Sanyo M-X960K motor getting very hot

[tutanxamon]

Hello fellow boombox lovers!

I recently purchased M-X960K in great cosmetic condition. However it was not working at all when I bought it and after I replaced blown fuse radio section started to work. Cassette deck wouldn't react to any button presses. Quick voltage check at motor terminals indicated that power is being delivered to the motor which meant that something is wrong with capstan motor. It is a 4 terminal capstan motor with external pitch control resistor (rotary on the front panel). After taking capstan motor apart I discovered that cylinder that I thought is the motor itself is just an external shell with PCB and actual motor inside. There was an obviously burnt component on the PCB. I was lucky that marking on it was still legible and couple of weeks later I had few replacement LA5512 delivered. I soldered the new IC and put everything back together. Motor was now spinning and casette mechanism was mostly working. I used a bluetooth casette adapter for test purpose. I've not used cassette players for a very long time and frankly I was underwhelmed by the audio quality. However this was not the main issue. Motor was getting quite hot very quickly which was concerning. I got busy with other priorities and abandoned my little project for a while. Few weeks ago I decided to dig a bit deeper and try to identify the root cause of the motor getting hot. I've ordered a 3000hz test tape to check capstan speed. Using test tape I got around 4500-5000Hz which I couldn't fully adjust using both pitch control knob and variable resistor on the motor control PCB. Then I decided to test motor parameters and it certainly doesn't look alright, especially temperature and current consumption.

Voltage on the motor terminals: 10.5V
Current consumption: 0.6A
Control IC temperature: Heated from 25degC to 80degC in 1 minute.
Motor armature heated to 60degC within 1 minute of operating time.
Winding resistance: 6.1Ohm, 6.3ohm, 9.2ohm.

Armature winding looks discolored. Can I conclude that this motor is cooked and need to be replaced?
If yes, what kind of motor can I replace it with?

shaft diameter d 2mm
shaft total length L 37mm
shaft protrusion l 10mm
inner can outer D 32mm
inner can length L 22mm
outer can inner d 35mm
outer can outer D 38mm
outer can length L 36mm

 

[tutanxamon]

This is internal motor PCB schematic BTW.

 

[Superduper]

Several thoughts come to mind. If the motor was overheated, the insulation between the windings might have broken down which will reduce motor efficiency. Motors could run hotter than they should because of insufficient voltage or excessive load (imparted by mechanism issues and friction, etc.). The friction issue is self explanatory -- increased friction = increased effort to spin everything = increased load. This is especially something that needs to be considered if the mechanism has had belt degradation issues. Sanyo boombox belts tends to deteriorate by melting into a sticky goo, unlike some others that tends to degrade by getting stiff and slip. If this has occurred, it is imperative that all evidence of this goo be carefully removed and all pivoting/rotating/sliding and levers be checked for free and clear of excessive binding or friction. Now, the voltage issue is that if the voltage supplied to it is too low, the motor tries to make up the watts by increasing current, and current = heat. This may seem counter to ohms law but you need to understand that motors do not behave like pure resistive DC load.

Seeing that you are only getting 10.5V at the motor terminal, that is way undervoltage to the motor. This motor should be supplied rail voltage and seeing that this is a 15V boombox, it's entirely possible to see 20+ volts when powered on AC mains. It's unclear why you are having this undervoltage condition, you'd need to investigate this and examine the conditions for this low voltage. Are you using depleted batteries? Are you supplying power using the DC jack with an underpowered or incorrect voltage DC adapter? Do you have an issue with the internal power supply, and/or is the voltage settings correct if this is a multi-voltage set? Perhaps you have failed components or excessive contact and switch corrosion?