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Friday, May 27, 2022

DLK Model 1 1/2 Loudspeaker Restoration

Unit: Loudspeaker System
Manufacturer: DLK
Model: 1 1/2
SN: 12994 & 13463

I bought these speakers a year ago from the original owner. The cabinets and grills are in excellent cosmetic condition. According to the owner, the woofers were re-foamed several years ago at Midwest Speaker Repair in Roseville, Minnesota. I didn't get a chance to test them before buying, but the seller claimed these speakers sound great. After some negotiations, we made a deal.

DLK_Model 1 1/2_After Restoration

I have read a lot of good reviews about DLK speakers on various audiophile forums but I have never owned them. DLK speakers were designed by Don L. Kliewer and built in Minnesota. To the best of my knowledge, there were many variations of the 1 1/2's over the years of production. This pair has woofers with rubber surrounds, DLK dust caps, phenolic ring tweeter, black foam padded grills, and solid wood cabinets.

There are also two versions of the 1 1/2 model with an impedance of 4 ohms and 8 ohms. This pair of speakers have a 4 Ohm impedance. 

I believe this is the 1st generation of DLK 1 1/2  speakers with the round terminal plate. Below is a specification for model 1 1/2 from the DLK brochure.

Woofer: two 8'' high compliance with low-mass cones
Tweeter: one 1 1/2'' direct-radiating
Frequency response: from 40Hz to 18.5kHz
Crossover network: 1st order series type. 6dB/octave slope. The 3000Hz crossover point is above the most critical midrange frequencies
Impedance: 4 Ohms
Recommended minimum amplifier power: 15 Watts
Recommended maximum amplifier power: 80 Watts
Cabinet dimensions: 24'' (H) x 14 1/2'' (W) x 12'' (D)
Weight: 38.5 pounds

So, before hooking up, I measured the impedance of each speaker to make sure there weren't any surprises. The measured impedance was about 4 Ohms. I then hooked them up to my Pioneer SX-770 receiver that I had recently restored, and... And I was very disappointed! The sound was coming only from the woofers but the tweeter was silent. I twisted the L-pad back and forth a few times but didn't notice any change. I pulled out the tweeter from each speaker and tested it. As expected, both tweeters are blown. I also pulled out the woofers and tested them as well. Fortunately, all woofers are still good.

I decided to restore these speakers and bought new tweeters from the Midwest Speaker Repair. The replacement part number is MT-4107-4. This is an exact replica of the original tweeter for this model. 

The original L-pads and capacitors in the crossover were also upgraded. I bought new L-pads from the Parts Express. The replacement part number is 260-255 (50W Mono, 1'' shaft, 8 Ohm). 

The original bipolar capacitor (10uF/50V) was replaced with a new MPT (metalized polypropylene film) cap made by SuperTech. This is a self-healing capacitor, rated from -40°C to 105°C with a capacitance tolerance of +/- 10%. 

Finally, all original wires were also replaced since I noticed the oxidation on several crimps. I used the 18 AWG stranded wires. I believe the original wires are 20 AWG stranded wires (no marking on them). New crimps are TE Connectivity, the part number is 42710-2.

The cabinets are still in perfect condition, with no scratches or dents. So, I just treated the cabinets to some Restore-a-Finish and Feed-n-Wax.

Below are some photos I took during my restoration process. The final result is amazing! The sound is smooth, with great detail, strong and tight bass, good midrange, and clear highs. These are very underrated speakers from the 70's. Please watch a short demo video at the end of this post. Thank you for reading.

DLK 1 1/2 Network wiring schematic

DLK_Model 1 1/2_Network Wiring Schematic

Both tweeters are blown

DLK_Model 1 1/2_Twetter_Blown

Original woofer with green frame and green cone - still in excellent shape

DLK_Model 1 1/2_Woofer_Original with green frame

Original crossover

DLK_Model 1 1/2_Crossover_Original

Crossover with new L-Pad and MPT capacitor

DLK_Model 1 1/2_Crossover_New L-Pad and MPT capacitor

Round terminal plate

DLK_Model 1 1/2_Terminal Plate

Replaced parts

DLK_Model 1 1/2_Replaced parts

DLK Model 1 1/2 - after restoration

DLK_Model 1 1/2_After Restoration

DLK_Model 1 1/2_After Restoration_with grills

DLK_Model 1 1/2_Back Side

Demo video after repair & restoration

Sunday, May 1, 2022

Pioneer SX-950 Receiver Restoration

Unit: AM/FM Stereo Receiver
Manufacturer: Pioneer
Model: SX-950
SN: XH2906404S

Today I'm showcasing a Pioneer SX-950 vintage receiver that came in for restorationThe SX-950 is a high-end offering from Pioneer's most famous product line, the SX-X50 series. It was initially introduced in 1976 and made through 1978. It delivers a continuous power output of 85 watts per channel into an 8 ohms load from 20 to 20 kHz with no more than 0.1% total harmonic distortion. 

