AKAI AM-2400

The AKAI AM-2400
Integrated Amplifier

Another ebay purchase at a good price.

All that was required was to dry-brush and vacuum-clean the inside of the amplifier, contact-clean the switches, apply lubrication to the switch action, and carefully wash the fascia, controls, and 'work the buttons' with diluted metal polish.

Apart from a few scratches, it's looking good.

Tone Control Board

I've replaced all the capacitors on the tone board; there were (and still are) electrolytic capacitors valued at 0.15uF and 0.22uF that don't need to be electrolytic. The said board was also re-transistorized; replacing the 2SC1213 NPN types with Motorola BC549B types. I am not advising anyone reading this to do the same, although (probably) the 2SC1845/KSC1845 would be a better solution, as the pins are configured in a similar (but reversed) way as the originals. My choice was purely based on my large stock of BC549Bs, and since I wanted to lower my stock in numbers, that's what I did.

I replaced these 2SC1213 transistors in the hope that low level background tone amplifier transistor and general thermal noise could be reduced - you can hear it with headphones when the volume potentiometer has been turned fully anti-clockwise. Later, I hope to replace all carbon resistors within the said board with metal film types. Hopefully this will reduce tone control amplifier noise?

The reader may get the impression that this amplifier is 'noisy', the truth is - it is not. The reason I can hear the thermal or Johnson Noise when the volume potentiometer is turned fully anti-clockwise is that the volume control sits before this audio stage! This means that the push-pull Class A-B power amplifier is receiving the full output of the tone board all the time, and that includes any natural thermal noise.

The only difference the current transistor replacements made are - noise is balanced between left and right, where previously it was more leaning towards the right side. Subjectively, the noise may indeed be lower?, but it's difficult to confirm absolutely.

Below we see the original service manual schematic highlighting the 2SC1222 series as the main audio transistors.

The tone amplifier employs a mixture of Current-Shunt,
and a switchable Voltage-Series feedback topography.

Original 2SC1213 transistors on the tone control board

Note - the alignment of the BC549B on the tone board.

Note - the necessary alignment of the BC549B on the tone board.

Tone Control Board Alterations: (22/09/2022)

Sometime later, there were random 'noise bursts' in the left channel at a very low level, almost imperceptible, but nevertheless they persisted. Later I discovered the apparent source - a dirty contact within the main volume potentiometer VR2, and after cleaning this (again), the problem disappeared. 

Just prior to this work I decided to exchange the BC549B transistors for Fairchild's KSC1845FTA, and KSC1815-GR on this board.

Now we have: TR1=KSC1845,  and TR2=KSC1815

Also of note, the main audio board which serves the power amplifier and voltage regulation has been re-populated with new electrolytic capacitors.

23/09/2023.  This page may be updated if anything relevant to the working, or modifications to the AKAI AM-2400 are undertaken.