First time trying to fix an amp (Akai AM-2800)

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rocketpark

New Member
I bought this used and it worked great for a week and then stopped outputting any sound. No sound from speakers or headphone jack. The power light is on and the meters light up but don't move. It was in use when it shut off and there was a burning rubber smell right after it stopped working. I'm totally new at trying to troubleshoot an amp and would appreciate any pointers you vets may have.

-Nick
 

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What is the status of those three fuses in the left rear in the first photo?

The 'burning rubber smell' is not a good omen.
 
Fuses look fine to me. Attached a photo (not sure why it rotated 180 when I uploaded it)
 

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When the volume is turned all the way up, you can hear the music coming out of the right side (heatsink?) of the actual amp. Nothing out of the speakers though.
 
right then ok protection is on .. set your meter to 200v dc black lead on metal chassis ..red lead on one of the coils that are coated in varnish ..one is right in front of the relay box other to left of it .. you need to scrape a little varnish of till its clean copper .. if you get very low reading next try on 20v reading ..if still low go to 200mv setting .. report back your findings ..
if you get one that's high reading like around 40volts stop testing .report back
 
Thanks Pete, forgive my ignorance with a bad question, should the amp being plugged in and powered on while testing these coils?
 
yes it needs to be plugged in switched on . just be aware of high voltages ..
what you are testing here is speaker output before the relay as its not activating ..i am thinking if you heard sound from one side heatsink only its going to be the other channel that's gone bad ..
the relay wont activate if high dc is present ..maybe anything over 300mv .
so i am expecting high dc on the left coil .
 
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The left coil read 54.8 on 200
Right coil under the relay box wouldn't show anything until 2000m and then it was only .1 to .2
 
just as expected ...next suspect is the output transistors ..these are to be found on the heatsink . if any are shorted as i suspect then the driver transistors need checking ...these obviously are the left side only .. amp turned off and unplugged from now on ..leave it for 10 minutes then carefully check voltage on the main filter capacitors we want this around zero volts . or just pull the fuses ..
can you see the 3 legs on each output transistors ? if so set meter to 200 ohms .check for zero or very low ohms 3 ways on each transistor .. if low reading you need to unsolder both transistors then use diode test on meter to test them ...
this should keep you busy for a while .
before you ask the capacitors are next to the transformer to the right .
 
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i have just been looking at the picture and it appears the output transistors are at the end of the blue yellow red wires so should be easy to test .. one transistor per each 3 wires .. check them at the board next to heat-sink ..
 
Thanks for all your help!

Are these the output transistor legs I'm looking for? When testing on them at 200ohms my meter shows a couple numbers near 75-100 then goes to 1. Any other setting in ohms on my meter just displays 1.
 

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yes them are those ... you need to disconnect them both and test again ..i cant tell if they are sockets or soldered in ...try undoing the screws ..if they pull out its sockets if not they are soldered and need unsoldering ..make a note of the numbers and which goes where .. or better still get a pic of the side with the numbers on ..
 
next thing is has your meter got diode test ? if so we need to establish bce " base collector and emmiter " the case will be collector ...get the transistors out and report back with the numbers that are wrote on them ..
 
i am trying to read the schematic here that i cant see the numbers of the transistors .. i will wait until you tell me the numbers
 
Yep my meter has a diode test. I was able to get the transistors off and attached photos.

Numbers are 2SD424 and 2SB554 on each transistor.
 

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2sd424 is npn 2sb554 is pnp .. ..
i have no data but the case is collector on both ,,, we need to find the base so diode test black lead on case for npn .. try both pins one should read about 500 .. keep red on that pin and put black on other pin should get around 500 .. pnp is the other way round so red on case to start ...
 
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