Motorola car radio and 8-Track player - Help!

[Chris in FL]

Hello to all!

I bought a couple of months ago a US Motorola car radio from the 70's

A so called „Motorola TF852AX AM FM Radio and 8 Track tape player (stereo)“.

Pristine cosmetically but unfortunatelly not working – or at least not fully.

The radio was clearly messed up some day in the past and I succeeded to fix the mechanics of the tape player. It is even working now.

The issue I am facing now is that the left speaker has (let’s say) a max. 10% of the volume of the right speaker.

The sanity check of swopping speakers to check if something in the speakers itself or their wiring was done with negative results. Everything fine there.

Going with the balance from end lock to end lock results in one end having zero volume on the „ill“ left speaker and 100% volume in the „healthy“ right speaker; and in the other end having 10% volume in the "ill" left speaker and zero on the "healthy" right speaker.

I conclude I do not have a balance setting problem but just an output power issue on the left speaker.

All this said and based on the diagram that thankfully came with the radio and I attached here below, would you coach me please towards the most suspect components being able to cause such symptoms?

I am not an electronics guy but I yes can solder and I have a conventional multitester and very basic understanding.

I have recapped a couple of old-timers and often had success so I was tempted to replace those C131 & C132 capacitors (twice 10V 1000mF) at the exit of the louspeaker outputs. Would a defective capacitor there bring the volume down to 10% of nominal?

If there is anything else I can check (I have the cabinet already open), please let me know!

Many thanks in advance and kind regards!!!

Chris

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[Station X]

Hello and welcome to the forums.

As always with this type of fault involving a stereo system, you should check and compare voltage readings for the two channels. This will give you an idea as to where the fault is.

The diagram you posted appears to show what voltages you should expect.

[Chris in FL]

Indeed! Thanks, Graham. The diagram says at "vtvm 10%, no input signal"... what does vtvm means? Just volume? Thx, again!

[paulsherwin]

VTVM normally means Vacuum Tube Volt Meter in American schematics, but the context should confirm this. A modern digital multimeter should give similar readings. I don't know what it refers to though.

[Chris in FL]

Thanks! An the "10%" attached to the VTVM? What could be the meaning of this?

['LIVEWIRE?']

The speaker coupling capacitor could be at fault, but, as Graham says in post #2, check and compare the voltages between thw working channel and the faulty one. The amplifiers are DC coupled, so (e.g.) if one output transistor was faulty, this could have a 'knock-on effect' on the other devices in that channel.

[Chris in FL]

Duly noted, Gentlemen. I will do my homework and report back the results. Thanks!

[Chris in FL]

I measured voltage at both sides of the electrolytic capacitors at each loudspeaker output. Readings are almost identical and compliant or slightly above those to be found in the diagram (car battery at 13.0V). Is this measurement conclusive of anything? I can check further voltages and I would appreciate any help to tell me what to check exactly for next... I have to admit that finding the other points it's less easier than those above as these are in the PCB and to find them is challenging...

After the mesurements no change: the right loudspeaker outputs max. 25% of the volume of the left one. Balance set of course "in the middle"...

I will thank any lead to check for! I attached a small note with the few points I could measure so far... Many thanks to all!

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[Tiggers]

I know my limitations, so this could be a very naïve comment; damage in the wire to the ‘quiet’ speaker; have you tried disconnecting and jury rigging a replacement? If a speaker wire is down to its last or chafed on bodywork somewhere the output will earth out and not reach the speaker. Please don’t shout if this is a really stupid suggestion

[julie_m]

It's a stereo amplifier and one channel is working properly; so you have a known good set of readings. Also, if the fuse did not blow instantaneously when power was connected, that means the output transistors probably are intact (they usually fail short-circuit).

What you will have to do now is identify each transistor on the circuit board; and measure the voltages on each lead -- collector, base and emitter -- of each transistor in the working amplifier channel, starting at the input and working towards the output. Then repeat the same measurements at each lead of each corresponding transistor in the faulty channel. Once you have found a discrepancy between the two channels, you know the fault must lie somewhere nearby there.

