AKAI GXC-760D: Introduction

An AKAI cassette deck! From 1976 I believe. This is one of the good AKAI cassette tape decks. It is equipped with three GX heads, closed-loop dual capstan, a massive AC servo motor for capstan drive. In addition, it has direct drive reels powered by DC motors.

I finally managed to raise money to buy one of these. It was barely working but in great cosmetic condition. Some of the incandescent bulbs were dead on arrival and the capstan belt was replaced but with mismatched size. The pinch rollers were defective as well. The left one has this weird wobble which also causes poor sound reproduction. At least one transistor has been replaced in the past with a genuine Japanese part. But the work looks like amateur. Thankfully nothing else was damaged. The PCB tracks are all OK. Cables as well. All seems original.

Even with these small inconveniences, the price was a premium. But it worths every cent. I like the sound. It resembles that reproduced by their old reel to reel tape decks. The more recent GX cassette decks have a more analytical sound. But this one sounds vintage.

The construction of this unit is way overengineered. And the looks... Have you seen one of these in reality?

Disclaimer

The following articles are not to be treated as do-it-yourself tutorials on how to fix, restore, rebuild, or improve the unit in cause. This was not my initial intention. But you can consider this whole content as a general guideline, should you decide to launch into such an adventure.

The entire documentation is just a reflection of my work and I cannot be held responsible if you damage your unit, or even harm yourself in the process.

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AKAI GXC-760D: Technical Data

This cassette deck has the following technical characteristics.

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AKAI GXC-760D: Parts List

I have assembled a parts list for this cassette deck.

Descriptions and Parts Listings

The Power Supply and Oscillator P.C. Board is coded CB-5002 and is located on the lower right part of the unit as you look at it from above. This board implements the power supply -- obvious -- and the record pre-magnetization oscillator used to erase the tape when in record mode.

The Switch P.C. Board is coded CB-2001 and is located in the lower middle position of the steel chassis as you look at it from above. It is mounted with the tracks side upwards.

The Noise Filter P.C. Board is coded CB-2027 and is located above the two flywheels. It is a small board by all means and it houses four filter coils and four capacitors. The reel motor wires are soldered to this board.

The Relay P.C. Board is coded CB-5001 and is located underneath the steel chassis. You can recognize it quickly due to the big relay and the single electrolytic capacitor.

The Servo P.C. Board is coded CB-2025 and is located in the upper left corner of the unit as you look at it from above. It is located near the big AC capstan motor.

The Peak Meter P.C. Board is coded CB-5031 and is located behind the two VU-Meter indicators.

Parts need to be ordered twice for this board.

The SYSCON P.C. Board is coded CB-5004 and it occupies the upper middle part of the steel chassis. You can't miss it as it is the first big printed circuit board that you see when you take the wooden lid off. In my unit there was an extra unmarked capacitor in the extreme lower-left side of this board as you look at the component side from above. You can find it in the list below marked as CX.

The Preamplifier P.C. Board is coded CA-5205 and it occupies the upper middle part of the underneath of the steel chassis. You can't miss it as it is the biggest printed circuit board that you see when you look at the steel chassis turned upside-down. It is also the most complicated board of them all.

Parts need to be ordered twice for this board.

From an electronics point of view, this deck has a complicated construction. It rises big problems for a full maintenance job. There is an insanely large number of electrolytic capacitors. Where it was possibly I have chosen a film or a tantalum capacitor. The density of electronic parts on these printed circuit boards is very high for a 1970s design. Oh, I almost forgot: there is no silkscreen. This means you need to double and triple check the position and polarity of each capacitor.

As always, let me throw a piece of advice. Do not hurry, take your time and do the job once. And do it well.

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AKAI GXC-760D: Replacing the Lamps

This cassette deck suffers from 40+ years old burnt lamps syndrome. Time to change all of them. There are five miniature bulbs rated 24 V / 40 mA in the command buttons. Another two axial lamps are in the cassette compartment. The remaining two axial lamps are above the VU meters, behind a prism. All axial lamps are rated 8 V / 200 mA. I sourced all these bulbs then proceeded to disassembling the unit.

First off, I have addressed the control buttons bulbs. The rewind and pause bulbs were still functioning. As a best practice, I replaced all light bulbs in this machine.

This is the controls block.

