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Old 19th June 2007, 04:50 PM   #1
vtgolf is offline vtgolf  United States
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Default newbie pc amplifer (blown or fixable)?

Hi, I'm not sure if this is the right pace to post this, but I don't have much information on the product in question.
I have recently been given a 2-channel pc amplifier to use for headphones, media players, etc.
Micro Multimedia Labs, Inc.
model: TC 1490
input power: 180 watts
ac supply: 117 v 60 hz
I was told it is an 80 watt amp and it says 40x40 on the front.
When I first received the amp I replaced the 1/4" fuse in the back as it was blown.
I turned the power to the amp on and the equalizer meter maxed out on both channels with nothing plugged in to the unit.
I attached some older speakers and turned the unit on and a horrible loud hum emitted from the speakers.
I tried the headphone and mic inputs, same result.
I took the top off and looked for more fuses to swap and found none.
Clearly, I am new to this, and was just wondering if there is anything I can do to identify the problem or is the unit blown?
I've included a pic, don't know if it is close enough?
Any ideas or suggestions would be appreciated.
thanks
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Old 19th June 2007, 06:14 PM   #2
JCoffey is offline JCoffey  United States
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While I dont have an answer to your problem, I just want to say that I owned the same amp, and for the most part LOVED it. Its a nice little integrated for the money I paid for it in 98. ($100)

The only problem I ever had with it ws a blown capaciter, so i'd check for one near the chips that it buldging or completely blown.

And since this uses the 3876TF chips this would be the right place to post this question.

Man I miss that amp.

Here's a thread I started a few years ago about mine, and removing hum, with pics attached
How to get rid of hum?
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Old 19th June 2007, 07:21 PM   #3
gootee is offline gootee  United States
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I'm not an expert. But I have successfully repaired quite a few oscilloscopes and other pieces of test equipmnt.

I usually always check the power supply, first, in any kind of failed equipment. (And, in this case, it sounds like it just might be something in the rectifier section, if you're lucky.) [However, if a power supply voltage is low, remember that it might be either a PS problem OR something downstream, "dragging" it low.]

First, inspect everything closely, by eye. Look for scorched parts or traces, bulging, leaking, or exploded electrolytic capacitors, bad connections, etc. (If there are pluggable connectors, unplug and re-plug each of them, several times, to possibly scrape away any oxidation and re-establish a better connection.)

NOTE: If you're not extremely familiar with the DANGER involved in servicing AC-Mains-driven equipment under power, and the proper safety precautions to use, then DO NOT attempt to power up the unit with the cover open!

You will need to have at least a multimeter or volt-ohm meter, for the following.

It's usually not a good idea to even power-up something that's not working correctly. But, since you already did... If you can identify the power supply output traces and ground, OR the chipamps' power supply and ground pins, try measuring the voltage between each of the power pins and ground.

You can also measure the AC and DC voltages across each of the rectifier diodes and each of the large capacitors, just to see if anything looks totally-out-of-whack.

While you've got the unit on (but not for too long at a time!), see if any of the power supply (or other) components are getting hot-enough to burn your fingers. (But stay well-away from the AC Mains voltages!)

With power OFF, if you can test the power supply's large capacitors' ESR, and test the large rectifier diodes, those would be good things to try. If not, consider just replacing all of them. They're relatively cheap. Since there are apparently four of each, even if you don't know how to test them, you could start by (after discharging the big caps with a resistor, or a screwdriver if necessary) just measuring the resistance across each of them, in both directions (polarities), and comparing each one's readings to the others'. If something looks "questionable", you might have to de-solder and remove them (or at least one side of each one) and test them out-of-circuit, to be more sure.

Let us know what you find and we can go from there.

(If you don't have an ESR meter, but you do have access to an oscilloscope and a square wave generator, you can quickly set up a simple but very effective ESR-testing system, like this one: http://www.fullnet.com/~tomg/esrscope.htm .)

Good luck. Remember: Be careful.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html
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Old 19th June 2007, 10:37 PM   #4
jaycee is offline jaycee  United Kingdom
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The PCB looks dreadful, but that is quite a score!

Check those 4 blue caps - they look like power supply capacitors.

If it's knackered you can always gut it and build a nice Gainclone of your own in there
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Old 20th June 2007, 02:09 AM   #5
vtgolf is offline vtgolf  United States
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Thanks very much for the replies. I'll look into what you've suggested. My knowledge is limited, so I'll proceed with caution.
I like the amp because of the small footprint and multiple inputs, so here's hoping I can get it working.
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Old 20th June 2007, 02:55 AM   #6
poynton is offline poynton  United Kingdom
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Hi.

Hum on both channels could well be a power supply problem.
The caps don't look blown or bulging but they may be underneath.

One other thing it could be is one of the rectifier diodes blown.

Also the photo looks like something has been spilled inside or maybe leaked.

Andy
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