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Akai AC 3750L Repair/Restore

AC3750L

Novice Member
Hi All,

I am new to this forum, i have been joining forums left right and centre to try and find someone who can help me with my Akai

I was given one of these when i was 4 years old, i was amazed by it, I had it in pieces all the time looking how it worked and watching the insides when i turned the radio dial wondering how it was changing station, watching the record mechanism work, absolutely fascinated by it, unfortunately it was destroyed in a bad flood we had when i was 10 years old :(

A year or so i bought another one, luckily as they are extremely rare now, it was listed as working, other than the tape and record belts, which i have replaced.

Unfortunately i was using 1 set of speakers for 2 hifi's, one set of leads from the akai to the mordaunt short speakers, then one set from there to the cambridge azure.

I powered the akai on, not realising that id left the cambridge one on too, the cambridge audio one went into protect mode instantly, and the akai blew the 5a internal fuses, I remember from being a child and messing with it so much, that im SURE it used to blow the 1a glass fuses inside when i connected the speakers up wrong, or shorted the wires out or something, but now its just blowing the 5a glass fuses inside without anything connected, if i plug a speaker into it with new fuses, it buzzes for a second through the speaker and blows the fuses...

Please can someone help me with this? I am desperate to get it working again, i love this hifi, its just so nice and brings back good memories :)

I have made videos, and taken multiple pictures if that helps anyone, someone on another forum said it could be a transistor or something? I am capable of re capping a board, as i have done many times in TV's and washing machines and stuff, so this should be a problem to me whatsoever if i can just start somewhere in diagnosing the problem



Thanks all - Joe
 

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AC3750L

Novice Member
Im wondering if one of the diodes has gone that is joined to the fuses that keep blowing, those things i say in the video "whatever they are" if a signal has gone into the unit and blown one of those, would that cause a short circuit?
 

AC3750L

Novice Member
Here is another video, testing the amplifier transistors as best i can, does this mean anything what im doing? Am i even doing it right?

 

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noiseboy72

Distinguished Member
OK, so to start off with, never connect 2 amplifiers to the same set of speakers. You must use a switch or physically change the cables over.

I would start with the output devices - the large transistors on the heatsink and work from there. Best bet is to meter them out and see what readings you get. You then need to look at the drivers for these and possibly the power supply, but diodes are usually pretty bomb proof. Again, you can measure them and see what readings you get.
 

AC3750L

Novice Member
Yes i understand i shouldnt have done it without a switching box, stupid of me, how do i go about testing the transistors, as you can see in the video, i tried to test them but im not certain what im doing :)
 

noiseboy72

Distinguished Member
Each "pair" should read the same, so the 2 black transistors and 2 green transistors should each have the same reading. If your meter has a diode testing position, the transistor when out of circuit should give a high reading between between pins in 1 direction and a lower reading in the other. If you get a short circuit in both directions, then the transistor has failed. You may get different results with the transistor in circuit.

The transistors must be insulated from the heatsink,so be careful if you remove them and fit the insulators when you replace them.
 

AC3750L

Novice Member
Ok so if i take them off and test them all, if they are ok they should give a reading, if they are short circuit will the meter just beep as continuity and diode testing are the same function on this meter i believe, then it has ohms resistance and voltage DC and AC

I have just tested the transistors, and i get readings from all of them, other than the one shown here, it justs beeps continuity at 0 the rest give a reading, dependant on how i test them, although the left black one gives a reading of around 75, and the right black one gives a reading of 720, or 1800 or something, ill have to test them all again and write down what they were
 

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AC3750L

Novice Member
ok i shall remove all the transistors from the board, and re test, hopefully i will have found my fault soon, my box of 100 varied amperage fast blow fuses will be here soon too, so if the transistor is the issue and ive replaced it and tested it, i should be good to go, is it possible to remove the bad transistor, or a pair at a time, and test again with just one channel? 2 are for right and 2 are for left, or will that blow sometihng else haha
 

noiseboy72

Distinguished Member
If you remove a pair, that should be OK.

It might be the transistor that drives the output devices, as sometime this blows when the output stage goes.
 

AC3750L

Novice Member
Im convinced that thats the problem now, unfortunately on recieving my soldering iron back from my friend just now, its totally burnt out and been left switched on.... so ill have to wait until tomorrow now, but i get what seem to be correct readings from the right hand chanel transistors, but the black and green left hand chanel ones, the black one is LOW like 75... and the green one is a dead short all across, shall i just snip them off the board and re test see if it blows the fuses? I can put lower amerage fuses in it, like 1 or 2 amp fuses, as with no load, no music and no direct short in theory it shouldnt blow them
 

AC3750L

Novice Member
YOUR A GENIUS!!!!!

I cut the left hand transistors out of it, and it works a TREAT now just on right channel!!!!

 

AC3750L

Novice Member
Someone on vintage radio forum is now telling me that if i put new transistors in this it might blow them again due to another load of transistors that have failed? Look:

@@The problem is that lots of other transistors may have failed too. If you just change the output pair without identifying the others then they will go pop as soon as you power up. That's why these designs are so difficult to fix. @@

Should i keep testing the smaller transistors? not sure where to go from here other than buying the broken transistors, but so glad to hear it play music again

 

AC3750L

Novice Member

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AC3750L

Novice Member
Ive ordered those transistors, they seem to be right, and a direct toshiba replacement, will there be any noticable audio difference from one channel to the other with having diferent transistors? I assume there will be, i didnt even think about that to be honest, If these work, I will find the exact replacements for the akai, Next step is to move onto my Aiwa D91M stack system, working amp (or was) and tuner unit, i stupidly threw the twin tape deck away and CD player for it, but ive managed to get my hands on those just about, for a really decent price too, as they are proprietary leads to interconnect the whole lot together, so no one without the other bits wants them if they come across them, just got the double cassette deck for £24, auto stop, pause detect, fully electronic i remember them being really good, and ive got a set of original speakers on ebay too im buying, i once again stupidly threw mine away, soon i will have a fully working Akai AC3750 and Aiwa D-91M stack system, then onto my vintage computer collection to start repairing, cant wait!!!

Thanks for all your help
 

AC3750L

Novice Member
just out of interest why would this have happened, i turned the dial to cassette, pressed play, turned it back to radio, the sound was biblical and the right hand output transistors and the white thing infront of it got UNTOUCHABLY hot, i turned it off, left it on radio, turned it back on and it was fine, was it because the tape deck sent a left hand chanel output to the place where the transistors arent putting more stress on the right hand channel?
 

AC3750L

Novice Member
Its fixed now! new transistiors in, Both channells are working, record player is working, tape is working, everything is working perfectly, thank for your help!
 

AC3750L

Novice Member
Here is the finished product, working an absolute treat!!

Thanks all :)
 

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