This block is disassembled by removing the two screws holding the black mask. Then you carefully remove each button by gently pulling it out. A spring is beneath every button. Remove these as well. Then again three screws are securing the lights block to the chassis. Remove the top two screws and the bottom screw. The entire plastic holder gets out. Now another two screws are holding the printed circuit board to the plastic holder. Remove these as well. And...

Clearly someone has been here before me. Check out the 3 mm LED in place of the record bulb. Dirty job. The guy even managed to break two printed circuit tracks. Which I fixed as well with miniature copper tape. I also cleaned the lights board with isopropanol. The result is as follows.

Here are the new miniature light bulbs.

I have then cleaned all the old AKAI corrosive glue with D509 thinner. Lengthy job. And smelly too. Well ventilated room is a must. Then I have inserted all the five new miniature bulbs and soldered them in place.

Next, follows the trimming of the bulb leads. This is how the bulb array looks on the other side. All nice and clean.

Detailed miniature bulb view.

All good on these control buttons bulbs. I have reassembled everything in reverse order.

Next up, is the replacement of the cassette compartment bulbs. You can see here that only the left original bulb is still lighting. The other one is burnt.

Removing this steel part is easy enough. Two screws are securing it in place. Here is the steel holder with the old lamps. New lamps are visible also.

I have then reassembled the steel part in reverse order and moved on to the VU meters illumination.

Just above them there is a small screw that is holding up the light dispersing prism. I have removed this and put the prism in a safe place. Each VU meter has its own lamp mounted on independent printed circuit boards. Each board is secured in place with two screws. I removed the screws and desoldered the lamps. Unfortunately the same corrosive glue attacked two of the screws. I gave them a good bath in a mild acid that eats rust. The screws came out clean.

Before reassembling the prism I fired the unit up. The lights came on as expected.

I have secured the prism back in its place then I have proceeded to the reassembly of the unit. This is the lamp aftermath. All junk.

Here are some final pictures.

I have chosen not to replace the lamps with modern LEDs because I really like the warmth of incandescent light. I hope the new bulbs will last at least as long as the old ones.

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AKAI GXC-760D: Replacing the Pinch Rollers

Because the pinch rollers on my unit were worn out, I have acquired new parts custom built for this model. Here are the old rollers versus the new ones.

You can clearly spot the worn out surface of the old pinch rollers in the pictures below.

I think that the old ones were not original. Why is that? Because most certainly the original rollers had a bronze inner bush. Exactly like the new custom built ones.

Pinch roller replacement is a fairly easy operation on this machine. First, I have removed the springy metal C-clips and then I have pulled out the old rollers. I used isopropanol to clean the shafts of old grease and residue. I have applied a slight amount of quality fine grease to the metal shafts and then installed the new rollers. I have used some nylon washers between on each side of the rollers. Next up, I have secured the rollers in place with the C-clips.

All good. Deck plays perfectly now. I can play again 100 min. BASF tapes without fears of tape skewing.

However there is a design drawback. When the deck is idling, like in 90% of its life, the left pinch roller is pressed by the tape guidance mobile armature. There are two springs behind this armature. One is regulating the tension on the pinch roller and the other smaller one is regulating the tension on the mobile armature.

So this is how the roller appears is pressed when idling. This is not good on the longterm. The rubber on the tape contact path will deform and distortions due to wow & flutter will increase.

I have found a silly simple solution to this problem. Thus, I have installed a piece of standard printing paper folded in six directly between the tape guidance arm's resting piece and the metal shaft where it rests. It is hard to explain but simple to figure out while you are looking at the tape transport with a pen flashlight.

Now the pinch roller is no more pressed by the tape guide when the deck is idling.

Here is the paper.

And the assembly in play position.

Up until I will come back with a better solution, I am happy with this.

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AKAI GXC-760D: Replacing the Belts

Next, follow the belts. In this unit there are only two rubber belts. One is for the closed-loop dual capstan drive system. And the other one is for the counter and auto-stop mechanism.

The capstan belt is wider. Here is how it looks like.

The counter belt has a normal cassette tape deck belt profile and it looks like this.

The replacement is a relatively simple operation. Once you get out the mecha-block steel backplane that holds the capstan drive AC servo motor. As for the counter belt, I have just persuaded the old belt to exit via space between the reel hub and the metal cassette assembly frame. The new one was mounted in reverse.

