1s1555 Diode Equivalent
Athenasby Regalado
Lifted tracks. Ripped tracks. Mismatched diodes. Weird transistors. Mismatched hFE small signal transistors. Main filter capacitors replaced. Why? I have never seen an ELNA large electrolytic capacitor malfunctioning. Tantalum capacitors in the signal line on the phono preamplifier. PCBs dirty by mishandling flux and solder wire. Crazy solder joints. You name it. However I could sense the audiophile syndrome, which I cannot embrace. The former owner restored it with a mixture of good quality electrolytic capacitors from Panasonic. Namely the series FM, FC, and another one which I cannot remember. The power supplt ripple filter capacitors are Kendeil. Some say these are good. I will replace them anyway. Some of the small signal transistors were even very close 2SA493 or 2SC1313 equivalents.
Oh and about the sound. When the packet arrived I have coupled a set of speakers and fired it up. I was totally disappointed. A thin sound that was lacking bass and the high frequencies were hissing. I couldn't believe my ears that a Sansui could sound that bad. Especially an AU-x500 unit. But this is going to change.
The following articles are not to be treated as do-it-yourself tutorials on how to fix, restore, rebuild, or improve the unit in cause. This was not my initial intention. But you can consider this whole content as a general guideline, should you decide to launch into such an adventure.
I have assembled a parts list for this amplifier. My restoration targets mainly the electrolytic capacitors and almost all small signal transistors. Even though I consider these transistors irreplaceable for an authentic Sansui sound, I am aiming for maximum reliability and low maintenance. I don't believe that transistors actually contribute to any specific sound signature. Below you will find lists consisting of the various capacitors in this amplifier.
Besides the capacitors and transistors up-mentioned, I would not normally touch other parts in this amplifier. But when a former technician exchanges components high and low using sub-mediocre replacements, I prefer to watch things from a different perspective. So I will be listing all kind of replacement parts in the tables below: capacitors, (variable) resistors, (Zener) diodes, and transistors.
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. The BP inscription signifies a bipolar capacitor. 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.
There are some capacitor bleeding resistors underneath the steel chassis. These are soldered directly on the big capacitor terminals. In my unit they were cracked. Not by misuse, but by a careless technician before me.
The Power Supply Block is coded F-2013-1A and is located on top of the metal chassis, mounted in a central position. You can quickly recognize it due to the stabilizer transistor cooled by a small aluminum radiator and the four small current diodes that form a rectifying bridge.
The Protector Block is coded F-2041 for stock no. 7591230 and F-1215-A for stock no. 7591300. That's right, two boards for one schematic. This is a particularity shared by both AU-6500 and AU-7500 series. F-2041 is located on the left side of the chassis mounted directly on it. You can quickly recognize it due to the big speaker coupling relay. F-1215-A is located beneath the steel chassis directly under F-2041. There are three wires connecting the two boards. Both boards are easy to service.
There is one combined audio driver board in this amplifier responsible for both left and right channels. It is coded F-2034-A and can be found mounted on the chassis just beneath the power transistors array. The four variable resistors are subject to change as well. You can find them designated in the table below.
The Equalizer Block is coded F-2028-A and implements the Phono preamplifier and the RIAA curve corrector. This board is right positioned, under the large Faraday cage metallic shield. This board is socketed. Note that the TC notation signifies a tantalum capacitor.
The Tone Control Block is coded F-2045 and is located in front of the unit, between the steel chassis and the front part of this amplifier. Of them all, this board is the most complicated to work on because there are a lot of electrolytic capacitors and small signal transistors. Also, a lot of wires on the solder side. The mounting position is crazy as well so disassembly is needed. The capacitors and transistors are listed below.
As you can see, there are a lot of electrolytic capacitors to be replaced. While this amplifier desires a lot of respect, I cannot but advice you to take your time and observe the electrical schematics and my capacitor lists. You can stick to the original values and types or you can follow my list. It is up to you. But the results will be similar. Your amplifier will shine again and will produce that great Sansui sound. After all, this is a top performer and will surely reward your ears.
This is the first amplifier restoration that I did using a new approach for spacing various ceramic and film capacitors from the printed circuit board. I am using glass beads. For certain diodes I took a similar spacing approach, this time using sub-miniature glass tubes. This method might look largely unnecessary. But as I previously said: if it is worth doing it, it surely worths overdoing it.
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 clean and reapply grease. 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. I did not took pictures in a before state. But here is the power transistors array after the service.
These two filtering capacitors and their bleeding resistors follow up next. Somebody replaced the original ELNA parts with these Kendeil ones. They might be of good quality but I really don't know anything about them. Or if they were new or used when they were installed.
When mounted on the chassis, the metal clips don't fit the pre-drilled holes. That must be one of those weird cases when new capacitors of the same rating, are bulkier than old ones. I also had to use longer screws to tighten the clips over the capacitors.
I don't like that the new capacitors don't fit correctly on the chassis. At the same time I am thinking that maybe the metal clips are the problem. Maybe I will investigate whether there are metal clips that fit. But that is a lower priority project on my list.
The power supply printed circuit board is the most damaged board of them all. The former technician worked carelessly and lifted tracks. He also used inappropriate replacement parts. Here is the board before the restoration.
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. The ceramic capacitors received glass beads as spacers from the printed circuit board.
Transistor TR001, originally 2SB507 was replaced by the former technician with a BD244 part. I replaced it with KSA940. Transistors TR002 and TR003, originally 2SA678 were replaced by the former technician with 2N3904 parts. I have used KSA733 parts. Be sure to order the transistors with the C suffix. This means center collector.
Zener diode ZD was replaced with a 15 V / 0.5 W Zener by the former technician. I replaced it with a Nexperia 14.1 V / 1.3 W modern part. The original was a 14 V / 1.5 W part. Diodes D002 through D005 were formerly replaced with 1N4004 parts. I ditched them and replaced with 1N5392 parts. These appear to be more of an equivalent to the original 10D-1 parts. Diodes received glass tubes as separators. Why? Rectifier diodes produce heat. While it is true that in this case, the heat is not sufficient to damage the printed circuit board, good engineering practices say to use separators. Germanium diodes are susceptible to heat. A longer lead will act like a heatsink preventing damage to the junction when soldering. Then again I have never damaged a Germanium diode or transistor by soldering since my childhood when I built my first diode-detector radio. I guess this comes with practice.
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