68 - 69 am/fm stereo question

[My68SS]

I understand that the multiplexer box gave you left/right and front/rear fade control, what were the other two controls for?
Was it fed with left/right speaker out from the radio, or is the fade box also a stereo power amp fed with line level from the radio?

[JohnZ]

<<I understand that the multiplexer box gave you left/right and front/rear fade control, what were the other two controls for?  >>

Here's the control in the Estes Z/28 convertible. 

[My68SS]

Cheers John,
Having volume and tone controls would strongly suggest that it also has its own stereo power amp, or perhaps even a 4 channel amp.
That would also strongly suggest that it was driven by line level from the radio, with the signal being picked up before the vol, tone, balance controls in the radio.

I assume the radio still had it's own stereo power amp that you could connect a pair of speakers to? and then just use the radios own controls? so the radio could be used in stand-alone mode and you would still have am/fm stereo?

[JohnZ]

Cheers John,
Having volume and tone controls would strongly suggest that it also has its own stereo power amp, or perhaps even a 4 channel amp.
That would also strongly suggest that it was driven by line level from the radio, with the signal being picked up before the vol, tone, balance controls in the radio.

I assume the radio still had it's own stereo power amp that you could connect a pair of speakers to? and then just use the radios own controls? so the radio could be used in stand-alone mode and you would still have am/fm stereo?

What I know about radio/stereo circuitry you could put in a thimble and still have room for your thumb; maybe Mark C will chime in - he really knows that stuff.  :-)

[My68SS]

From pics I have seen of the whole setup, the radio is still just mono output as there is only 1 output transistor [DS501]
The multiplex unit that bolts up under the dash has a similar heatsink as the radio with another DS501 transistor mounted on it and must also be the left/right mpx decoder.
The console mounted controls may well be just a bunch of dual gang pots with an r/c network for tone. Mark C, where are you? I/we need help.

What was the first chevy to have am/fm stereo as a single unit receiver i.e. all in one, no external bits?

[Mark]

They 68 and 69's) are completely different animals, 69s send the unamplified mono signal over to the multiplex unit, where it splits it up, amplifys it and sends it back to the main head unit so that the tone, balance and fade controls can be used.  68s are one of two configurations, one works pretty much like the 69 version, the other is an inline multiplex adapter wher ethe unamplified signal goe thru a remote multiplex unit, then a remote tone, volume, balance, and fad control unit (like pictured above) and then out to the speakers.  eitherway you can't use a muliplex radio without the muliplex unit.

[My68SS]

Hi Mark, glad you dropped in here 
Am I right in assuming that for 68's e.g. the right channel was fed back to the power amp in the head and the left channel went back to the multiplexer to use its power amp?
Where was the fade control in 69's - not enough shafts on the head?
Do you have any functional block diags. that show the interconnections and signal paths between the 3 components in 68's?
Cheers

[Mark]

Sound like a controls engineer, functional block diagram.  Yep I have them.  These are the 68 setups, one using the radio knobs for adjustment, one using the remote head n the console for adjustment.

[Mark]

And I guess tone is not the correct term for adjusting these radials, all they have is volume, fade and balance.  There was no way to adjust treble and base, as they were not truly a stereo radio system, the were basically a mono radio with a splitter and delay circuits that gave a fake stereo sound.  There was only one channel input going to the multiplex modules from the radio head units.

This is the actual 68 remote control multiplex setup, this one is slightly different from the one in the circuit above because it has 2 cables that connect the remote head to the multiplexer and the speaker harness attaches to the multiplex unit, while the diagram has the speakers connected to the remote head. but they are essentially the same.

[My68SS]

Thanks heaps for the pics Mark, has cleared the muddy water quite a bit!
With regard controls, if we just consider the volume control for the 68 setup with remote controls, we end up with two volume controls, one on the radio and one on the remote panel. Did both volume controls affect the volume level?

I'm pretty sure the multiplexer is a legit mpx stereo decoder, not simulated stereo.
The grey lead running from the head to the multiplexer in you pic above is muticore [from other pics I have seen] and I'm sure it would have carried the composite demodulated audio [L+R, L-R and 19khz pilot] from the radio to the multiplexer for demuxing to left and right stereo.

The other give-away is the green stereo light on the remote controls, if that is off when your not tuned on a fm station or the station signal is weak and on when you tune a fm station of good signal strength, then that's pretty good evidence that it is a true mpx stereo decoder that's driving that light.

The audio path required to include front/rear fade when you only have two output stages, is rather convoluted I must admit, especially since the two outputs are located in different places!

I think the difference your seeing between your pic and the top wiring diag. is that your pic is of the setup required when you have D55 - console, whereas the wiring diag. is what you have when you don't have a console - multiplexer and controls all in one unit.
This is also borne out by the fact that it says on the controls for the top diag. "Remote control head - Part of 7305221" which is the multiplex adapter.
So the speakers are still connecting to the multiplex adapter, it's now just in the same box as the controls.

