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

Pioneer SX-680 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-680

SN: ZH3676461Y

Another classic Pioneer receiver from the famous SX-x80 product line is on my bench today.

The SX-680 was manufactured from 1978 to 1980. It produces 30 watts per channel into an 8-ohm load with a total harmonic distortion of no more than 0.1%. The damping factor at 8 ohms is 25. 

According to the Pioneer database, this particular unit was manufactured in August 1979.

Note: the service manual for this model, available online, has been scanned in very poor quality. I have tried my best to identify each electronic component as accurately as possible, but some errors may still be present.

Main Board Assembly (GWM-117)

Power Supply Circuit

The power supply circuit has 8 aluminum electrolytic capacitors (C403, C404, C405, C406, C407, C408, C415, C416) and two main filter caps (C413, C414).

All original e-caps (except two filter capacitors) were replaced with low-impedance Nichicon UPW/UPM caps. Pay attention to the electrolytic capacitor C416. It is 1uF/50V for models intended for the American market (type KU), and 2.2uF/50V for models intended for the Canadian market (type KC).

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 circuit:

C403: rated capacitance – 220uF, measured – 151uF, ESR – 1.43Ω, deviation: -31%

C404: rated capacitance – 220uF, measured – 230uF, ESR – 1.19Ω, deviation: +5%

C405: rated capacitance – 330uF, measured – 142uF, ESR – 1.18Ω, deviation: -57%

C406: rated capacitance – 220uF, measured – 186uF, ESR – 1.86Ω, deviation: -15%

C407: rated capacitance – 47uF, measured – 53uF, ESR – 2.4Ω, deviation: +13%

C408: rated capacitance – 100uF, measured – 98uF, ESR – 3.2Ω, deviation: -2%

C415: rated capacitance – 220uF, measured – 169uF, ESR – 1.63Ω, deviation: -23%

C416: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 6.1Ω, deviation: 0%

The main filter capacitors C413 and C414 were upgraded to Cornell Dubilier 10000uF/63V capacitors.

Test results on the original filter capacitors:

C413: rated capacitance – 8000uF, measured – 7094uF, ESR – 1.55Ω, deviation: -11%

C414: rated capacitance – 8000uF, measured – 7746uF, ESR – 0.88Ω, deviation: -3%

The two bleeding resistors (R408 and R409), which were connected in parallel to the main filter capacitors, were replaced with new KOA Speer metal oxide resistors.

Both original fusible resistors (R401 and R410) were replaced with new KOA Speer flameproof carbon film resistors.

I also replaced two voltage-regulating transistors (Q401 and Q402) and a Zener diode D408 to improve the reliability of the power supply circuit.

Below is a list of original and replacement parts that I have used.

Q401: NPN, 2SD712 (original), replaced with a new Fairchild KSC2073TU

Q402: NPN, 2S1384 (original), replaced with a new Fairchild KSC2690AYSTU

D408: MZ-140 (original), replaced with a new Onsemi 1N5244B

Note that the original 2SD712 transistor has degraded significantly over time; its current gain is only 19, whereas the datasheet specifies a minimum of at least 55.

Test results on two original voltage-regulating transistors

Q401: hfe - 19, Vbe - 0.662V 

Q402: hfe - 135, Vbe - 0.708V

Tuner Circuit

The tuner circuit has 3 low-leakage capacitors (C138, C139, C141) and 13 aluminum electrolytic capacitors (C122, C124, C125, C129, C131, C132, C142, C143, C150, C151, C164, C165, and C167).

All original e-caps with a nominal capacitance 1uF and lower were replaced with film polyester WIMA caps. The original low-leakage capacitor C139 was replaced with a modern low-leakage Nichicon UKL cap. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.

