Onkyo TX-2500 MKII Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Onkyo

Model: TX-2500 MKII

SN: 88107956 

Look what I have on my workbench today - a magnificent Onkyo TX-2500 MKII receiver! This model was manufactured from 1979 to 1980. It produces 40 watts per channel into 8 ohms with no more than 0.1% total harmonic distortion. The damping factor at 8 ohms is 40.

For some reason, Onkyo receivers tend to be less popular among vintage audio collectors and are often priced significantly lower than comparable Marantz, Sansui, or Pioneer models from the 1970s. Personally, however, I am a huge fan of the Onkyo MKII series. There is something almost magical about them: their distinctive aesthetic and characteristic sound possess a unique charm that truly draws me in. Currently, my personal collection includes two models from this lineup: the TX-1500 MKII and the TX-2500 MKII. Both are in excellent cosmetic condition. I hope that, one day, I will manage to acquire the remaining three models from this series as well.

Power Supply Board (NCPS-473)

The power supply board has 5 aluminum electrolytic capacitors C905, C910, C911, C914, and C917.

All original e-caps were replaced with low-impedance Nichicon UPW/UPM/UHE caps. 

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

Test results on original capacitors removed from the power supply board:

C905: rated capacitance – 220uF, measured – 261uF, ESR – 1.69Ω, deviation: +19%

C910: rated capacitance – 220uF, measured – 219uF, ESR – 3.6Ω, deviation: -1%

C911: rated capacitance – 220uF, measured – 221uF, ESR – 2.2Ω, deviation: +1%

C914: rated capacitance – 470uF, measured – 479uF, ESR – 1.61Ω, deviation: +2%

C917: rated capacitance – 220uF, measured – 295uF, ESR – 2.4Ω, deviation: +34%

The original Zener diode D912 was replaced with a new ON Semiconductor Zener diode to improve the reliability of the power supply circuit.

Power supply board - before and after

Tone Amplifier Board (NAAF-471)

This board is the most difficult to service due to the abundance of wires. I attempted to release it from the chassis, but doing so without removing the front chassis panel seems impossible. Nevertheless, it is still possible to service this board without removing it.

The tone amplifier board has 4 low-leakage capacitors (C353, C359, C453, C459) installed in the signal path, and 8 ordinary aluminum electrolytic capacitors (C365, C366, C369, C912, C465, C466, C470, C913).

The original low-leakage capacitors C353 and C453 were replaced with modern low-leakage Nichicon UKL capacitors. The other two original low-leakage capacitors, C359 and C459, as well as ordinary e-caps C366 and C466, were replaced with film polyester WIMA capacitors. The ordinary e-caps C365 and C465 were replaced with modern low-leakage Nichicon UKL capacitors to reduce noise in the circuit. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPM capacitors.

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

C353: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 3.5Ω, deviation: +5%

C359: rated capacitance – 1uF, measured – 1uF, ESR – 3.4Ω, deviation: 0%

C365: rated capacitance – 22uF, measured – 24uF, ESR – 2.3Ω, deviation: +9%

C366: rated capacitance – 1uF, measured – 1.5uF, ESR – 3.1Ω, deviation: +50%

C369: rated capacitance – 220uF, measured – 265uF, ESR – 0.76Ω, deviation: +21%

C370: rated capacitance – 220uF, measured – 268uF, ESR – 1.02Ω, deviation: +22%

C453: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 3.3Ω, deviation: +5%

C459: rated capacitance – 1uF, measured – 1uF, ESR – 3.2Ω, deviation: 0%

C465: rated capacitance – 22uF, measured – 24uF, ESR – 2.3Ω, deviation: +9%

C466: rated capacitance – 1uF, measured – 1.4uF, ESR – 3.2Ω, deviation: +40%

C912: rated capacitance – 470uF, measured – 533uF, ESR – 0.79Ω, deviation: +13%

C913: rated capacitance – 470uF, measured – 567uF, ESR – 0.86Ω, deviation: +21%

Tone amplifier board - before and after

Main Amplifier Board (NCDA-472)

The main amplifier board has 15 aluminum electrolytic capacitors C501, C504, C508, C511, C512, C513, C601, C604, C608, C612, C613, C703, C704, C915, C916.

