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Old 25th February 2008, 11:06 PM   #21
Alexx49 is offline Alexx49  Russian Federation
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Default 2 Mikewong

Hi Mikewong. I`m Novice on This site. I can`t send You mail directly. I`m owner of Akai Gx-95. Can You send me schema of This device?. Best Regards.
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Old 3rd March 2008, 07:39 PM   #22
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Ok - was going to post this query separetely..
Here goes..
Have 4x Akai GX-52 - all fine - an old GX-38 (made super FE recordings..& still OK).

My GX-8 (the one with dbx) has playback levels which are fairly low (minus ~6dB both channels equally) compared to a 0VU tone recorded (accurately) on another deck
Can't find any pots that would adjust the level - but logically there should be??

Next - a GX-R88..display kept failing - but giving it a thump restored it - until I needed to take off the front panel to restore - and 'sparks flew' - which left the deck without motive power to the heads/various leds - but the display partially lights.
I've the huge Service Manual - but no real idea where to start (& don't have test gear..).

The GX-R88 might be a goner (a shame) - but possibly I could 'correct' the GX-8's foible - which might be impacting on the Dolby B performance - as I tend to prefer the GX-52's 'extra clarity'??

TIA if anyone has an idea or two..

[Won't ask about the Beocord - 2x 8004 & 9000 that don't provide power to the cassette..possibly lots for the scrap-heap though...]
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Old 6th March 2008, 02:37 AM   #23
anatech is offline anatech  Canada
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Hi frankwm,
You need a half decent digital meter to troubleshoot with. It's also useful for many other things. Buy one. A cheapish Fluke is far better than an iffy meter with many functions. A good used Fluke is tremendous. If you get an 87 - you're doing great!

A scope may be needed for other troubleshooting. You can use audio as a signal source, buy I like a sine wave oscillator. Stick to music for now.

Do not adjust anything!! This is not your problem unless someone else was in there. You need test equipment and test tapes to set levels and EQ. Switches may be one problem. Clean them. A dirty tape path could be your other issue. Use Methyl Hydrate on a cotton swab. If everything looks clean, clean anyway. Do not try to demagnetize your heads unless you know what you are doing. It's possible to make things much worse.

Test one. Make sure your capstan bearing is okay before attempting to do anything.

-Chris
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Old 6th March 2008, 08:31 AM   #24
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Hello Chris.
Thanks for replying (thought the thread might be near-dead when I checked the previous dates..)
My tape transports/heads are always 'surgically-clean' - use Isopropyl for that - also the Maxell HE-44 to demagnetize (frequently - no ill-effects that I've ever noticed..)
I did recently get a (non-auto ranging) MM - but find it pretty 'useless' - has transistor test on top of 'the basics' - but appreciate that oscilloscopes/etc are required.

Just thought that I might have missed something in the GX-8 that adjusted the tape playback level - as my old JVC KD-A5 had a set of I/O pots (and I guess they're also on my same vintage KD-A8 too....whose 'computer' adjusts (amazingly accurately) for tape sensitivity as well as bias.).
I've made some test-tapes from CD frequency-tones on a JVC TD-V711 - which I feel comfortable with - and the HXPro in-circuit enables some extended HF to be accurately recorded.

The mismatch in record/replay levels is presumably something that can affect Dolby-B tracking - and between tape decks playing the same off-air tape - hence my concern regarding the GX-8 - and my wish to remedy...although a ~ -2dB drop on replay (on the same deck) seems quite typical - so possibly a manufacturers' fault, generally.
Don't think my 'ex-eBay' decks have ever been 'tinkered with' previously..most having been pretty-well 'under-used' in the first place!

Frank
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Old 6th March 2008, 11:54 PM   #25
anatech is offline anatech  Canada
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Hi Frank,
The very first thing most bad techs and experimenters do when encountering a problem is .... twiddle with the controls. Never bet that a machine has not been interfered with. In over 30 years of audio service, tape machines are one of the worst serviced.

