The meters!

Technics SE-A3MK2 Introduction & Description

Technics SE-A3MK2 SN:AH5118A002No matter what kind of fast car you may drive, regardless of the many important specifications that describe the car, the one single thing that will impress everyone is the top speed...  Everybody understands what it means as they laugh and point at the speedometer!  With the high end stereo power amplifier, the equivalent specification is the RMS output power per channel.  No other amp than theSuper Audio Caps! elegant Technics SE-A3MK2 has the style to attract such attention in the first place but then to simply blow people away once they look confused and disbelievingly at the fantastic power VU meters that stretch all the SE-A3MK2 SN:AH5118A002 running in my main systemway to 1000 watts per channel!  For more about the wonderful meters, see my SE-A5 page.  In fact, the monster SE-A3MK2 4-stage darlington output stage has an instantaneous power handling capacity of 1600 watts per channel and can produce 300 watts RMS per channel, both channels driven into any load between 4 ohms to 8 ohms all day long unlike the SE-A3 that can only do this with a 4 ohm load (when it is working ).  This is achieved through the use of an impedance detection circuit to prevent meltdown with low impedance loads. Even with this aggressive power, the SE-A3MK2 has simply stunningly low levels of harmonic and intermodulation distortion that can shame any present day equivalent (if there was such an elegant equivalent with this power).  Although the famous and even more rare Technics SE-A1 can push 350 watts RMS per channel, the Technics SE-A3MK2 is the most elegant and sophisticated high end power amplifier ever Click here for the operating instructionsproduced by Technics and is still the mostSE-A3MK2 Distortion Chart sought after.  Using of course a twin monoblock design with separate power transformers and two awesome 105 Volt 22,000uF capacitors for each channel, the SE-A3MK2 is built like a tank and weighs in at 86lbs!  In addition, this is the only 1980's power amp I know of that uses an active microcontroller based system to control the output transistor bias according to temperature and signal load which Technics called Computer Drive.  It sounds and looks simply fantastic and every one who enters the room where it is housed is immediately drawn to it as if it were a classic 59 Cadillac or a new Ferrari whether first noticed by sight or sound!

Computer Drive Circuit

Detection Points
  60o C Thermal (L or R)
100o C Thermal (L or R)
130o C Thermal (L or R)
1.3V Signal (L and R)
9.0V Signal (L and R)

Computer Drive Block DiagramThe computer drive circuit is a system that controls the biasing (ICQ) of the outputMN1421STA Computer Drive! stage of the SE-A3MK2 by using sensors to monitor temperature and signal and then to adjust the bias current based on that data according to a predefined pattern and also to provide a quick warm up, startup muting and operation of protection.  This then is similar to the concept of digital electronic ICQ Control Circuitfuel injection in an internal combustion engine as apposed to a carburetor and analogue ignition rather like the Honda CX650 Turbo built shortly after this amp!  The actual computer is a Technics MN1421STA microcontroller.  While I have no data sheet for it, this looks like a custom made chip for the SE-A3MK2 and runs at a rather slow 400kHz compared to the 12MHz 8031/51 chips of the day. Both the temperature sensors and signal level detection are connected to the computer drive via comparators and inverters to give the simple digital inputs shown in the table above to the right with a resolution of just 1 bit each, in other words, just binary 1 or 0 representing True or False that each condition exists or not.  The temperature input values for the left and right channels are simply combined (ORed) together with diodes. So which ever channel crosses a temperature threshold first, both channels are biased the same due to the temperature change.  Then both channels will need to fall below the threshold before the change will be noted by the computer drive.  The signal values for left and right are connected independently.  The bias level outputs provided are simply Low, Med and High.  The Low output is combined for both channels and the Med and High are separate for each channels.  So all in all, this is a very very simple system with just 7 binary inputs and 5 binary outputs with the outputs being mutually exclusive.  This setup suggests that the Med and High biasing due to signal can be set on a per channel instantaneous basis.

