Technics SA-600 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Technics

Model: SA-600

SN: AJ9611F124

This is a receiver I restored some time ago and it comes from my private collection. I bought it a year ago from a kind older lady for a fair price and in excellent cosmetic condition. The wood case, top grill, faceplate, and knobs were almost flawless.

The Technics SA-600 is a great mid-range receiver. It was manufactured from late 1978 to around 1979 and produces 70 watts per channel into 8 ohms with no more than 0.04% total harmonic distortion. It has the classic Technics look with dark insets around the knobs and the warm glow of the tuning dial. The build quality is excellent.

A very nice feature of the Technics SA-600 is its sliding wooden top, held by two small brackets. The top slides off once the two screws are removed, providing access to the inside of the receiver. However, the restoration of this Technics SA-600 was not an easy task, as I will explain further below.

Below are some photos I took during my restoration process, complete with comments and recommendations. This receiver had no electronic issues, so the purpose of my restoration was to improve its general performance. I did so by replacing the old electrolytic capacitors with new high-quality capacitors and replacing the old transistors which were prone to create noise or work improperly. I also refreshed the thermal compound between every power transistor and heat sink and adjusted the factory settings to be sure that the receiver would function well for the next twenty or so years.

Dial Lamps

I replaced 3 old dial lamps with new incandescent bayonet base 6.3V, 0.25A bulbs. When replacing dial lamps, I typically install warm white LED lamps to maintain the receiver’s original look and decrease the production of heat. However, some models will have a more authentic look with original incandescent bulbs and this SA-600 is no exception.

Power Transistors

Four power transistors Q613 (x2), Q614 (x2) are mounted on a large heat sink. These transistors are very easy to remove and replace if the L-shaped supporting plate is removed from one side of the heat sink. All power transistors were removed, cleaned, and tested with Atlas DCA55 semiconductor analyzer. The measured DC current gain was in spec according to the datasheet. However, it should be noted that Atlas DCA55 semiconductor analyzer provides the accurate reading for DC current gain only on low-power transistors. A high-power transistor requires a much higher collector current and collector-emitter voltage to accurately measure its current gain. However, it is still a very useful device for comparing transistors of a similar type for the purposes of gain matching or fault-finding.

I applied a fresh thermal compound and replaced the old thermal pads with new Mica ones which is always a good idea when working on any vintage receiver. The old grease can frequently dry, causing the transistors to suffer from poor heat dissipation. This can result in overheating and the destruction of the device.

Heat sink with four power transistors - L-shaped supporting plate removed

PNP type power transistors were removed, cleaned, and tested

NPN type power transistors were removed, cleaned, and tested

Power transistors tested with Atlas DCA55 semiconductor analyzer

Power transistors mounted on a heat sink with new Mica pads and fresh thermal compound

Power Level Display and Display Amplifier

The power level display circuit board (#14170D) and display amplifier circuit board (#14170C) are mounted beneath the reflector plate. Due to a lack of space and engineering design, it was a bit tricky for me to remove these two boards, but being patient and documenting my steps assisted the process.

The power level display board only has two e-caps (C809/C810, 10uF/16V), which I replaced with high-reliability low impedance Nichicon UPW caps. The display amplifier board has nine e-caps. Low capacitance e-caps C801/802 (1uF/50V) and C811 (0.47uF/50V) were replaced with high-quality film polyester WIMA MKS2 caps and the other e-caps with Nichicon UPW. The new Nichicon e-caps are longer by ~1mm than the original blue e-caps and I slightly bent them before mounting the insulation shield. A nonconducting stripe inside the metal shield prevents any electrical contact between different e-caps.

Power level display circuit board (#14170D) - before and after

Display amplifier circuit board (#14170C) - before and after

This point during the restoration is the best time to clean and lubricate the speaker switches (main and remote) and two pots (bass and treble). I would strongly recommend doing it before mounting the power level display and display amplifier boards. It is impossible to reach these switches and pots without removing those two boards once again. I used the DeoxIT 5% contact cleaner and DeoxIT FaderLube 5% spray for cleaning and lubricating, respectively.

