Pioneer SX-780 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-780

SN: ZI3608397Y

I restored this receiver about a year ago but unfortunately didn't take a lot of "as is" photos. However, I thought it would still make sense to write a post about my restoration process on this beautiful classic receiver. 

The Pioneer SX-780 is a great mid-range receiver from one of Pioneer’s last series of receivers before they switched to digital meters and black faceplates. It features a "high output, low distortion power amplifier, a high fidelity equalizer amplifier with low noise and high gain, a high stability tuner section, tone controls with tone defeat function, built-in protection circuits and independent power meters". The Pioneer SX-780 was manufactured from 1978 to around 1980. It was initially built in Japan but later production switched to South Korea as evidenced by the varying tags on the back of the unit. It produces 45 watts per channel into 8 ohms with no more than 0.05% total harmonic distortion. The list price in 1978 was $325.00. According to the Pioneer database, this unit was manufactured in September 1979.

I bought this receiver from the second owner and it had all the typical issues associated with dirty pots and switches. The right channel was very scratchy and the left channel had no sound at all. I was afraid that the Darlington power pack from the left channel was dead but fortunately, it was just a VERY dirty volume pot. The receiver was serviced at least once in the past but only two filter capacitors C501/C502 were upgraded from 8000uF/50V to 10000uF/50V Nichicon UKW series. Everything else looks original and untouched on all boards. Below are some photos I took during my restoration process, complete with comments and tips.

AF Amplifier Board GWK-118

Three transistors (Q19, Q20, and especially Q25) in the power supply circuit are running very hot and can even unsolder themselves. Those transistors are mounted on heat sinks but the size of each heat sink is not enough to efficiently dissipate the heat. I would strongly recommend carefully investigating a PCB around those transistors for any signs of excess heating if you need to service this model. Be aware that those heat sinks are electrically live.

I decided to replace all three transistors with modern Fairchild MJE transistors due to better thermal characteristics in comparison to original transistors. A new silicone thermal compound (Wakefield-Vette, 120 series) was applied between each transistor and the heat sink.

Three transistors Q19, Q20, and Q25 in the power supply run very hot - heat sinks are electrically live!

Three original transistors Q19, Q20, and Q25 were replaced with Fairchild MJE transistors

New Fairchild MJE transistors installed

All electrolytic capacitors in the power supply and protection circuits were replaced with low impedance and high-reliability Nichicon UPW caps. Those e-caps are designed for switching power supplies and can operate in the temperature range from -55 to +105 C. The protection circuit in SX-780 is responsible for muting when the POWER switch is turned ON and OFF. The delaying action is determined by the time constants of the timing capacitor C317 and the constant current circuit. The transistor Q26 is turning the relay S7 on and off thereby protecting the speakers.

I found an error in the schematic concerning the e-cap C311. The schematic shows this capacitor as 100uF/16V despite the original capacitor installed on the board being 220uF/16V. I replaced this e-cap with a Nichicon UPW capacitor, 220uF/25V.

Another issue in the schematic is related to four e-caps C324 thru C327 installed in the power meter circuit. The unit I own was manufactured in South Korea and there are only two capacitors C324 and C327 installed on the PCB. Two jumpers are installed instead of C325 and C326. I believe this is a result of production optimization which was not documented by Pioneer. So, I replaced those two capacitors with new Nichicon UPW caps.

Only two capacitors C324 and C327 installed on the PCB in the power meter circuit

This board has 12 low leakage electrolytic capacitors installed in the signal path (all orange caps). These are C103/C104, C117/C118, C207/C208, and C217 thru C230. I replaced two e-caps C207/C208 with high-quality film polyester WIMA MKS2 caps. The other two e-caps C227/C228 were replaced with film polyester Nichicon caps. And the remaining 8 capacitors were replaced with low leakage Nichicon UKL caps.

