Vintage Audio
this section covers some restoration work that I did on my vintage HI-FI gear
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Article #1 | 06:00 PM Sunday 01/10/2016

PIONEER SA-9100: Introduction

A Pioneer amplifier that knows how to play music. From my experience, Pioneer gear manufactured during the 1970-1980 decade are refined units that sound full and rich. Some would even say warm. I had very few Pioneer units during my life because I strongly disliked their sound. I can remember the CT-F950 or the CT-F400 cassette decks. Spectacular looks but dull and cold sound. So when I saw this SA-9100 unit on the local market, I decided to give it a chance. It was cheap because it was sold as defect.

And the seller was right. I plugged the thing in, switched on the power and nothing. No power light and no sound at all. From time to time, the protection would trigger and light the red indicator on the front panel. I opened it up and surprise: it was missing the original supply rail filtering capacitors. Instead some axial capacitors of mixed brands were floating on the chassis, soldered directly to the wire terminals. What a mess. I opened the bottom cover only to find another two capacitors of the same axial type floating around. So in total there were four short-circuit prone parts floating in the chassis.

I have temporarily insulated the wire leads with electrical tape and decided to try to power on again. This time, the thing started correctly. Relay was acting properly as well. However I was initially mislead by malfunction due to the front power indicator being burnt out. Then I decided to connect a set of speakers and a signal source. I have played a few tunes. Despite the supply buzz caused by the semi-leaking axial capacitors, the sound was powerful and warm. Now wait a minute, this one is a keeper.

This is how I decided to restore and keep this unit.

Article #2 | 11:00 PM Sunday 03/01/2016

PIONEER SA-9100: Technical Data

This amplifier has the following technical characteristics.

TECHNICAL PARAMETERS
ParameterValue
Continuous RMS power (both channels)60 W + 60 W (8 Ω at 1,000 Hz)
Total Harmonic Distortion<0.1% (at rated output) and <0.04% (at 1 W + 1 W)
IHF power bandwidth (each channel driven at 8 Ω)5 Hz - 40,000 Hz
Frequency Response (at normal listening level)7 Hz - 80,000 Hz
Load Impedance4 Ω - 16 Ω
SemiconductorsTransistors: 63 / Diodes: 30
Power Voltage110 V, 120 V, 130 V, 220 V, 240 V, at 50/60 Hz
Power Consumption430 W (max)
Dimensions / Weight138 mm (H), 430 mm (W), 341 mm (D) / 13.6 kg

Article #3 | 12:00 PM Tuesday 05/01/2016

PIONEER SA-9100: Parts List

I have assembled a parts list for this amplifier. My restoration touches almost all of the original transistors. Normally these transistors contribute to the authentic Pioneer sound. But modern equivalents are better in all aspects. I am also interested in replacing all the electrolytic capacitors. So this is what you will find listed below.

The schematic value corresponds to what normally can be found in the electrical schematics. The recommended value is what I replaced the former part with. Where I found appropriate, I have chosen a film capacitor replacement instead of an electrolytic. My reasons are reliability in time. Less electrolytic capacitors, less time-ticking bombs.

Descriptions and Parts Listings

The power supply rail filtering capacitors are mounted on the chassis. Should they test bad, they need to be replaced. However in my case, they were missing altogether so I am obliged to add them.

Chassis
IdentifierSchematicRecommendedMouser Number
C7115.000 uF / 50 V75-36DY153F050BM2A
C7215.000 uF / 50 V75-36DY153F050BM2A
C71 mount hardwaremetal bracket75-1245860036A
C72 mount hardwaremetal bracket75-1245860036A
RLXXXDC24V MY-4 RelayOMRON MY4F-DC24653-MY4F-DC24

The Equalizer Amp Assembly is coded AWF-007-0 and implements the Phono preamplifier and the RIAA correction curve. You can find it constructed on dual-mono topology in the front-right part of the steel chassis. In order to operate changes, you need to extract the two printed circuit boards. The schematic doesn't correspond to the actual circuit. Nor does the layout of the printed circuit board correspond to the foil side pictured in the service manual. Some capacitors have different markings and some are even recent additions of a new design. Please note that the actual PCB assemblies installed in this unit are of type ANP-201-A and not AWF-007-0.