Computer Drive Monitor

Pin 4 3 2 15 16 17 23
Name Low Med High Therm Signal Auto Preheat
0.4 sec " " " " " " "
0.4 sec " " " " " " "
0.4 sec " " " " " " "
0.4 sec " L H " H " "
0.4 sec " " " " " " "
0.4 sec " " " " " " "
0.4 sec " " " H L " "
0.4 sec " " " " " " "
0.4 sec " " " " " " "
0.4 sec L H " " H " "
0.4 sec " " " " " " "
0.4 sec " " " " " " "
 14 sec H " L L L "  
~ " " H " " L H
Computer Drive MonitorThe Preheat, Auto, Signal & Thermal lamps in the display are directly driven by the Computer Drive.  The High, Med and Low are connected to the left channel bias control outputs from the Computer Drive.  As described above, there are in fact 3 temperature sensors per channel and 2 signal level sensors per channel and of course the right channel can also be independently driven to Med and High bias, so clearly this display while cute, is not coming very close to displaying what is really going on.  At power up the display runs through a routine as shown in the table to the right ending with the Thermal, Signal and Auto lamps lit, and that is all I have ever seen it do!  The H & L in the table indicate the logic level on the Computer Drive pin and as indicated by the yellow blocks, each lamp is ON when the respective pin is Low as the sequence runs top down in the table.  As this startup routine runs the output stage is indeed exposed to the effects that the display suggests which is to force a high ICQ current through the output transistors to heat them up quickly to the optimum temperature.  Click here to see a video of the sequence.

Speaker Impedance Monitoring

Another unusual feature of the SE-A3MK2 is hinted at in the specifications.  It says that the maximum power output is 300 Watts per channel when driving either an 8 ohm or 4 ohm load.  How can that be?  The power should always be more with a 4 ohm load.  If the load impedance is reduced then more current will flow which will cause more power to be dissipated in the load right?  That's basic ohms law.  But the SE-A3MK2 is smart enough to catch the case when the load is changed from 8 to 4 ohms and it does this with an Auto-Load Impedance Detector as it is called in the marketing description.  When the power is first turned on or when the speaker selection is changed, a load detection circuit measures the impedance of the total connected load to see if it is more or less than 6.5 ohms.  If the load is less than 6.5 ohms there are transformer tap select relays that operate on both channels to actually reduce the AC output of the power transformer from 68.3V to 57V thus reducing the maximum power once again to the rated 300W per channel.  The reason for implementing such a design is that the amplifier can be run at maximum output current when driving an 8 ohm load but power is limited with a 4 ohm load to prevent the amp from blowing up under higher load conditions, yet still allow it to operate at maximum current with a higher load (lower impedance).  You can think of it like a rev limiter on an engine that prevents damage at high revs while in a low gear, yet still allows maximum revs in a higher gear.  So this means that maximum output potential can still be achieved with an 8 ohm load and running with 4 ohm speakers won't allow you to suck any more power out of the amp.  So in fact the SE-A3MK2 is really no more powerful than a regular SE-A3 as far as maximum output current goes with an SE-A3 actually specified at 320 watts per channel RMS with a 4 ohm load.  With all the other flashing lights that the SE-A3MK2 has, you would think that Technics would have added an indicator to show the position of the tap select relays, but unfortunately this is not the case.  So the next question then is why are Technics SB-M1 monitors, the speakers most often paired up with the SE-A3MK2 by Technics back in the day, designed with a 6 ohm load?  With 6 being very close to 6.5, if you are driving SB-M1's with your SE-A3MK2, the adjustment of the load impedance detection circuit had better be correct!

