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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 17th Nov 2017, 1:33 pm   #1
monaro0162
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Default AKAI GXC-46D Cassette recorder/player

Hello,

I have one of these machines, it is in very good condition and plays cassettes very well, but has one issue: A 50hz mains hum (lowish level) can be heard in the right channel when the motor is engaged, the left channel is fine. When the motor is disengaged the hum disappears. I'm assuming that the motor is loading the power supply which in turn is causing the hum, may be there is a cap going or the psu voltage is low? Could anybody advise where the power supply rail (or rails) can be probed with a scope so as I can see what is going on?
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Old 17th Nov 2017, 1:40 pm   #2
Nickthedentist
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Default Re: AKAI GXC-46D Cassette recorder/player

Is it definitely a low voltage DC motor? Some of these early hifi cassette decks used big AC mains motors.
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Old 17th Nov 2017, 2:10 pm   #3
Ted Kendall
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Default Re: AKAI GXC-46D Cassette recorder/player

I have one of these - I'm sure it is a mains motor. Just about every mains deck of this period had one.
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Old 17th Nov 2017, 4:37 pm   #4
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Default Re: AKAI GXC-46D Cassette recorder/player

Even if it is not a mains motor, many decks had a control board built into the top of the motor. So it might be the motor board that is the problem.
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Old 17th Nov 2017, 7:01 pm   #5
monaro0162
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Default Re: AKAI GXC-46D Cassette recorder/player

It looks like a mains motor. It has an 'outside' flywheel attached to it and it is fed by 3 wires that lead to what looks like a control board. Seems to be running at the correct speed with no wow and flutter. I've found the power supply pcb, after 4 screws are removed it lifts up enough to gain access. But I haven't got a service manual, and I'm still wondering why engaging the motor is causing this noticeable LF hum on just one channel??
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Old 17th Nov 2017, 7:51 pm   #6
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Default Re: AKAI GXC-46D Cassette recorder/player

Hifiengine have the schematics (circuit diagrams) for this machine, and they show a capacitor-run mains motor. No help with the fault, I guess.
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Old 17th Nov 2017, 7:53 pm   #7
Ted Kendall
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Default Re: AKAI GXC-46D Cassette recorder/player

The motor runs continuously. If you are getting pure 50Hz on play, perhaps this is induced into the head as it engages - the right channel is at the bottom of the head.
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Old 17th Nov 2017, 8:04 pm   #8
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Default Re: AKAI GXC-46D Cassette recorder/player

Again according to that circuit diagram (I don't have an actual machine) there is a muting switch which shorts the audio signals to ground except in record and play modes. So the hum might be there all the time, but only audible when the muting switch opens.
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Old 17th Nov 2017, 9:43 pm   #9
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Default Re: AKAI GXC-46D Cassette recorder/player

Seems to have been a ground or earth loop problem - connected it up to a different amp and now it's fine. So the cover has gone back on and I've crossed my fingers.

It's a nice machine. My Dad had one in the 1970's, I got this one on e-bay a while back for just 15 and I'm very pleased with it. It's quite heavy and looks well built inside. I think the motor must be an ac synchronous type similar to the motor in the 4000DS.

It has a nice smooth sound. Time to review the 500 or so Cassettes that I accumulated over the years - glad I never threw them out when CD arrived!

eta - when I said engaging the motor - I actually meant engaging play - the motor runs full time as stated.
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Old 17th Nov 2017, 11:04 pm   #10
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Default Re: AKAI GXC-46D Cassette recorder/player

Schematics for the GXC-46D

http://www.vintageshifi.com/repertoire-pdf/Akai-3.php
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Old 17th Nov 2017, 11:14 pm   #11
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Default Re: AKAI GXC-46D Cassette recorder/player

Quote:
Originally Posted by Ted Kendall View Post
The motor runs continuously. If you are getting pure 50Hz on play, perhaps this is induced into the head as it engages - the right channel is at the bottom of the head.
Actually on the cassette format the left channel is at the bottom of the head.
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Old 19th Nov 2017, 9:12 pm   #12
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Default Re: AKAI GXC-46D Cassette recorder/player

