Akai M8 Schematic – just a simple tube amp
The Akai M8 Schematic
Note that we have also details on this marvellous amplifier and deck on our sister website’s pages: Analog Ian
So just what makes this tape recorder so iconic? To be honest, I don’t know. Sentimentality definitely. The fact that it has a tube amplifier in it – just possibly. The fact that it is a single stage class A EL84 is another reason. Audio gurus will rave about the EL34 though and let’s not go into the American tubes with low down grunt. The bottom line is that tube amplifiers are in demand only dampened by the exorbitant costs. The mystery deepens when one finds that there are engineers modifying these amplifiers for the preamplifier only – i.e. the microphone amplifier and making good money out of it. It’s no hidden secret that the output transformer is what costs the DIYer, the preamplifier here is just a voltage amplifying device, no snake oil and gecko juice. With a little bit of modification and a payback of U$1 000 one understands the demand. Throw in a VU meter and then we have a discussion piece. I do believe the ART Pro MPA2 is also made pretty well and without modifications looks better. Does it sound better though? Well the Akai M8 schematic beckons.
[For all our readers please note that American manufacturing company Roberts also manufactured these machines under licence which was known as the 770X. There were some tiny differences, possibly improvements on the original design].
The left channel schematic is the same as the right except for the bias oscillator, V4 6AR5 pentode Hartley oscillator which feeds the erase and crossfield heads. There is nothing out of the ordinary here. We have a microphone, line and pickup (turntable) input. We have a 6X4 tube for rectification which the audio geeks either remove and replace with fast switching UF series diodes or leave as is for vintage reasons. Tubes used in rectifiers suffer from droop, silicon diodes conduct when the anode is about 0.7V positive to the cathode. Maybe there is merit here but the diehards prefer to not make the sacrifice and keep the tube in place. Different strokes for different folks.
As a standard rule all electrolytic capacitors should be changed starting with the high voltage cans across the anode (plate) supplies. With the 2 M8s I have to tinker with the anode rails are all over the place. Bear in mind emission of these tubes may not be all that great and these rails will be highly dependent on the tube quality – seen at the low end of R22 and R15 or across the cathode bias resistors.
If you are keeping the transport mechanism check the bias oscillator which runs at +/- 60kHz. This is usually scoped at the erase head to get the waveform and using a DFM for frequency. With modern equipment the DIYer can fault trace far quicker than in the 60s but if in doubt get someone who knows his way around a scope and DFM. The quality of your recorded signal depends on this.
If the tape transport mechanism is going to the junk yard (pity) and you are only using the preamplifier bear in mind that there is a massive slider switch which must be pushed into record mode to reproduce signal from the preamplifier.
Do not test these (or any tube) amplifier without a load. Flashover between tube electrodes or output transformer windings may occur is there is an input source, likewise spurious oscillation. I used to do this when I was a youngster to hear the tubes and output transformer “sing”. Very clever.
Clean all switch contacts and sockets – anything of this vintage will have cracks and pops when changing mode or jack plugs.
The head and motor switching is straightforward, the bigger issue being mechanical alignment of the heads when changing tracks. The motor start capacitor can be dud and needs to be replaced if the motor torque is low. C306 22uFd. Clean all switches but do not touch head alignment – this can be very tricky to set up without a test tape.
Below we have 002a Erase Head, 002b Rec/PB Head and 002c the famous Crossfield Bias Head. Rotating 003 moves the PB and erase heads up or down. Readers have tried to adjust what they thought were misaligned heads with disastrous consequences. Always check tape path first.
If these recorders came out in 1963 one must marvel at the construction quality. The aluminium faceplates are top class, the print is still 100% and the control knobs solid and well, the whole darn thing is just so well put together.
Our next page on the Akai M8 schematic will cover all the images we could muster, what to do, what not to do and if all things go well, the voltages at those important points.Please be careful when testing these decks and amplifiers because of the high voltages and charged capacitors.
Note that the Akai M8 Schematic is available for download at numerous forums & associated websites. There is a whole service manual out there as well.