Hi ! This post is about my Vintage AKAI AM-2250. It has failed sometime ago.
My question is the following: is it worth to repair it or to hack it ?
This amp is from the late 70's begin of 80's, it is a AB Class amp with 25W RMS per channel. I bought 20 years ago this one when I was a student for 30 € or something like this. I just enjoy it for several years, nice little amp with correct sound.
What is cool with such Vintage amps, there are very good sources of documentation:
When I open it, I saw a blown fuse on the audio side, and I saw it was rated too high, probably replaced before, verdict STK463 dead: I checked the DC offset voltage on only one channel and I have -35V... it should be 0 to 50 mV maximum. I can replace it, the price is ok, the reliability that is another story. In general there is another bunch of bad components, it is a 40 years old amp. I did not see any burn resistance, still you need to fully check everything with the schematic component by component. The PCB looks ok there are very light touch of rust. The power supply seems fine with -35V/35V, associated caps not sure, look like it leaked a bit. What is cool also with those amps, there are only discrete components that are still easy to find, easy to solder and to access, the only ICs is the STK463 and the LA-3122S for the Phone input. The limit is you have too many failed components and you need to salvage another amp which does not make sense.
Look like both power caps leaked on the PCB, that may explain partially the failure
There is an interesting document about how to choose resistors and capacitors for replacement:
http://www.condoraudio.com/wp-content/uploads/Reviews/How-to-Choose-Replacement-Capacitors-and-Resistors.pdf
I was thinking of an alternative way, check if the simple pre-amp is ok directly catching up on the pin 1 and 16 of the STK463 and if it is ok, I can simply bypass the existing power stage by putting instead two Class-D amps with its own PSU, may be I will need to change a few things at the output of the preamp and check if there is a DC voltage offset problem too. Now, there are some chips on the Class-D providing a good sound, those ICs are small and inexpensive. Also, I want to keep its look and feel: when your 3 old daughter turn the big metal potentiometer like crazy you know it will not be an issue. I will receive soon a pair of TPA3118 and PAM8610 for another project, I can perform a quick test.