These are really good decks. I love the Akai cassette decks from this period - I've got a GXC40D and GXC46D. With both the main issues were with corroded contacts in the record/replay switch banks (servisol sorted that), stretched belts, hardened pinch rollers, and occasionally noisy transistors. Once sorted they can sound excellent if using decent tape and the heads are as hard as nails and seldom wear out.
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Old 20th Nov 2017, 2:01 am   #13
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Default Re: AKAI GXC-46D Cassette recorder/player

Quote:
Originally Posted by monaro0162 View Post
But I haven't got a service manual
I have a full service manual for the GXC-46D if you need it.
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Old 20th Nov 2017, 11:38 pm   #14
monaro0162
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Default Re: AKAI GXC-46D Cassette recorder/player

It does seem to have a problem with mains hum. I listened to it again through different amps, but I am hearing more or less the same mains hum in the background with each amp, and more hum on one channel. Its not that loud, but is loud enough to be noticeable in the background whilst playing music.

So I connected the output directly to a PC audio card running audacity and made a recording - definitely seeing more hum on one channel. If I swap the L&R line out leads from the GXC, into the audio card on the PC, the fault reverses on the audacity display, so the problem looks to be coming from the GXC. Attached is a screen shot of the Audacity analysis. The gain has been turned up to capture the hum, and more hum can be seen on one channel compared to the other.

I'm wondering if this is just a ground earth loop problem, rather than a failing power supply? I haven't actually scoped the power supply yet, I'll will try later on in the week.
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Old 21st Nov 2017, 3:19 am   #15
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Default Re: AKAI GXC-46D Cassette recorder/player

Hi, it's certainly worth trying the ground loop theory. When I had Akai machines in the past I always disconnected the earth wire on the Akai's mains plug to stop hums when they were connected to a system.

That's assuming it is a loop because the amp you're using is earthed already. An easy test would be to pull the phono plugs half way out so only the centre pin is making contact in the socket and see if the hum changes. Ofcourse if your amp does not have it's own earth then this won't apply.

I don't know why there would be a difference in the channels but anything's possible with the high gain preamps and one side could just be a bit closer to the mains TX or the motor.

David.
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Old 21st Nov 2017, 9:10 am   #16
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Default Re: AKAI GXC-46D Cassette recorder/player

...or try it just plugged into the mains and with a pair of sensitive headphones.
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Old 22nd Nov 2017, 3:11 am   #17
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Default Re: AKAI GXC-46D Cassette recorder/player

I agree with Nick's original point about the mains motor. Compared to the cheap and lightly constructed DC motors that replaced them, the AC motors were often sturdy and long lasting, but induced hum, especially when located close to the head.

I had a nice dual capstan early Akai model but I felt the hum, probably even from new, was unacceptable. That was even with a factory fitted hum shield around the motor.

As an elimination test you could try temporarily shielding the play head by holding a piece of suitable metal in front of it to see if that reduces the hum.
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Old 26th Nov 2017, 5:58 pm   #18
monaro0162
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Default Re: AKAI GXC-46D Cassette recorder/player

Spent a few hours this morning continuing the investigation .... turns out it is a very simple problem to eliminate. The problem is related to how far the unit is physically located from the amplifier that it is connected to. I've been running the deck through a Rotel RA-310 and an AKAI AA-8030 receiver, and they seem to emit mains radiation that the AKAI deck picks up. I had the deck placed on top of each amp. Move the deck about a foot away from the amps and the hum goes May be it won't pick up as much hum if a connect it to my (much newer) Arcam amplifier ... i'll try it and see.
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Old 26th Nov 2017, 7:48 pm   #19
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Default Re: AKAI GXC-46D Cassette recorder/player

Ah, that old chestnut! Well done for finding the cause.

You can get a similar situation with a magnetic cartridge in a turntable situated too near an amp.

Amps with toroidal mains transformers are better in this respect.

Nick.
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Old 27th Nov 2017, 12:29 am   #20
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Default Re: AKAI GXC-46D Cassette recorder/player

I had a portable radio-cassette player that would hum badly about half the time it was used, yet behaved impeccably the rest of the time; it turned out to be fussy about the supply polarity. Having the outer end of the transformer primary winding to neutral must have acted as a crude electrostatic screen .....
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