Test tapes.
You can not create a test tape by yourself. It can not be done, and this is why.

A test tape is created on a special machine that is full track. Azimuth is checked with another test tape, or more probably with magnetic particles and measured to be perpendicular with the edge of the tape. A two head machine never records with correct azimuth if it's adjusted properly. Many three head machines don't either. No matter, you still have tape path and gap issues due to a multi track head.

The machines that create the test tapes are horribly expensive. The speed is more tightly controlled, as is the tape path. Levels and azimuth are NIST traceable. So, you can not create your own test tape that is correct. Not possible.

Cleaning.

Some tapes shed binder. It's not visible to the naked eye and only methyl hydrate removes it that I know of. We've tried many things and we do know that isopropyl is not effective at all. This is what may cause a Nakamichi deck to squeal if you've ever heard that. Same thing with open reel machines.

You will need AC voltmeters, a 'scope and signal generator plus a set of test tapes - real ones. This is a minimum.
Quote:
The mismatch in record/replay levels is presumably something that can affect Dolby-B tracking
Absolutely! However, you need to access the "dolby test points" rather than the output jacks. Some level shift is designed in to some machines and you can not test that way.
Quote:
so possibly a manufacturers' fault, generally.
Well, no. You need the service manual to understand how this works. If you don't have the service manual, and do not have enough properly trained experience, you are dead in the water.
Quote:
whose 'computer' adjusts (amazingly accurately) for tape sensitivity as well as bias.).
A good technician can get things much closer than the auto systems. Nakamichi had the best ones though. The 1000ZXL was easily the best machine. Then the lower models, but still better than any other brand on the market.
Quote:
and the HXPro in-circuit enables some extended HF to be accurately recorded.
Oh man! If you only knew the truth! HX messes with the bias level to reduce self erasure. The rec levels will shift. You shouldn't be that hot anyway as there are other problems occurring.

Anyway, I'm not trying to stomp on what you are doing, but you do have some misconceptions. There are some errors being made in the way you are doing things. I think you should get the service manuals for your equipment. If you are even considering working on your own stuff, you need the following at the very least.
A 20 MHz 'scope, dual trace.
An AC voltmeter. DVMs are often inaccurate by the time you reach 400 Hz. The dual pointer ones are nifty.
New, fresh test tapes as required. You will need at the minimum
Take up and back tension tape.
Fast wind tension tape.
Head height gage block.
Mirror tape (you can make a cut away one).
--- Now the mechanical stuff is covered, electronic stuff next -----
A Dolby level test tape (To set levels)
A Frequency Response test tape (to set EQ)
An Azimuth test tape (to set head azimuth).
Fresh blank tapes of the type you use (this should be easy)
There are times some of these may be combined into one tape. The decent ones are not.

There is no discussing this. You really do need these things. The test tapes will cost more than the 'scope. I know. I had at least five different sets of test tapes for cassette. Don't ask about R-R test tapes. A cheap, new test tape is vastly superior to what you are doing now.
The recorded signal strength may not be the same as your required tape. They are specified in nWb. You need to correct the levels in the manual to agree with the tape. So if it calls for 580 mV and your test tape is 2 dB hotter, you adjust the level to be 2 dB hotter.

So, there is the truth about tape machines and alignment.

-Chris
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Old 23rd June 2013, 01:21 AM   #26
berrins is offline berrins  Germany
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Hello Friends I have an old Akai dual well cassette deck. The playback and pb/rec heads are kinda worn out. It's not a GX model, so my Akai doesn't have Gx head. But I have a small collection of heads. Alps Gx and super metal heads, sendust, metal and many more.

Should I blindly go for the head with a matching impedance? The Alps glass head and super metal heads are not upto the impedance of original Akai heads. Sendust comes almost near the required impedance. The playback only head is 300+ ohms. Playback/record head is around 210 ohms. sendust head I have reaches around 290 ohms and one metal head is around 200 ohm.

Any help regarding this matter would be greatly appreciated. Thank you
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