Test results on original capacitors removed from the tuner circuit:

C122: rated capacitance – 4.7uF, measured – 4.4uF, ESR – 6.2Ω, deviation: -6%

C124: rated capacitance – 1uF, measured – 1.2uF, ESR – 4.8Ω, deviation: +20%

C125: rated capacitance – 47uF, measured – 51uF, ESR – 6.8Ω, deviation: +9%

C129: rated capacitance – 1uF, measured – 1.1uF, ESR – 7.2Ω, deviation: +10%

C131: rated capacitance – 220uF, measured – 208uF, ESR – 2.8Ω, deviation: -5%

C132: rated capacitance – 4.7uF, measured – 4.7uF, ESR – 3.2Ω, deviation: 0%

C138: rated capacitance – 1uF, measured – 0.9uF, ESR – 8.1Ω, deviation: -10%

C139: rated capacitance – 6.8uF, measured – 7.3uF, ESR – 3.4Ω, deviation: +7%

C141: rated capacitance – 1uF, measured – 0.9uF, ESR – 8.2Ω, deviation: -10%

C142: rated capacitance – 1uF, measured – 0.9uF, ESR – 4.3Ω, deviation: -10%

C143: rated capacitance – 220uF, measured – 215uF, ESR – 3.4Ω, deviation: -2%

C150: rated capacitance – 0.47uF, measured – 0.54uF, ESR – N/A, deviation: +15%

C151: rated capacitance – 0.47uF, measured – 0.55uF, ESR – N/A, deviation: +17%

C164: rated capacitance – 220uF, measured – 231uF, ESR – 1.1Ω, deviation: +5%

C165: rated capacitance – 4.7uF, measured – 4.7uF, ESR – 3.9Ω, deviation: 0%

C167: rated capacitance – 3.3uF, measured – 3.0uF, ESR – 2.4Ω, deviation: -9%

Two PNP transistors (Q7, Q8) installed in a low-pass filter are notorious 2SA726 transistors. These transistors get very noisy over time. I replaced them with modern low-noise Fairchild KSA992 transistors. The new transistors were carefully matched by current gain and base-emitter voltage. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

There is a lot of dried glue around the main filter capacitors. This glue must be removed before installing the new components to ensure reliable operation of this receiver in the future.

Old dried glue (main filter capacitors removed)

Old glue was removed

Main board assembly (power supply and tuner circuits) - before and after

Phono Equalizer Circuit

The phono equalizer circuit has 6 low-leakage capacitors (C203, C204, C205, C206, C211, C212), and 4 ordinary aluminum electrolytic capacitors (C209, C210, C219, C221).

All original low-leakage capacitors were replaced with modern low-leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW caps.

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

C203: rated capacitance – 2.2uF, measured – 1.9uF, ESR – 4.8Ω, deviation: -14%

C204: rated capacitance – 2.2uF, measured – 1.8uF, ESR – 4.6Ω, deviation: -18%

C205: rated capacitance – 47uF, measured – 52uF, ESR – 3.2Ω, deviation: +11%

C206: rated capacitance – 47uF, measured – 51uF, ESR – 2.4Ω, deviation: +9%

C209: rated capacitance – 100uF, measured – 105uF, ESR – 5.1Ω, deviation: +5%

C210: rated capacitance – 100uF, measured – 117F, ESR – 3.6Ω, deviation: +17%

C211: rated capacitance – 2.2uF, measured – 1.9uF, ESR – 4.6Ω, deviation: -14%

C212: rated capacitance – 2.2uF, measured – 1.7uF, ESR – 6.1Ω, deviation: -23%

C219: rated capacitance – 4.7uF, measured – 4.2uF, ESR – 5.1Ω, deviation: -11%

C221: rated capacitance – 10uF, measured – 11uF, ESR – 4.2Ω, deviation: +10%

The original PNP transistors (Q203 and Q204) installed in phono equalizer circuit are notorious 2SA725. The aging of these transistors usually leads to shot noise. I replaced both of them with a modern low-noise Fairchild KSA992. Note the pinout of the new transistors. The original transistor has a BCE pinout, and the new one has an ECB pinout.

The original NPN transistors installed in phono equalizer circuit are 2SC1919. This transistor is not on my list of notoriously unreliable transistors prone to failure. However, the 2SC1919 is complementary to the 2SA725, so it makes sense to replace it with the KSC1845, which is complementary to the KSA992. Note the pinout of the new transistors. The original 2SC1919 transistor has a BCE pinout, while the new one has an ECB pinout.

Tone Control & Power Amplifier Circuits

The tone control & power amplifier circuit has 8 low-leakage capacitors (C301, C302, C329, C330, C507, C508, C511, C512), and 10 ordinary aluminum electrolytic capacitors (C309, C310, C311, C312, C335, C336, C501, C502, C513, C514). Also, two aluminum electrolytic capacitors (C333 and C334) are installed in the power meter circuit.

