Sansui AU-6500 #1: 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 Fairchild KSA992 transistors as their terminals are designated as ECB. The old 2SA493 or 2SA726 are BCE. Same for KSC1845. So the replacements must be mounted in reverse.
Working on this unit exposes you to electrical hazards. There are lethal voltages inside.
Severe accidents and possibly death by electrocution might occur. I am qualified and skilled with electronics and I have been doing audio gear repairs for over 20 years. If you lack experience, please take these articles as just a knowledge base. Do not attempt to repair something that you cannot handle as there is a high chance of doing further damage while also possibly suffering accidents.
Good tools are a must for a quality restoration. I use eutectic soldering alloy and a temperature-controlled soldering station equipped with various tip shapes. I a standard and a precision desoldering pumps and desoldering wick in various widths. To clean the flux, I use isopropyl alcohol and high purity acetone.
Empirically, I found that working with a temperature of exactly 300 °C is safe for these vintage printed circuit boards. I have never lifted any pads and I never wait more than a couple of seconds with the hot tip on any pad. While working on the chassis, I use between 360 and 440 °C. Flux fumes are extremely toxic and should be avoided at all costs.
Every replacement part is brand new, from a reputable manufacturer, ordered from the U.S.A., Japan, or Germany. In addition, I only use parts that are suitable in specific circuit sections, after inspecting and comprehending the original schematic diagrams. Last but not least, I have years of experience backing up my choices and actions.
On the chassis there are only the power transistors that I am aiming to restore. Thankfully there is not much to do here since the transistors are all original. However they are dirty and the old mica is very brittle as it breaks when I touch it. This is how the radiator looks without the power transistors. In this picture it is semi-clean.
This is how new mica looks like.
Under each transistor and under the mica itself there is a drop of non-conductive computer-grade thermal paste. This helps with the heat transfer.
F-2013-1A Power Supply Board Restoration
Easy to work on. Here it is in a before state.
Very messy on the tracks side.
Crazy round shape diode. Looks nice if you ask me.
The board has been stripped of parts. The brownish spot is due to regulator transistor overheating. Using a small radiator is not part of a good engineering design.
I cleaned the pads. Nice and shine.
But somehow I managed to mecanically lift a pad on the filtering capacitor. Don't ask how but I did. With all the care I employ in these restorations, it still happens from time to time.
This is the insanely small radiator.
Which I replaced with a bigger copper radiator. Copper is a better heat conductor than plain steel. I wanted to go even bigger than this but board real estate is prohibitive.
For C009, C010, C012 I have used Nichicon KZ MUSE series rated 10 uF / 100 V. C014 was replaced with a Nichicon KZ MUSE modern counterpart rated the same as the original. Normally I don't touch ceramic capacitors but this time I decided to replace C011 with a WIMA film capacitor rated 47 nF / 63 V. C013 was replaced with a modern MLCC capacitor rated 10 nF / 100 V. This operation was absolutely unnecessary though. The original KCK ceramic capacitors were still good. Even though the originals measured slightly off their marked values, it doesn't really matter that much: they are not in critical circuit positions.
I still need to solder the filtering capacitor designated C014.
On the track side I will use one leg of C014 to make a good electrical contact with the big corresponding pad.
Ripple filter capacitor is in and the board is secured on the steel chassis. Power section is ready to go.
F-2041 / F-1215-A Protector Board Restoration
F-2041 is shared with the AU-7500 model and is still a joy to restore. A few capacitors and the coupling relay are targeted here.
This is before.
C901 was a 47 uF / 10 V bipolar capacitor and it was replaced with a Nichicon ES bipolar series modern part rated 47 uF / 16 V. C902 acts as the relay turn-on delay and it was rated 220 uF / 6.3 V. I replaced it with a modern part rated 220 uF / 25 V. C903 is a supply rail filter capacitor for this circuit. I replaced it with the same 100 uF / 50 V rating capacitor from the Nichicon KZ MUSE series. For C906 rated 1 uF / 50 V I have used a Panasonic stacked film capacitor of the same rating. C907 is a 1 uF / 50 V bipolar capacitor that was replaced with a Nichicon ES series counterpart of the same rating. The two relay driver transistors were replaced with KSC1845 parts. Even though these are small signal audio semiconductors, they can handle the relay load as well.
The relay can be ordered by the OMRON MY4-02-DC24 part number. This fits perfectly in place of the old relay. In fact it is identical despite the 40+ years that have passed.
Everything went away in the first place. After cleaning, this board appears in almost new condition.
The track pads cleaned well.
All new parts are mounted in their own places. The 1 uF bipolar capacitor is missing in these pictures. But you will see it at the end.
On the tracks side everything looks ready for the next four decades.
F-1215-A is very easy to work on because there is exactly one capacitor to be replaced. Removal is easy as well. Only one screw to remove. Here is a picture before the job.