The parts are listed for only one channels. So double ordering is required. For this chart, I have switched the Schematic notation with Installed notation for the reason stated above. Capacitors that don't appear on the schematic but are in the circuit are marked with C symbol followed by * characters.

Equalizer Amp Assembly (ANP-201-A)
IdentifierInstalledRecommendedMouser Number
C12.2 uF / 25 V2.2 uF / 50 V BP647-UES1H2R2MDM
C447 uF / 25 V47 uF / 35 V647-UFG1V470MPM
C73.3 uF / 50 V647-UFG1H3R3MDM
C910 uF / 6 V10 uF / 50 V647-UFG1H100MDM
C10470 uF / 6 V470 uF / 16 V647-UFG1C471MPM
C*2.2 uF / 25 V2.2 uF / 50 V BP647-UES1H2R2MDM
C**22 uF / 50 V647-UKZ1H220MPM
Q12SA774KSA992512-KSA992FTA
Q22SA774KSA992512-KSA992FTA
Q32SC1313KSC1845512-KSC1845FTA
Q42SC1313KSC1845512-KSC1845FTA
Q52SA726KSA992512-KSA992FTA

The Control Amp Assembly is coded AWG-018-A and implements the preamplifier and the tone control circuits. You can find it in front of the steel chassis under the black Faraday shielding cage. In order to operate changes, you need to extract the printed circuit board. This is the long board between the equalizer amp assemblies and the power supply circuit B printed circuit board.

The parts are listed for both channels.

Control Amp Assembly (AWG-018-A)
IdentifierSchematicRecommendedMouser Number
C11 uF / 25 V1 uF / 50 V FILMN/A
C21 uF / 25 V1 uF / 50 V FILMN/A
C747 uF / 16 V47 uF / 25 V647-UFG1E470MEM
C847 uF / 16 V47 uF / 25 V647-UFG1E470MEM
C93.3 uF / 25 V3.3 uF / 50 V647-UFG1H3R3MDM
C103.3 uF / 25 V3.3 uF / 50 V647-UFG1H3R3MDM
C1147 uF / 16 V47 uF / 25 V647-UFG1E470MEM
C1247 uF / 16 V47 uF / 25 V647-UFG1E470MEM
C134.7 uF / 25 V4.7 uF / 50 V647-UFG1H4R7MDM
C144.7 uF / 25 V4.7 uF / 50 V647-UFG1H4R7MDM
C1710 uF / 25 V10 uF / 50 V647-UFG1H100MDM
C1810 uF / 25 V10 uF / 50 V647-UFG1H100MDM
C21100 uF / 16 V647-UFG1C101MPM
C221 uF / 25 V1 uF / 50 V FILMN/A
C231 uF / 25 V1 uF / 50 V FILMN/A
C241 uF / 25 V1 uF / 50 V FILMN/A
C251 uF / 25 V1 uF / 50 V FILMN/A
C261 uF / 25 V1 uF / 50 V FILMN/A
Q12SA774KSA992512-KSA992FTA
Q22SA774KSA992512-KSA992FTA
Q32SA774KSA992512-KSA992FTA
Q42SA774KSA992512-KSA992FTA
Q52SC1312KSC1845512-KSC1845FTA
Q62SC1312KSC1845512-KSC1845FTA
Q72SC1312KSC1845512-KSC1845FTA
Q82SC1312KSC1845512-KSC1845FTA
Q92SC1478KSC1845512-KSC1845FTA
Q102SC1478KSC1845512-KSC1845FTA

The Filter Amp Assembly is coded AWK-021-0 and is located under the steel chassis in between of the switch boards and the power transformer. You can recognize it easily since it houses two big filter coils beneath an aluminum cage -- on some models the shielding cage is missing.

The parts are listed for both channels.

