RadioShack 8 Track Car Stereo Radio - not working

[TomRock]

Hello all,

I have a "Radioshack Realistic" 8-Track Stereo Tape and Radio in my car that has stopped working.

I am inexperienced with electronics and although I appreciate this isn't a very special piece of equipment I would appreciate any advice on how I could get it working again.

The radio gets a good 12v to the positive and the chassis is well grounded. When it has power the internal light turns on, and on insertion of an 8 track the motor spins. Everything appears to work, but there is no audio output.

I have tested it on and off the car with speakers that I know are good. I put an oscilloscope on the speaker outputs (with no speaker load) and the output on both is periodic waveform with about 0.03v p2p, which I assume is small interference from the motor.

Any advice would be appreciated, even if the advice is that I should pronounce it dead!

Cheers

Tom

[julie_m]

OK, so we know some power is getting to the unit, and it has worked before and stopped working as opposed to never having worked.

First question: Do you get any noise -- a thump, a crackle or a burst of static -- from the speakers, when power is applied or removed? If so, then at least the output stages probably are working. If not, then the fault probably is in the output stages. This could potentially be fatal, but it's still always worth a quick shuftie inside just in case there is an obvious fix like a blown internal fuse or fusible resistor.

If you get power-up noises from the speakers, try the set on radio. Just poke the stripped end of a few metres of wire in the aerial socket, as a makeshift aerial for testing. Even if you can't pick up any signals clearly, just interference, that proves the radio side probably is good.

If you can get a signal on radio, is the tape being pulled around? Tape heads won't generate any signal at all unless the tape is moving past them. These units usually have a belt drive from the motor to a flywheel which is attached to the capstan. If the belt has perished or snapped, there will be no tape motion, ergo no sound.

Are your eight-track cartridges OK? If the foam pressure pads behind the tape have perished, the tape will not make proper contact with the head, and you could well end up getting nothing from the speakers.

[Norman Raeburn]

Have you had the machine open, it's not something silly like a broken or stretched belt that has came off. Norman

[Mike. Watterson]

Likely the drive belt.
If you leave in tape and it never swiitchs track, that's likely.

[TomRock]

Thank you all very much for your responses.

No, there is no noise from speakers at all. Not even interference or say a squeak as the 8 track was pushed in and started. I remember before, when it was working, it used to make a squeak noise as the 8 track was pushed in. I have had a look for anything "ropey" looking on the inside but my untrained eye cant spot anything.

I have tried the radio (with aerial) but still unfortunately no noise (radio or static) either.

The belt is good and turns the big flywheel looking thing smoothly (not sure what this is called). I can also see the tape is being moved past the pickup head.

I have attached a couple of photos to see if that would help.

Once again, thank you, the help is very much appreciated.

I have remembered the previous owner had shorted the 2 (left and right) positive speaker outputs into one to power a single speaker with the mono button pressed in. Could this have lead to the failure? I could well have accidentally deselected the mono mode while listening to stereo radio. Would this damage the system?

There was no resistor or anything between channels (which I believe is how it should be done). They were simply in the same connector that went to one of the speaker terminals.

[coopzone]

I used to sell these, many moons ago, when i worked for Tandy.

I remember most of the kit we sold had a circuit diagram in the instructions - it may help people if took a decent photo of that and posted it.

Thanks

Derek

[TomRock]

Good idea! Please see schematic attached. It was in the back of the manual. I've studied it, but I dont really know what I'm looking at

I am not sure if the schematic will be legible with the compression, if not if it could help anyone I could email it to them if they gave their email in private message.

[coopzone]

By the time it's been uploaded to the forum the resolution is not too good, can't make out the part numbers etc. Perhaps you could upload it some place else then put a link on here?

Anyway from what I can see, and assuming you have a multimeter since you mentioned a scope before, you could start to check for continuity from the speakers heading back to the PCB near the two output IC's (two triangles middle of the bottom half of the diagram.

You could also check for signal using your scope on the inputs to the same IC's from what I would assume is the volume control, to the midle-left of the circuit

Referring to you first pictures, I would say the output IC's and connections are on the PCB left of the motor. I would think the yellow/blue/black wires are from the volume/tone control? you need to check for yourself - but you should see audio around here on your scope.

Start to check for voltages being present on the PCB as well.

My best guess so far would be the output IC's are kaput? What is the recent history. Did it really just stop working or did other thingS also happen on the car? New battery ? other electrical work etc etc.

Derek

[AD360 Rob]

Originally Posted by TomRock

I have remembered the previous owner had shorted the 2 (left and right) positive speaker outputs into one to power a single speaker with the mono button pressed in. Could this have lead to the failure? I could well have accidentally deselected the mono mode while listening to stereo radio. Would this damage the system?

There was no resistor or anything between channels (which I believe is how it should be done). They were simply in the same connector that went to one of the speaker terminals.

This shorting of the loudspeaker positives together to drive one speaker will undoubtedly be the reason you are now getting no audio. The output devices will have been damaged and will need replacing along with possibly the driver stages too as output device failure can cause a cascade effect taking other devices with them. The mono button usually is only active in radio mode on FM so that weak stations can be received without hiss.

[julie_m]

Originally Posted by TomRock

I have remembered the previous owner had shorted the 2 (left and right) positive speaker outputs into one to power a single speaker with the mono button pressed in. Could this have lead to the failure?