[My68SS]

Had a realization about a couple of things.
For 68 at least, U69 am/fm and U79 am/fm stereo is still the same radio 7303241. U79 just had the extra bits to go with the mpx decoder [multiplexer] 7305221 w/D55 console [console mounted controls] or 7304621 w/o D55 console [mpx and controls in one unit under dash]

So it seems that for those radios without the stereo multiplexer, there's still a [dummy] plug in the multiplexer adapter socket on the underside of the radio, to link the composite audio back to the audio amp in the radio.
This plug can be seen in many/all pictures of the am/fm U69 radio you find on the net.
If that's true, then any U79 radio can be made to work 'stand-alone' as per U69 so long as you insert the dummy plug or figured out which two pins [holes] to link together.

I also suddenly realized that auto radio designers would have suddenly found themselves with their backs to the wall when fm stereo hit the airwaves.
The concentric dual gang pots for volume and tone were not going to work in the stereo world, you needed quad gang - two for volume on the inner shaft and two for tone on the outer shaft - all getting a bit cumbersome in what is already a tight space inside an auto radio.
No wonder Delco decided on separate controls for fm stereo. Other radio designs had separate controls built in to the radio, but sitting side on 'thumb wheel' style.

So the volume/tone in the radio must not do anything if a multiplexer is plugged into it.

Edit: I've just noticed that there is a conflict of multiplexer p/n's and wiring diag. arrangements as referred to in the last paragraph in my previous post.
If the wiring diag quoted 7304621 ilo 7305221 I'd be happy, but it doesn't....hmmm

[My68SS]

Mark, your pic is unique, all the pics I have stored of 68 U79  console controls show the speaker connections coming from the controls box.
The 68 AIM is not totally clear, but does seem to show this - U79 A3 item 4.
The 69 AIM looks another minefield with regard U79 and tape player, will navigate that later! 

[Mark]

I also have a picture of a 221 remote head with the speaker connectors coming off of it and a single composite cable coming to it from the multiplex unit, and this one with two composite cables out and back to the remote head, and the speaker connection coming off the multiplex unit.  Its probably an early/later production kind of thing, don't think its a console, non console thing, becuae if you don't have a console, where do you mount the remote head?  If you didn't have a console you had to use the 621 radio instead of the 221 remote setup.   It was probably easier and would allow you to use the same underdash harness as the 621 radio setup if the speaker wire connector was at the remote multiplex unit which mounted up next to the steering column under the dash, then all the way out next to the remote head in the console so I think the one I have posted is a later version than the one in the diagram.

[My68SS]

Yes, I initially thought that the diag. was showing the multiplexer with integrated controls - under dash style = no console, simply on the basis that it says  "Remote control head - Part of 7305221".
However, the diag. clearly shows a harness connecting from the multiplexer to the console mounted controls and has the word console in brackets at the top.
Was getting late at night here - 'what was I thinking'! lol

I think your right that the one you pictured is a later revision. Maybe the delco designers decided that a single cable with power output audio [speaker level] running through it was too much of a crosstalk problem, so split the single harness into two harnesses, or perhaps they decided that two harnesses with a 6 pin plug on the ends was easier to manufacture than a single harness with an 11 pin plug on it.

Also just as a side note, when I use the word 'composite' I'm not particularly referring to the type of harness [multicore] but rather the signal coming from the radio to the multiplexer. Only one shielded single core cable is needed to carry the composite L-R signal from the radio to the multiplexer.
The other wires in that harness [radio to multiplexer] would be for carrying one channel of the decoded/volumed/toned/L-R balanced signal back to the power amp in the radio and + - power.
The other channel is fed to the power amp in the multiplexer.
Then those power outputs would have to be fed back to the controls for front/rear fading, at least this is how I'm understanding it so far.

So to summarize U69 [am/fm] and U79 [am/fm stereo] for 68:

U69 am/fm
Radio pn 7303241 - could be ordered with aux. rear speaker U80, but not rear antenna U73.
U80 also includes the fitting of a front/rear fader to the tuning shaft.

U79 am/fm stereo
Radio pn 7303421
Stereo multiplexer with integrated controls pn 7304621 [for vehicles without D55 console]
Stereo multiplexer with separate console mounted controls pn 7315221 [for vehicles with D55 console] [two versions of wiring harness have been noted]
4 speaker system [2 on rear parcel tray, 2 in front kick panels]

edit: Ah, I see now that the 7304621 arrangement is drawn at the bottom of the other page in your pics

[My68SS]

What was the first chevy to have am/fm stereo as a single unit receiver i.e. all in one, no external bits?

Have found a part answer to this question. Seems that 73 Camaros had am/fm with integrated stereo and fader - 4 speaker connections on the rear.