Sunday, January 3, 2021

Pioneer SX-434 Receiver Restoration

Unit: AM/FM Stereo Receiver
Manufacturer: Pioneer
Model: SX-434
SN: WB3943648Y

Today I'm showcasing a Pioneer SX-434 receiver that came in for restoration. The SX-434 is Pioneer's budget-priced stereo receiver. It produces only 15 watts per channel into 8 ohms but has an excellent build quality and a very sensitive FM tuner with low noise FET and 3-gang frequency linear variable capacitor. The total harmonic distortion is less than 0.8%. The SX-434 was manufactured from 1974 to 1976. It is a very nice-looking receiver with a warm and rich sound. According to the Pioneer database, this unit was manufactured in February 1976.

Pioneer SX-434_After Restoration_Side view

Power Supply Board

The power supply board (#AWR-062) has 6 electrolytic capacitors C7 thru C12. All of them were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. This board is easy to service but one should be careful with old nylon snap lock supports. They become stiff over time and can be easily broken if too much force is applied to release the board from the chassis.

Power supply board - before and after

Pioneer SX-434_Power Supply board - Before servicing

Pioneer SX-434_Power Supply board - After servicing

AF Amplifier Board

The cleaning and lubricating of all pots and switches are routine work on any vintage gear. Some make/models have very easy access to pots/switches by design, and it usually takes just 10-15 minutes to thoroughly clean all of them. However, in some models, this routine work could be a real challenge. The Pioneer SX-434 is one of them. It is impossible to clean the pots and switches in this model without disassembling the AF amplifier board from the chassis. In addition, it is a little bit tricky to take it out due to the large heat sink with an unusual shape. The function switch was loosened and the headphone jack was removed from the chassis to facilitate this process. Moreover, the power supply board was also removed from the chassis since one screw holding the heat sink is located just under the power supply board! Anyway, after the board was successfully disassembled from the chassis I thoroughly cleaned all pots and switches with DeoxIT 5% contact cleaner and lubricated them with DeoxIT FaderLube 5% spray.

The original coupling capacitors installed on the AF amplifier board (#AWK-034) are the notorious sky blue Sanyo e-caps. These e-caps become very leaky over time and I always prefer to replace them with modern film capacitors. For more information about sky blue Sanyo e-caps refer to my previous post on Pioneer SX-828 restoration. I replaced all sky blue Sanyo e-caps C1/C2, C17/C18, C23/C24, C35 thru C38 with high-quality film polyester WIMA MKS2 caps. Two low leakage e-caps C11 and C12 were replaced with modern low leakage Nichicon UKL caps. The remaining ten e-caps C41 thru C44, C51, C52, C55 thru C58 were replaced with low impedance and high-reliability Nichicon UPW caps.

The differential pair amplifier in each channel consists of two notorious transistors 2SA725. Those transistors Q10/Q12 (left channel) and Q11/Q13 (right channel) become very noisy over time and should be replaced. A modern substitution for 2SA725 is the Fairchild KSA992 transistor. I replaced all 4 transistors and carefully matched each pair of KSA992 transistors by current gain and base-emitter voltage before installation.

AF amplifier board - disassembled from the chassis

Pioneer SX-434_AF Amplifier board

AF amplifier board - before and after (two orange e-caps are low leakage, ten sky blue are Sanyo e-caps)

Pioneer SX-434_AF Amplifier board_Before servicing

Pioneer SX-434_AF Amplifier board_After servicing

Dial and Tuning Meter Lamps

Replacing the dial and tuning meter lamps in SX-434 is a very easy task. To release a lamp box holding the dial and meter bulbs one should just unscrew two small screws on the top of the lamp box. And then carefully remove it from the chassis. I replaced the original dial bulbs with warm white LED lamps to maintain the original look and decrease heat. The original incandescent bulb installed for the tuning meter usually overheats the plastic on the meter and discolors it. As a result, the plastic often becomes yellowish. I replaced the original tuning meter bulb with a cool blue LED lamp to overcome this issue and better match the tuning meter to the rest of the dial.

The stereo indicator bulb burned out in this unit and I replaced it with a new incandescent bulb. This is a very delicate work since the original rubber holder dries out over time and eventually becomes very brittle. I used an X-ACTO #2 Knife to remove the original bulb from the holder carefully trimming around the bulb. The diameter of the new incandescent bulb is smaller than the original one and to compensate for the difference I put on a heat shrink on a new bulb. After that, the new incandescent bulb was inserted into the holder and secured with super glue.

The dial pointer bulb burned out in this unit as well and it was probably the most challenging work for me in this model. It is not so easy to replace a burned-out dial pointer bulb in Pioneer SX-434. Ideally, the dial pointer holder should be removed from the dial panel before bulb replacement. But in that case, the holder should be disassembled from the dial string. I didn't want to break the factory sealing and decided to replace the burned bulb without disassembling the dial pointer holder from the dial string. Well, I wouldn't say it is an easy task but it is doable with extra care and with the right tools. I used an X-ACTO #2 Knife again and small pliers to release a plastic dial holder from the metal part. Be extra cautious during this step if you need to replace a dial pointer bulb in this model. It is very easy to break a plastic needle. I was patient and lucky. The plastic needle was not broken and I successfully replaced the old bulb with a new one!  

