Akai AM-2400 Stereo Integrated Amplifier Restoration

Unit: Stereo Integrated Amplifier

Manufacturer: Akai

Model: AM-2400

SN: 20642-1763

Another vintage integrated amplifier came on my bench for a full restoration. The Akai AM-2400 was manufactured from 1976 to 1979. It produces 40 watts per channel into 8 ohms with no more than 0.15% total harmonic distortion. The damping factor is 60.

Note: both the schematic and service manual available online for this model contain many errors. In addition, the scan quality is very poor. I have tried my best to identify each electronic component as accurately as possible, but some errors may still be present.

Main Amplifier Board (M2-5432A)

The power supply, protection circuit, and main amplifier are all located on a single PCB. This board has 15 aluminum electrolytic capacitors (C5a, C5b, C7a, C7b, C14, C15, C20, C21, C22, C28, C29, C30, C31, C32, C33) and 3 solid sky-blue Sanyo e-caps (C2a, C2b, C21). 

The Sanyo capacitors have a very bad reputation for drying out, losing capacitance, and eventually becoming conductive. I replaced two Sanyo caps (C2a, C2b) installed in the signal path with high-quality film polyester WIMA caps. The C21 capacitor was also replaced with a WIMA cap. And the remaining aluminum capacitors were replaced with low impedance Nichicon UPW/UPM caps.

The original e-caps were tested with an Atlas ESR70 capacitance meter and the results are below.

Test results on original capacitors removed from the main amplifier board:

C2a: rated capacitance – 0.47uF, measured – 0.63uF, ESR – N/A, deviation: +34%

C2b: rated capacitance – 0.47uF, measured – 0.61uF, ESR – N/A, deviation: +30%

C5a: rated capacitance – 47uF, measured – 56uF, ESR – 0.61Ω, deviation: +19%

C5b: rated capacitance – 47uF, measured – 58uF, ESR – 0.63Ω, deviation: +23%

C7a: rated capacitance – 33uF, measured – 38uF, ESR – 0.24Ω, deviation: +15%

C7b: rated capacitance – 33uF, measured – 34uF, ESR – 0.24Ω, deviation: +3%

C14: rated capacitance – 3.3uF, measured – 4.0uF, ESR – 1.52Ω, deviation: +21%

C15: rated capacitance – 3.3uF, measured – 4.1uF, ESR – 1.94Ω, deviation: +24%

C20: rated capacitance – 220uF, measured – 239uF, ESR – 0.26Ω, deviation: +9%

C21: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 2.8Ω, deviation: +23%

C22: rated capacitance – 470uF, measured – 604uF, ESR – 0.14Ω, deviation: +29%

C23: rated capacitance – 470uF, measured – 528uF, ESR – 0.08Ω, deviation: +12%

C28: rated capacitance – 3.3uF, measured – 3.8uF, ESR – 2.1Ω, deviation: +15%

C29: rated capacitance – 33uF, measured – 32uF, ESR – 0.48Ω, deviation: -3%

C30: rated capacitance – 100uF, measured – 111uF, ESR – 0.12Ω, deviation: +11%

C31: rated capacitance – 33uF, measured – 31uF, ESR – 0.86Ω, deviation: -6%

C32: rated capacitance – 33uF, measured – 34uF, ESR – 0.42Ω, deviation: +3%

C33: rated capacitance – 330uF, measured – 264uF, ESR – 0.08Ω, deviation: -20%

The first stage of the main amplifier circuit is a differential amplifier consisting of two amplifying transistors with a common emitter: TR1a/TR2a (left channel) and TR1b/TR2b (right channel). The original PNP transistors installed in these positions are 2SA640. These transistors are not on my list of infamous transistors that are prone to failure. However, I recently serviced a Sherwood S-7210 receiver and found that one pair of these transistors was badly matched. The two transistors installed in a differential amplifier must be precisely matched to work properly in the circuit. So, it makes sense to test each pair of original 2SA640s to make sure they are closely matched. 

Testing showed that the two transistors in the left channel are perfectly matched in terms of current gain within 1.4% (TR1a_hfe=301, TR2a_hfe=297). The other two transistors installed in the right channel matched slightly worse, but still within the acceptable range of ~3.5% (TR1b_hfe=283, TR2b_hfe=293). Despite this, I decided to discard all original 2SA640 transistors and replace them with modern Fairchild KSA992 transistors. Before installation, each pair of KSA992 transistors was carefully matched by current gain within 1%.

I also decided to retire the original voltage-regulating transistor TR18 and replace it with a new Fairchild MJE15032G. The original transistor installed in this position is 2SD313 and it is very reliable. However, when I removed it from the heat sink to refresh the thermal paste, I didn't find any traces of the old paste. The back side of this transistor was absolutely dry and clean. So, probably it was initially mounted on the heat sink without any thermal paste or it has been completely dried out. I don't know. But this transistor is running rather hot under normal working conditions and probably needs to be replaced after many years of working under thermal stress. The new MJE15032G transistor is bulletproof due to superior thermal characteristics over the original 2SD313. Of course, a new thermal paste was applied between the transistor and the heat sink to make its life even easier.