Filter Amp Assembly (AWK-021-0)
IdentifierSchematicRecommendedMouser Number
C14.7 uF / 10 V4.7 uF / 50 V647-UFG1H4R7MDM
C24.7 uF / 10 V4.7 uF / 50 V647-UFG1H4R7MDM
C547 uF / 10 V47 uF / 25 V647-UFG1E470MEM
C647 uF / 10 V47 uF / 25 V647-UFG1E470MEM
C74.7 uF / 10 V4.7 uF / 50 V647-UFG1H4R7MDM
C84.7 uF / 10 V4.7 uF / 50 V647-UFG1H4R7MDM
C1122 uF / 10 V22 uF / 25 V647-UFG1E220MDM
C1222 uF / 10 V22 uF / 25 V647-UFG1E220MDM
Q12SC1345KSC1845512-KSC1845FTA
Q22SC1345KSC1845512-KSC1845FTA
Q32SC1345KSC1845512-KSC1845FTA
Q42SC1345KSC1845512-KSC1845FTA

The Power Amp Assembly is coded AWH-023-B. There are two identical boards that are bolted to the big black radiators for the output power transistors. You recognize them quickly as they occupy most of the visible area that you see when you first remove the top cover. These boards are hard to get out without a proper angled screwdriver.

The parts are listed for only one channel.

Power Amp Assembly (AWH-023-B)
IdentifierSchematicRecommendedMouser Number
C10.22 uF / 25 V220 nF / 50 V647-QYX1H224JTP
C20.47 uF / 25 V0.47 uF / 100 V647-UFG2AR47MDM
C333 uF / 16 V647-UFG1C330MDM
C633 uF / 16 V647-UFG1C330MDM
C12470 uF / 6 V470 uF / 16 V647-UFG1C471MPM
VR1330 Ω500 Ω652-3306F-1-502
VR2100 Ω652-3306F-1-101
R2310 Ω 1 W10 Ω 3 W71-CPF310R000FKEE6
R2410 Ω 1 W10 Ω 3 W71-CPF310R000FKEE6
R25150 Ω ½ W150 Ω 3 W71-CPF3150R00JKE14
R26150 Ω ½ W150 Ω 3 W71-CPF3150R00JKE14
R274.7 Ω ½ W4.7 Ω 3 W71-CPF3-J-4.7-E3
R284.7 Ω ½ W4.7 Ω 3 W71-CPF3-J-4.7-E3
R3110 Ω 1 W10 Ω 3 W71-CPF310R000FKEE6
Q12SA774KSA992512-KSA992FTA
Q22SA774KSA992512-KSA992FTA
Q32SC869KSC1845512-KSC1845FTA
Q42SC869KSC1845512-KSC1845FTA
Q52SA628KSA992512-KSA992FTA
Q62SC1451KSC3503512-KSC3503DSTU
Q72SC1451KSC3503512-KSC3503DSTU
Q82SD358KSC2073512-KSC2073TU
Q92SB528KSA940512-KSA940TU
D11S24731N414878-1N4148
D21S24731N414878-1N4148
D31S24731N414878-1N4148
D51S24731N414878-1N4148
D61S24711N414878-1N4148

The Protection Circuit Assembly is coded AWM-025-A and is located beneath the steel chassis on the right, next to the power transformer.

The parts are listed for both channels. The starred (*) part does not exist on the schematic nor does it exist on the actual circuit board. I have installed it additionally to provide protection to the relay driver transistor from inverse transient voltages from the relay coil.

Protection Circuit Assembly (AWM-025-A)
IdentifierSchematicRecommendedMouser Number
C10.22 uF / 25 V220 nF / 50 V647-QYX1H224JTP
C20.22 uF / 25 V220 nF / 50 V647-QYX1H224JTP
C3330 uF / 6 V330 uF / 16 V647-UFG1C331MPM
C4330 uF / 6 V330 uF / 16 V647-UFG1C331MPM
C54.7 uF / 25 V4.7 uF / 50 V647-UFG1H4R7MDM
C6100 uF / 16 V647-UFG1C101MPM
Q12SC857KSC1845512-KSC1845FTA
Q22SC857KSC1845512-KSC1845FTA
Q32SC945KSC945512-KSC945CGTA
Q42SC945KSC945512-KSC945CGTA
Q52SA733KSA733512-KSA733CGBU
Q62SC945KSC945512-KSC945CGTA
Q72SC1384KSC2690512-KSC2690AYSTU
D*1N5392625-1N5392-E3/54

The Power Supply Circuit-A Assembly is coded AWR-031-0 and is located in the center of the unit. You can spot it between the big power transformer and the metallic Faraday cage that houses the small signal audio preamplifier boards and the power supply circuit B board.

