Vintage Audio
this section covers some restoration work that I did on my vintage HI-FI gear
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Article #1 | 10:47 AM Thursday 08/02/2018

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 to 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?

Article #2 | 11:00 AM Thursday 08/02/2018

AKAI GXC-760D: Technical Data

This cassette deck has the following technical characteristics.

Track System4 track 2 channel stereo system
Tape Speed4.76 cm/sec
Wow and Flutter<0.06% WRMS
<0.17% (DIN 45500)
Frequency Response30 Hz - 15,000 Hz (±3dB) - Fe2O3 tape
30 Hz - 16,000 Hz (±3dB) - CrO2 tape
30 Hz - 19,000 Hz (±3dB) - FeCr tape
Distortion<1% (1,000 Hz / 0 VU) - Fe2O3 tape
Signal to Noise RatioBetter than 51 dB (tape peak of +5 dB)
Erase RatioBetter than 70 dB
Bias Frequency100 kHz
Heads1 x GX record head
1 x GX playback head
1 x ferrite erase head
Motors1 x AC servo outer-rotor motor for capstan drive
2 x DC motor for reel drive
SemiconductorsTransistors: 83 / FET: 6 / Diodes: 129
Power Voltage220 V at 50 Hz
Dimensions / Weight142 mm (H), 440 mm (W), 306 mm (D) / 11.1 kg

Article #3 | 20:00 PM Thursday 08/02/2018

AKAI GXC-760D: Parts List

I have assembled a parts list for this cassette deck.

The schematic value corresponds to what normally can be found in the electrical schematics. The recommended value is what I replaced the former part with. The BP inscription signifies a bipolar capacitor. Where I found appropriate, I have chosen a film capacitor replacement instead of an electrolytic. My reasons are reliability in time. Less electrolytic capacitors, less time-ticking bombs.

Descriptions and Parts Listings

Article #4 | 10:30 AM Saturday 10/02/2018

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. A a best practice, I replaced all 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.

Article #5 | 07:30 PM Friday 16/02/2018

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 brass 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.

Article #6 | 10:00 AM Saturday 17/02/2018

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.

Article #7 | 14:50 PM Saturday 24/02/2018

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.

Copyright © 1998- Alexandru Groza