It's worth taking the time to print out the wiring diagram and some photos of the PCB, taken from a few angles, in advance, and getting some coloured pens or pencils which you can use to mark them up. Then you can work out at your own leisure which transistor is which, and plan a schedule of tests. That way, you will not need to keep the set powered up for any longer than strictly necessary; just as a precaution, in case the existing fault is quietly over-stressing some other components. You don't want two things wrong with it.

Post your readings here, and someone is bound to be able to make sense of them.

['LIVEWIRE?']

Originally Posted by Tiggers

I know my limitations, so this could be a very naïve comment; damage in the wire to the ‘quiet’ speaker; have you tried disconnecting and jury rigging a replacement? If a speaker wire is down to its last or chafed on bodywork somewhere the output will earth out and not reach the speaker. Please don’t shout if this is a really stupid suggestion

IMHO, that is a very good suggestion. A check with a multimeter (analogue or Digital) between the Negative(speaker terminal connection) of the faulty channel's output coupling capacitor and ground will almost certainly show a lower resistance that the 3 or 4 ohms of a good speaker. If there is an intermittent short to ground, the resistance measured may vary as the speaker leads are wriggled. Use of an analogue meter will also produce 'clicks' in the speaker. Another possibility is a high resistance connection in one or other of the speaker leads or the terminals to which they are connected. Please note that the above tests should be carried out with the stereo switched off I once had a similar problem with a later model car stereo whose output leads sit at approx. 6.5v wrt ground. This set worked perfectly on the bench, but when refitted to the vehicle, one channel was silent, due to one of the leads in the car being shorted to earth by a screw in the trim under which the wires had been run.

[Chris in FL]

Hello everybody!

Sharing here a pair of extra facts with the hope of narrowing root causes:

- I read stable 3.5ohm in both speakers and by swapping them, both work fine
- I checked continuity of all 4 wires to the speakers. Also everything fine there
- Wriggling wires while the radio powered on cause any effects. Continuity of cables seem solid

Thanks, Livewire. Thanks, Tigger.

Julie: I understand by your statement that having 5.2V upstream of each of the last electrolytic capacitors (10V, 1000mF) at each speaker output doesn't mean much... You say now voltages between N and both Ps of each transistor should be measured and search differences... Is my understanding correct?

I have a very difficult access to the transistors (see picture) so I will try to remove the baseplate holding the transistors from the chassis and have a better access to the pins. I think I will need to earth also the baseplate if I remove it...

I will post results!

Many thanks to all!

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[steve1010uk]

Could be worth checking C119

Steve

[Station X]

Originally Posted by Chris in FL

I have a very difficult access to the transistors (see picture) so I will try to remove the baseplate holding the transistors from the chassis and have a better access to the pins. I think I will need to earth also the baseplate if I remove it...

I think you'll find that base plate is a heat sink. It may be insulated from the rest of the chassis.

Slip some sleeving (old wire insulation) over all but the very tip of your meter probe. You may then be able to probe the transistor leads without shorting them to something else.

[Chris in FL]

C119 at the gate of the defective channel! That could be one, Steve! How should I test that capacitor?

In the meantime I did some homework. I think the easiest to understand this ignorant is to better see the attached pictures.

I measured all voltages in all ten transistors in the final stage before the output to the speakers. I did not checked absolute values against the diagram but I yes can say that a have a very high simmetry between both channels and one of them works properly so I am lost again.

Specially remarkable was that by just touching with the multimeter the central contact of two symmetrical transistors (see second pic), by touching the one in the defective channel, the volume in the speaker greatly recovered (without distorsion) and by touchin exactly the same contact in the good side, a horrible crackling like a gun machine was to be listened in the good speaker. Is this giving any hint?

Thanks, Station X, for that advice. I exactly did that and took care to isolate the body of the heatsink with a piece of rubber to avoid silly shortcuts...

Thanks again for the patience!

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[camtechman]

Just to clarify..... is your 'balance' problem the same on both the radio & the 8 track ?

[Chris in FL]

Yes, it is!

['LIVEWIRE?']