The new belts have no notable improvement over playback quality. The machine was operating fine with the old ones as well. But knowing that new rubber parts have been installed, should keep me on the safe side for years to come.

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AKAI GXC-760D: Adjusting Playback Speed

This cassette deck has a FG servo controlled AC capstan motor. Thus, it has a small printed circuit board that performs the task of the speed controller.

Adjusting the playback speed is done by connecting the output of the deck to the input of the frequency counter while playing a reference tape containing a 3,150 Hz sine-wave recorded at 0 dB.

I had to use an insulated screwdriver to rotate the trimmer. With a metallic screwdriver there were some unusual anomalies in motor speed control. I found out that the FG servo circuit is fairly stable. Less stable than my aging Hewlett Packard sine-wave signal generator. But this is as expected.

Playback speed is now accordingly set thus I can enjoy good music reproduction on this unit.

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AKAI GXC-760D: Servicing the Reel Motors

While using this tape deck I have noticed that the tape transport stops completely from time to time. As I have changed the only two belts in this machine there is nothing else suspect but the reel motors. In my case the take-up reel motor. So it is time to dismantle it and see whether I can fix it.

In this deck, the reel motors are of type GSM-300 and have three functions: motor, generator, and magnetic brake. These motors are now no longer available (NLA). So fixing them is imperative in order to use the GXC-760D machine. Otherwise another solution would be to find a similar deck for spare parts and recover the motors hoping they won't exhibit the same issues.

Let's go with the problematic reel motor first. I won't detail the removal procedure since it is straightforward if you study the mechanical block.

This is the motor outer shell. There are two screws that secure the bottom cap in place.

This is the motor inner shell. There are three screws that secure the bottom cap in place.

This is the stator with the beautiful copper wire. You can clearly spot the carbon and oxidation build-up on the collector. I am going to remove this altogether and clean everything.

The bottom cap contains the carbon brushes. In this motor the brushes are quite worn. But there is nothing I can do to straighten this up. What I can do however is to replace the dampening sponge material between each brush arm and the tensioning spring. Well it is not quite a spring but it works similarly. This material is worn already and it easily disintegrates when touching it. I have cut two pieces from a similar material that I had around and then glued them in place of the old dampening material.

This is the restored motor stator.

The motor is reassembled.

I have reapplied a drop of quality grease to the bronze bushings. The motor was installed back from where it came from.

Since I was at this point, I decided to work on the supply reel motor as well. The steps are identical so I will just put some pictures.

The motor servicing operation is lengthy and requires patience. It took me about one hour per motor from disassembly to reassembly. You need an additional hour to disassemble the mechanical block and about the same time to reassemble it as it was.

While working on these motors I also preventively changed two electrolytic capacitors on the motor power supply distribution printed circuit board that is bolted to the mechanical block. These are rated 0.47 uF / 50 V. I have replaced them with Kemet modern parts rated 470 nF / 50 V. Normally I would have changed these on a full capacitor replacement job. But given this PCB is hard to reach and I already had the mechanical block disassembled, the operation seemed straightforward.

This is the completed board.

Next, I was eager to test the serviced motors. I have reassembled the mechanical block and loaded a cassette. The two motors accelerated very quickly and snapped my tape in a split-second. I observed that both the supply and the take-up reels were spinning in reverse. I cannot explain why this was happening since I noted carefully the way the wires were originally soldered. Upon re-soldering I have carefully respected these notes. Total disaster. After inverting the connections to both motors it appears that everything works as it should. Play mode delivers constant torque. Fast forward and fast rewind is smooth again.

Now I will throw-in my 2 cents opinion. One should never ever run these motors on full speed without a tape loaded. The motors will accelerate very fast and the brushes will quickly deteriorate. The collector will become clogged in carbon dust and spark debris. In the end the motors will either work intermittently or loose torque. Or both at the same time. I would go even further to say that it wouldn't be advised to fast forward or fast rewind tapes on this old deck. Once the brushes are completely deteriorated it will be very hard to replace them. Assuming that this operation can be done.

That's about it. Off I go to listen some tapes now.

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AKAI GXC-760D: Wow & Flutter Curiosity

This evening I loaded a Sony Metal cassette tape in the GXC-760D machine. After the first couple of seconds I was like jaw drop: an insane amount of wow & flutter was omnipresent. I mean what the heck! New pinch rollers, serviced motors, new capstan belt, clean and lubricated mechanism. And junk mechanical performance.