Power Supply Circuit-A Assembly (AWR-031-0)
IdentifierSchematicRecommendedMouser Number
C5470 uF / 63 V AXIAL1000 uF / 63 V AXIAL594-2222-120-18102
C6470 uF / 63 V AXIAL1000 uF / 63 V AXIAL594-2222-120-18102
D1SR3AM-81N5408863-1N5408RLG
D2SR3AM-81N5408863-1N5408RLG
D3SR3AM-81N5408863-1N5408RLG
D4SR3AM-81N5408863-1N5408RLG
D5SIB01-021N5392625-1N5392-E3/54
D6SIB01-021N5392625-1N5392-E3/54
D7SIB01-021N5392625-1N5392-E3/54
D8SIB01-021N5392625-1N5392-E3/54

The Power Supply Circuit-B Assembly is coded AWR-032-0 and is located in front of the amplifier, under the black Faraday metallic cage. This board is located just left of the control amplifier board. You can recognize by the two regulator transistors that have undersized radiators.

Power Supply Circuit-B Assembly (AWR-032-0)
IdentifierSchematicRecommendedMouser Number
C1100 uF / 63 V100 uF / 80 V647-UBT1K101MPD
C2100 uF / 63 V100 uF / 80 V647-UBT1K101MPD
C510 uF / 16 V10 uF / 50 V647-UBT1H100MPD
C610 uF / 35 V10 uF / 50 V647-UBT1H100MPD
C710 uF / 50 V647-UBT1H100MPD
C810 uF / 50 V647-UBT1H100MPD
C910 uF / 50 V647-UBT1H100MPD
C1247 uF / 25 V47 uF / 50 V647-UBT1H470MPD
C1347 uF / 25 V47 uF / 50 V647-UBT1H470MPD
C1433 uF / 25 V33 uF / 50 V647-UBT1H330MPD
C1533 uF / 25 V33 uF / 50 V647-UBT1H330MPD
C2010 uF / 50 V647-UBT1H100MPD
R1470 Ω ¼ W470 Ω 3 W71-CPF3-G-470
R2470 Ω ¼ W470 Ω 3 W71-CPF3-G-470
R3820 Ω ¼ W820 Ω ½ W71-RN60D8200F
R4820 Ω ¼ W820 Ω ½ W71-RN60D8200F
R58.2 kΩ ¼ W8.2 kΩ ½ W71-RN65D-F-8.2K
R75.6 kΩ ¼ W5.6 kΩ ½ W71-RN60D-F-5.6K/R
R12390 Ω ½ W390 Ω 3 W71-CPF3-G-390-E3
R13390 Ω ½ W390 Ω 3 W71-CPF3-G-390-E3
R148.2 kΩ ¼ W8.2 kΩ ½ W71-RN65D-F-8.2K
R158.2 kΩ ¼ W8.2 kΩ ½ W71-RN65D-F-8.2K
R185.1 kΩ ¼ W5.11 kΩ ½ W71-RN65D-F-5.11K
R195.1 kΩ ¼ W5.11 kΩ ½ W71-RN65D-F-5.11K
R20390 Ω ¼ W390 Ω ½ W71-RN60D3900F
R21390 Ω ¼ W390 Ω ½ W71-RN60D3900F
R24180 Ω ½ W180 Ω 3 W71-CPF3180R00FKE14
Q12SD313KSD526512-KSD526Y
Q22SB507KSB596512-KSB596YTU
Q32SC945KSC945512-KSC945CGTA
Q42SA720KSA1013512-KSA1013YTA
Q52SC1318KSC2690512-KSC2690AYSTU
Q62SA720KSA1220512-KSA1220AYS
Q72SC1318KSC2690512-KSC2690AYSTU
Q82SA720KSA1220512-KSA1220AYS

The Switch-B Assembly is coded AWS-039-0 and is located under the steel chassis. It is home of the Tape Monitor and Duplicate switches. Easy to spot on but hard to work on.

The parts are listed for both channels.