The original low-leakage capacitors installed in the input signal path (C301 and C302) were replaced with film polyester WIMA caps. Other original low-leakage capacitors were replaced with modern low-leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW caps.

Test results on original capacitors removed from the tone control & power amp circuits:

C301: rated capacitance – 0.47uF, measured – 0.42uF, ESR – N/A, deviation: -11%

C302: rated capacitance – 0.47uF, measured – 0.41uF, ESR – N/A, deviation: -13%

C309: rated capacitance – 22uF, measured – 27uF, ESR – 3.4Ω, deviation: +23%

C310: rated capacitance – 22uF, measured – 28uF, ESR – 3.8Ω, deviation: +27%

C311: rated capacitance – 47uF, measured – 47uF, ESR – 1.14Ω, deviation: 0%

C312: rated capacitance – 47uF, measured – 45uF, ESR – 1.08Ω, deviation: -4%

C313: rated capacitance – 100uF, measured – 108uF, ESR – 3.2Ω, deviation: +8%

C314: rated capacitance – 100uF, measured – 101uF, ESR – 3.8Ω, deviation: +1%

C329: rated capacitance – 10uF, measured – 11uF, ESR – 10.2Ω, deviation: +10%

C330: rated capacitance – 10uF, measured – 11uF, ESR – 10.8Ω, deviation: +10%

C333: rated capacitance – 3.3uF, measured – 3.0uF, ESR – 4.1Ω, deviation: -9%

C334: rated capacitance – 3.3uF, measured – 3.1uF, ESR – 4.8Ω, deviation: -6%

C335: rated capacitance – 47uF, measured – 43uF, ESR – 2.6Ω, deviation: -9%

C336: rated capacitance – 47uF, measured – 47uF, ESR – 3.5Ω, deviation: 0%

C501: rated capacitance – 3.3uF, measured – 3.0uF, ESR – 3.6Ω, deviation: -9%

C502: rated capacitance – 47uF, measured – 47uF, ESR – 1.65Ω, deviation: 0%

C507: rated capacitance – 4.7uF, measured – 6.4uF, ESR – 4.2Ω, deviation: +36%

C508: rated capacitance – 4.7uF, measured – 6.3uF, ESR – 4.1Ω, deviation: +34%

C511: rated capacitance – 2.2uF, measured – 1.7uF, ESR – 4.2Ω, deviation: -23%

C512: rated capacitance – 2.2uF, measured – 1.7uF, ESR – 4.1Ω, deviation: -23%

C513: rated capacitance – 100uF, measured – 113uF, ESR – 1.45Ω, deviation: +13%

C514: rated capacitance – 100uF, measured – 110uF, ESR – 2.1Ω, deviation: +10%

Main board assembly (phono equalizer, tone control and power amp circuits) - before and after

STK Modules Upgrade

During the restoration process, I noticed that the STK modules in this receiver looked different from each other. The STK module installed in the right channel appears to be original, while the STK module installed in the left channel raises my suspicions (see photo below). The original Sanyo power modules are obsolete and no longer available on the market. And the "new" STK-0039 modules available on eBay are usually counterfeit from China.

The "original" STK modules (the one on the left seems suspicious to me)

After some consideration, I decided to replace both STK modules with custom-made modules using discrete transistors from Pyramid Audio. The replacement modules have the same mounting holes as the original STK modules and, after minor modifications, can be installed on a heatsink. The new modules with discrete transistors will significantly increase the reliability of the power amplifier circuit.

Replacement modules with discrete transistors from Pyramid Audio

These pins (circled in red) must be removed in order to install the new replacement modules

New replacement modules installed

Dial Lamps

The old dial bulbs in this unit were replaced with new incandescent lamps. To reflect the light and diffuse the heat coming from these bulbs I attached a strip of foil tape inside the top cover. It substantially reduces the heat coming from incandescent bulbs and protects the top cover and veneer from heat damage.

Output Power and Total Harmonic Distortion Tests

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 18.02 VRMS (left channel) and 18.05 VRMS (right channel). It corresponds to the output power of 40.6W on the left channel and 40.7W on the right channel.