Mechanical Construction

Inside view from bottomInside top viewUnlike the Technics SE-A5, the both the SE-A3 and the SE-A3 MK2 weigh so much that an ordinary frame with a simple stiff back and front with a cover plate bolted to the bottom and an upside down U shaped cover for the top just won't suffice to manage all this weight!  The frame is more like the kind of thing that you would find on a truck!  With each power amp board having two massive heat sinks each, the two enormous power transformers and the many other boards that are packed in, everything is bolted to everything else to hold it all together.  Even to remove enough of the case to just take a peek at the power transistors is a major operation and many pages of the service manual as dedicated to describing how this should be done!  The top cover is actually made up of several solid and grilled plates that are supported by U shaped steel supports that bolt to the frame.  The sides are separate castings that bolt to the side of the frame.  All this prevents the unit from twisting under it's own weight if it is lifted unevenly.  One ultra important thing to note about the case is that the front, sides and Any colour you likeback are all solid allowing no air flow.  All the air flow is intended to go up through the case from bottom to top.  On no account shouldSolid sides any other equipment be located above the amp!  Not adhering to this very important note in the user guide is the cause for all those SE-A3's and SE-A3MK2's out there with blown output stages as it is excessive heat over a prolonged period that causes power transistor breakdown.  Sometimes people ask me exactly what color the case is on a Technics SE-A3 or SE-A3MK2.  As you can see from the picture to the left, it is any color you like!

SE-A3MK2 Protection circuits

Protection light on!The SE-A3MK2 has a vast array of protection systems all vying to light the infamous yellow protection indicator at the slightest hint of trouble.  Anything from over input, low speaker impedance, invalid DC balance to over heating will kick it into action.  Some of these conditions will result in the protection circuit resetting automatically while others require that the amp be powered down in order to reset the condition.  Just because you see an SE-A3MK2 with the protection active all the time, don't assume that the output transistors are blown until you have followed the Measurement and Adjustment procedures below and checked all the output transistors by removing them and checking each one in turn.  Many issues can be solved with some simple servicing.  One case I haveNo books or SU-A4MK2's allowed on top!!! found that really annoys an SE-A3MK2 is to drive a pair of QUAD ESL-63 speakers.  When the speakers get overdriven (happens easily with an SE-A3MK2) their protection circuit slaps a short circuit across the input and leaves a meager 1.5 ohm load for the amp.  While a Technics SE-A5 will happily drive such a load all day without a blink, the load impedance detection of the SE-A3MK2 has a different opinion and will shut down the whole amp until the power is reset.  So if you are looking for a Technics amp to drive QUAD ESL-63 speakers then stick with the SE-A5.  I have still yet to see an SE-A3MK2 that has all the output transistors blown.  If you have a defective unit and want to send it to a good home, please contact me as I am interested in buying it.  The flowchart below can be used to identify protection circuit problems.

      Click here for the SE-A3MK2 Schematic
Set power switch to OFF and change the input terminal on the back from "DC" to "NORMAL".  Next set the power switch to "ON" and check if the protection indicator lights up NO
As the equipment (pre-amp side) other than this unit is out of order, DC power is applied to the input causing the protection circuit to operate.  So it is not a sign of trouble in this unit    
Check if Computer Drive Monitor indicates "auto" is lighted NO
The temperature detection circuit may be operated.  Check if the radiating vents are blocked with a book or other audio equipment. NO
The over current and overload detection circuits are operated.  Check if the line is short circuited between speaker terminal and speaker side terminal
YES       NO
The DC detection circuit is operated.
Check if DC voltage is present at terminal 133 of power amplifier PCB E.
Check muting relay drive IC301 and its adjacent circuits.   Check transistors Q114, Q119 and Q120 of the temperature protection circuit
YES       NO
Check the positive (+B) and negative (-B) voltage of power amplifier PCB E, synchrobias PCB D, and previous stage differential amplifier PCB C. ABNORMAL
Regulator circuit is defective or fuse is blown, so check regulator circuit.   Check power transistors Q125 ~ Q138
Remove the synchrobias PCB D, then connect terminal 8 led to Q125 and Q126 base of power amplifier PCB E, terminal 7 led to Q135 and Q138 base to ground line, and then check if DC voltage is present at terminal 133 of power amplifier PCB E. YES
Poweramplifier PCB E is defective so check power transistors Q125 ~ Q138   Check transistor Q130 and diode D311 of over current detection circuit.
NO       NO
Connect synchrobias PCB D to poweramplifier PCB E and remove front stage differential amplifier PCB C.  Connect terminal 3 of lead wire 121 to ground and check if DC voltage is present at terminal 133 of poweramplifier PCB E. YES
Synchrobias PCB D is defective, so check Q115 ~ Q124.    
Previous stage differential amplifier PCB C is defective.  Check each transistor. NO
  Send the amp to me!