The Switch-B Assembly (AWS-039-0)
IdentifierSchematicRecommendedMouser Number
C11 uF / 25 V1 uF / 50 V FILMN/A
C21 uF / 25 V1 uF / 50 V FILMN/A
C510 uF / 16 V10 uF / 50 V647-UFG1H100MDM
C610 uF / 16 V10 uF / 50 V647-UFG1H100MDM
Q12SC1313KSC1845512-KSC1845FTA
Q22SC1313KSC1845512-KSC1845FTA

The Switch-C Assembly is coded AWS-040-0 and is located under the steel chassis. It is home of the Muting, Low Filter, Tone Defeat, High Filter, and Loudness switches. There are a lot of wires connecting this board to other electrical sub-assemblies. Since servicing this board is difficult, replacing the electrolytic capacitors with film counterparts would be good for future proofing.

The parts are listed for both channels.

The Switch-C Assembly (AWS-040-0)
IdentifierSchematicRecommendedMouser Number
C90.47 uF / 10 V470 nF / 50 V FILM
C100.47 uF / 10 V470 nF / 50 V FILM
C110.47 uF / 10 V470 nF / 50 V FILM
C120.47 uF / 10 V470 nF / 50 V FILM

The construction of this amplifier is very complicated due to a lot of wires and complicated layout. Thus the restoration is not exceptionally easy. In fact it is on the hard side. The wires cannot be removed and in some places you need to use your contortionist skills.

As always, let me throw a piece of advice. Do not hurry, take your time and do the job once. And do it well.

Article #4 | 01:00 PM Tuesday 01/01/2019

PIONEER SA-9100: Restoration

As always before I start working on a vintage unit restoration, I organize all the parts that will be changed. Further more I am sorting transistors based on the hFE factor. This is a boring job but it pays for itself. Careful with new Fairchild transistors legs disposition. Thankfully the Pioneer engineers designed these boards with triangle transistor legs dispositions. This means that it is extremely easy to mount every new transistor in place of the old ones. However one needs to carefully observe the board markings and the electrical schematics first.

This Pioneer amplifier is hard to work on due to the multitude of wires that are all wire-wrapped. These cannot be unwrapped and de-soldering the wire-wrap posts is next to impossible. The posts were mechanically fixed before being soldered. This means that you have to work on the boards in place, carefully bending all the wires at once. This operation is risky and I wouldn't bend the wires more than a few times. So a lot of patience is required.

General Considerations

I should begin by saying that working on this amplifier presents a very big health hazard. There are lethal voltages inside. Not knowing what you are doing might result in severe accidents and possibly death by electrocution. I am very skilled in electronics repair and I have been doing such repairs for almost 20 years. This qualifies me to work in this field. But if you do not have experience, please take this information just as a knowledge base. Do not attempt to repair something that you cannot handle as there is a big probability to severe it further while also suffering accidents.

Good working condition tools are also necessary for this restoration. I am using a quality temperature regulated soldering station with multiple tips for every situation that I could possibly encounter. Also I am using a good solder pump and quality desoldering wick in two sizes. Isopropyl alcohol is handy to decontaminate boards of old flux and other residues. I also use eutectic formulation quality solder. Every replacement part is brand new, from a reputable manufacturer, ordered from U.S.A., Japan, or Germany. Also I only use parts that are suitable as replacements in various sections of the amplifier, after inspecting and comprehending the original schematic. Last but not least I have the years of experience backing up every action that I take while working on this unit.

I have found out that working with a temperature of between 300 -- preferred -- and 323 degrees Celsius is sane for these vintage ELNA printed circuit boards. I have never lifted a pad with this temperature. But it is also true that I never wait more than three seconds with the soldering tip on a pad. While working on the chassis, I am pumping up between 360 and 440 degrees Celsius in the soldering iron.

Flux fumes are extremely toxic and should be avoided at all costs. Especially toxic are the fumes released while working on these old Japanese electronic boards.

I have not taken a lot of pictures during this restoration because the boards are hard to position for good lighting.

Chassis Restoration

On the chassis there are a few things to be done. The power supply rails filtering capacitors, the front panel indicator bulbs, and the speaker protection relay. It appears that there is not much to do here. But this assumption is plain wrong. Let's proceed then.