The SX-950 looks very similar to SX-850 which I serviced a few months ago. The only difference is the number of speaker terminals. The SX-950 has three speaker terminals (A, B, and C) while the SX-850 has only two (A and B). Both units have the same dimensions and similar weight. Moreover, all boards except the tuner and power amplifier assembly have the same circuit design.

According to the Pioneer database, this particular unit was manufactured in August 1977.

The restoration of the smaller brothers of SX-950 can be found on my blog here:

Pioneer SX-950_After Restoration

Initial Evaluation and Troubleshooting

The protection relay in this unit was bypassed for some reason. Obviously, this is not a smart decision and should be fixed. Moreover, the previous owner (or whoever serviced this receiver before) added two additional capacitors (6800uF/71V) parallel to the main filter capacitors (22000uF/63V). Again, not a good idea. I know some technicians like the idea to upgrade the main filter capacitors with higher capacitance but I am not in favor of this modification. It potentially can increase the stress on the circuit and reduce the overall reliability. I always prefer to stick with the original circuit design. 

Also, look at the photos below. Did you notice how badly this modification was done?! Check this out: a lot of extra soldering alloy was used, the insulating sleeve on both original filter capacitors was touched with a soldering iron and melted, the insulation on some wires was also melted, and the electrical tape was used instead of shrink tubing. It's sloppy work! Well, everything should be reworked here to bring it back to its original configuration.

Protection relay bypassed, wire insulation was melted, electrical tape was used instead of shrink tubing

Pioneer_SX-950_Protection Relay Bypassed

Two additional capacitors (6800uF/71V) were added (not a good idea and sloppy work)

Pioneer_SX-950_Additional Filter capacitors

I removed the original protection relay and checked its contacts. To my surprise, all contacts were still in perfect condition. I didn't notice any degradation due to arcing. I also tested the power supply and protection unit and didn't find any substantial issues. So, not sure why the protection relay in this unit was bypassed.

I replaced the original relay with a new Omron relay (part # LY2F-DC24) and wired it according to the schematic. Then, I powered up this receiver with my Dim Bulb Tester. The light bulb flashed for a second on bright and then dimmed out almost completely. The protection relay engaged after a few seconds and a normal sound was restored. So, the unit works properly again.

A new Omron protection relay was installed

Pioneer_SX-950_Protection Relay_New Omron

After discussion with the owner, it was decided to remove additional capacitors and restore the original circuit. The original filter capacitors (22000uF/63V) were tested with Atlas ESR70 capacitance meter and are still within the factory capacitance tolerance with almost zero ESR: 18100uF/0.01Ω and 18230uF/0.01Ω. So, I rewired them and also installed new metal oxide resistors (KOA Speer, 2.7kΩ/2W). All wires with melted insulation were also replaced. The electrical tape was replaced with shrink tubing. Now, it's time to move to routine restoration.

Original (22000uF/63V) filter capacitors - rewired

Pioneer_SX-950_Original Filter Capacitors_rewired

Power Supply Board (AWR-101)

The power supply board (AWR-101) has twelve aluminum electrolytic capacitors C10 thru C16, C18 thru C21, and C23. All of them were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. The max operating voltage was increased one step up on several e-caps.

The original e-caps removed from this board were tested with an Atlas ESR70 capacitance meter and the results are below. Only two out of twelve original e-caps are outside the factory capacitance tolerance +/- 20%. Not too bad in comparison to the SX-850 which I serviced a few months ago.

