly to replace.
Nothing is very easy on this deck, in fact, except the replacement of the cassette belt. Ultimately though, you will find that there are so many bad solder joints in one of these, you might as well overhaul the whole thing in one big job.
Now, if you are only replacing illumination bulbs, you do not have to pull the whole thing apart. This can be done with the mainboard and faceplate carrier still attached to the deck. First, you obviously unscrew the multitude of small Phillips screws holding the faceplate PCB to the deck. Now, we must open up the top and remove the upper PCB tray.
This is the colorful and somewhat intimidating view once you remove the top panel. This deck has connectors galore, and most of them will have bad solder joints right underneath them, if there are any at all.
I haven't bothered to indicate most of the connectors with arrows because they all need to be unplugged to get the upper tray out. Most of these connectors cannot be plugged into any other connector, but be warned now - if you do not make note of where these connectors go, and you do plan to address the bad solder joints on the mainboard, you will face a confusing mess of them later. It is very easy to lose track of which go where, even with most of them being different from one another. We've already seen seven of them, and you have no idea what you're in for later.
Now, I have indicated one connector in white. This mess of wires runs all the way down to the cassette mechanism. I've indicated it thus because if you take this deck apart far enough to work on the mainboard, this cable may fall out and try to hide. It has connectors on both ends, and is easy to lose track of.
Up top, you will see a row of four connectors. Three of them are for the faceplate display - these may be pushed through the metal carrier to enable you to replace illumination bulbs. But before you do that, hold on for a second. There are more connectors on the mainboard you must remove and two wires you must de-solder before you can. More on them in a minute.
Meantime, the red arrows indicate the six screws that hold the tray in the deck. Yellow arrows indicate the two screws that hold the PCB to the tray. Go ahead and remove all the red arrowed screws now, but let's look at the next picture before removing the tray.
Here, we have some arrows to talk about. Yellow indicates two ground wires. The upper one must be de-soldered before the upper PCB tray will come out. The lower one must be de-soldered to remove the faceplate PCB.
Irritated yet? Hold onto your hat, because this deck only gets worse.
Should you be unlucky enough to get a flaky deck with the need to re-solder some bad joints, you will have to touch up some joints on the upper PCB. Remember those two screws indicated in yellow two pictures up? Remove those. Remove the two screws holding the antenna jack to the tray. Then, take your soldering iron and de-solder all the wires indicated in red, which are all ground wires attached to the tray.
Then, flip the tray over.
De-solder the two wires in blue. You will need to peel back the plastic insulator to get at the one on the right, but don't remove the insulator completely. It needs to be there when the deck goes back together.
With the tray and PCB separated, you can now address any bad solder joints you find. If your deck is anything like this one, there will be a ton of them all over the board. Take your time... make sure you get them all. You don't want to be inside this thing more than once. The blue arrows indicate the most problematic joints where the front panel PCB connectors are. Once you're done, put the PCB back on the tray and reconnect all those little ground wires.
Brace yourselves. You're about to find out what the mainboard looks like.
I do believe I just heard the sound of my collective readers having a heart attack. I cannot blame you folks, this deck is a rat's nest of wires and connectors. This is a very intimidating sight indeed. Fear not, I'll help you get through it.
If all you want to do is replace illumination bulbs, we are as far into the deck as we need to be. Observe the white arrows - these indicate the connectors for the faceplate PCB. Once you unplug these, you will have enough wire length there to allow you to pull the faceplate PCB away from the deck far enough to work on the illumination and solder joints on the faceplate PCB. You can also re-solder any bad joints on the two amplifier boards at the top of the picture. They will never be easier to access than they are now.
Let's look at that faceplate PCB.
That red arrow points to the ground wire that is normally soldered to the metal assembly. Don't forget to de-solder it before you pull the board forward.
Blue arrows indicate the twist lock bases for the illumination bulbs. You can find these 12V "grain of wheat" bulbs on eBay, or perhaps at a hobbyist store where they sell model train accessories. Some are missing in this picture, having been used by yours truly to repair other decks.
Before we move on to the mainboard, let's get the tape deck done. It must be removed anyway to get the mainboard and faceplate carrier out. Remove the bottom cover of the deck now.