I have replaced the old 220 uF / 35 V capacitor with a modern Nichicon KZ MUSE series counterpart rated 220 uF / 50 V. The old transistor with the corroded legs was replaced with a Fairchild KSC1845. And here is after the job.
While putting this board back, pay attention to the metal fixture. It has a tendency to run off and misalign with the screw hole. Nothing to worry about. With minimal care I have secured it back in place in no time.
Nichicon 1 uF / 50 V bipolar capacitor is in. Protector board is fixed on the steel chassis and it is good to go.
F-2034-A Driver Board Restoration
The driver board is just near the power transistors array. Very easy to service if you turn the unit upside down to have access to the soldering side while holding parts with your left hand from beneath the chassis. Mine was very dirty so I decided to remove it completely from the chassis.
This is the board prior to restoration.
The tracks side was also very dirty.
Input coupling capacitors C801 and C802 were originally rated 1 uF / 50 V. I have used same rating Panasonic stacked film capacitor replacements. C805, C806, C811, and C812 were rated 47 uF / 50 V and were replaced with same rating Nichicon KZ MUSE series modern capacitors. The bipolar capacitors in positions C807 and C807 were rated 47 uF / 10 V. I have replaced them with Nichicon ES bipolar series capacitors rated 47 uF / 16 V. Power supply rail decoupling capacitors C815, C816, C821, and C822 were rated 100 uF / 50 V and were replaced with the same type modern parts from the Nichicon KZ MUSE series. C819 and C820 are used for emitter decoupling of transistors TR805 and TR806 for static functioning point stabilization. These were 220 uF / 6.3 V and were replaced with Nichicon KZ MUSE series parts rated 220 uF / 25 V.
I have also replace all four 2SA493 transistors in positions TR801, TR802, TR803, and TR804. hFE matched Fairchild KSA992 transistors were used instead.
This is halfway through.
The board came up really clean. Remember the old gunk?
Tracks side was cleaned with copper wire wick and 97% isopropyl alcohol.
Nice 40+ years old tracks.
Cleaning individual resistors is a very boring job. Replanting them is a little bit more rewarding. But everything looks nice in the end.
Next, comes the other halfway through.
All done. Nice and clean.
Tracks side looks good now. Good quality eutectic solder should make for very strong joints that will last for at least another four decades.
The bipolar Nichicon capacitors are in place as well. This is one good looking power amplifier section.
This board resembles the F-2034 board that can be found in Sansui AU-7500 units. However some parts are different and some are mounted in different other places.
F-2028-A Equalizer Board Restoration
The equalizer board is socketed and can be found in the rightmost side of this amplifier. It has a thick steel Faraday cage protecting the circuit of electromagnetic radiation. Removal of the shield is done by removing the two screws securing it in place. Gently pull the board out of its socket presents you with this. In my unit there was a F-2028 board, sans the "-A" designation. I have inspected the F-2028 board installed in the AU-7500 series amplifier and while they are identical, certain parts have slightly different values. Furthermore some parts are missing on the board installed in AU-6500 series.
Prior to restoration.
Input capacitors C601 and C602 were of tantalum type, rated 3.3 uF / 25 V. However, on my board, they were of 2.2 uF / 25 V type. I have replaced these with Nichicon FG series modern parts rated 3.3 uF / 50 V. C603 and C604 in the RIAA correction loop were polarized parts rated 10 uF / 10 V. They were replaced with Nichicon FG series parts rated 10 uF / 50 V. C609 and C610 are rated 47 uF / 16 V. I have replaced these with Nichicon KZ MUSE series capacitors rated 47 uF / 25 V. C611 and C612 were rated 33 uF / 10 V and were replaced with Nichicon KZ MUSE series modern parts rated 33 uF / 25 V. C623 and C624 were marked as 10 uF / 25 V and were replaced with Nichicon FG series rated 10 uF / 100 V. Output coupling is done through C625 and C626, rated 1 uF / 50 V. I replaced them with Panasonic stacked film capacitors rated 1 uF / 50 V. All fault-prone transistors were replaced with modern Fairchild parts, hFE matched. Transistor replacements, as follows.
- TR601, TR602, TR605, and TR606, originally 2SA726 (2SA493 on my board), with Fairchild KSA992.
- TR603 and TR604, originally 2SC1313 (2SC1000 on my board), with Fairchild KSC1845.
While inspecting this board I have found out the level of dirt on it was unbearable for my tastes. Thus, I have decided to attempt a full scale clean-up. This means everything must go off first. Then I will use isopropyl alcohol to clean the old dirt that embedded itself within the flux. Next, follows the re-planting of all the parts on the clean printed circuit board.
I have removed the components halfway through.
Then I have thoroughly cleaned the pads.
All shiny and clean.
Replanting the various passive parts is a boring work.
One channel is done.
Both channels are done.
Solder side is looking good as well.