A technician installed some axial electrolytic capacitors: two Setton and two Roederstein -- marked ERO. The mounting hardware is missing as well. Oh no! This is how the former capacitors looked like.

Now please check the electrical connections the former technician did.

Junk capacitors are all out. One of the Roederstein parts has vented and upon measurement it was revealed that actual capacity was about 970 uF out of the rated 4700 uF. ESR was 8.1 Ω and loss was 31%.

Thankfully I was able to find Sprague Powerlytic 36DY large can series capacitors that will fit this purpose. In fact these have identical electrical specifications to the old parts. That means 15.000 uF / 50 V.

Mounting hardware is available as well. And it fits perfectly the old machined holes.

However I had to manufacture connector wires since the old ones were cut and floating around the chassis protected by electrical tape. So I took 1.5 mm stranded copper wire and soldered terminals designed for screw mount capacitor leads.

I have used thermo tube to isolate the potential short-circuit prone zoned.

I have soldered the wires using a very resistant technique and then covered the joints with thermo tube.

This is how the work looks from below.

And in the end this is how the capacitors look like from above.

Moving on to the front panel indicator bulbs now. The power indicator bulb is burned out. The other one works without issues. I have decided to remove the bulbs and replace them with LEDs: yellow 5 mm LED for power indication and red 5 mm LED for protector indication. LEDs require specific current and rectified voltage. I have used a simple circuit for each LED. The parts involved are: a 390 R / 0.5 W resistor in series with a 1N4148 diode in series with each LED. I have decided to replace the wires as well since the old ones were very short. I have decided to solder each diode as close to the LED as possible and cover it with thermo tube. This makes for a very clean approach. The 390 R resistor is left uncovered and is mounted directly on the wire. Can you spot the tiny DALE resistors?

Original bulbs were Stanley 8 V / 50 mA. Very nice! Stanley is still in business now and makes automotive tail lights and bulbs. Should you want to take an original restoration approach, please find similar bulbs. I didn't have any similar bulbs around and 5 mm LEDs fit the rubber casing very well.

Final result.

Next up is dealing with the speakers protection relay. It is located in the lower left part of the steel chassis, next to the Power Supply B assembly. In addition to extreme heat exposure, it is also probably unusable by now due to the contacts being clogged with oxide and carbon deposits resulted from electric arcs. These arcs are generated when the mobile armature opens or closes the circuit while music is being played at a consistent volume.

Mounting a new relay requires a little bit of ingenuity. But everything is possible as you will quickly see.

Equalizer Amp Assembly Restoration

The equalizer amplifier is built on two different printed circuit boards which are located on the far bottom-right side of the steel chassis. It is very difficult to work on these two boards because of the connection wires that are wrapped on two diametrically opposing sides. I decided to de-solder the top four wire wrapping posts of the right outer PCB, taking great care not to damage them.

The schematic doesn't entirely correspond to what can be found on the printed circuit boards but I am posting it here anyway.

This is before.

And after.

Control Amp Assembly Restoration

The control amplifier implements the tone controls and can be found under the black front main Faraday shielding cage.

Here is the electrical schematic.

Originally I thought this would've been the hardest board of them all since the number of wrapped wires is just outstanding. An entire array of wires spanning throughout the entire PCB length. But once I removed the two screws I was able to flip the board upside-down and get access to all the parts. I have shotgun replaced every transistor and capacitor on this board.

Pictures as follows.

Working my way through the component forest.

While adding small glass beads on stacked film capacitor leads.

And tubular glass beads on the collector terminal of each small-signal transistor.

All good on the solder side.

Metal can transistors are off as well. Modern Fairchild replacements are in place.

Filter Amp Assembly Restoration

The filter amplifier implements is accessible from underneath the unit. There are four transistors and a few capacitors to replace.

Here is the electrical schematic.

Removing it from the plastic securing posts is easy if you have patience. Servicing this PCB does not raise any problems.

Before the restoration.

After the restoration.

And this is the solder side. On this board I have not touched any other solder joints than necessary.

Power Amp Assembly Restoration

There are two power amplifier printed circuit boards in this unit. Both are bolted to their appropriate power transistor heatsink. There are four screws per board. The two on the top are easy to remove but in order to get out the two on the bottom you need to disassemble the heatsink and get out the power transistors themselves. The wires leading to these boards are fairly short and it is difficult to work on the boards and also to take pictures.