Test results on original capacitors removed from power supply board:

C10: rated capacitance – 470uF, measured – 370uF, ESR – 0.02Ω, deviation: -21%
C11: rated capacitance – 470uF, measured – 399uF, ESR – 0.02Ω, deviation: -15%
C12: rated capacitance – 2200uF, measured – 2120uF, ESR – 0.01Ω, deviation: -4%
C13: rated capacitance – 1000uF, measured – 1072uF, ESR – 0.01Ω, deviation: +7%
C14: rated capacitance – 47uF, measured – 53uF, ESR – 0.38Ω, deviation: +13%
C15: rated capacitance – 470uF, measured – 374uF, ESR – 0.01Ω, deviation: -20%
C16: rated capacitance – 47uF, measured – 51uF, ESR – 0.71Ω, deviation: +9%
C18: rated capacitance – 47uF, measured – 34uF, ESR – 2.6Ω, deviation: -28%
C19: rated capacitance – 47uF, measured – 56uF, ESR - 0.24Ω, deviation: +19%
C20: rated capacitance – 100uF, measured – 101uF, ESR – 0.06Ω, deviation: +1%
C21: rated capacitance – 47uF, measured – 52uF, ESR – 0.78Ω, deviation: +11%
C23: rated capacitance – 100uF, measured – 114uF, ESR - 0.16Ω, deviation: +14%

All transistors on this board were also replaced to improve the reliability of the power supply. They should be retired after working hard for the last 45+ years. Below is a list of original and replacement transistors I used. Four transistors Q1, Q2, Q8, and Q9 are mounted on the heat sink. The old thermal pads were replaced with new Mica ones. The old thermal compound was refreshed with a new silicone thermal compound (Wakefield-Vette, 120 series).

Q1: NPN, 2SD313 (original), replaced with a new Fairchild KSC2073TU
Q2: PNP, 2SB507 (original), replaced with a new Fairchild KSA940TU
Q3: NPN, 2SC1384 (original), replaced with a new Fairchild KSC2383YTA
Q4: PNP, 2SA720 (original), replaced with a new Fairchild KSA1013YBU
Q5: NPN, 2SC1384 (original), replaced with a new Fairchild KSC2690AYSTU
Q6: PNP, 2SA720 (original), replaced with a new Fairchild KSA1013YBU
Q7: NPN, 2SC1384 (original), replaced with a new Fairchild KSC2383YTA
Q8: NPN, 2SD313 (original), replaced with a new Fairchild KSC2073TU
Q9: NPN, 2SD313 (original), replaced with a new Fairchild KSD526Y
Q10: NPN, 2SC869 (original), replaced with a new Fairchild KSC2383YTA

Power supply board - before servicing

Pioneer_SX-950_Power Supply_AWR-101_before servicing_01

Pioneer_SX-950_Power Supply_AWR-101_before servicing_02

Power supply board - after servicing

Pioneer_SX-950_Power Supply_AWR-101_after servicing_01

Pioneer_SX-950_Power Supply_AWR-101_after servicing_02

Protection Unit (AWM-062-0)

The protection unit in Pioneer SX-950 (as well as in SX-850) has the same design as in Pioneer SX-1010 introduced to the market in 1974. There are two coupling capacitors (C1, C2) and four aluminum electrolytic capacitors (C3 thru C6) installed on this board. Two coupling capacitors are the notorious sky blue Sanyo e-caps. I replaced them with high-quality film polyester Kemet caps. The remaining aluminum e-caps were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from protection unit board:

C1: rated capacitance – 0.22uF, measured – 0.28uF, ESR – N/A, deviation: +27%
C2: rated capacitance – 0.22uF, measured – 0.28uF, ESR – N/A, deviation: +27%
C3: rated capacitance – 330uF, measured – 354uF, ESR – 0.21Ω, deviation: +7%
C4: rated capacitance – 330uF, measured – 305uF, ESR – 0.19Ω, deviation: -8%
C5: rated capacitance – 4.7uF, measured – 5.3uF, ESR – 1.57Ω, deviation: +13%
C6: rated capacitance – 100uF, measured – 114uF, ESR – 0.21Ω, deviation: +14%

The relay driver transistor Q7 suffers from the current spike every time the relay turns off. Eventually, it fails due to severe degradation. I unsoldered and tested this transistor with Atlas DCA55 semiconductor analyzer. The measured current gain was only 57. According to the datasheet, the minimum gain for this transistor is supposed to be at least 120 (2SC1384, rank R). I replaced the original 2SC1384 transistor with a new Fairchild KSC2690. The new transistor has the same pinout as the original one. Also, a fly-back safety diode (1N4004G) was added to the circuit to prevent potential output damage. The safety diode was soldered between pins 9 (anode, "+") and 10 (cathode, "-").