Output power test

I also tested the total harmonic distortion (THD) of this receiver with the new STK modules, and the results were impressive. According to the service manual, the THD should not exceed 0.1% at a continuous output power of 30W. The measured THD at a continuous output power of 30W was 0.031% (left channel) and 0.032% (right channel). Thus, the result is almost three times better than with the original STK modules!

THD at a continuous output power of 30W - left and right channel

As usual, all the knobs and the front panel 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. This receiver works perfectly now and sounds amazing! Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-680 - after restoration

Demo video after repair & restoration

Pioneer SX-626 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-626

SN: TF3744403

Today I'm showcasing a Pioneer SX-626 vintage receiver that came in for restoration. The SX-626 was manufactured from 1972 to 1974. It produces 20 watts per channel into 8 ohms with no more than 1% total harmonic distortion. The damping factor at 8 ohms is 40. The retail price in 1974 was $329.95 (Ref. Stereo Review, Stereo Directory 1974, page 48).

According to the Pioneer database, this particular unit was manufactured in June 1973.

Power Supply Board (AWR-006D)

The power supply board has 4 aluminum electrolytic capacitors: C5, C9, C10, and C12. 

All of them were replaced with Nichicon UPW/UPM low-impedance capacitors.

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:

C5: rated capacitance – 100uF, measured – 115uF, ESR – 4.1Ω, deviation: +15%

C9: rated capacitance – 330uF, measured – 298uF, ESR – 1.46Ω, deviation: -10%

C10: rated capacitance – 330uF, measured – 325uF, ESR – 1.21Ω, deviation: -2%

C12: rated capacitance – 100uF, measured – 111uF, ESR – 1.45Ω, deviation: +11%

I also replaced all transistors on this board to improve the reliability of the power supply. Below is a list of original and replacement transistors that I have used.

Q1: NPN, 2SD313 (original), replaced with a new Fairchild KSC2073TU

Q2: NPN, 2SC945 (original), replaced with a new Fairchild KSC2383YTA

Q3: NPN, 2SD313 (original), replaced with a new Fairchild KSC2073TU

Power supply board - before and after

Head Amplifier Board (W21-004-B)

The head amplifier board features two solid tantalum capacitors (C1, C2) installed in the input signal path, two sky-blue Sanyo coupling capacitors (C7, C8) installed in the output signal path, and four aluminum electrolytic capacitors (C9, C10, C15, C16).

The original tantalum capacitors were replaced with low-leakage Nichicon UKL caps. Two coupling capacitors were replaced with high-quality film polyester WIMA caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.

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

C1: rated capacitance – 10uF, measured – 11uF, ESR – 3.4Ω, deviation: +10%

C2: rated capacitance – 10uF, measured – 12uF, ESR – 2.9Ω, deviation: +20%

C7: rated capacitance – 0.47uF, measured – 0.51uF, ESR – N/A, deviation: +9%

C8: rated capacitance – 0.47uF, measured – 0.44uF, ESR – N/A, deviation: -6%

C9: rated capacitance – 3.3uF, measured – 3.9uF, ESR – 2.8Ω, deviation: +18%

C10: rated capacitance – 3.3uF, measured – 3.6uF, ESR – 2.2Ω, deviation: +9%

C15: rated capacitance – 100uF, measured – 110uF, ESR – 1.4Ω, deviation: +10%

C16: rated capacitance – 100uF, measured – 109uF, ESR – 1.1Ω, deviation: +9%

The original NPN transistors installed on this board are the infamous 2SC1312. The aging of these transistors usually leads to static noise. I replaced them with modern, low-noise Fairchild KSA1845s. Note the pinout of the new transistors. The original transistor is BCE, and the new one is ECB.

Head amplifier board - before and after

Control Amplifier Board (AWG-008-A)

The control amplifier board has 12 aluminum electrolytic capacitors (C3, C4, C7, C8, C11, C12, C19, C20, C23, C24, C25, C26), and two sky-blue Sanyo coupling capacitors (C27, C28) installed in the output signal path.

The original coupling capacitors were replaced with low-leakage Nichicon UKL caps. The ordinary electrolytic capacitors C11 and C12 were also replaced with low-leakage Nichicon UKL capacitors to reduce noise in the circuit. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.

