Tape Recorder Restoration

For the three and more legged things
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Mike H
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#1 Tape Recorder Restoration

Post by Mike H »

Some peeps might be vaguely interested in this.

I only put it here cos it's solid state electronics, if Nick wants to move it to somewhere else OK :D

This is an Akai GX-370D reel-to-reel recorder from about the mid 1970's near as I can deduce. Bought it off a mate quite a few years ago for fifty quid. Was not without its problems though which subsequently revealed themselves over the passage of time.

It's built like an air-raid shelter and weighs an absolute ton. Hence 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 can also run speakers. When the tape runs out it has both auto-stop and auto-shut-off (turns itself off at the mains).

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

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The back...
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The front...
 
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Mike H
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#2

Post by Mike H »

With lower front panel removed (a very nice alloy casting BTW), reveals the so-called 'amplifier block' ...

... which (second pic) can be pulled out like a drawer.

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#3

Post by Mike H »

General rear view of interior...

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Mike H
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#4

Post by Mike H »

Power supply section.

Round black things (only left one visible) are the direct drive spool motors and are run straight off the 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 :lol:

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#5

Post by Mike H »

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.


Dinner time now, more later!

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#6

Post by Mike H »

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.

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#7

Post by Mike H »

Five tape heads ~ left, erase & record, 'normal' forwards direction; right, record & 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 it physically moves to the other side of the tape for reverse. Actuated by a solenoid.

Oh yeah it can do playback while recording too.

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.

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#8

Post by Mike H »

The play head moving solenoid.

Plus below the channel swapping relay, so L & R channels still come out the right way round for reverse play.

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#9

Post by Mike H »

#1. Tape speed selector board includes frequency EQ switching for the playback and record amps.

#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), can't remember what the pull strength is offhand but it's like kilograms. IIRC...

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

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#10

Post by Mike H »

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 had split down the back, so the central contacts were 'intermittent'. Previous attempts to solve this mostly consisted of spraying it with different concoctions of switch / contact cleaner type stuff, and god knows what else such that the board ended up looking like it had been dipped in 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 'bowed' effect on it while it was actually plugged in...

New bulbs for Play / Rev. buttons

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 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 requiring the S.R.T. ('Special REcording Tape') selection.

Appendix ~ at the same time equalised play head azimuth; on dual-trace 'scope, set for zero phase shift @ 1 kHz. Was slightly off

Replaced 'Stop' button microswitch (worn contacts causing spontaneous 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.

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 stopping. Actually this was a bit complicated because many microswitches have especially low-force actions, I had to do a bit of swapping around. The standard V3 microswitches won't work for most of these because the required operating force is much too high. 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.

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DSJR
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#11

Post by DSJR »

I shudder to think how much an item like this would cost to make today with current labour rates. A heck of a lot of work in assembling and soldering up one of these I reckon..
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Mike H
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#12

Post by Mike H »

My thoughts exactly! Really is remarkable, lots of fiddly wiring and that. All the looms tied up with string and everything.

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#13

Post by Dave the bass »

Mike H wrote: All the looms tied up with string and everything.

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It's a line from a song... I know it... I know it.........


"....and snow flakes that fall on my nose and eye lashes..."

:-)

DTB.

PS. Bung it in the skip :lol: Cassette is the future!
"The fat bourgeois and his doppelganger"
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Mike H
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#14

Post by Mike H »

Image

"Tiny valves with faces all aglow"

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#15

Post by Mike H »

Dave the bass wrote: PS. Bung it in the skip :lol: Cassette is the future!
Funny you should say that just so happens I've got a fairly decent and unused JVC top loader mechanism waiting for a project build of some description, latest thought was to use valves. Not sure that's been done with a cassette recorder before :?:

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"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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