Protection unit - before and after

Pioneer_SX-950_Protection Unit (AWM-062)_before servicing

Pioneer_SX-950_Protection Unit (AWM-062)_after servicing

Tone Amplifier Board (AWG-039C)

The tone amplifier board installed in this unit is AWG-039, revision C. Note that two electrolytic capacitors (C29 & C30) are bypassed in revision C. The tone amplifier board has two solid tantalum capacitors (C25, C26), eight low leakage electrolytic capacitors (C13, C14, C15, C16, C19, C20, C23, C24), and two aluminum e-caps (C21, C22).

The original tantalum capacitors were replaced with film polyester Kemet caps. All original low leakage e-caps were replaced with modern low leakage Nichicon UKL caps. Finally, two aluminum filtering capacitors (C21, C22) were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from tone amplifier board:

C13: rated capacitance – 2.2uF, measured – 1.8uF, ESR – 3.9Ω, deviation: -18%
C14: rated capacitance – 2.2uF, measured – 1.8uF, ESR – 5.2Ω, deviation: -18%
C15: rated capacitance – 2.2uF, measured – 1.9uF, ESR – 4.4Ω, deviation: -14%
C16: rated capacitance – 2.2uF, measured – 1.8uF, ESR – 4.1Ω, deviation: -18%
C19: rated capacitance – 100uF, measured – 118uF, ESR – 0.26Ω, deviation: +18%
C20: rated capacitance – 100uF, measured – 110uF, ESR – 0.31Ω, deviation: +10%
C21: rated capacitance – 47uF, measured – 58uF, ESR – 0.11Ω, deviation: +23%
C22: rated capacitance – 47uF, measured – 61uF, ESR – 0.11Ω, deviation: +30%
C23: rated capacitance – 2.2uF, measured – 1.7uF, ESR – 4.3Ω, deviation: -23%
C24: rated capacitance – 2.2uF, measured – 1.9uF, ESR – 3.7Ω, deviation: -14%
C25: rated capacitance – 0.22uF, measured – 0.23uF, ESR – N/A, deviation: +5%
C26: rated capacitance – 0.22uF, measured – 0.23uF, ESR – N/A, deviation: +5%

Two original NPN transistors (Q1 and Q2) installed on this board are 2SC1312. This transistor is known to become very noisy over time. I replaced both original transistors with modern low noise Fairchild KSA1845. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Tone amplifier board - before and after

Pioneer_SX-950_Tone Amp Board_AWG-039C_before servicing

Pioneer_SX-950_Tone Amp Board_AWG-039C_after servicing

Flat Amplifier Board (AWG-038)

The flat amplifier board has four low leakage e-caps installed in the signal path (C1, C2, C5, C6), four solid tantalum capacitors (C17, C18, C23, C24), two low leakage e-caps (C25, C26), and three aluminum filtering e-caps (C13, C14, C27).

Two low leakage e-caps (C1, C2) installed in the input signal path were replaced with film polyester WIMA MKS2 caps. The other four low leakage e-caps and four tantalum capacitors were replaced with modern low leakage Nichicon UKL caps. The remaining three aluminum filtering capacitors (C13, C14, C27) were replaced with low impedance Nichicon UPW/UPM caps.

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

C1: rated capacitance – 2.2uF, measured – 1.8uF, ESR – 4.2Ω, deviation: -18%
C2: rated capacitance – 2.2uF, measured – 1.8uF, ESR – 4.1Ω, deviation: -18%
C5: rated capacitance – 4.7uF, measured – 5.3uF, ESR – 1.9Ω, deviation: +13%
C6: rated capacitance – 4.7uF, measured – 5.4uF, ESR – 2.2Ω, deviation: +15%
C13: rated capacitance – 220uF, measured – 199uF, ESR – 0.02Ω, deviation: -10%
C14: rated capacitance – 220uF, measured – 198uF, ESR – 0.89Ω, deviation: -10%
C17: rated capacitance – 10uF, measured – 10uF, ESR – 0.55Ω, deviation: 0%
C18: rated capacitance – 10uF, measured – 11uF, ESR – 0.64Ω, deviation: +10%
C23: rated capacitance – 4.7uF, measured – 4.7uF, ESR – 2.1Ω, deviation: 0%
C24: rated capacitance – 4.7uF, measured – 4.7uF, ESR – 1.31Ω, deviation: 0%
C25: rated capacitance – 100uF, measured – 109uF, ESR – 0.26Ω, deviation: +9%
C26: rated capacitance – 100uF, measured – 106uF, ESR – 0.28Ω, deviation: +6%
C27: rated capacitance – 100uF, measured – 101uF, ESR – 0.04Ω, deviation: +1%