SE-A3MK2 Measurement and Adjustment

SE-A3MK2If you have reason to to change out any major components in the circuit at any time or if the protection circuit is unstable, you will need to adjust a few items to make sure the amp is running correctly and these are the procedures that need to be followed.  All you will need is a DC voltmeter that can measure down to 0.1mV DC and an oscilloscope if available.  Also, you need to be able to generate a 400Hz tone.  If you don't have a tone generator handy, you can click here for a 400 Hz wav file (right click, Save Target As...) that you can play from your PC or burn to a CD.  Refer to the location of adjustment and test points below.  Note that on some later model SE-A3MK2's TP2 is right next to TP1 and is not on the side of the power amp board.

Click for top view of SE-A3MK2 adjustment and test points  Click for bottom view of SE-A3MK2 adjustment and test points Volt meter

Voltage Regulator Adjustment (After repair of voltage regulator circuit)
Turn the voltage regulator adjuster VR201 to minimum position before turning power supply on, and connect nothing to the speaker terminals.
Connect a DC voltmeter between TP5 (+) and TP4 (Ground) and adjust VR201 so that the voltage is +90V.
Next connect a DC voltmeter between TP3 (+) and TP4 (Ground), then make sure the value is between -89 to -92V.

DC Balance Adjustment (After repair of power and drive amplifier circuit)
Turn the sound volume and VR103 to the minimum positions before turning power supply on, and connect nothing to the speaker terminals.
Connect a DC voltmeter between TP2 (+) and TP4 (Ground), and adjust VR103 to 0mV in the 30mV range.
If it is not adjustable, pull out J101 and make the adjustment again.

Clock Adjustment (After repair of computer drive microcontroller circuit)
If an oscilloscope is available, connect it to TP701 and adjust VR701 so that the period of the waveform is 3.3u seconds (400kHz).
If no oscilloscope is available, set TP701 to the center position and check that the computer drive sequence finishes in about 14 seconds.

Idling Current and Clamp Voltage Adjustment (After repair of power and drive amplifier)
Turn the sound volume to the minimum position before turning power supply on, and connect nothing to the speaker terminals.
Turn ICQ adjusting VR104 (both L and R channel) and VR105 (both L and R channel) to the minimum positions.
Connect a DC volt meter between TP1 and TP2 (Note that on some later model SE-A3MK2's that TP2 is right next to TP1).
Connect a low frequency oscillator to the input terminal and apply 400Hz, 300mV signal.
With power turned on, adjust VR104 so that the voltage is approx 19mV about 10 minutes after the computer drive monitor "auto" lights.
Next, adjust VR105 so that the voltage between TP1 and TP2 is approx 20mV.
Make sure to wait the 10 minutes so that the adjustment is not made while the preheat is active and with the amp warm.  Note that another way of getting the 300mV input level correct if you are using the 400Hz tone noted above is to adjust the level until the power meter reads two marks past 1 Watt as shown here.  Make sure to check the meter adjustment first!  When you are done, let the amp run for about an hour with no signal.  Make sure that the heat from the large heat sinks of both channels is cooler than the heat from the rear and front of the amp.

Load impedance detection circuit adjustment (After repair of load impedance detection circuit)
Set the speaker select switch to "main" and set the sound volume to minimum.
Connect a 6.5 ohm (1 Watt) resistor to the "main" speaker terminals.
Connect TP601 and TP602 with lead wire.
Turn power supply on and then turn VR602 counterclockwise (the sound of the tap relay will be heard).
Connect a DC voltmeter between TP601 and TP605 and then adjust VRVR601 so that the voltage is -200mV (-80mV to -400mV).
Connect a DC voltmeter between TP601 and TP604 and then adjust VRVR602 so that the voltage is -200mV (-80mV to -400mV).
Disconnect the lead wire that connects TP601 and TP602, then make sure the tap relay is switched.