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

C3: rated capacitance – 47uF, measured – 63uF, ESR – 1.78Ω, deviation: +34%

C4: rated capacitance – 47uF, measured – 61uF, ESR – 1.52Ω, deviation: +30%

C7: rated capacitance – 47uF, measured – 49uF, ESR – 1.68Ω, deviation: +4%

C8: rated capacitance – 47uF, measured – 50uF, ESR – 1.84Ω, deviation: +6%

C11: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 2.8Ω, deviation: +21%

C12: rated capacitance – 4.7uF, measured – 5.6uF, ESR – 2.8Ω, deviation: +19%

C19: rated capacitance – 10uF, measured – 14uF, ESR – 1.9Ω, deviation: +40%

C20: rated capacitance – 10uF, measured – 14uF, ESR – 1.4Ω, deviation: +40%

C23: rated capacitance – 100uF, measured – 133uF, ESR – 1.73Ω, deviation: +33%

C24: rated capacitance – 100uF, measured – 134uF, ESR – 1.67Ω, deviation: +34%

C25: rated capacitance – 47uF, measured – 62uF, ESR – 1.3Ω, deviation: +32%

C26: rated capacitance – 47uF, measured – 60uF, ESR – 0.86Ω, deviation: +28%

C27: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 3.7Ω, deviation: +9%

C28: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 1.6Ω, deviation: +23%

The original PNP transistors (Q3 and Q4) installed on this board are notorious 2SA725. The aging of these transistors usually leads to shot noise. I replaced both of them with a modern low-noise Fairchild KSA992. Note the pinout of the new transistors. The original transistor is BCE, and the new one is ECB.

The original NPN transistors (Q5 and Q6) installed on this board are the infamous 2SC1312. The aging of these transistors usually leads to static noise. I replaced them with modern, low-noise Fairchild KSA1845s. Note the pinout of the new transistors. The original transistor is BCE, and the new one is ECB.

Control amplifier board - before and after

Power Amplifier Board (AWH-002-D)

The power amplifier board has 11 aluminum electrolytic capacitors (C1, C2, C3, C4, C5, C6, C7, C8, C11, C12, C21) and two coupling capacitors (C13, C14).

The ordinary e-caps installed in the signal path (C1, C2, C5, C6) were replaced with low-leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps. I left the two original coupling capacitors on this board since their measured capacitance is within ±5% with low ESR.

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

C1: rated capacitance – 3.3uF, measured – 4.8uF, ESR – 3.4Ω, deviation: +46%

C2: rated capacitance – 3.3uF, measured – 4.7uF, ESR – 2.7Ω, deviation: +42%

C3: rated capacitance – 100uF, measured – 116uF, ESR – 0.78Ω, deviation: +16%

C4: rated capacitance – 100uF, measured – 115uF, ESR – 0.41Ω, deviation: +15%

C5: rated capacitance – 3.3uF, measured – 5.0uF, ESR – 3.4Ω, deviation: +52%

C6: rated capacitance – 3.3uF, measured – 4.4uF, ESR – 3.4Ω, deviation: +33%

C7: rated capacitance – 100uF, measured – 113uF, ESR – 0.58Ω, deviation: +13%

C8: rated capacitance – 100uF, measured – 112uF, ESR – 1.08Ω, deviation: +12%

C11: rated capacitance – 100uF, measured – 103uF, ESR – 1.47Ω, deviation: +3%

C12: rated capacitance – 100uF, measured – 102uF, ESR – 1.65Ω, deviation: +2%

C13: rated capacitance – 2200uF, measured – 2108uF, ESR – 0.43Ω, deviation: -4%

C14: rated capacitance – 2200uF, measured – 2127uF, ESR – 0.48Ω, deviation: -3%

C21: rated capacitance – 100uF, measured – 115uF, ESR – 1.75Ω, deviation: +15%

Power amplifier board - before and after

FM/AM Tuner Board (AWE-011)

Be aware, that replacing the original capacitors in the tuner section may require subsequent tuner alignment. I do not recommend servicing the tuner section of any vintage receiver unless you have all the necessary equipment and sufficient experience in tuner alignment.

The tuner board has 5 sky-blue Sanyo capacitors (C53, C55, C57, C66, C67) and 13 aluminum electrolytic capacitors (C23, C27, C29, C30, C33, C36, C38, C45, C54, C58, C61, C65, C73).