The power level display and display amplifier boards are removed - it's the best time to clean and lubricate the speaker switches and two pots

 

Main Amplifier & Power Supply Board

The circuit board #14170A contains the power supply, main amplifier, tone control, and speaker protection circuits. This board is fairly simple to work with, excluding the speaker protection part which is located under the small board with fuses and near two large filter capacitors. I replaced two signal path capacitors C601/C602 (4.7uF/50V) with low leakage Nichicon UKL caps. The remaining e-caps were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. I recommend paying attention to e-cap C520 on this board if you need to service it. The schematic shows this position as 33uF/16V despite the original capacitor being 100uF/6.3V. I replaced this cap with another 100uF/16V.

Other than that, I found nothing abnormal on this board. This board has no transistors prone to failure or noise. So, no transistors were replaced here. However, I noticed that many of the original capacitors are leaky and have corrosive leads due to electrolyte leakage. I highly recommend replacing all electrolytic capacitors in vintage receivers because they are most susceptible to aging and degradation.

Main amplifier & power supply circuit board (#14170A) - before and after

C520 is between transistor Q602 and the ground screw

 

Speaker Protection

The speaker protection circuit is part of the main amplifier & power supply circuit board (#14170A) and has four electrolytic capacitors. One of which is a bi-polar cap (C904, 220uF/16V). This is another tricky place to work on due to the lack of space. I had to remove a small board with fuses and the supporting plate, which helps hold two large filter caps but still had very limited access to those capacitors. A special tool shown in the picture helped me to remove the old e-caps and install new ones. Note that e-cap C902 (2.2uF/50V) is a low leakage cap and should be replaced with a low leakage type. I replaced it with low leakage Nichicon UKL cap. The C903 and C905 e-caps were replaced with low impedance Nichicon UPW caps. I didn’t replace the bi-polar C904 cap because it was in spec and marked as a critical safety component on the schematic. If the cap had been defective, I would have used the Nichicon UEP cap as a substitute.

Small board with fuses next to the power transformer

Supporting plate for two large filter caps removed

Speaker protection circuit - very limited space to work on

A special tool used to remove the old e-caps and install new ones on the speaker protection circuit

FM/AM Tuner and Equalizer

The circuit board #14210A contains an FM/AM tuner and equalizer amplifier. The orange e-caps on this board are low leakage capacitors installed in a signal path. I replaced all e-caps with the capacitance of 1uF or less with high-quality film polyester WIMA MKS2 caps. The orange e-caps with the capacitance of 3.3uF and 4.7uF were replaced with low leakage Nichicon UKL caps. The rest e-caps were replaced with low impedance Nichicon UPW/UPM caps.

Transistor Q751, located behind the volume pot, is mounted on a heat sink. This transistor is running hot under normal operating conditions. Some PCB discoloration can be seen around a heat sink. I decided to refresh the thermal compound between this transistor and the heat sink. To my surprise, I found no old thermal grease when I removed the heat sink from this transistor. The back side of the heat sink was completely dry. What the heck?! The technician forgot to apply thermal grease? I tested this transistor with Atlas DCA55 semiconductor analyzer and it was Okay. I applied a fresh thermal paste to the transistor and mounted it back onto the heat sink to make its life easier.

Transistor Q751 mounted on a heat sink - no old thermal grease was found

Bias Adjustment

Finally, I checked and adjusted the Bias on the power amplifier. This procedure is straightforward and clearly described in a service manual. I adjusted the bias to 7.4mV on each channel. 

All the knobs and the faceplate were cleaned with dish soap and slightly polished with the Mothers Mag & Aluminum polish paste. All controls have been cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray.

Bias on the left and right channels after restoration

Now, the final result is amazing! The receiver looks and works great, and I enjoy listening to it. Please watch a short demo video at the end of this post. Thank you for reading.

Demo video after repair & restoration