Two original PNP transistors (Q5 and Q6) installed on this board are notorious 2SA725. The failure of this transistor usually results in a shot noise. I replaced both of them with a modern low noise Fairchild KSA992. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Flat amplifier board - before and after

Pioneer_SX-950_Flat Amp Board_AWG-038_before servicing

Pioneer_SX-950_Flat Amp Board_AWG-038_after servicing

Switch Assembly (AWS-094)

Only one aluminum electrolytic capacitor (C1) is installed on this board. I replaced it with a low impedance Nichicon UPW cap. Don't forget to clean and lubricate all switches on this board before mounting it back to the chassis!

Test results on the original capacitor removed from the switch assembly

C1: rated capacitance – 220uF, measured – 298uF, ESR – 0.22Ω, deviation: +36%

Switch assembly - before and after

Pioneer_SX-950_Switch Board_AWS-094_before servicing

Pioneer_SX-950_Switch Board_AWS-094_after servicing

Equalizer Amplifier Board (AWF-011B)

The equalizer amplifier circuit in the Pioneer SX-950 is a three-stage direct-coupled design using a one-stage PNP transistor to achieve high gain amplification. Distortion is kept low by applying sufficient negative feedback. 

The original PNP transistors (Q1 and Q2) installed in the first stage are the notorious 2SA725. The failure of this transistor usually results in a shot noise. I replaced both of them with a modern low noise Fairchild KSA992. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB. Also, keep in mind that these transistors are supposed to be gain-matched (orange dot on the top). Below are test results on original transistors removed from the board along with test results on new closely matched transistors. Note that the original transistors are poorly matched.

Test results on original transistors 2SA725:

Left channel: Q1 (gain - 411, Vbe - 0.769V)
Right channel: Q2 (gain - 568, Vbe - 0.773V)

Test results on new closely matched transistors KSA992:

Left channel: Q1 (gain - 461, Vbe - 0.760V)
Right channel: Q2 (gain - 464, Vbe - 0.759V)

The original NPN transistors (Q3 thru Q6) installed on this board are 2SC1313. These are also known as noisy/leaky transistors. I replaced them with modern low noise Fairchild KSA1845. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Two coupling capacitors (C1, C2) installed in the input signal path are notorious sky blue Sanyo e-caps. I replaced them with film polyester WIMA MKS2 caps to improve the signal-to-noise ratio. Two low leakage e-caps (C11, C12) were replaced with modern low leakage Nichicon UKL caps. And the remaining aluminum electrolytic capacitors (C7, C8, C17, C18, C*, C**) were replaced with low impedance Nichicon UPW/UPM caps. Note that revision B has two additional electrolytic capacitors (C*, C**) rated at 100uF/6.3V.

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

C1: rated capacitance – 1uF, measured – 1.3uF, ESR – 5.5Ω, deviation: +30%
C2: rated capacitance – 1uF, measured – 1.2uF, ESR – 5.1Ω, deviation: +20%
C7: rated capacitance – 330uF, measured – 394uF, ESR – 0.23Ω, deviation: +19%
C8: rated capacitance – 330uF, measured – 388uF, ESR – 0.14Ω, deviation: +18%
C11: rated capacitance – 3.3uF, measured – 4.7uF, ESR – 4.8Ω, deviation: +42%
C12: rated capacitance – 3.3uF, measured – 4.5uF, ESR – 5.5Ω, deviation: +36%
C17: rated capacitance – 100uF, measured – 108uF, ESR – 0.07Ω, deviation: +8%
C18: rated capacitance – 220uF, measured – 241uF, ESR – 0.08Ω, deviation: +10%
C*: rated capacitance – 100uF, measured – 102uF, ESR – 0.45Ω, deviation: +2%
C**: rated capacitance – 100uF, measured – 103uF, ESR – 0.43Ω, deviation: +3%

Equalizer amplifier board - before and after

Pioneer_SX-950_Equalizer Amp (AWF-011)_before servicing

Pioneer_SX-950_Equalizer Amp (AWF-011)_after servicing

Power Amplifier Board (AWH-050C)

The SX-950 power amplifier is an all-stage direct-coupled pure complementary circuit. It is an OCL circuit with a balanced positive and negative power supply and center point DC potential kept at 0V.