The ordinary e-caps installed in the input signal path (C501 and C601) were replaced with modern low-leakage Nichicon UKL capacitors to reduce noise in the circuit. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM/UHE capacitors.

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

C501: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 3.1Ω, deviation: +5%

C504: rated capacitance – 47uF, measured – 48uF, ESR – 2.6Ω, deviation: +2%

C508: rated capacitance – 47uF, measured – 54uF, ESR – 1.2Ω, deviation: +15%

C511: rated capacitance – 470uF, measured – 456uF, ESR – 1.3Ω, deviation: -3%

C512: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 3.3Ω, deviation: +9%

C513: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 3.2Ω, deviation: +9%

C601: rated capacitance – 2.2uF, measured – 2.0uF, ESR – 4.1Ω, deviation: -10%

C604: rated capacitance – 47uF, measured – 46uF, ESR – 2.3Ω, deviation: -2%

C608: rated capacitance – 47uF, measured – 55uF, ESR – 1.2Ω, deviation: +17%

C612: rated capacitance – 4.7uF, measured – 5.0uF, ESR – 3.1Ω, deviation: +6%

C613: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 3.4Ω, deviation: +9%

C703: rated capacitance – 220uF, measured – 265uF, ESR – 2.2Ω, deviation: +20%

C704: rated capacitance – 220uF, measured – 238uF, ESR – 0.9Ω, deviation: +8%

C915: rated capacitance – 47uF, measured – 33uF, ESR – 1.7Ω, deviation: -30%

C916: rated capacitance – 220uF, measured – 247uF, ESR – 2.1Ω, deviation: +12%

The original trimming resistors R515 and R615 were replaced with new Bourns potentiometers.

Main amplifier board - before and after

AM/FM Tuner and Equalizer Amplifier Board (NCRFE-470)

Equalizer Amplifier Circuit

The equalizer amplifier circuit has 2 low-leakage capacitors (C301 and C401) installed in the input signal path, and 6 aluminum electrolytic capacitors (C305, C309, C311, C312, C405, C409).

The original low-leakage capacitors were replaced with modern low-leakage Nichicon UKL capacitors. The ordinary aluminum e-caps (C309 and C409) installed in the output signal path were replaced with film polyester WIMA capacitors. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW capacitors.

Test results on original capacitors removed from the equalizer amplifier circuit:

C301: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 3.7Ω, deviation: +5%

C305: rated capacitance – 47uF, measured – 45uF, ESR – 2.1Ω, deviation: -4%

C309: rated capacitance – 0.47uF, measured – 0.56uF, ESR – N/A, deviation: +19%

C311: rated capacitance – 3.3uF, measured – 3.6uF, ESR – 4.1Ω, deviation: +9%

C312: rated capacitance – 3.3uF, measured – 3.7uF, ESR – 3.4Ω, deviation: +12%

C401: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 4.4Ω, deviation: +9%

C405: rated capacitance – 47uF, measured – 49uF, ESR – 2.4Ω, deviation: +4%

C409: rated capacitance – 0.47uF, measured – 0.59uF, ESR – N/A, deviation: +26%

AM/FM Tuner Circuit

The tuner circuit has 5 low-leakage capacitors (C118, C119 C122, C123, C124), and 15 aluminum electrolytic capacitors (C106, C108, C109, C113, C116, C127, C130, C131, C132, C134, C168, C169, C172, C173, C175).

The original low-leakage capacitors were replaced with film polyester WIMA and Kemet capacitors. Two ordinary aluminum e-caps (C109 and C172) were replaced with film polyester WIMA caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM capacitors.

