Akai GX-95

[Mikewong]

This Akai GX-95 which is the first generation GX-95 from Akai and does any one of you know its bias frequency?

 

[anatech]

I don't know the frequency but 105 KHz was popular. The best way to figure it out is monitor the record traps on the audio side and tune the oscillator up and down (note the starting frequency first). Note the frequency of minimum output for each channel. The proper frequency should be close and would be an integer value.

I hope there has not been too much "gerfingerpoken" already.

-Chris

 

[Mikewong]

Thank you anatech for you information and as I know from the Service manual that the GX-95Mk2 is having a bias frequency of 105khz but the the tuning range of GX-95 doesn't go over 102khz!
That's the problem, it seems that the value should be lower than this and I don't have any information for this.

 

[anatech]

Do you have any way of testing the capacitors? I have found in servicing that the caps in this circuit can either open partially or short. The circuit does run somewhere around 60 ~ 90 VAC. That can be hard on the caps.
It's not unusual to see the bias oscillator dead either on the older machines. Same cause. A defective cap may take out a transistor as well.

-Chris

 

[Mikewong]

Today I checked the cap and even subtitute with a new cap having the same value that`s 150 pf and checked that the adjustable frequency range is around 82khz-95khz,and the previous value 102khz posted should be incorrect.
And 95mk 2 should be an improved version of this earliest 95 version and usually having a higher bias frerquency in this case 105khz.

 

[anatech]

But where do you see the minimum signal on the audio side of the traps? That will be very accurate, no matter what anyone says.

-Chris

 

[Mikewong]

apples

The lowest mean value as I measure the bias traps is -6.7V!
at 90khz.

 

[anatech]

This would be an AC value. I use an oscilloscope as the average meter will not indicate a null easily at that frequency. If you can't get an oscilloscope, you can make an RF detector probe for your meter. Essentially it's a signal diode in series with your probe and a small (0.001uF) cap from the probe to ground. Have a look on the web to make sure I have it right.

So you need to find the lowest point, or null. 95KHz was another common frequency used.

-Chris

 

[Mikewong]

I did solve the bias problem for Akai GX-95.
Now I have to bring out another problem found in the other GX.95 mk2 that's during calibration,the level functions but the bias has no response!?
and I have checked against another working GX.95 mk2 and found that the measured voltages are correct and I just couldn't get it right.
And I suspect the problem is caused by the calibration switch.
Any comment.

 

[anatech]

Hi Mike,
The bias control should vary the bias level to the record head. You can see this with an oscilloscope or an RF probe for your meter.

Your calibration switch should be working or you wouldn't see the record level change. Can you post a picture of the front panel controls? This might give us a clue how it's set up.

So how did you figure out your bias problem and what was the frequency?

-Chris

 

[Mikewong]

There isn't neither any bias level nor any change in bias level when Turning the bias control.
Even trying to vary the CAL bias VR652b has nothing effect to the bias level.
And the bias frequency measured across C786 is 105khz.

 

[anatech]

Hi Mike,
Interesting, what are the levels now? Typically level is done at 400 Hz and bias is done somewhere between 6 and 12 KHz. You should check the signal generator in the deck to make sure it's working.

Now if wish we had a schematic! Can you post the schematic or email it to me. A digital picture might work. This would have to be sent to my email (too big to post). My link works, bhome at sympatico dot ca.

The signal generator will only operate in Record - Calibration mode. Did you say you had a 'scope?

-Chris

 

[Mikewong]

I just send you full audio schematic of Akai GX-95mk2, size 12mb.
a return notification:The user(s) account is temporarily over quota.
You might try to subscribe to www.gmail.com where you are given en email space as large as 2.5g.

 

[anatech]

Hi Mike,
Yeah. The family certianly can chip away at the 10M limit I get. Doesn't matter since even that's under your size.

Thanks for trying, I will figure something out and e-mail you.

-Chris

 

[rotovator]

help needed

Mike Wong.

Hello from Spain

I'm new here. I've searched for help to repair my Akai GX-95, and I've found this web (Which I already was registered) and this thread.

As I'm new here, I may not send you an email.

I think I broke my deck when recording a high frecuency signal (about 18 Khz). It seems amazing, but after doing that test, It records a lot of distortion on chrome and metal tapes at low frecuencies.

I need to find out what is wrong, and I would like to have the schematics of that tape to follow the signal path with an oscilloscope.

I've measured the distortion using a silly pc-based oscilloscope and a pc-based signal generator.

The board has very few indicators about what stage

Could you, please, get in touch with me??

llagona (at) hotmail

Thanks

 

[bawang]

Chris and Mike, I own an Akai GX-52 and love it to death. Bought it during my student days in UNB, Fredericton, New Brunswick, Canada, at a store named Magic Forest.

I've amassed a huge collection of recorded cassettes, mostly encoded with Dolby B/C HX PRO. Recently I've decided to convert this collection to CD but discovered that my deck is malfunctioning! Specifically the Dolby circuit. Played without Dolby, it's normal, but with Dolby, all high frequency sound gone.

What can I do?

The deck is mechanically sound...... I am at my wit's end.....

 

[anatech]

Hi rotovator,
You did not break your deck recording an 18 KHz tone. Something may have failed, but you didn't break it. Check that the record oscillator is putting out a sine wave. Sometimes caps fail in the bias oscillator or rec / play switches get dirty.

Hi bawang,
It's unlikely that your deck is okay mechanically. Head wear and tape path problems can both do this. Clean your head with Methyl Hydrate (Canadian Tire, paint section) to start.

Guys, perma alloy heads (AKA Glass heads) wear as well. It's hard to see though. The head gets wavey and the gap actually chips so it's wider. These heads also overload easily. Metal tapes and Memorex will tear any head of any deck, believe it.

-Chris

 

[bawang]

Chris, the deck is rarely used. It used to sound fine (with and without Dolby NR) the last time it was used, about a year ago. I still switch it ON once in a while, but it was not used to play cassettes, just power ON. The problem just appear recently, when I took it out of retirement....... Help.......

 

[rotovator]

thanks anatech for your reply

I know It shouldn't broke when recording a 18Khz signal. But it worked ok before and bad after that test... it's strange, maybe the fault was there before and something has brought it into life. I realized it one or two days after the test, so maybe...

I'm waiting for the oscilloscope to arrive. I will check bias signal, and many other things, but I also suspect that the problem could be in the record amplifier. I suspect this because recording on normal (Ferro) tapes is ok, no distortion observed... Is this a reasonable thought?
Does changing from Type I tapes to Type II or Type III affect only to the bias signal and level?? Is the recording circuit unique for all three types of tape??

wait!...

I'm having a look a tandberg 3034 schematics and the recording amplifier appears to have partial different internal signal paths for different tape formulations. So I answer myself a little.


Thanks bye

 

[anatech]

Hi rotovator,

So I answer myself a little.

Yes. Bias level will change. You may have any number of faults. A distorted bias signal will look like a distorted audio signal. Audio rec level changes as well.

-Chris