No matter what, the Sony Metal cassette had this massive wow & flutter over the recorded music. Especially on the beginning of the tape. I loaded then a 100 minute BASF CrO₂ tape and there was absolutely no audible wow & flutter. I really cannot explain what is wrong.

The weird thing is that this Sony Metal cassette works flawlessly in my Sony TC-K850ES recorder. It is obviously a mechanical problem but I cannot identify it. Pinch roller pressure is good, otherwise it would chew up longer tapes such as 100 and 120 minute cassette tapes.

I guess this machine doesn't like metal tapes. Unfortunately I cannot postulate this theory since I only have two metal cassettes: both being cheap Sony Metal XR types.

At this very moment I am like 20 minutes deep into side A of the 100 minute BASF cassette and everything is perfect sound-wise. You can never be bored with vintage stuff. Either you repair one module then another one fails elsewhere either there are incompatibilities. Oh the joy!

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AKAI GXC-760D: Wow & Flutter Follow-up

In a madness access I have disassembled the mecha-block and cleaned the capstan sintered bearings and shafts. Then I applied 0W20 fully synthetic oil with the aid of a needle connected to a 5 ml syringe. The oil was applied through the oil hole under each bearing housing. Each capstan assembly ate about 1.5 ml of oil. I don't know where this oil is now -- obviously inside the bearing housings. It doesn't drip outside via the oil hole nor does it escape though the microscopic space between the capstan and the sintered bronze housing.

I put a tiny drop of the same oil inside each sintered bronze pinch roller bearing for better lubrication. The fine grease that I used last year when I mounted the new pinch rollers was already dried up and evaporated for some reason. The oil is here to stay. Hopefully the capillary action will draw and hold the oil molecules inside the bronze bearings for a long time.

The result is that now even the junk Sony Metal XR tape doesn't have noticeable wow & flutter while listening on the headphones.

What have I learned? Never trust the vintage HI-FI sellers. The guy from whom I bought this deck back in 2018 said the mecha-block was fully serviced and oiled up. I think he was dreaming as the capstan bearings were running dry. I was mislead by what he said and I thought the mecha-block was OK. Apparently it really needed maintenance.

12.25 AM Later Edit: Here I am deep into the night listening to a BASF Chrome Super II old tape with selections of The 3^rd and the Mortal. I almost forgot how atmospheric this band was. I don't know if it still exists at the moment. I played the 90 minutes tape twice in a row.

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AKAI GXC-760D: Restoration

Finally I managed to complete the replacement of worn out electronic parts in this deck. This is an insanely tedious work. While this unit is very serviceable from a technical point of view, it is also very not service-friendly from any point of view. Work goes on very slow. Slower than any other unit I have worked on. Ever.

As a matter of fact I wanted to sell this deck many times. But each time I played -- or even recorded -- a tape with it, I repeatedly gave up on the idea of selling it. Sound-wise this machine is the closest to how a good reel to reel tape deck sounds. It doesn't even sound like an AKAI RTR. Well maybe it sounds a bit like an old AKAI GX-635DB after all. In my opinion, this deck sounds more like a STUDER machine. Or, like I said, an early AKAI RTR model.

The inside is filled with a crappy spaghetti of wires with zero connectors. Every wire is either soldered or wire-wrapped. And almost all of them are way too short and don't allow for much space when working inside. I don't know how they managed to make a deck with such a low noise floor after all. There are a couple of PCBs scattered around the chassis. All of them are connected with big wire looms. It seems the engineers went a zero cost cutting way for a consumer-class cassette deck. I have seen my share of wire chaos in other decks and electrical gear, but nothing like this. It is even very hard to take good pictures of the PCBs and the serviced sections. Then again there is the sound it produces...

In the end I haven't took any pictures since I was kind of pissed off by the wire forest and the many capacitors that I had to replace. I think I spilled out the most swears ever while repairing an electronics device in my entire life.

But you have to believe me that I went up until the end with this restoration. And I am fully enjoying this cassette deck as it matches with almost any cassette formulation recorded on almost any other deck. I don't even use the tape formulation selector switches. On normal it plays back all tapes perfectly, be them Fe₂O₃, CrO₂, or Metal.

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Please note that all the work presented herein this site is non-commercial. This is my hobby and I am doing this in my spare time. Through this page I freely share my knowledge with you. But if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!

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