Could this mean that the speaker coupling capacitor, C131, has lost capacitance or is virtually open circuit? A quick way to test this would be to temporarily connect another 470 or 1000uf capacitor in parallel with it, making sure the polarity of the added cap. is correct -i.e. +ve to the transistors. Similarly, if you suspect C119 is either o/c, or has lost capacitance, connect a similar known good capacitor across it. In either case if the volume & quality of the sound is now similar to that in the other channel, then remove the original capacitor and fit a permanent replacement.

[Station X]

Originally Posted by Chris in FL

Specially remarkable was that by just touching with the multimeter the central contact of two symmetrical transistors (see second pic), by touching the one in the defective channel, the volume in the speaker greatly recovered (without distorsion) and by touchin exactly the same contact in the good side, a horrible crackling like a gun machine was to be listened in the good speaker. Is this giving any hint?

Could this be a mechanical problem such as a dry soldered joint? Try touching the same points with an insulated object and see if you can get the same result.

[julie_m]

The plot thickens in our little detective story

We know that the DC voltages are the same at corresponding points in both channels, yet the AC performance is different between the two channels. What behaves differently to AC and DC? Capacitors! (And inductors; but there are none in this circuit.) When the circuit has been powered up with no signal for long enough to reach stable equilibrium, all the capacitors will be charged to a steady voltage. In fact, if you disconnected all of the capacitors, the final voltage would not change; because no current is flowing into or out of any of them, so they have no effect on the steady-state conditions. So if a capacitor was faulty, depending on what was wrong with it, it could conceivably have no effect on DC conditions. If it was leaking current, then it would appear to have an extra resistor in parallel with itself, and the DC voltages would be affected. But if it had just lost capacitance or gone high-resistance, this would not show up on DC voltage tests.

So let us return to the circuit diagram, and this time pay attention to the capacitors and how they could affect the circuit's behaviour if low-capacitance or open-circuit.

C131 (C132) is the loudspeaker coupling capacitor. This blocks the 5.2 V DC at the junction of R139 / Q25 (R140 / Q26) from reaching the loudspeaker, but admits any changing voltage. Electrolytic capacitors do die of old age, and if this was faulty then it would certainly result in loss of volume. C119 (C120) performs a similar function at the input; blocking the DC voltage at the base of Q17 (Q18) from being affected by the volume control R113 (R114).

Other likely candidates are C121 (C122) and C125 (C126), the emitter bypass capacitors of Q17 (Q18) and Q19 (Q20) respectively.

A small current flowing into the base of Q17 (for argument's sake; but the same logic applies to all of Q18 - 20, with appropriate substitution of passive component numbers) causes a larger current to flow into the collector. This will cause the voltage across R125 to increase; so there is then less voltage between Q17 collector and earth. But. All the current flowing into the base and collector has to flow out of the emitter (Kirchoff's first law). If C121 was not there, the increased emitter current would also be flowing through R123, so increasing the voltage across it; and you can see, this increase in voltage at the emitter is acting to oppose the reduction in voltage at the collector. C121 prevents the emitter voltage from changing suddenly. So all the extra current can only flow into it, and it has enough capacitance to ensure that the voltage change will be very small (and in any case, if the next half-cycle is a mirror-image of the previous one, then that will just undo the change). So the voltage change at the collector for a given change in base current will be greater with C121 in circuit than without, because it prevents the transistor from fighting against itself.

In case the above caused you or , just remember that a capacitor at the emitter of a transistor gives it more gain.

Now, capacitors are cheap enough, so I would suggest just to replace all of C131, C119, C121 and C125; in that order, and testing between doing each one and the next. That way, if you make a mistake and introduce a new fault, you will know about it straight away. Don't be surprised if the replacement capacitors are smaller than the ones in your set; that is just due to the cumulative effects of years of improvements in manufacturing techniques. Modern electrolytic capacitors also usually have both connecting leads brought out of the same end, which is a preferrable arrangement for robotic assembly, so you may need some creativity to wire them in. Just make sure you get the positive and negative the right way around (except for C119, which is not an electrolytic capacitor and so will work O.K. either way around). 470 nF (0.47 µF) is the nearest commonly-available modern value to 0.5 µF, and will work fine as a replacement.

Also, you probably will end up having to replace C132, C120, C122 and C126, if the repaired channel ends up sounding better than the other channel!