If all you need to do is replace a broken cassette belt, heave a sigh of relief now. This deck, while being the hardest to disassemble, actually has the easiest cassette module to work on of any other Chrysler deck I've seen. You can replace the belt almost the second the bottom cover comes off. Two screws, and presto - it's right there. You will have to loosen up that metal plate covering the upper reel pulley to get a new belt in there, which does require removal of the module, but compared to the rest of this deck that's easy stuff.
Blue arrows indicate the screws holding the mechanism in, while red indicates the three connectors that must be disconnected to remove the module.
With the module removed, you can see another reason that I like this deck so much. The pinch rollers, capstans (blue), and tape head (red), could not be easier to access than they are in this deck. There is endless space to get in there and clean all these items. For a deck with as many wires and connectors to keep track of as this one, the cassette module is a real joy to work on.
Ok, let's get to the hard stuff - the mainboard. Again, this deck had a multitude of bad solder joints throughout, and many of them were on the mainboard. This is where it becomes absolutely critical to keep track of all the wires and connectors. Mark them with markers, use masking tape as labels, scratch markings into the connectors with a knife... just keep track of them somehow, or this deck will make you see red for days trying to get it back together.
This is the view with the cassette module out. Disconnect all the connectors arrowed in blue. Note the one arrowed in purple - that is the one that runs from the tape deck up to the upper PCB tray, and the one that will try to escape once you pull the mainboard and faceplate carrier off.
Also note the red arrow. As you pull the carrier forward, some wires at the joystick control will want to tangle around that metal support. Go slowly, so that you can free them up.
Before you actually pull the carrier away, turn the deck back over for a moment. Disconnect every last connector on the topside of the mainboard. Note where they go. If you don't need to remove the faceplate PCB, or have already done your work on it, you can leave those three connectors plugged in and the faceplate PCB attached to the carrier. There will be some wires up there that don't have a connector and are soldered down - don't worry about those. You will have already disconnected them on the other end by now.
Got them all unplugged? Good. Remove the two front center screws on each side that hold the carrier in. Refer back to my pictures of the side panels if you need a reminder. Then, lift up just enough to clear the tabs on the sides beneath where the screws go, and just pull the whole thing forward as a unit. Again, do it slowly so you can free up any wires.
And there you go - you now have access to all the solder joints on the mainboard. You can also get at the joystick assembly for cleaning from here, too. I'd show you how, but it's not hard to figure out and I've spent enough time on this unit.
Here's the topside of the mainboard, showing you once again the three connectors for the faceplate PCB in green. Make sure you clean both potentiometer assemblies as well while you have access to them.
Now, we'll get to some slightly easier stuff.
We're going to remove the power and filtering module to solder up the joints on it. This part is very easy from here. Remove the two screws holding it on (red arrows), and pull it out. There will be one connector to disconnect.
Here's the module with the solder joints exposed. If you need to replace parts, there are a few twist tabs holding the metal housing on to de-solder. Otherwise, this module is very easy to work on.
The module from the inside, arrowed in red.
It's time to remove the inner sleeve to have a look at the amplifier chips. Remove all remaining screws around the case, noting the longer ones used to hold the amp chips to the case. Then, take the back panel in one hand, and that metal support in the other, and just pull them right apart.
The inner sleeve slides right out, like so.
Now, we can work on the amp chips. Sharp eyes will have spotted one missing next to the gray connector. This one was actually taken from here to repair the Infinity deck you saw earlier in this article. That's right, they use the very same amp chips.
Here's one of them now, a Hitachi HA1388. These are good for 18 watts at four ohms with 10% THD. These are actually pretty common parts, and you can even find them as I type this on eBay.
So, do you remember where everything goes? Excellent. Double check all the solder joints in the deck before you reassemble it. Like I said, you will not want to take this unit apart more than once if you can help it.
Before I close this article, some pictures of my Infinity deck in operation, since my black one is currently not functional. This is the deck playing a cassette with the clock defeated. This particular deck was found on eBay, having been taken from a 1987 New Yorker. It showed up with a bad amp chip and a barrage of bad solder joints.
Finally, the clock re-enabled.
You might be thinking to yourself, wow that seems like an awful lot of work for a mid eighties cassette deck with no music search or auxiliary input functions. But, the fact of the matter is, I don't know of another deck Chrysler offered in the eighties that sounds as good as this one does. Even the problematic Infinity II sounded like a step backwards to my ears, and that unit was technically the next iteration of these decks. As long as you're willing to deal with the solder joint issues that plague these, you will end up with a deck that will make you happy for a long, long time.
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