Good source for small DC motors?

[weirdlookinguy]

Hey guys. My amp (Technics SU-G91) started overloading again today. I broke out the multimeter and found that the small DC motor in the fan assembly is bad, sometimes it doesn't seem to want to spin when I know it should be spinning and my multimeter confirms it is getting +3vdc. Heavy bass will kick in and the fan will just twitch sometimes instead of spinning like it should. I think it's a 5v, as my amp never seems to feed it more than 3.5v (it fluctuates with the volume of the music).

The date code on it is August 1992, so I guess it's time. I took it apart and lubed it. No help. A 12v computer fan would not only have to be hacked into the amp, but it won't move enough air at 3 volts. I tried it.

The fan motor is a Matshushita MDN-4RB4MRC. According to MCM electronics, it can be subbed by a Panasonic MDN-4RB4MXA (same shit probably). I think any 5v DC motor of the right size will do. Does anybody have a good source? I can get the original Matsushita replacement for $20 here:
http://www.encompassparts.com/products/?MSCMDN-4RB4MRC

but there's gotta be a cheaper alternative.

[Wizard]

Re: Good source for small DC motors?

This is unusual amp that utilizes the motor's momentum to monitor it's spinning correctly by pulsing it and read the dc voltage between power pulses, if not ampifier control IC shuts down. Use very close match motor.

Cheers, Wizard

[weirdlookinguy]

Re: Good source for small DC motors?

You guys think I can get something that works locally?

My parents are gonna flip if I spend almost $30 (after shipping and everything) for the motor. If I buy it locally, they don't have to know how much it cost.

[weirdlookinguy]

Re: Good source for small DC motors?

Wizard was right.

I tried a generic Radio Shack replacement (same size but a little longer). It spun, but not as much as the original and even though it was spinning the amp shut off. I need a very close match to (if not) the original

I put in the original and sure enough, proper airflow and no more shutdown. But the original stalls sometimes. Leave a song with a groovy bassline on, and after a minute or two the fan stops spinning, starts twitching, and before it unsticks the amp shuts off. I pulled it apart and tried cleaning the insides, put a little lube inside. Maybe I didn't put in enough?

Suggestions on reviving the original?

[Wizard]

Re: Good source for small DC motors?

reason for this stoppages is crud in the commutor slots, Go easy and carefully clean slots out with stiff paper or something and razor blade, go real easy, just to "scrape" in the slot but not to nick or burr the edges of commutor.

The bearings and shaft, I usually clean them with toothpicks and alochol or acetone. Use light oil. Sewing oil is not what you want, it's not refined enough and evaporate too quickly.

Common in CD players too.

Cheers, Wizard

[weirdlookinguy]

Re: Good source for small DC motors?

Which are the commutor slots?

[Wizard]

Re: Good source for small DC motors?

http://www.stefanv.com/rcstuff/qf200212.html

Copper or silver looking with slots as pointed out has lines that is really cut slots to seperate it into individual segments is commutor.

Crud build up in the slots and interferes with by too much electrical leakage due to low voltage operation of these motors in that mode (around 1.5V to 2V or so depending on how quick it's spinning. High current motor for torque doesn't suffer from that at all as sufficient power overcomes the leakage thru crud in the slots.

Cheers, Wizard

[weirdlookinguy]

Re: Good source for small DC motors?

Well there was no crap in the commutors, but I took the whole thing apart and threw all the pieces into some 70% alcohol. Twirled all the parts around a bit, then put them in the toaster oven for 10 minutes at 200F to dry. Put a little oil on it (SAE 10W40 car oil, it's the only oil we had) and put it back together.

Problem SEEMS to be solved, but I can't crank it too loud for too long since it's nightime over here. Will find out tomorrow when I can crank it loud for a decent amount of time.

It was shutting off about halfway through "I Can't Wait" by Sleepy Brown before. Good song for testing because it has booming bass that cuts in and out over and over, making the fan pulse. I have never had my amp make it through that song while playing loud because the fan would stick in between the pulses about halfway through the song. It made it through the song three times in a row after I cleaned out the fan tonight :/

But I'm wary on saying that the problem is fixed and I can enjoy my music without further worries. I've said that about 4 times with this amp. I need to see it pump out the tunes LOUD for at least a half hour before I can start building confidence in it.

[Wizard]

Re: Good source for small DC motors?

When I said crud in the gaps/slots, it is surprising little that can screw up. That why we gently scrape clean in these slots. Simply washing doesn't always do.

Cheers, Wizard

[weirdlookinguy]

Re: Good source for small DC motors?

It went back to Goodwill today for store credit. I followed your suggestion of cleaning the commutor slots again and it seemed to go for a few more days, but it started overloading again. I've had it with this amp. I guess it's back to my crappy 20WPC Mini HiFi system until I can save up the money for this beauty:

http://www.crutchfield.com/p_022RX49...+RX-497&tp=179

I've had it with used amps. That's two amps and $80 down the drain so far. I could have had a flawless new Yamaha by now if I hadn't been wasting time and money trying to fix old crap. Damn shame it didn't work out with this Technics since it's all Matsushita parts inside.