Test results on original low leakage capacitors removed from AF Amplifier board:

C103: rated capacitance – 2.2uF, measured – 2.5uF, deviation: +14%

C104: rated capacitance – 2.2uF, measured – 2.5uF, deviation: +14%

C117: rated capacitance – 2.2uF, measured – 2.5uF, deviation: +14%

C118: rated capacitance – 2.2uF, measured – 2.6uF, deviation: +18%

C207: rated capacitance – 1uF, measured – 1.07uF, deviation: +7%

C208: rated capacitance – 1uF, measured – 1.03uF, deviation: +3%

C217: rated capacitance – 4.7uF, measured – 5.7uF, deviation: +21%

C218: rated capacitance – 4.7uF, measured – 5.7uF, deviation: +21%

C227: rated capacitance – 0.22uF, measured – 0.25uF, deviation: +14%

C228: rated capacitance – 0.22uF, measured – 0.23uF, deviation: +5%

C229: rated capacitance – 2.2uF, measured – 2.4uF, deviation: +9%

C230: rated capacitance – 2.2uF, measured – 2.4uF, deviation: +9%

All other electrolytic capacitors on this board were replaced with low impedance and high-reliability Nichicon UPW caps. I found one extra e-cap on this board that was not marked on the circuit board schematic. It is just to the right of e-cap C309 and the value is 4.7uF/35V. It looks like this is also a result of production improvement which was not documented by Pioneer. I replaced this cap with a new Nichicon UPW.

Extra e-cap which is not marked on the circuit board schematic - 4.7uF/35V (right of e-cap C309)

New Nichicon UKW series filter capacitors C501 & C502 - upgraded by the previous owner

Tuner Board AWE-099

The tuner board has two transistors Q7 (2SA726) and Q8 (2SA726) which are prone to create noise. These transistors are gain matched (blue paint dots on the top of each). I replaced them with modern low noise Fairchild KSA992. The new transistors were gain matched within 1%. I didn't change any capacitors on the tuner board.

Tuner board - transistors Q7 and Q8

Darlington Power Pack

As I mentioned earlier this receiver came in with a very scratchy right channel and the left channel had no sound at all. The first suspect was a faulty STK-0050 Power Pack which is a fairly common issue in this model. Any speaker shorting results in excessive current and can easily blow these power packs. So, I checked the operating voltages on all pins and compared them against the schematic. The voltages on all pins were very close to those on the schematic. This was a good sign meaning that both STK-0050's packs are not defective. Indeed, after a deep and intensive cleaning of all switches and pots with DeoxIT the sound in both channels was restored. Yep, sometimes those old gears require just a very good cleaning and lubrication. I carefully unsoldered both Darlington packs, removed the old thermal paste, and applied a new silicone thermal compound between each pack and the heat sink. It is always a good idea to refresh the thermal paste to improve the heat dissipation on any output devices.

Original STK-0050 power packs

STK-0050 power pack removed from the PCB

Dial Lamps

Initially, I installed modern warm white LED lamps instead of old incandescent bulbs but they look too bright on this model. So, I moved to new incandescent bulbs. To reflect the light and diffuse the heat coming from these bulbs I installed a strip of foil tape inside the top cover. It substantially reduces the heat coming from incandescent bulbs.

Tip: there is a small opening at the center of each lamp socket. I used a small Allen wrench and gently pushed each lamp out of the socket. In that case, the brittle socket tabs won't be damaged or broken.

Push each dial lamp out of the socket with a small Allen wrench

DC Balance & Power Meters Adjustments

At the end of my restoration, I checked and adjusted the DC balance of the power amplifier as described in the service manual. No dummy load or input signal is required for this adjustment. The DC balance was adjusted to ~7mV on each channel.

An AC voltmeter and function generator are required to adjust power meters. A sine-wave signal of 1 kHz should be applied to the AUX terminals and the level of this signal should be adjusted so that the voltage on the SPEAKERS terminals read 20 VRMS. Then, the trimming resistors VR7 and VR8 should be adjusted so that the power meters read 50W.

DC balance adjusted to ~7mV on each channel after restoration

Output Power Test

The final output power test was conducted at the end of the restoration. Two low inductance 8Ω/100W resistors connected across each speaker terminal were used as a dummy load. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 20.51 VRMS (left channel) and 20.75 VRMS (right channel). The output power on each channel can be calculated based on the following formula: P = (VRMS x VRMS)/8. The max output power (before clipping) is 52.6W on the left channel and 53.8W on the right channel. This corresponds to the factory specifications of this model and even slightly exceeds them.

Output power test - perfectly symmetrical output sine-wave signal with no clipping up to ~53W

As usual, all the knobs and the faceplate were gently cleaned in warm water with dish soap. I also slightly polished all knobs by Mothers Mag & Aluminum polish to remove some small spots of aluminum oxidation. All pots 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 sound is wonderful, very warm, and extremely clean. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-780 - before restoration

Pioneer SX-780 - after restoration

Demo video after repair & restoration