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[cam.in.head]

hi .no jts not that model .apparently and confusing the san diego dj name was used on different models.i did struggle to find a schematic when i was trying to sort a previous issue of distortion on cassette only .i can find a pic and email it to you if that would help ?

[LC0112G]

The circuits seem to cover lots of models - all basically the same, but followed by another city or state name. Cleveland, Dresden, Boston, Madison, Freiburg, Las Vegas, Canberra, San Diego, Essen, Alicante, Lausanne, Konstanz, San Remo, Santa Cruz, Dusseldorf, Kansas, Bologna, Hawaii, Maryland, Munchen or Modena.

Is there a numeric part number on the radio? I think the San Diego DJ RCR 127 should be "8 646 713 310"

[cam.in.head]

on the side it says
7 646 764 310

[cam.in.head]

ok so slight update on this !
the unit has worked fine in the house every time and could  continue for some time (or go faulty tommorow !)
so .as i have a few of these units i went back to trying to solve one of their other common issues which is distortion on the cassette playback .( i have 3 that do this)
this started years ago and i never solved it then (just bought a replacement off ebay)
the radio section is fine and the cd control section is all fine and crystal clear . the cassette is distorted .my replacement unit has gone the same too !
i have spent every evening with a perfect deck and a faulty deck swapping components but to no success .
so far i have swapped 1st stage preamp chips ,second stage transistors and virtually every electrolytic including all the coupling ones in the sound path between stages .i have even swapped the tda4748 main ic (that was a bugger) between units andcthe distortion still stays on same unit .
tried tapping the left and right channels off into my domestic hifi amp and the signal is crystal clear .
so its crystal clear where it goes into the tda ic but the tda distirts it but the ic is ok ?
ttied powering the cassette motor from a 9v  battery too !
messaged dave dnd fof advice but not interested !
so any other pointers  OR are there any radio repair engiineers who i could actually visit and go through it with me ?
help !!

[Marks DTM Calib]

The cassette motors generally have a sub board in them to regulate the speed, so check the motor (or swap it) and also check the mechanism for dry grease, tight spindles etc

[cam.in.head]

tried that .the complete decks were the first thing i swapped !
its as if the coupling capacitors are faulty but swapping them unit to unit doesnt swap the fault !
its very frustrating !

[cam.in.head]

one more update (for anyone whos interested!)
ive just found and bought 2more units off ebay .one works fine .other has slight cassette distortion !
also i tried a  previously perfect new old stock unit that i fitted into my carlton a few years back and lo and behold the cassette is very bad on that one now as well !
so even if its being affected by the cold now im 100% sure they never used to be so theres something very weird going on especially to affect so many identical models all with an identical issue.   i did message dave dnd at the begginning of this thread as im sure he would have worked on many of these sets in the day but he wasnt too helpfull !   fair enough hes running a business and doesnt seem to be on this forum anymore .
my next move is to either buy one of the small preamp boards of try to idolate ond from another unused different old radio cassette !

[LC0112G]

Can't see it being related to motor speed. That would normally result in either slow/fast playback or more likely a wow-ing type variation in the audio output as the motor speed hunts up and down.

My first guess would have been it's related to tape pickup head alignment. It doesn't take much misalignment on the tape/head to cause a muffled type sound. However, if I understand correctly you're saying you've picked off the signal prior to the main amplifier and played it through a different amp, and it sounds ok? If that's correct, then probably not alignment.

Have you got a signal generator and an oscilloscope? If so you can usually inject a signal somewhere close to the tape pickup heads and then follow it through the circuit using the scope to see where the distortion is introduced. I'd go with a 1KHz signal at low amplitude (perhaps a few mV) to start with and crank up the amplitude once you're happy. Distortion on pure sine waves is usually pretty easy to spot by eye (on a 'scope), and even easier to hear.

There are some fairly complex frequency compensation circuits on cassettes due to the different charicteristics of various metals used in the tapes, plus other compression stuff (Dolby B/C for example). Don't rule out faults in these compensaiton circuits.

Haven't got time to look deeper today - need to go to Street/Clarks Outlet centre tonight. Shops close at 7pm and parking is only free after 6pm so a narrow window to get in and out without paying.  Not looking forward to it. 

[cam.in.head]

hi .no its definately not a deck issue ive swapped decks ,that was the very first thing i did years ago and now as well .i dont have a signal generator or oscilloscope but yes if i seperate the sound circuit after the head preamps and the main input ic (at the coupling caps) then i get a good sound out AND a good sound if i feed into the main ic so this should be impossible .one way should be faulty but it isnt .im back on it now and looking at dome smd caps see if changing them makes a difference !

[LC0112G]

Ok, survived Clarks village - just.