Five new warm white LED lamps and one cool blue LED lamp installed

Pioneer SX-434_New warm white LED lamps installed

New stereo indicator lamp installed

Pioneer SX-434_New Stereo Indicator Lamp

Original dial pointer bulb - removed from the plastic holder

Pioneer SX-434_Dial Pointer Bulb_Removed from the holder

New dial pointer bulb installed - we have light!

Pioneer SX-434_New Dial Pointer Bulb Installed

DC offset and Bias Adjustments

The service manual describes step-by-step the power amplifier alignment but these instructions could be confusing for someone who is not familiar with this model. The problem is that this receiver has no trimmers to adjust the DC offset or Bias. Instead, it has three jumpers per channel which should be cut to make a correct alignment. 

The first step is to check a DC offset. To do this I loaded this receiver with two low inductance 8Ω/100W dummy resistors in each channel and connected my DC voltmeter between terminal #27 and ground. The measured voltage was ~13mV which is perfectly normal. Then, I connected the DC voltmeter between terminal #24 and the ground. The measured voltage was ~5mV which is again in an acceptable range. Ideally, the measured voltage should be as close as possible to zero volts. But in real life, any DC voltage between 0V and ~30mV would be just fine. So, I didn't cut any jumpers and left them alone. However, if the measured voltage is above 40-50mV then the correct jumper lead should be cut to get the DC offset close to zero volts. Jumper C should be cut if the DC voltmeter reads a positive voltage between terminal #27 and ground (i.e. > +50mV). Jumper E should be cut if the DC voltmeter reads a negative voltage between terminal #27 and ground (i.e. > -50mV). Jumpers D or F should be cut if a positive or negative voltage is measured between terminal #24 and the ground.

DC offset on the left and right channel after restoration

Pioneer SX-434_DC Offset_Left channel

Pioneer SX-434_DC Offset_Right channel

The second step is to check a Bias.  According to the service manual, the bias should be above 5mV otherwise the jumper lead should be cut. I connected my DC voltmeter between terminals TP1 and 27. The measured voltage was 7.6mV which is above the minimal voltage stated in the service manual. The measured voltage between terminal number TP2 and 24 was 8.3mV which is again in an acceptable range. So, no jumpers were cut here as well. If the measured voltage between terminal number TP1 (TP2) and 27 (24) is less than 5mV then the jumper A (B) should be cut.

Bias on the left and right channel after restoration

Pioneer SX-434_Bias_Left channel

Pioneer SX-434_Bias_Right channel

As usual, all the knobs and the faceplate were gently cleaned in warm water with dish soap. All knobs were also slightly polished by Mothers Mag & Aluminum polish to remove some small spots of aluminum oxidation.

The final result can be seen in the photos below. As I mentioned earlier this receiver has a very warm and rich sound. And it looks very cool too! Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-434 - after restoration

Pioneer SX-434_After Restoration_01

Pioneer SX-434_After Restoration_With replaced components

Pioneer SX-434_After Restoration_Side view

Demo video after repair & restoration


  1. Oleg seems up with this masterfully restored 434. I had a good sense about his passion from reading through this blog and once I saw the receiver, I knew he was the real the deal. The quality and detail Oleg put into this unit shows and I feel very lucky to have come across him.

    If you’re wondering where to find someone who really understands vintage hifi receivers, look no further. I would trust anything this man’s hand graces.

    Thanks again!

    1. Thank you Paul. I am glad that you are happy with this receiver.

  2. I have exactly this Pioneer unit and the tuning needle that shows the frequency has broken would you be able to help me finding another one possibly?

    1. You might want to contact Oak Tree Vintage. They have a lot of spare parts from different models. This is a link to Pioneer spare parts: http://www.oaktreevintage.com/Pioneer_Stereo_Parts_Salvage_Units.htm

      Occasionally the dial pointer for this model is also available on Ebay.

  3. Thanks for some great advice here. I am replacing the transistors Q20, Q21, Q22 and Q23 in the power amp. These are the ones fixed to the large heat sink, and you do not mentioned replacing these. Now they have produce a continuous loud crack and popping sound when I reconnect it. I wonder if I've not used the correct equivalents. For D313D I used 2n5298 and for 507D I used TIP42L. The pinouts on both are BCE from L to R. One comment in Badcaps site for this suggests MJE15033G and MJE15032G. Any further advice on this, please?

    1. Hello Ric, the original 2SD313 and 2SB507 transistors are complementary pair. These are designed for the output stage of low power AF amplifiers. I would recommend to use the following modern substitutes: KSC2073TU for 2SD313 (Q20/Q21) and KSA940TU for 2SB507 (Q22/Q23). The modern KSC2073 transistor is complement to KSA940. I serviced four SX-434's in the past two years and one of them had toasted output transistors. I replaced the toasted transistors with KSC2073 and KSA940, and had a great result.

    2. Great, thanks Oleg! Will follow this and report back. Best rgds, Ric

  4. Nice job there Oleg. I have this receiver and the phono seems a little muddy. I think it can sounds better. Any recommendations?