Peak power meter adjustment (After replacement of meter amplifier circuit or peak power meter)
Set the speaker select switch to "main" and set the sound volume to minimum.
Before power ON, completely turn VR403 and VR404 counterclockwise.
Connect a low frequency oscillator to the "DC Input" terminal and an AC voltmeter to the speaker terminal.
Apply a 1kHz signal from the low frequency oscillator and adjust the volume so that the AC voltmeter reads 48.99V.
Adjust VR403 (Left) and VR404 (Right) so that the meter reads 300W.

Overload detecting protection circuit check
Set the speaker select switch to "main" and set the sound volume to minimum.
Connect a 0.33 ohm (30W) resistor to the remote speaker terminal.
Connect a low frequency oscillator to the "DC Input" terminal and an AC voltmeter to the speaker terminal.
Apply a 1kHz signal from the low frequency oscillator and adjust the volume so that the AC voltmeter reads 5V.
With the speaker selector is switched to the "Remote" position, if the relay turns off, then the protection circuit is in normal operation.
Switch power OFF, wait and then switch ON to reset protection circuit.

SE-A3MK2 ProblemsNo books or SU-A4MK2's allowed on top!!!

Click here for the SE-A3MK2 SchematicJust about every SE-A3MK2 that I have come across has had the idling current incorrectly set.  It is important that this is properly set according to the instructions above otherwise you will either have no class A at all or the poor output stage will be stressed to death with the heat.  Apart from the usual issues of blown output stages caused by people placing other equipment on top of the amp and blocking the cooling vents, I have found some other SE-A3MK2 issues which I'll describe here.  Before I start, let me say that the best place to search for original Technics/Matsushita/Panasonic replacement parts is  Also you can find many equivalent devices to the many items of old stock no longer available.  Checkout the pages for the power transistors that everyone is always asking about as shown below.  Just when will people learn that Yahoo is the best search engine?

2SC3264-Y (Q125 - Q128)    2SA1295-Y (Q135 - Q138)

One of my SE-A3MK2's that came to me via eBay from a seller that said he was real great and had tested the amp meticulously, was not working properly when I received it!  All this after paying top dollar for what was supposed to be a perfectly working amplifier!  When I turned the amp on, there was a constant crackling from the right channel accompanied by reciprocal activity of the right power meter.  Only a total dunce could have missed this problem and it could obviously not have been caused by any shipping damage especially as the amp was in perfect cosmetic condition.  Meticulous my ass!  As SE-A3MK2's are so rare and this unit was in excellent cosmetic shape and had a working output stage I decided to keep it although it sat waiting for my attention for more than 3 years. 2SC2291-G Problems like this that effect only one channel are quite easy to find in a twin mono block amp as it is just a matter of moving through the boards swapping them from side to side until the crackling moves from one side to the other.  Then once the board had been identified, going through all the transistors one at a time until the problem moves back to the other side again.  This system showed that Q103 in the cascade amp of the right channel was defective and while kind of working,Q103 2SC2291-G had begun to break down.  This is a 2SC2291-G, a funky device with 5 legs containing two matched back to back NPN/PNP transistors with a common base terminal, so an exact replacement would be needed.  Luckily UED has a page for the device and it was an easy repair in the end although it took me all those years to get to fixing it!  The moral of this story is not to trust anyone on eBay just because they say they are great and have sold hundred's of items of stereo equipment unless it is me of course...  This device is also used in the Technics SE-A5.

Meter Lamps

Technics Pilot LampsReplacement LampsThe correct original part for the meter light pilot lamps is part number XAM43P, order number NR57515-01 and you may be able to order them here.  They are very easy to replace compared to an SE-A5 and there is no soldering required.  All you have to do is remove the two side plates and the top front cover strip.  Then there are 3 screws to remove from the top of the white lamp cover.  The lamps are then exposed to be changed as shown here.

