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Capacitors are good at causing hum, but it could be something like a semiconductor. Since it's in both speakers you are looking for something common to both channels. So could be in the power supply part. Another possible culprit is an IC where both channels go in. IC and Transistors that cause problems as the get hot can be traced by blasting the device with Servisol Freeze Spray. You wait while it's at its worse. Then spray the suspected devices one at a time. If the hum, goes when sprayed you should have found the faulty part!
Take these measurement and either look on the MCM Electronics web site or Mat Electronics web site. The Diameter does not help much the measurement they use is Inner circumference or IC. The IC is near double the length id the belt is pinch and measured on a scale. Also allow for the belt being stretched so you may need a belt 5-10% less than the old one. If you can not figure this out then just contact Marrs Communications and tell him the model and he will have the belt kit but this is more expensive.
Often a micro switch can break that cuts off the sound, look around for them.
Otherwise it sounds like a amp problem. It is also something that common to both channels. Maybe an IC that both channels goes into, or perhaps the IC is not getting power.
Sounds like either the head is open circuit or more likely the pre-amp for the 8-track has gone. Follow the leads from the head and it should lead you to the PC board of the amp. Check for broken connections first. You should get the pre-amp to buzz by touching the terminals where the head leads join. As you are getting no sound from either channel it might be the power to the board. But if you see a single IC it might have gone. For the power option (if the channels are seperate) suspect the largest electrolytic capacitor on the pre-amp board first.
By the way if you get a buzz from both channels the head has gone.
Follow the left channel lead to where it joins with the PCB. Use a pointed tool and prod connections if you get a buzz then that stage in the amp is working. I suspect you will find it leads to a IC that might cause the fault. If you find it buzzers on the right but not left, then it's gone. PS don't short pins. If you can get hold of a signal injector it will make things easy.
Be careful and good luck.
Tape decks have a record amp, depending on the age of the machine it will probably be an IC. Yours might have failed. See if you can identify it or try and get a C diagram. To locate it follow the leads from the head. If your lucky it might have 'Rec Amp' on the board. Their maybe a preset marked 'rec level' for each channel. The (IC) box with legs - near them will most likely be it. Don't adjust the presets unless somebody has tampered with them before. (they will look to be sitting around the middle if they are OK).