JVC RC-M50C

I recently bought a RC-M50C, and it was working when I got it, although it needed some work. This morning, I tried to use it again, but the tape motor won't turn and the speakers make a humming sound. The speakers also make a popping sound when the power switch is turned off. I haven't tried line in, but it seems the radio is no longer working either. When I first noticed, it was plugged into AC in the wall, but the same symptoms appear with batteries. Could this be due to a failed power supply? If not, what could it be?

Update: Line in and radio work properly, but the capstan motor doesn't turn.

Update 2: Capstan motor is getting power, but there's too much friction for it to start on its own.

Could be the belts slipped off the main flywheel.

JVC Floyd said:

Could be the belts slipped off the main flywheel.

Click to expand...

I wish it was this easy. The belt it on the pulleys and flywheel, and is still in good shape. The motor gets power, but there's too much friction for it to turn without me helping it. Maybe it needs to be oiled?

Deck might need lubing or the belt might be too tight

Northerner said:

Deck might need lubing or the belt might be too tight

Click to expand...

Ok, I tried removing the belt, and the motor still doesn't turn. I don't think the belt is too tight, but lubing the moving parts is a good idea.

Good news! There ended up being a loose wire in one of the connectors, so everything is running as it should now!

Definitely spoke too soon, should have tested more. While I did fix the loose wire, it appears that wasn't the problem. While trying to troubleshoot, I discharged the capacitor next to the capstan motor. It's the only thing I did differently this time than the other times I've been troubleshooting. It was working for a bit after discharging the capacitor and fixing the loose wire, but the capstan motor is not turning anymore.

Does it sound like that capacitor is bad?

Was it plugged in to AC power at the time?

Fatdog said:

Was it plugged in to AC power at the time?

Click to expand...

Yes, it was plugged into AC power when it worked, and later when it stopped. It was not plugged into AC when the capacitor was discharged.

Another update

I took it apart to replace the capacitor next to the motor to see if it would help the problem, and it didn't. After pulling the capacitor out, it tested well with the multimeter, but I used the new one anyway.

I tested the incoming voltage to the motor, and it was around 3V when it should be 12V. I started trying to follow the circuit back to the power supply and test along the path. Most of this was on a circuit board that the power supply and motor both connect to. The wires from this circuit to the motor had about 3V going through them, as well as a 16V capacitor next to it that was labeled 'Motor'. The circuit was too complicated for me to go through each step, but the 50V capacitor directly next to the incoming wires from the power supply measured about 17V.

After these measurements, my box stopped powering on, so it seems that the power supply failed. I cannot tell if this could have been a related issue to the capstan motor or not. The power supply could have not been supplying ample power for the capstan motor in theory, but I don't know enough about how that works.

Any ideas?

I've never owned the m50 but what I would do is look for a fuse on the power supply board.

Thank you, JVC Floyd! Fuse is definitely blown, I'll replace it when I can find one.

Yet another update

Fuse has been replaced and the box powers on again. However, the original problem remains. The motor is only receiving 3V when it should receive 12V, and I can't seem to find out why.

maybe try cleaning the tape / radio switch.

I would check the voltage at the main Leaf switch that engages power to the deck it's usually located on the deck itself.

Thanks for the suggestions, I will try both of these next week

I checked the leaf switch behind the tape deck and it was getting 0V with play engaged. I noticed the brown wire led to a 50V .47 uF capacitor and when I checked the voltage going through the capacitor, the power supply fuse blew again.

Unfortunately I have not been able to properly clean the Tape/Radio/Line In switch yet.

You should stop & think about your testing methods & techniques. You are blowing fuses meaning that you are introducing shorts. So many members have probed around a live Boombox circuit & blew something (not fuses) and have deeply regretted it. I constantly warn against this but does anyone listen? Nah. You have been lucky so far, as the fuses have protected you. But don’t push your luck. The most likely cause of low voltages is dirty contacts in the switches.

I had thought that a multimeter set to measure voltage wouldn't have a very high potential for damage, but I may be wrong. I am a little out of my element here, and certainly no circuit expert.

Thanks for the warning and info, I will clean the switches soon.

The problem is that even a momentary slip of a probe will be enough to blow something. It only takes a millisecond. Capacitor leads often bridge a positive rail & ground.