While electronics can and do work with serious amounts of dirt on them, I like to see clean units. This is better for future service and maintenance operations. And I also recognize the smell of dirty electronics. This is a no-no situation. Thus, I have performed this complicated cleanup operation.
F-2045 Tone Control Board Restoration
This board was the most complicated to work on of them all. This is because of wire forest that is soldered directly on the solder-side of the board. I had to tag all the wires first so that I will know where they connect. There is a handful of capacitors and a eight small signal transistors that are subject to replace. This printed circuit board was not that dirty. But since I took a no-reserve disassemble and clean-all operation, I have removed it as well. Naturally every electronic part had to go off the board, get cleaned up then re-planted.
Here is a picture of this board prior to servicing.
C703 and C704 were rated 47 uF / 16 V and were replaced with Nichicon KZ series parts rated 47 uF / 25 V. C707 and C708, rated 33 uF / 16 V were replaced with Nichicon KZ MUSE series capacitors rated the same as the originals. Coupling capacitors C709 and C710 were rated 4.7 uF / 50 V and were replaced with same rating Nichicon ES series bipolar parts. C717, C718, C725, and C726 were 1 uF / 50 V polarized capacitors. I have replaced these with Panasonic stacked film 1 uF / 50 V capacitors. C719, C720, C723, and C724 were 10 uF / 16 V and were replaced with 10 uF / 50 V Nichicon FG series parts. All transistors were replaced with modern Fairchild parts, hFE matched as follows.
- TR701, TR702, TR705, TR706, TR707, and TR708 originally 2SA493, with Fairchild KSA992.
- TR703 and TR704, originally 2SC1000, with Fairchild KSC1845.
Working my way through the resistor forest.
And by magic: all done. And by hazard: one error. TR701 is misplaced in these pictures. It should be in reverse to meet ECB standards. I have spotted this error upon testing of the unit. No sound was coming on that channel. I have corrected the error but didn't bothered to take new pictures because of the wire forest that needs to be desoldered and soldered back again.
Solder side looks good again.
F-2036 Accessory Board
Normally there is nothing to restore on this particular board. But of them all, this was the most dirty and oily. I can only imagine a previous technician spraying high and low in this unit with his magic spray solution. Dirty... fullstop.
Here are some picture of this board prior to servicing.
Then everything went out for a clean.
I have slightly mecanically lifted a track pad with my small pliers. But it is an easy fix. Scroll down a bit to see how I improvised with the resistor leg.
All parts are back on the printed circuit board.
And the solder side. Track pad lift belongs to the past now.
All back, nice and clean.
Power plug. What the fuck! The former owner thought that the junky salad of old capacitors, transistors with corroded legs, all soaking in a bath of oily deoxit substance inside this amplifier would sing better music if he'd changed the original power plug with this monstrosity. It looks like the power plug of a small concrete mixer. Look at it dwarfing the large power transformer. It must go away, that is for sure.
And please check this particular capacitor that is burned out by the heat radiated by the power supply regulator transistor.
The restoration cannot be complete without performing the electrical settings as illustrated in the service manual. Thus, there are two main settings that I need to touch before putting the case back on.
- Output of Power Amplifier Section
- Current Alignment of Power Amplifier Section
The procedure for setting the output of the power amplifier DC voltage is very simple. First, commute the Speakers selector to SYSTEM-A position. Then connect a DC voltmeter to the SYSTEM-A left terminals. Set the scale of the DC voltmeter to 20 mV. Adjust VR801 up until the voltmeter reads 0 mV ±10 mV. Repeat this step for the right channel. This time adjust VR802.
Adjusting the idling current of the power amplifier is done by removing the speaker fuse cover on the back of the unit. The next step is to remove fuse F002. Turn Speakers selector to OFF position. Connect DC ammeter on the terminals of F002 fuse. Adjust VR803 for a reading of 30 mA ±2 mA. Repeat the procedure for the right channel. This time remove fuse F003 and adjust trimmer VR804.
Needless to say that you need to power the unit off each time you remove the fuses. It can be done with the unit powered but there could be a risk to short-circuit terminals and blow off the nice output power transistors.
Cleaning the Front Panel
A tedious work. I have dismantled the panel and cleaned it with a cotton rag and some natural BIO degreasing substance. This is after the job.
I particularly like these small orange light jewels. And this one is in a very good condition.
On these series, Sansui used to put the serial number of the faceplates under the volume knob.
Between the faceplate and each up-down switch there is some kind of felt. This was very dirty soaked in that oily deoxit substance. I had to wash it thoroughly with isopropyl alcohol.
Cleaning the knobs and switches is a long boring work.
But in the end, all is nice and clean, ready to play some music.
I think this is about it for the restoration of this unit. I was expecting a lot of work but nothing like this. At least not for an AU-6500. But hey, it is a Sansui and it deserves some respect, right?