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

It should be noted that the original 2SD712 transistor has degraded significantly over time, and its current gain is only 19, while the minimum value, according to the datasheet, should be 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