Akai M8 Schematic – just a simple tube amp

Akai M8 Schematic - head and motor switching.

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.

 

Akai M8 Schematic - left channel
Akai M8 Left channel with bias oscillator

[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.

Akai M8 Schematic - head and motor switching.
Head and Motor switching

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.

M8 Head Block and switching
M8 Head Block and switching

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.

Akai M8 Restoration

M8 Left Channel

As promised we move on to the inner workings of this single motor transport deck and tube amplifiers.

Note that we have also details on this marvellous amplifier and deck on our sister website’s pages:  Analog Ian

Restoring an Akai M8

This tape recorder came onto market about 1965 sometime (HiFi Engine lists this as 1963). It was preceded by the M7 and then afterwards, the transistorised M9.  Both these models also proved to be very popular. Enthusiasts veer towards the tube amplified version, discarding the deck.  If the deck is still in good condition I would not advise this as much of the parts can still be purchased through eBay albeit at astronomical prices or if you are fortunate enough, repair machined locally.

I purchased two of these units over the last two months. The first unit was not in bad condition, the amplifiers still worked but the head switching mechanism (mechanical) was missing. The pinch roller shows capstan burn. Capstan/flywheel belt is loose. Counter belt also loose but has enough tension. A catch was missing from the lid as well as not having the metal rear cover.

The second unit was the conundrum. The photos on Gumtree did not do it justice. When received I found it to be in many ways better than the first deck although I was going to use this for spares to get deck number one running.  Both M8s cost about 200 U$ which I felt was pretty much realistic for what I was getting.  Paying more for these recorders may leave one frustrated if there is an issue with a tube, mechanical cam etc. Although everyone sells these as being in excellent working order guaranteed there’s going to be a hiccup – if not hidden in a sea of lies maybe through shipping damage. Tubes do not travel well. This deck also had a dodgy pinch roller (dried and cracked). The heads still looked very good though.

For those buying these recorders for the tube preamp and class A single Pentodes must note that these amplifiers run off 110V and not 220 or 240V. There is an auto transformer mounted in the case which supplies the correct voltage to these amplifiers – so you will need another 2:1 step down transformer for mono block usage. These amplifiers have their own 110V (primary) isolation transformers.

Both recorders had major pinch roller slippage (as received). Back tension also felt very high. The metal cams used in the FF/REW which everyone complains about was still intact and in great condition. Cleaning and de-greasing required. Start capacitors for motors seemed to still be good, both decks had very strong motors and FF/REW pull was like new. I don’t have test tapes – this is next on the agenda but right now we are aiming to restore only one M8 and do the other one later.

At this stage I must mention that although both these machines are over 50 years old the transport mechanisms were still in very good condition (bar the rubber).  There’s a few words of caution which must be heeded when working on any vintage circuits (actually all goods running from mains power):

  • The supply rails to tube amplifiers are lethal. We are not talking about starved anode supplies but the real M’Coy.
  • Discharge all capacitors after powering up and switching off.
  • Always remove the mains plug from the outlet.
  • When working live make sure you follow the one hand rule. (keep one hand behind your back if need be).

Electrolytic Capacitors

Electrolyte dries out of older cans and these caps need to be replaced. Enthusiasts call applying a low current source at the working voltage of the capacitor reforming. In many cases electrolytics should be replaced with new units. Sometimes we can get our hands on NOS (New Old Stock) cans and because we want to keep the equipment vintage we stick to these. It’s not always the best solution. Electrolytics can blow up entirely or blow smoke and flame through the vent, not a pleasant sight, neither smell. Electrolyte is conductive and will damage other circuitry – it’s just not worth it. (a certain Chinese brand of computer power supply was notorious for catching fire so be careful of the quality of these electrolytics – and this is not to say that all manufacturers in China make shoddy products, some are excellent).

If you can, power up through a variac or have a current limiting solution like a light bulb, possibly a 60W for high voltage and a regulated supply for low voltage reforming.

Mains Transformer

The mains transformer in the Akai M8 is not an isolation transformer but an auto variety. This is a multi-tapped transformer. A while back a reader asked how to connect this to his amplifiers because the wire colouring did not match that of the diagram. The rule of thumb here would be to have an accurate resistance meter do the work for you. The highest reading between the windings on resistance check should be the 220V leg. Midpoint would be 110V. A variac or lightbulb in series is the best way to check after you have done the resistance checks to prevent the transformer from burning out. If the voltage is too high the transformer will start sizzling – they can be resilient beasts but always be very quick on the switch. Never apply power to a transformer if you aren’t sure of the winding configuration with a load applied.

Before powering up

Always clean the insides. A spider or roach resting across the terminals of a high voltage source may catch fire putting an end to your project. Smell, look and feel.  Something that doesn’t smell right is not alright. The previous owner may have cooked the mains transformer, a capacitor may have leaked, a resistor may be burnt.

Make sure the voltage setting is correct. Especially if you live in a 220V country.

Next up, some circuitry to look at:

M8 Left Channel
Akai M8 Left channel with bias oscillator

Next Page:  The circuits

Tape Bias – Why and How

Otari MX80 Professional - sophisticated bias control

Before attempting to repair any tape deck or RTR one needs to understand one of the most critical aspects to magnetic recording – actually two:

Tape Bias and the Tape Path

Although a tape recorder is a relatively simple device there are a few criteria which has to be met to reproduce good quality recordings. The decks or reel mechanism needs to pull the tape across the recording or playback path at very high tolerance levels, with near zero speed fluctuation, the heads need to be totally demagnetised and lastly, there needs to be a bias signal applied to the recording head when recording. Oh yes, we also have pre-emphasis and de-emphasis – largely used in FM broadcasting but very important in the recording industry.

Otari MX80 Professional - sophisticated bias control
By Salvador Calyso – Own work, CC BY-SA 3.0

The electronics in an open reel tape recorder (we refer to RTR here) may look simple on paper but there’s an awful lot going on under the hood. By taking the output of an amplifier directly to the recording head and modulating the magnetic particles of a tape we notice two things on playback – it sounds bloody awful: no bass and lots of distortion. Unfortunately the transfer characteristics from head to tape is not linear. Engineers worked out a way to combat this problem – by applying DC bias to the recording head. By careful control of this direct current the working point was pushed to the center of the upper or lower parts of the transfer characteristic.  Due to the symmetry of the transfer area the recorded signal contained 2nd order distortion, poor signal to noise and strong tape noise generated by the magnetised head. Early and cheap mass produced cassette decks all used some or other form of fixed magnet erase heads seated close to the recording head. Cheap dictaphones used this format.

HF bias which is in the range of 50kHz to 150kHz is superimposed on the input signal to the recording head. The amplitude is usually in the magnitude of 5 times that of the input audio signal. This method of biasing was found to be vastly superior to DC biasing, resulting in good signal to noise ratios, superior frequency response and lower distortion. The negative aspect to this is that HF bias, if not set correctly to each tape type and capstan speed  would result in poor frequency response, low output and distortion.

Further Reading:

Hysteresis – http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html

Pre-emphasis – boosting high frequency during recording

De-emphasis – boosting low frequency during playback

One further note: The cross-field or X-field – Wiki

Return:  Restoration of an Akai M8