For so long I have been trying to find the time to update this SE-A3 page although my SE-A5 and SE-A3MK2 pages are all done and thank you to everyone who has contributed to and commented on those pages! Mainly the reason I never get around to this SE-A3 page is because my SE-A3 is stuck in the UK and I am not! Sometime I'll get it over to Florida and I can finish up this page! Until I do, the following paragraphs answer the most often asked question I get about the SE-A3, so I hope that this helps some of you out. If anyone would like to contribute any further comments or photos, please let me know, it would be good to get this page in shape! Also, if anyone has an SE-A3 that needs fixing, please let me know as I would like to do another so I can photograph the process.
OK, so before going any further, what exactly is a Technics OD503A-P Dual Linear Power Transistor?
Its it A: A funky six legged thing that the Technics Marketers thought would Sex-Up the SE-A3 and blow every audiophiles mind in desperation to have some?
Is it B: A monumental design failure that caused Technics great embarrassment because so many SE-A3's were returned under warranty?
Is it C: Just two supposedly matched no name 230 Volt 15 Amp 180 Watt PNP and NPN bipolar power transistors stuck together in a plastic cap?
Is it D: The single most common point of failure in the Technics SE-A3 design?
Well, the fact is that the Technics DLPT OD503A-P power transistor is All of the above! I even heard a story once from a guy that was a soldier in the Danish army who was posted to Greenland. They had a radio station there that was setup in the early 80's and in true Western European style, the radio station managers ordered the biggest and very best, sexiest rack of floor to ceiling power amps they could! Yes, a whole stack of Technics SE-A3's! Two weeks after install, they were all busted! All of them, every single one... One can only imaging the tears and heartbreak that must have rippled through the Technics world... I heard that Technics came to some arrangement with the radio station due to the immense cost of shipping the things back to Japan. Some of them are probably still stacked up there, who could bring themselves to trash them? And the same story has been the fate of just about every Technics SE-A3 I have ever seen. In short, there are two kinds of Technics SE-A3's. First, the ones that have had blown DLPT's, and second, the ones that will, the latter category being in a very small minority. You see, the problem was that back in the early 1980's, bi-polar power transistor production was in the stone age compared with today. Those matched pairs were just not matched well enough. A blow out in one, causes a ripple effect all through the amp ending in terminal failure until the whole set can be replaced and the little yellow protection lamp is the only one to get any glory. So at the end of the day, you don't want any more Technics DLPT OD503A-P power transistors, what you need is a more modern alternative that will provide greater reliability and a smoother frequency response with which to rebuild the entire amp and bring the performance up to the full Electric-Sex level implied by the original intent of the design. I have also heard of some SE-A3's being fitted with OD503A-Q devices and there may be even more revisions, but of course, the only way I hear about them is because they are blown...
Modern Toshiba Replacement Transistors
OK, so now we know all this, finding replacements is as easy as picking up any Toshiba power transistor product list, and looking for what we need. Ah, yes, right there on page 53, the Toshiba 2SA1987 and the Toshiba 2SC5359 will fit the bill because they are 230 volt, 15 Amp, 180 Watts, complementary to each other and intended for audio power amp applications up to 100 Watts each! The Technics SE-A3 design uses doubled up pairs to make the 200 Watt output of the design. In fact it is that doubling up that is the cause of most of the problems where the transistors working in parallel are not properly matched between different OD503A-P devices. Most any pair of these modern Toshiba transistors will be closely enough matched for the SE-A3. Just make sure that they are real Toshiba products with the proper markings. It's best to replace all sixteen of the transistors and get them all from the same vendor at the same time. Do not mix & match! Now just search for them on the net and abracadabra, I found some right here in about 2 seconds! If that link is down, just search for another. Just incase you don't know, the Toshiba Semiconductor Company is one of the premier electronics component vendors for Hi-Fi equipment and I defy anyone to look in their stereo gear and declare that they have no Toshiba components in there! These Toshiba transistors will serve well in the Technics SE-A3 and bring all those old broken units back to new life for many more years to come!
