Akai AM-2250 fault

Neilz33

Member
2020-02-11 3:47 pm
Hi, I have an old Akai AM-2250 amp. Some time ago it stopped working through one channel. After asking around (on this forum a while ago) it was suggested that the STK amp unit was probably the culprit, so I got a new STK465 and recently got round to replacing it. Having done so, the same problem persisted.


I then noticed that a fuse had blown - there's a bank of three fuses in the middle of the board, and one of these had failed. It's a small 20mm 1.25A fuse, so I bought a pack and replaced that. When I turn it on, there is a loud buzzing coming through the channel that didn't work, which lasts for about 2 seconds, then the fuse blows again. I tried it with two new fuses, same each time.


I'm not technical and at the end of my expertise with this - this is a last attempt to find out what could be causing one channel to fail and the fuse to blow. Any suggestions on what to look for, or what could be the issue?


Thanks for reading :eek:
 
Clearly the output is convinced that the place to be is near one of the voltage rails - promptly blowing the mains fuse once a load is connected. One more reason why the first thing to connect to the output after a repair should be the multimeter.

The STKs you can buy these days tend to be either pulls or inferior replicas, so the new one might well have been bad to begin with as well. However, it is also possible that the actual fault is not within the hybrid but rather its surroundings - like an open feedback resistor. Check R20 (33k), and preferably all the rest of the half-dozen resistors per channel as well. Presumably you will have downloaded the service manual for the unit at this point.

On these STKs you can usually poke around with the multimeter and compare things on the left and right channel with the diode test function. Make sure that there is no short between pins:
* 4 and 5 (12 and 13)
* 5 and 6 (11 and 12)
* 7 and 9 (10 and 9).
Try it on the old one first, should be easier.
 

Neilz33

Member
2020-02-11 3:47 pm
Hi, I really appreciate the reply, but sadly this is way over my head! I can replace parts, but circuit testing is not my thing. So unless you're able to suggest which particular component could be at fault, there's not a lot I can do with that info, sorry :(