All sky-blue Sanyo capacitors (except C53) were replaced with low-leakage Nichicon UKL caps.  Capacitor C53, as well as capacitors C27, C30, C38, C58, and C61, were replaced with high-quality WIMA polyester film capacitors. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.

Test results on original capacitors removed from the FM/AM tuner board:

C23: rated capacitance – 4.7uF, measured – 5.2uF, ESR – 3.4Ω, deviation: +11%

C27: rated capacitance – 1uF, measured – 1.2uF, ESR – 1.56Ω, deviation: +20%

C29: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 3.8Ω, deviation: +23%

C30: rated capacitance – 1uF, measured – 1.3uF, ESR – 1.59Ω, deviation: +30%

C33: rated capacitance – 100uF, measured – 134uF, ESR – 1.06Ω, deviation: +34%

C36: rated capacitance – 10uF, measured – 13uF, ESR – 1.42Ω, deviation: +30%

C38: rated capacitance – 0.47uF, measured – 0.51uF, ESR – N/A, deviation: +9%

C45: rated capacitance – 10uF, measured – 13uF, ESR – 1.86Ω, deviation: +30%

C53: rated capacitance – 0.47uF, measured – 0.54uF, ESR – N/A, deviation: +15%

C54: rated capacitance – 4.7uF, measured – 6.1uF, ESR – 2.4Ω, deviation: +30%

C55: rated capacitance – 3.3uF, measured – 4.1uF, ESR – 5.2Ω, deviation: +24%

C57: rated capacitance – 3.3uF, measured – 3.9uF, ESR – 4.4Ω, deviation: +18%

C58: rated capacitance – 1uF, measured – 1.2uF, ESR – 3.1Ω, deviation: +20%

C61: rated capacitance – 0.47uF, measured – 0.52uF, ESR – N/A, deviation: +11%

C65: rated capacitance – 100uF, measured – 125uF, ESR – 2.4Ω, deviation: +25%

C66: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 5.9Ω, deviation: +9%

C67: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 3.1Ω, deviation: +23%

C73: rated capacitance – 47uF, measured – 59uF, ESR – 0.82Ω, deviation: +26%

The stereo lamp driver transistor Q19 was replaced with a new Fairchild KSC2383YTA. The original transistor installed in this position is 2SC968, and it often fails.

FM/AM tuner board - before and after

Filter Capacitor

The original filter capacitor was replaced with a new Nichicon LKG e-cap. The LKG series was designed for high-grade audio equipment and, in my opinion, is the best replacement for the original filter and/or coupling capacitors on the market. The new Nichicon LKG e-cap has the same diameter as the original, so the same clamp can be used to attach it to the chassis.

Test results on original filter capacitor:

C3: rated capacitance – 3300uF, measured – 3915uF, ESR – 3.8Ω, deviation: +19%

New Nichicon LKG filter capacitor

Dial and Meter Lamps

The original dial and meter lights on this receiver were replaced with warm white LED lamps to keep the original look.

Audio Adjustments

The service manual has no instructions for main amplifier adjustment. I used the following procedure to adjust the amplifier. 

First of all, I checked the voltage at pin +B1 on the main amplifier board. The measured voltage was 56.60V. Then I set the collector voltage of transistor Q11 to half the voltage measured at pin +B1 with trimming resistor VR1. Similarly, the collector voltage of transistor Q12 was set to half the voltage measured at pin +B2 with trimming resistor VR2. After that, the oscilloscope was connected across the speaker terminals, and a sine-wave signal of 1kHz was applied to the AUX jacks. The audio signal level was increased until the audio output on the oscilloscope began to clip. Trimming resistor VR1 (VR2) was slightly adjusted to ensure uniform clipping on each channel.

The Bias was adjusted to ~17mV across each emitter resistor R33 (R34) with the trimming resistor VR3 (VR4).

Voltage at pin +B1

Collector voltage of transistor Q11

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 13.11 VRMS (left channel) and 12.97 VRMS (right channel). It corresponds to the output power of 21.5W on the left channel and 21.0W on the right channel.

Output power test

As usual, all the knobs and the front panel 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 wood case was treated with Howard's Restor-A-Finish.

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

Pioneer SX-626 - after restoration

Demo video after repair & restoration