So the Toshiba 2SC5359 is our NPN transistor with the pins in the order, E, C, B and the Toshiba 2SA1987 is our PNP device with pins also in the order, E, C, B. The two transistors in the OD503A-P are mounted with the NPN on the left and the PNP on the right. This can be seen by comparing the Power Amp Layout to the left and the Power Amp Circuit to the right from the service manual. You can see in the Power Amp Circuit that pin G of the top connector goes to the base of an NPN transistor in both Q127 and Q128. Then referring to the Power Amp Layout, it can be seen that the track from pin G on the left connector leads to the base of the transistor on the left hand side (very left pin) on the OD503A-P on both Q127 and Q128 when viewed from the top of the board. So its 2SC5359 on the left and 2SA1987 on the right when comparing to an OD503A-P. Now there is a slight problem as you can see from the pin layout on the OD503A-P. The NPN transistor on the left is actually a complete mirror image of a normal power transistor and of the right hand side of the OD503A-P. i.e. the pins are in the order B, C, E rather than E, C, B. So this means, that when each of the the Toshiba 2SC5359's are mounted, the 1st and 3rd pins must be swapped around rather than soldered directly to the board like the 2SA1987 on the right side. Probably the neatest way to so this switcharoo is to break the tracks on the under side of the board to the Base and Emitter of the left side transistor and solder some wires on to cross them over. Remember though that the connection needs to be good and able to carry 15 Amps, so thin lead wire will not do! If anyone else has done that and can send me a picture to put here, I would appreciate it. Click here to see the SE-A3 Service Manual.
At the end of the day, under the covers of the SE-A3 as described above are bi-polar power transistors. They are what they are and they are NOT MOS-FETS because those just weren't invented back then. You need an SE-A100 for that! Bi-polar power transistors have a negative temperature coefficient of resistance which means that as the temperature increases, resistance reduces and more current flows leading to more heat and so on. There are of course temperature detection and biasing arrangements for this case in the design, but because the power transistors are doubled up in the SE-A3, if one of the pair gets a bit too hot and approaches this state, thermal runaway occurs in the device fast, and it starts to hog current that was intended for its parallel mate and Pow! This is usual the fate of the Technics DLPT OD503A-P transistors, mostly because of poor matching. Another thing that can cause this problem is the way the transistors are mounted. As indicated on the diagram to the left, the power transistors are mounted onto the heat sinks with insulating mica plates, but what it does not indicate is that it is vital for a liberal amount of Silicone Heat Sink Compound to be used on BOTH sides of the mica plate! Over the years since the SE-A3's were produced, if they are stored or operated in very low humidity, the original heat sink compound used by Technics can actually dry up leading to poor thermal conductivity between the transistors collector plate and the heat sink. Then, the first one to get too hot, starts off the chain reaction described above. So if you actually do have an SE-A3 with the original OD503A-P transistors, it may be a good idea to check the transistor mountings to make sure there is no trouble coming. Also of course, if you are fitting the new Toshiba transistors, make sure to use plenty of a good quality Silicone Heat Sink Compound when you put it all back together. Having said all that, the front line of defense to this problem is clearly written on page 2 of the SE-A3 user guide: "Because this unit is a high-power amplifier, its temperature will increase when it is used for long periods of time. Take care therefore, not to obstruct the ventilation holes (located on the top panel) by placing other audio equipment, books, etc. on top of this unit". Are we all understanding about this now?.. The SE-A3 is NOT for rack mounting and there must be NOTHING on top of it! Be good to your old SE-A3 and it will keep going for a lifetime! If you look closely at the pictures at the bottom of the page, you'll see that some of those SE-A3's have had the dust covers over the power amp heat sinks removed and in the SE-A3MK2 design, these are not fitted either. I would say that is a good idea if you are concerned about thermal problems with a stock SE-A3. Its easy to clean out the inside with a compressor once in a while like with all other amps.
SE-A3 Meter Lamps
SE-A3 Normal / DC inputs
The Technics SE-A3 is equipped with two sets of line level inputs marked "DC" and "Normal". The "Normal" inputs are coupled by a capacitor like with an ordinary amplifier. In this case, the amplifier operates as a normal AC amplifier with a low end cutoff frequency of about 2Hz. As a capacitor is a reactive device that can introduce phase changes, this is considered to be undesirable, so the SE-A3 has been designed as a DC amplifier where there are no capacitors in the signal path at all! So using the
SE-A3 Pictures & Reference
Technics SE-A3 Literature