Yeah, I finally decided to finally take the plunge and buy a used Philips-Magnavox D8300.  (They seemed to have reliable tape units and offered an unusual-looking monolithic design at a reasonable price.) I even have a service manual for it on order.  The Yellow Sub worked mostly fine out of the box, FM and AM come in clear and both tape decks play, but there are two little issues besides the usual aging electrolytic capacitors...

One:  Line-in isn't working (almost). 

I've followed the directions by other posters for using it, but the line in function seems dead.  Mostly dead, as driving it with a sufficiently hot source (my computer at full volume) produces a faint signal at high volume on the boombox, as if some transistors are being driven into their operating range.

Two:  The tape decks are both a semitone or so flat regardless of power source or tape used, indicating low tape speed.  Both running slow is a bit unusual, though...

I don't have the service manual for the RX-4 tape recorder components used by the D8300, so I really need some help here.  Belt tension seems fine and all of the parts are moving freely, at least.

I've done some exploratory surgery on the box, the boards and components look intact.  I did find a loose piece of nylon about the size of a fingernail loose in the casing, but it doesn't look like it snapped off of something.

Um...so, I'm wondering if any of you folks out there might know how to fix these problems.  (In addition, does anyone know a good place to look for a replacement shoulder strap?)

Chances are as it's a budget box..both decks only use one belt for the pair of them...hence both decks running slow...you can do 1 of 2 things...1 replace the belt or 2 you could take it apart..there will be a little hole in the back of the motor...you could turn this (cant remember wich way) as there is a speed ajuster inside that hole...1 way speeds things up..other vice versa...this may only be a temporary fix though..

to your other problem...i ain't got a clue..maybe sombody else can help...good luck dude..

Line-in only works when the cassette is in record mode as this was meant to be recording from something - hence the line in - Not as you percieve it to be as a Nother source to be able to play your what-Ever through this unit to be able to hear your "" Outside Source "" through this unit ... It is NOT gonna happen ... Did you happen to notice that there is "NOT" a dedicated switch for the line-in.... sorry ... yes you can hear a little when the cassette is in record mode with or without a cassette in the deck - provide you "know" how to do that - that is to fake it into thinking "it" has a cassette in it whilst it is recording... whew...

RR:  The belts look okay and aren't slipping, otherwise the two pitches would be different.  I'll pop the motor out, then.

EDIT:  Poo. No adjustment point. I should have bought that Panasonic.  Oh well.  (Or it's a bad capacitor on that motor, if that results in a series resistance...)  Then again if the workaround for issue 2 works, I really don't need to worry about it.

JCP:  I've figured that part about tricking the cassette mechanism by pressing the write-protect lever in.  Further exploration reveals that there's a single-in-line device with a switching mechanism moved by the record button mechanism.  Well, it should be moving, but it isn't, because the part that links it is missing...and it seems like the loose nylon piece floating around the casing might have been part of it.

Since I'm not planning to use too many tapes with this (most of my music is CDs and assorted digital audio), I came up with a workaround...a little bit of modeling sprue and electrical tape that holds that lever in the down position, enabling the line in.  Friction-fit, removable without damaging the board, yet won't fall out if the boombox is jarred.  Let's see if it works...

 Let's see if it works... .... and ????

...and it does.  Well, at the cost of rendering the second tape deck unable to play back anything (unless the bit of model sprue is removed), but hey, one working playback tape deck and a working line in jack and it's good enough for my purposes.  *remembers to stick note for next owner in his files for when he resells the unit...that five-speaker TDK unit will go on sale someday*

The tape speed issue...not going to worry about it.  I think I'm the only one that notices it, anyway.

EDIT:  And fiddling with the belts and rocking the motor's spindle back and forth a few times when I was messing around inside the boombox appears to have fixed that issue too, it's now more-or-less up to speed.  Yay!

Great that you got this one working... now also you may wish to know that this ^^^ above also applies to many 'other' magnavox's that have the same "ilk" uh, issue... as they weren't designed to work with a line in as present day people would like "Them" to do ( like for with their 'UGH !!' Ipid thingys thingy's n such )  when they know that there is a line-in and then find out that it is NOT a true line-in... Like on the MORE Quality Boombox's out there - I Know as I have one of these D8443, and the black-n-yellow 8300 magnavox's gee i think that there may even be a video out there about this very issue http://www.youtube.com/watch?v=-IwFVRIiabk WOW Look at that !!!

when you add that note you can add this video as well

http://www.youtube.com/watch?v=MaZP-Axr4Lw

Pink ...

*has it playing in the background*

Thanks for the help.

Moral:  I should have asked around a bit more...and probably just aimed for one of the nice Panasonics.

As far as i know...on this model both decks use one motor between them...good to know you have one deck working though....