Thinking about it on the drive home, it seems unlikely that you would get distortion on both left and right channels due to a single fault if the fault is in a capacitor or resistor. I mean all the circuits I've looked at the signal path is basically separate for L and R, so a single capacitor in the signal path going duff might distort one channel or t'other but not both. The chances of both L&R capacitors failing at the same time is tiny, and the chances of the same thing happening on multiple radios? Nahh, Occam's razor says it's something else.

You mention a TDA4748. Are you sure of that part number - or could it be a TDA7348? TDA7348 appears to be an audio processor & switch, switching the input between radio and tape inputs, and is used in lots of Blaupunkt stuff. TDA4748 is an obsolete switch mode power supply chip, and I can't see it on any of the circuit diagrams I've found.

The other possibility is power supply. When the tape deck motor is running the unit will probably be drawing 1A-2A more that when it's running on radio or CD. I'm wondering if this extra power is enough to cause the supply to drop and/or get noisy, enough to upset the supplies to the analogue sections of the tape and speaker drive circuitry. That would be enough to cause distortion on the 'tape' inputs, but allow the radio and CD to operate normally. It would also affect both L & R of the tape equally. It might also explain why if you pick off the tape signals and feed them into an external amp there is no distortion - the speaker drivers will also take a lot of power - perhaps there is only enough power to rune either the amps or the motors, and not both?

To test that theory I'd locate the supply pin to the tape deck - probably +12V - and power it externally from a bench supply. Need to be careful though - you don't want to blow up the cassette motors.

[cam.in.head]

hi .just checked yes sorry its a 7348 ic !.  im on a similar way of thinking and did power the drive motor seperate from a 9v battery ! still distorted!
this is why its driving me mad.if the input to the 7348 works fine (i swapped this chip from other unit) and the output before it is good when fed into another amplifier then the only common link there is the coupling caps but swapped them too . its theoretically working perfect on paper but not in reality .   
ai is totally useless on questions like this too as it tells me i could have a faulty tape,dirty or misaligned head ! all basic stuff which are eliminated by swapping deck complete .
please dont let this get into your head too and drive you mad as well ! its just something that shouldnt be happening and is common to a few units now so once i find the offending part it should mend them all !

[LC0112G]

I think it's going to be a struggle to diagnose without an oscilloscope.

The TDA7348 appears to have 3 identical stereo inputs. Pins 8 & 12 for the radio, 9 & 13 for the tape, and 10 & 14 for 'Aux' which I'm guessing is the CD in this instance.

The tape head pre-amp looks like it's a dual op amp - either an RC4558 or LM833. However, there is a mute circuit between the output of the preamp and the tape input of the TDA7348 which is controlled by the microprocessor. If the mute is being turned on and off rapidly it could sound like distortion and would affect both L & R channels.

So basically I'd want to 'scope the output of the preamp, the input to the TDA7348, and the mute circuitry. It should be obvious from these where the distortion is occurring.

[cam.in.head]

thankyou for that .il try work out the mute circuit and disable it .see what happens

[cam.in.head]

after further testing i have concluded that "some " rogue component in the second stage of the preamp (just a transistor in second stage)is causing the issue .even thou everything seems to test similar in values to a good unit .im going to try some freeze spray when i get some but in the meantime on one faulty unit i have fitted the standalone preamp board from a philips 752 this morning which has eliminated the problem completely .thus proving my theory that there something there which is either a design flaw or an age related one .either way this does solve it and make the xassette crystal clear again .needed slightly higher coupling caps (4.7 instead of 2.2uf)
 other idea which im contemplating is modding to a usb stick input as well as or instead of the cassette .might try this on one unit to see how it sounds .
the realy annoying thing here (althou its a learning curve) is the amount of parts /ic's etc that ive swapped over now and years ago when i knew less about what i was doing ! and didnt need swapping at all !?   hey ho !

[STEMO]

after further testing i have concluded that "some " rogue component in the second stage of the preamp (just a transistor in second stage)is causing the issue .even thou everything seems to test similar in values to a good unit .im going to try some freeze spray when i get some but in the meantime on one faulty unit i have fitted the standalone preamp board from a philips 752 this morning which has eliminated the problem completely .thus proving my theory that there something there which is either a design flaw or an age related one .either way this does solve it and make the xassette crystal clear again .needed slightly higher coupling caps (4.7 instead of 2.2uf)
 other idea which im contemplating is modding to a usb stick input as well as or instead of the cassette .might try this on one unit to see how it sounds .
the realy annoying thing here (althou its a learning curve) is the amount of parts /ic's etc that ive swapped over now and years ago when i knew less about what i was doing ! and didnt need swapping at all !?   hey ho !

Hallelujah !!!!