The original protection relay was replaced with a new Omron relay to improve overall reliability.

Main amplifier board - before and after

Tone Control Board (M2-5431)

The tone control board has 2 low leakage electrolytic capacitors (C6a, C6b), 8 solid sky-blue Sanyo e-caps (C1a, C1b, C7a, C7b, C8a, C8b, C15a, C15b), and 6 aluminum electrolytic capacitors (C2a, C2b, C4a, C4b, C5a, C5b). The original low leakage e-caps were replaced with modern low leakage Nichicon UKL caps. All Sanyo coupling capacitors were replaced with high-quality film polyester Kemet caps. And the remaining capacitors were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the tone control board:

C1a: rated capacitance – 0.1uF, measured – 0.13uF, ESR – N/A, deviation: +30%

C1b: rated capacitance – 0.1uF, measured – 0.13uF, ESR – N/A, deviation: +30%

C2a: rated capacitance – 47uF, measured – 62uF, ESR – 0.48Ω, deviation: +32%

C2b: rated capacitance – 47uF, measured – 59uF, ESR – 0.46Ω, deviation: +26%

C4a: rated capacitance – 100uF, measured – 86uF, ESR – 0.08Ω, deviation: -14%

C4b: rated capacitance – 100uF, measured – 85uF, ESR – 0.14Ω, deviation: -15%

C5a: rated capacitance – 47uF, measured – 55uF, ESR – 0.61Ω, deviation: +17%

C5b: rated capacitance – 47uF, measured – 58uF, ESR – 0.42Ω, deviation: +23%

C6a: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 1.48Ω, deviation: +9%

C6b: rated capacitance – 4.7uF, measured – 4.7uF, ESR – 1.31Ω, deviation: 0%

C7a: rated capacitance – 0.22uF, measured – 0.31uF, ESR – N/A, deviation: +41%

C7b: rated capacitance – 0.22uF, measured – 0.31uF, ESR – N/A, deviation: +41%

C8a: rated capacitance – 0.15uF, measured – 0.24uF, ESR – N/A, deviation: +60%

C8b: rated capacitance – 0.15uF, measured – 0.21uF, ESR – N/A, deviation: +40%

C15a: rated capacitance – 0.15uF, measured – 0.21uF, ESR – N/A, deviation: +40%

C15b: rated capacitance – 0.15uF, measured – 0.22uF, ESR – N/A, deviation: +47%

Tone control board - before and after

Equalizer Board (M2-5433)

The equalizer board in Akai AM-2400 is the most difficult board for servicing. Access to the back (foil) side of this board is limited without removing it from the chassis. And to release the equalizer board for servicing, you need to unscrew it from the back of the amplifier, remove the two shafts connected to the rotary switches, and remove the side metal clamp that holds the two rotary switches together.

Equalizer board - released from the chassis

The equalizer board has 4 low leakage electrolytic capacitors installed in the signal path (C2a, C2b, C9a, C9b) and 4 aluminum electrolytic caps (C6a, C6b, C10a, C10b). The original low leakage e-caps were replaced with modern low leakage Nichicon UKL caps. The remaining capacitors were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the equalizer board:

C2a: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 1.43Ω, deviation: +9%

C2b: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 1.41Ω, deviation: +9%

C6a: rated capacitance – 47uF, measured – 59uF, ESR – 0.42Ω, deviation: +26%

C6b: rated capacitance – 47uF, measured – 60uF, ESR – 0.55Ω, deviation: +28%

C9a: rated capacitance – 0.47uF, measured – 0.47uF, ESR – N/A, deviation: 0%

C9b: rated capacitance – 0.47uF, measured – 0.49uF, ESR – N/A, deviation: +4%

C10a: rated capacitance – 100uF, measured – 88uF, ESR – 0.12Ω, deviation: -12%

C10b: rated capacitance – 100uF, measured – 86uF, ESR – 0.15Ω, deviation: -14%

Equalizer board - before and after

Main Amplifier Adjustment

The idling current (or Bias) is measured across two emitter resistors: R19 & R20 (0.47Ω, 5W). The DC voltmeter is connected between pins 11 and 12 (left channel), and pins 28 & 29 (right channel). The voltage drop across two emitter resistors should be adjusted to ~30mV DC with the trimming resistor VR1a (left channel) and VR1b (right channel). The bias of ~30mV corresponds to the idling current of ~32mA.

Bias on the left and right channels after restoration

Output Power Test

The final output power test was performed at the end of my restoration. The amplifier was loaded with a low inductance 8Ω/100W dummy resistor for each channel. The oscilloscope was connected across the speaker terminals and a sine-wave signal of 1kHz was applied to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 19.70 VRMS (left channel) and 19.66 VRMS (right channel). It corresponds to the output power of 48.5W on the left channel and 48.3W on the right channel.

Output power test

As usual, all the knobs and the face plate were gently cleaned in warm water with dish soap. All controls have been cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray.

The final result can be seen in the photos below. The amplifier looks very cool and sounds awesome. Please watch a short demo video at the end of this post. Thank you for reading.

Akai AM-2400 - after restoration

Demo video after repair & restoration