Soundstream MC 140x any good?

[Dr Zeus]

I just got this little guy in. The owner said it works -ok- but supposedly the switches on the bottom have some bad connections. Needless to say I'll probably try my best to restore this little guy!

Soundstream MC 140x
Made in the USA
Supposedly 35w Rms, its a 4 channel amp

Looks like this little guy may have independant power for each side. 4 x 3A fuse holders. The amp currently has 5A fuses in it.

Was this a good little find?

[Perry Babin]

I don't have any opinion of the amp itself. Switches are a problem on virtually every amp they ever built that had switches. For this one, you may be able to simply clean the switch without doing anything else if it's working perfectly otherwise.

[Dr Zeus]

I like this little amp. Its very clean and clear. It has cleaner lines on the scope than my PPI PC275. I cleaned up all the switches so they dont make much noise. All the way through I couldnt find much of a problem with this amp on the bench so I decided to give it a whirl on my car for my door speakers. I set this amp to run it's four channels bridged down to two and wired it up to my door speakers.

With car off the sound was pretty crisp and clean, more than I would have expected. Problems only started when I turned the key one past ignition - then I started to hear my car's fuel pump whine through the speakers. I put my song on mute and sure enough it was apparent. I didnt get as far as to starting the engine; but I did notice that in bridged mode the gain controll for channel 3 wasnt working. The other 3 were. Then just 30 seconds later the soundstream popped channel 4's 5A fuse - I thought she was a goner but it's working again on the bench.

Does this amp maybe have a ground problem?

I also notice someone maybe even at the factory scraped through the trace under the PCB between channel's 1 and 2 RCA shields.

[Perry Babin]

Try the amp in the car with a test speaker (use no speaker wiring in the vehicle). Try each channel individually to see which channels produce noise or if all produce the noise.

[Dr Zeus]

Well now this amp is playing some tricks on me. It sisnt make the noise in the car with test speakers, nor did it make the noise when I switched back to the door speakers. Yet another 5A fuse blew on me from channel 4 after I increased the volume to 2/3 maximum listening level (about 50% up on the HU).

After the fuse pops I smell burnt silicon. I'm having troble finding where its coming from but will keep looking.

[Perry Babin]

You may have an intermittent short on one of your speaker connections (wires, terminals...) in the vehicle.

[Dr Zeus]

Theres a ghost in this little amp. now, channel 3 doesnt have any audio on it. I think also channel 4's output transistors have seen better days. They smell like old-burnt silicon but they still play. I tried tracing a sin wave signal with my scope on Channel 3 and theres something just not quite right with how far the signal is going.

Perry,
Do you know what the 6 pin IC is on the non-solder side of the PCB? They look like little opamps or something, and are about 1/4 inch x 1/8 inch with 6 little spider legs coming off. Each channel has one of them. Can I test these with a dvm? They seem to measure the same between them, but for some reason I dont think my source signal is being passthrough right on Channel 3 through this little piece.


I'm also thinking of just buying all new output transistors for this amp, since Channel 4's smell funky. They are original, and I think they are TIP101/102 and TIP106/107 but will have to confirm after removing them.

[Perry Babin]

Move the switches back and forth to see if channel 3 returns.

The 6-legged transistor may be a 2SC3067:
http://datasheet.octopart.com/2SC306...eet-102206.pdf

You can check it with a meter but it's unlikely to be defective with the symptoms you described. Do you have audio on this transistor? If so, you may have a blown fuse or a problem with a relay.

Can you post a photo of the inside of the amp? I've never had to repair one of these so I don't know exactly what you have.

[Dr Zeus]

Fuses look good. I'll have to check through the relay and switches. This is a finicky amp for sure.

[Dr Zeus]

Looking back at this amp for now. I notice that several National MPSA42 transistors on the 3rd channel are measuring 6v AC from general ground, where the other 3 channels have a measurement of 12v AC throughout. This is with RCAs connected sending a sign wave frequency.