Test results on original capacitors removed from the tuner circuit:

C106: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 2.8Ω, deviation: +9%

C108: rated capacitance – 10uF, measured – 15uF, ESR – 2.8Ω, deviation: +50%

C109: rated capacitance – 0.47uF, measured – 0.62uF, ESR – N/A, deviation: +32%

C113: rated capacitance – 47uF, measured – 61uF, ESR – 1.3Ω, deviation: +30%

C116: rated capacitance – 470uF, measured – 656uF, ESR – 0.8Ω, deviation: +40%

C118: rated capacitance – 0.1uF, measured – 0.1uF, ESR – N/A, deviation: 0%

C119: rated capacitance – 0.1uF, measured – 0.1uF, ESR – N/A, deviation: 0%

C122: rated capacitance – 0.33uF, measured – 0.34uF, ESR – N/A, deviation: +3%

C123: rated capacitance – 0.47uF, measured – 0.44uF, ESR – N/A, deviation: -6%

C124: rated capacitance – 0.22uF, measured – 0.21uF, ESR – N/A, deviation: -5%

C127: rated capacitance – 3.3uF, measured – 3.6uF, ESR – 2.9Ω, deviation: +9%

C130: rated capacitance – 10uF, measured – 11uF, ESR – 2.7Ω, deviation: +10%

C131: rated capacitance – 10uF, measured – 12uF, ESR – 2.3Ω, deviation: +20%

C132: rated capacitance – 100uF, measured – 95uF, ESR – 1.1Ω, deviation: -5%

C134: rated capacitance – 100uF, measured – 106uF, ESR – 2.2Ω, deviation: +6%

C168: rated capacitance – 10uF, measured – 13uF, ESR – 2.4Ω, deviation: +30%

C169: rated capacitance – 220uF, measured – 332uF, ESR – 1.7Ω, deviation: +51%

C172: rated capacitance – 1uF, measured – 1.4uF, ESR – 2.9Ω, deviation: +40%

C173: rated capacitance – 3.3uF, measured – 3.8uF, ESR – 2.6Ω, deviation: +15%

C175: rated capacitance – 100uF, measured – 95uF, ESR – 0.8Ω, deviation: -5%

The failure-prone VD1212 dual diode installed in position D102 was replaced with a pair of 1N4148 diodes in series.

AM/FM Tuner and equalizer amplifier board - before and after

Filter Capacitors

The original filter capacitors (8,200uF/50V) were upgraded with new Kemet ALT22 series capacitors (10,000uF/63V). The new filter capacitor has the same diameter as the original.

New Kemet filter capacitors

Dial, Meter and Function Lamps

This receiver is equipped with two axial dial illumination lamps, positioned at the edges. I replaced them with warm white LED lamps.

Two meter and three indicator bulbs were replaced with new incandescent lamps. I recommend replacing the indicator bulbs from the front bracket side. The original rubber caps on the bulbs often degrade and become brittle over time. So, be careful not to damage them during the replacement process.

Dial lamps - original and warm white LED

Two meter lamps

Three indicator lamps

Bias Adjustment

The bias on each channel is measured between ID and CT terminals. The voltage should be adjusted to ~20mV with the trimming resistor R513 (left channel) and R613 (right channel).

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 19.14 VRMS (left channel) and 19.09 VRMS (right channel). It corresponds to the output power of 45.8W on the left channel and 45.6W on the right channel.

Output power test

I usually gently clean the faceplate in warm water with dish soap. But this is not the case for this receiver (as with any other Onkyo receiver from this series)! The lettering, silk screened onto the front aluminum faceplate can be easily flake off by improper cleaning. I had a very bad experience many years ago with an Onkyo TX-4500 MKII and I learned my lesson. The safest method is to gently, and I emphasize VERY gently, wipe the panel several times with a damp cloth. Be careful: do not ruin this magnificent receiver by using an unsuitable cleaning method.

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 receiver looks awesome and sounds great! Please watch a short demo video at the end of this post. Thank you for reading.

Onkyo TX-2500 MKII - after restoration

Demo video after repair & restoration