Bowtie-Bri

Looking at the back of my radio I noticed a harness coming out of the back with a round multi pin plug on the end of it. Is a harness that would of been used with a factory under dash 8 track player or is it for something else? Did all AM-FM radios come with this harness?

Yellowstatue

That plug is for the multi-plex (stereo) unit which is to be calibrated with the radio. If there is a sticker on it with a model number, we could give you a model year.

classicmuscle442

Could be a 70 radio that had a separate amplifier

Yellowstatue

It's a stereo multi-plexer not an amp. The book calls it a 'stereo multiplex adapter'.

quaddriver

there is a sep amp box and it includes the MPX circuit I think. I have one on the bench for a 70 toro I need to get into

oldspackrat

The separate amp box was mounted above the glove box insert. It's a pretty tight fit & may still be in your car? As far as I know, 1970 was the last year for the separate piece. They integrated it into the radio in '71 & onward.

quaddriver

I had a 74 in college that had this box behind the dash, factory amfm. I yanked it for the sony XR??R du jour and never paid it much mind, but from looking at the state of things, this setup is for stereo 68-75ish GM wide.

in another thread I am working on the 'gm radio project' in which I am deciphering the hows whats to get these working. I will cover the divorced amp and the changes to the FM section in it. problem is, the radio and amp pop the breaker on my 4 amp power supply. it appears that if the bias is off....oh heck, tech talk for the other thread. anyways, stay tuned.

rcorrigan5

Here are some shots of the multi-plex unit in my '69 4-4-2. The glove box is out so it's easier to see how the unit is mounted in the space above the glove box.

Randy C.

joe_padavano

Technically, the factory stereo adapter IS an amplifier. The stock stereo radio of this period uses the mono amp in the basic radio head to drive one channel for the front speaker. The "adapter" is just a second copy of that amp and is used to drive the other stereo channel for the rear speaker.

Interestingly, the factory 8-track has it's own pair of stereo amps built in and so completely bypasses the radio amps and drives the speaker directly. The switching takes place inside the 8-track unit when you insert a tape. This is why you could get the stereo 8-track even with a mono radio (though the rear speaker option was required).