Last Modified: 20 Apr 2013





Compiled from forum topic



OME PEEPS MIGHT BE vaguely interested in this. This is an Akai GX-370D reel-to-reel recorder from about the mid 1970's near as I can deduce. I bought it from a workmate quite a few years ago when we both worked for Maplin Electronics, for fifty pounds.

It was not without its problems though, which subsequently revealed themselves over the passage of time.

It's built like a battleship and weighs an absolute ton. Hence it lives permanently on the floor! It features plug-in cards type circuit board assembly as I think was popular at that time. 0.2 inch pitch edge connectors and gold-plated 'fingers' on the boards.

It plays forwards and backwards and does auto-reverse (but only if tape has the special conductive metal strips spliced into the end leader). It has independently adjustable left and right channels or can do four-track mono. Also has an automatic record level control called 'Compute-O-matic'. This uses a servo-driven stereo potentiometer.

Apart from line-in, the only other inputs are for dynamic microphones, but the two sources can be mixed for dubbing. Line-out buffering is done by some sort of primitive, 4-legged op-amp, indeed these are the only IC's in the whole thing. The only other output is for headphones which, interestingly, uses an emitter follower driving a step-down transformer. Apparently this was a bit of a deviation as it seems most other models could also run speakers. When the tape runs out it has both auto-stop and auto-shut-off (which means turns itself off at the mains!).


Sorry pictures are a bit rough, I seem to have lost the originals:









With the black lower front panel removed (a very nice alloy casting by tthe way!), reveals the so-called 'amplifier block' ...




... which can be pulled out like a drawer.





General rear view of interior...



After of course getting it out of its enormous plywood case. The best procedure is put it face down on something soft yet sturdy (a mattress is good), then take the case off the chassis (rather than try lift the chassis out of the case). In any event this method provides the best access to the huge self-tap screws (8 off) which hold the case on and incorporate the rubber feet (four on the bottom for upright use and four on the back for horizontal use).



Power supply section.

The round black things (only left one visible) are the direct drive spool motors and which run straight off the 240V mains. Tape tensioning and take-up is achieved by putting a small amount of input to these motors through massive coffin style ceramic dropper resistors (top left / top right). You can dry your socks on the top of it after it's been running for an hour or so





Direct drive capstan spindle motor is a fixed armature type, which was a new-fangled gizmo at the time. It means the armature stands still while the whole outer cage whizzes around it. Hence it's its own flywheel.

Notice gear toothed periphery and what looks like a tape head beside it (that's cos it is!), this is part of the speed controller system, which interestingly uses a FM disciminator type circuit.





Up-stream of the tape heads, the additional passive flywheel, spun by the actual tape.

Below, the transport control board, comprising of a number of bistable latches and relays. The bistables are activated by the one-touch piano key type levers on the front panel. Includes a time delayed auto-stop function so you can go straight from say rewind to play without having to hit 'stop' first.

The relays are 24V, 4-pole changeover with gold-plated contacts.





Five tape heads ~ left, erase and record, 'normal' forwards direction; right, record and erase, reverse direction.

Centre is the play head of Akai's own particular 'glass and crystal ferrite head' construction. It's mounted on a parallelogram tilting cradle, so that it physically moves across to the other side of the tape for reverse play. This is actuated by a solenoid.

Oh yes it can playback what it's recording too (monitoring).

The two plain pillars either side also move to push the tape off the play head for any mode that isn't play or record, as the play head is connected to its pre-amp all the time.





The play head moving solenoid.

Plus, below it is the channel swapping relay, so that L and R channels still come out the right way around when reverse play is selected.





#1. Tape speed selector board includes frequency EQ switching for both the playback and recording amplifiers.




#2. A view of the top deck with cover removed.

Just above the left hand spool may just be seen the end of a plunger with a coil spring on it (bit of green locking varnish on the nuts) ~ the capstan pinch wheel pressure is applied by a socking great solenoid (this is the moving end of it); the pinch wheel pressure is 1.5 kilograms.

To save electrical power, it has an activating winding which is then shorted out by a microswitch to leave just a holding winding.





Tinkering History

June - July 2004
New edge connector (an original Vero 22-way, 0.2" pitch double-sided no less, which I just happened to still have in the spares box!) for the System Control Board (transport controller). This board plugs into two 22-way edge connectors and one of them had split down the back, so that the central contacts were 'intermittent' (the centres of each side were bowed out by the contacts' pressure, lifting the centre contacts off). Previous attempts by previous owners to solve this mostly consisted of spraying it with different concoctions of switch cleaner and contact cleaner type stuff, WD40, and god knows what else such that the board ended up looking like it had been dipped in used engine oil! Failure to recognize the actual fault is, of course, entirely due to the simple fact that the split in the connector closed up and disappeared whenever the board was taken out! Think it was quite by chance that I just happened to notice a curious 'bowing' behaviour of the socket while the board was actually being plugged in...

New bulbs for Play / Reverse buttons (the five transport control buttons [RWD - REV - STOP - FWD - F.FFD] act upon industrial V3 style micro-switches).

October 2004
Calibrated record / replay levels and VU meter. Plus "Com. Detector" ('Compute-O-matic'). Not having Akai's special prerecorded test tape immediately to hand, to do this I made the max. play level = 70mV r.m.s. @ TR2 collector of the playback pre-amp.

This was based on a tested clipping level of 140mV @ TR2 collector when using Agfa-Gaevart tape @ 3.75 ips speed (a 'medium strength' tape). I usually use high-energy Maxell UD tape which (it eventually dawned upon me!) requires the S.R.T. ('Special Recording Tape') tape type switch selection.

Appendix ~ at the same time I equalised the play head azimuth; using a dual-trace oscilloscope, monitor the playback both channels simultaneously while recording 1 kHz sine, and set the azimuth screw for zero phase shift @ 1 kHz. It was slightly off.

Replaced the 'Stop' button microswitch (worn contacts causing spontaneous self-stopping).

October 2007
Undo previous owner's modification which was to effectively move Play output level pot. to between Line Out and output phono sockets, so that it does not also change the input to the VU meter. The wiring is now as original. <BR><BR>Remove two 8n2F cap.s, for Record Amp EQ @ 3.75 ips speed, from the Tape Speed Selector board. This is to reduce the slight remaining treble pre-emphasis when the Maxell UD tape is used even with the S.R.T. setting.

November 2007
Further to task Oct. 2004, replaced the 'Auto-Stop' microswitch, this, it turns out, was additional to the 'Stop' microswitch problem, worn contacts causing spontaneous self-stopping. Actually this was a bit complicated because this one, like some of the other microswitches, has an especially low-force action, so I had to do a bit of swapping around. The standard V3 microswitches won't work in place of most of these low-force types because the required operating force is much too high. For the auto-stop, the spring of the tape tension arm is too weak to depress even 1 pin of a standard microswitch. Consequently the new one ended up replacing the one being worked by the pinch wheel solenoid.


Get the Akai GX-370D Service Manual here!

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