Yes. If one amplifier is trying to drive its output high while the other one is trying to drive its output low, and the two are connected together, there is going to be a very low resistance current path through the top transistor of one amplifier, the shorting link between the two speakers and the bottom transistor of the other amplifier. That's not going to do anything any favours.

Originally Posted by TomRock

I could well have accidentally deselected the mono mode while listening to stereo radio. Would this damage the system?

If the "mono" button was engaged, joining the two amplifiers at their inputs, then in theory the two outputs would always be trying to do the same thing. Disengaging the mono button would potentially allow the outputs to go apart.

However, wired up like that, with two outputs joined together, it was never going to last very long anyway .....

We might be looking at blown output amplifier ICs, and the only likely source for an exact match is a donor unit (they might have been used in other Tandy sets). Still, get some voltage measurements first so we can be sure before condemning anything.

[TomRock]

Cheers for all the replies. I have followed your advice and had a good play around with it. There doesn't seem to be any continuity problems from the speakers to the amp and I am fairly sure there is good signal going into the amp. It all points towards the output ICs being blown.

Derek, I am unsure of exactly when it stopped working as for a period of time I had no antenna and no 8 track cassettes that I liked very much! In that time I had done a few electrical jobs, but nothing I thought should have done any damage (same battery, same dynamo).

Thanks to everyone for the help but for the moment I am going to give up and possibly look for a replacement.

[Nickthedentist]

Not so fast!

What type are the output chips? Can't quite read the diagram. TA....?

[boxdoctor]

As Julie said, the output I.C.s have probably been ruined. If one went short-circuit, the other one would be loaded by a short, or near short circuit. That would put paid to that one too.
I notice the driver chip is AC coupled to the output I.C.s, so there would be unlilely to be any damage done to that one assuming the above situation to be the case.
As Nick cautions, do not throw it away until you are sure it can't be repaired, as new ones are (I believe) not now made. Apart from this, the build quality of that machine will be better than any modern one, if any are still produced.
Replacement I.C.s will almost certainly be available from some source on the net. Tony.

[Herald1360]

Even if they're not, the signal from the player could be taken to an inconspicuous little modern amplifier hidden in the dash somewhere and powered via the tape player switch for "transparent" operation.

[julie_m]

I had another thought. The set runs from a single 12V supply and the speakers share a common earth return. This means there must be a large electrolytic capacitor between each amplifier IC and its speaker. And it's just about within the bounds of possible for those capacitors to have sacrificed themselves for the good of the power amp ICs. When one output drives high and the other drives low, the energy available will be limited by what's stored in those capacitors. There were some amplifier ICs that had a reputation for blowing up if you looked at them funny, but there were also some that were as tough as old boots.

If you have a suitable capacitor to hand (anything from 220μF to 2200μF, 25V or more will work for testing) you could try putting the negative lead of this in series with a speaker and the positive lead on the IC output where the positive of the existing speaker coupling capacitor is connected. Also check for anything on the power amplifier board looking as though it's been hot.

[Christoffrad]

Likewise I would favour using an external audio amplifier if the replacement ICs are not readilly available. It is worth checking that the existing amplifier is not drawing excessive current having gone faulty. Measure the current drawn when the machine is switched on but idle. Or, more crudely, feel the temperature of the heatsink for the amplifiers after a minute duration.

[Hybrid tellies]

This 8 track player is certainly worth saving. Tandy Realistic equipment is fairly well built and 8 track machines are getting rather rare. I agree totally with Julie m the previous owner has probably destroyed the output IC or IC's. Even though both outputs are fed to the speakers by electrolytic capacitors if the speaker leads have been shorted out the output IC can easily be destroyed. I have seen this happen many times.
Before you condemn the output IC or IC's you need to check the voltages around them. I see from the circuit diagram that they do mark up some of the pins with voltages but unfortunately the circuit diagram is not terribly clear. As well as checking voltages there are other tests that are done before replacements are sourced such as injecting a low level signal, from your figure via a screw driver, to the inputs of the IC and if a scope is available we would also check the waveforms on the input to prove the rest of the player is working ok.

['LIVEWIRE?']

The easiest thing to check-voltage wise would be to see if there is between about 6 & 7.5v on the output pin of each I.C. These pins go to the positive terminals of the two speaker coupling capacitors, which should be easy to identify. they are almost always 470 or 1000uf 10volt electrolytics, whose negative terminals go via wires to the speaker plugs. A simple resistance check, with no power applied to the unit should show, too, whether the two caps. are short circuit.

[TomRock]

Hello All!

Update on the 8 Track player:

I gave up for an amount of time, moved on to a modern mini Bluetooth amplifier ("Nobsound" NS-01G) simply to allow me to power my speaker in the car without a head unit.

I then turned my attention back to the 8 Track as I missed using the player and was delighted to see all the replies I had received on the forum!

As suggested by many it seems the amps are blown. After much experimentation I have now "fixed" my radio by simply connecting an external amplifier (the NS-01G) to the preamp signial by piggybacking off a connection inside the radio.

Although it hardly counts as fixing the broken radio, I am happy that I am still using the old radio and 8 track without further damaging it. It now has more power and Bluetooth, which is a nice touch.

Again, thanks to everyone for the helpful comments.

Tom

[Herald1360]

Congratulations! You now have your vintage gear doing what it was built for- providing ICE