I am targeting all small signal and medium power transistors, all small signal diodes, all variable resistors, and some of the standalone resistors. I chose Dale resistors with a higher wattage than the original specs. The variable resistors are Bourns. Transistors are either ON Semiconductor or Fairchild. Capacitors are all Nichicon FG series electrolytic types or Nichicon film types. The 1N4148 small signal replacement diodes have been mounted on glass standers. The replaced pre-driver transistors have received thermal compound between them and their heatsinks.

The Pioneer glue used to mechanically bind capacitors to the printed circuit board assembly is easily removable with 100% acetone and cotton swabs. No mechanical removal approach is required.

Here is the electrical schematic for one channel.

Let's start with the right channel which is located in the rightmost side of the steel chassis.

Before the restoration.

After the restoration.

The power transistors received new mica insulators and a drop of quality non-conductive thermal compound.

I am continuing with the left channel which is located in the leftmost side of the steel chassis.

Before the restoration.

After the restoration.

As before, these power transistors also received new mica insulators and a drop of quality non-conductive thermal compound.

Protection Circuit Assembly Restoration

This board implements the speaker protection circuit. It is easily serviceable and it is mounted directly underneath the big can power supply filtering capacitors. It is secured by plastic posts. On this board I am targeting all transistors and two capacitors. In my unit the two capacitors were 220 nF / 25 V tantalum capacitors. I have replaced them with Nichicon film capacitors rated 220 nF / 50 V. In addition I have added a flyback diode to protect the relay driver transistor from inverse transient voltages generated by the relay coil when it de-energizes.

This is the electrical schematic.

Before the restoration.

After the restoration.

Flyback diode.

Power Supply Circuit-A Restoration

I think this is the easiest board to work on from the entire amplifier. There are only two capacitors and eight diodes to replace. Normally the diodes are not prone to failure but since this amplifier is very hard to disassemble, I have chosen to overhaul it in terms of inaccessible semiconductors.

The electrical schematic as follows.

Before the restoration.

ON Semiconductor power diodes are mounted on the PCB assembly.

And mounted back in its place. You can spot the two axial capacitors.

Power Supply Circuit-B Restoration

This is probably the worst power supply construction that I have ever seen in an audio amplifier. Hot transistors, small heatsinks, cooked capacitors, scorched resistors, mismatched power dissipation coefficient on certain resistors. Drifted resistor values and cracked resistors. But even with all these design flaws the power supply was still running OKish.

I have shotgun replaced a bunch of resistors that appeared damaged or on their way out. I have chosen Dale resistors with higher power dissipation factors in order to ensure some derating. As for capacitors, all of them were replaced high temperature Nichicon BT series parts that can withstand temperatures of +125 °C while still doing their job. These capacitors also have an unusually long life if they are exposed to ambient temperatures -- think 30 to 40 °C -- while operating. The transistors were all replaced with better ones. The two power transistors used as regulators were moved somewhere else on the steel chassis so they can better dissipate heat.

The electrical schematic as follows.

Before the restoration. Notice the massive heat damage to the printed circuit board. I think these CEM (?) boards are 94V-1 flame retardant certified. It really stood the test of time and heat but it doesn't look pretty. However the tracks are intact and the solder pads are in excellent condition. Heat discoloration is the only issue I can see here.

New capacitors.

New resistors with a big power dissipation reserve.

After the restoration.

Power transistors have been moved away from the PCB assembly. Here they are now.

Switch-B Assembly Restoration

Easy to locate but damn hard to work on it. I used some medical pliers to extract and place the new components in position. It is also very hard to photograph. So there you have it: I took only one picture.

The electrical schematic as follows.

After the restoration.

Cleaning the Front Panel

I have dismantled the panel and cleaned it with a cotton rag and some natural BIO degreasing substance. This is after the job.

Detailed view.

This concludes the restoration of this unit. I hope it will work great for a long time.

If you will

Please note that all the work presented herein this site is non-commercial. This is my hobby and I am doing this in my spare time. Through this page I freely share my knowledge with you. But if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!

Copyright © 1998- Alexandru Groza