The first stage of the power amplifier is a differential amplifier consisting of two PNP transistors with a common emitter: Q1/Q3 (left channel) and Q2/Q4 (right channel). The original PNP transistors installed in these positions are notorious 2SA726. This transistor gets very noisy over time. I replaced them with modern low noise Fairchild KSA992. Before installation, each pair of KSA992 transistors was 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. Below are test results on original transistors removed from the board along with test results on new closely matched transistors.

Test results on original transistors 2SA726:

Left channel: Q1 (gain - 325, Vbe - 0.778V), Q3 (gain - 370, Vbe - 0.776V)
Right channel: Q2 (gain - 367, Vbe - 0.778V), Q4 (gain - 331, Vbe - 0.777V)

Test results on new closely matched transistors KSA992:

Left channel: Q1 (gain - 427, Vbe - 0.761V), Q3 (gain - 426, Vbe - 0.762V)
Right channel: Q2 (gain - 426, Vbe - 0.760V), Q4 (gain - 431, Vbe - 0.761V)

Four coupling capacitors C1 thru C4 installed in the signal path are notorious sky blue Sanyo e-caps. I replaced them with high-quality film polyester WIMA caps. The remaining four aluminum e-caps (C5, C6, C11, C12) were replaced with low impedance Nichicon UPW caps.

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

C1: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 11.4Ω, deviation: -5%
C2: rated capacitance – 2.2uF, measured – 2.5uF, ESR – 3.6Ω, deviation: +14%
C3: rated capacitance – 1uF, measured – 0.8uF, ESR – 4.7Ω, deviation: -20%
C4: rated capacitance – 1uF, measured – 1.2uF, ESR – 4.4Ω, deviation: +20%
C5: rated capacitance – 33uF, measured – 36uF, ESR – 0.62Ω, deviation: +9%
C6: rated capacitance – 33uF, measured – 36uF, ESR – 0.71Ω, deviation: +9%
C11: rated capacitance – 330uF, measured – 390uF, ESR – 0.16Ω, deviation: +18%
C12: rated capacitance – 330uF, measured – 394uF, ESR – 0.14Ω, deviation: +19%

The original trimming resistors VR1/VR2 (10 kΩ) and VR3/VR4 (100 Ω) were replaced with new Bourns potentiometers.

Power amplifier board - before and after

Pioneer_SX-950_Power Amp Board_AWH-050C_before servicing

Pioneer_SX-950_Power Amp Board_AWH-050C_after servicing

Power Transistors

Eight power transistors were removed, degreased, and tested with Atlas DCA55 semiconductor analyzer. The measured DC current gain on all transistors 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.

Dial Lamps

All original dial lamps in this unit were replaced with new warm white LED lamps (8V, wedge base). Each lamp is inserted into an individual lamp socket and is very easy to replace. 

DC offset and Bias Adjustments

At the end of my restoration, I adjusted the DC offset and Bias on the power amplifier according to the service manual. Before adjustment, two jumper plugs were removed and a 5.1kΩ resistor was connected to the POWER IN jacks.

The DC offset on the left channel is measured between pins 10 and 9. On the right channel, it is measured between pins 25 and 24. The DC offset should be adjusted as close to zero volts as possible with the trimming resistors VR1 and VR2.

The Bias on the left channel is measured between pins 12 (+) and 13 (-). On the right channel, it is measured between pins 27 (+) and 28 (-). The Bias should be adjusted to ~20mV with the trimming resistors VR3 and VR4.

DC offset on the left and right channels after restoration

Pioneer_SX-950_DC offset_left channel

Pioneer_SX-950_DC offset_right channel

Bias on the left and right channels after restoration

Pioneer_SX-950_Bias_left channel

Pioneer_SX-950_Bias_right channel

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 27.50 VRMS (left channel) and 27.55 VRMS (right channel). It corresponds to the output power of 94.5W on the left channel and 94.9W on the right channel.

Output power test

Pioneer SX-950_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 stained with Howard's Restor-A-Finish. I also slightly polished all knobs with Mothers Mag & Aluminum polish to remove some small spots of aluminum oxidation.

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

Pioneer SX-950 - after restoration

Pioneer SX-950_After Restoration

Pioneer SX-950_After Restoration_with parts

Demo video after repair & restoration