Normal / DC inputs

Normal & DC line inputs

The Technics SE-A3MK2 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-A3MK2 has been designed as a DC amplifier where there are no capacitors in the signal path at all!  So using the "DC" input gives a completely flat frequency response right down to 0 Hz with no reactive phase effects of any capacitors!  One possible issue with doing this is in case of a fault in the device connected to the input.  If for some reason there is a DC component in the input signal, this will result in the speaker cones being pushed and held there which will cause the voice coils to burn because of the very low DC resistance compared to about 8 ohms of AC resistance (or inductive reactance), although the internal output protection circuit will operate to save the speakers from this.  So if it is desired to operate with such an input, the "Normal" inputs are provided to ensure compatibility and are simply coupled to the "DC" inputs via a capacitor which eliminates the DC component of the signal.  When ever possible, the amp should be operated using the "DC" input and only one set of the inputs should be connected at one time.  Some other Technics power amp designs have just one set of inputs with a switch that has the "Normal" and "DC" settings.

SE-A3MK2 Specifications

Rated minimum sine wave RMS power output 20Hz~20kHz both channels driven with 0.002% total harmonic distortion 8 ohms. 300W
Rated minimum sine wave RMS power output 20Hz~20kHz both channels driven with 0.003% total harmonic distortion 4 ohms.  Note that because of impedance monitoring the 4 and 8 ohm values are always the same. 300W
1kHz continuous power output both  channels driven 0.001 total harmonic distortion 8 ohms. 320W
1kHz continuous power output both  channels driven 0.003 total harmonic distortion 4 ohms. 320W
Dynamic Headroom 8 ohms. 1.0dB
Dynamic Headroom 4 ohms. 2.0dB
Total harmonic distortion at rated power 20Hz~20kHz. 0.002%
Total harmonic distortion at half power 1kHz. 0.0003%
Transient Intermodulation Distortion. unmeasurable
Frequency Response DC ~ 200kHz -3dB
Frequency Response 20Hz~20kHz. +0dB - 0.1dB
Input Sensitivity 55mV (1V, IHF '66)
S/N (IHF A) 100dB
S/N (IHF '66) 125dB
Input Impedance 47k ohms
Low Frequency Damping Factor 8 ohms 200
Low Frequency Damping Factor 4 ohms 100
Total Load Impedance 4~16 ohms
Meter Reading Range 8 ohms 0.0001W~1kW
Meter Frequency Response from 10Hz~20kHz at more than -40dB +/-1dB
Meter Frequency Response from 10Hz~10kHz at less than -40dB +/-1dB
Meter Attack Time 50usec
Meter Recovery Time 759 ms (0db to --20dB)
Power Consumption 1200W, 1400VA
Dimensions (WxHxD) 16 15/16" x 8 3/16" x 19 31/32"
Weight 39 kg (86 lbs)

SE-A3MK2 Pictures & Reference

Click to enlarge to full size 3280 x 1846! Click to enlarge to full size 3280 x 1846! Click to enlarge to full size 3280 x 1846! Click to enlarge to full size 3280 x 1846!

Technics SU-A4MK2

Technics SU-A4MK2

Of course to make a complete amplifier out of the SE-A3MK2 you need a preamp and the Technics SU-A4MK2 is the one that goes with it.  Click here to see the user guide.  Of less use these days in big multi channel systems than the SE-A3MK2, the SU-A4MK2 is still a very nice preamp.

Technics SE-A3MK2's for Sale

I sometimes have SE-A3MK2's for sale that I have refurbished or acquired by other means.  If there are any, the following pages are what I prepare for eBay auctions and these items may or may not be currently listed for sale on eBay but you can find out by clicking here to see my current eBay auctions.  If you would like to buy one that is listed here please let me know.  I accept payments by credit card (PayPal), cashiers check or money order.  If you have a Technics SE-A3MK2 that you would like to sell in the USA, please let me know and I'll come and pick it up!

SOLD Technics SE-A3MK2 Serial Number AH5118A002 SOLD
SOLD Technics SE-A3MK2 Serial Number AH3L06A001 SOLD