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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 8th Dec 2023, 6:10 pm   #21
ButchJames
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Default Re: AKAI GXC-46D Service Manual

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I forgot to mention - there is no Type II bias, which is traditionally about 50% higher. This machine is old, and so Chrome tapes back then seem to be biased at the same bias level as for Normal tapes?, except that pre-emphasis (via circuit '(2)') was a little different if we study the circuit. And from the same period, I have a Sony TC-134SD (and a sack full of TC-161SDs) where both decks only offer the same bias and PB equalisation ('de-emphasis') for both tape types.

It appears also, that de-emphasis is the same for both Normal and Chrome.
Correction: The deck appears to raise its bias for Chrome according to the cct diagram, but is not reflected when I switch to 'Chrome'. Another part to fix! Apologies to any GXC-46D owner!
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Old 16th Dec 2023, 7:03 pm   #22
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Default Re: AKAI GXC-46D Service Manual

Just an update for any GXC-46D users and, or prospective repair guys. Be interesting to compare notes.

Dolby Board: Complete NPN/PNP transistor replacement, all electrolytics re-capped, new signal diodes, and new zener diode for the dynamic-filter (Dolby) controlling JFET. Previous zener voltage according to SM is '8.6v', although if I read it correctly, I think the actual ones were marked '8.5'v? Sorry, I've forgotten, as I've thrown them away. Replaced with 8.2v versions - no problem there. The only components not replaced so far: resistors (obviously), 2 x 2SK30A (GR type) N-Channel JFETs, and the filter (polyester?) capacitors - I took some out, they are very accurate and low ESR, ~ 2-4 ohms. So I'll leave them alone.

The service manual is a little confusing when instructing to calibrate Dolby pre-emphasis and de-emphasis, but I think I've translated it correctly. Anyway, for now, I've actually calibrated by trail and error and ear! Here I obtain about 8dB pre/de-emphasis at 10Khz at about -40dB (ref 2.7dB 0VU/Dolby Level) with both channel equal - no 'pumping' effect between channels. I can see why Sony and others made their Dolby board tamper-proof. later, I may swap out the 2SK30A J-Fets. It's such an old machine, it deserves to be completely re-built.

I have noticed that old Dolby encoded pre-recorded tapes (1970s ... 1990s) sound much better when the de-emphasis is not ~10dB at > 5Khz etc.

Erase Head: I have decided to raise the erase head slightly since the right channel (inner most track) doesn't always completely erase. Problem cured, but I am aware that the issue could be with the record head height!? I've not 100% checked that, but it seems good, since recording/PB to and from my other decks inc TEAC V8030S/Sony TC-K707ES/Nakamichi DR10/Revox B215 are well modulated, ie record mid-band tones (300Hz-500Hz) at Dolby Level (+2.7dB over 0VU) and that level is approximately reflected on the other machines.

The erase height has been raised using some cassette head shims I have in stock, not sure of their thickness - I need to get a good calliper!

Any comments, corrections, or advice is always welcome.

(Edited x 3)
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Old 16th Dec 2023, 8:55 pm   #23
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Default Re: AKAI GXC-46D Service Manual

A practical way to check the Dolby unit tracking is to record a signal of say ~13kHz at 30dB below the ref level (i.e. set recording level controls at 0VU with the gen's signal then reduce the recording level by 10dB and finally the signal from the gen by 20dB). During playback the difference with Dolby in-out must be about 7 to 10dB. It is obvious that firstly the deck must be correctly calibrated.
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Old 16th Dec 2023, 10:03 pm   #24
jamesperrett
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Default Re: AKAI GXC-46D Service Manual

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I have noticed that old Dolby encoded pre-recorded tapes (1970s ... 1990s) sound much better when the de-emphasis is not ~10dB at > 5Khz etc.
High frequencies tend to self erase slightly over time with cassettes, so I find that sending the Dolby decoder a slightly higher level will help to reduce the effects of this self erasure.
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Old 16th Dec 2023, 10:50 pm   #25
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Default Re: AKAI GXC-46D Service Manual

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A practical way to check the Dolby unit tracking is to record a signal of say ~13kHz at 30dB below the ref level (i.e. set recording level controls at 0VU with the gen's signal then reduce the recording level by 10dB and finally the signal from the gen by 20dB). During playback the difference with Dolby in-out must be about 7 to 10dB. It is obvious that firstly the deck must be correctly calibrated.
Thanks mitajohn, basically I did that or very similar at 10Khz.

Final Dolby calibration will be done at the very end of the restoration, but for now I am pleased.
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Old 16th Dec 2023, 10:52 pm   #26
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Default Re: AKAI GXC-46D Service Manual

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Quote:
Originally Posted by ButchJames View Post
I have noticed that old Dolby encoded pre-recorded tapes (1970s ... 1990s) sound much better when the de-emphasis is not ~10dB at > 5Khz etc.
High frequencies tend to self erase slightly over time with cassettes, so I find that sending the Dolby decoder a slightly higher level will help to reduce the effects of this self erasure.
Funnily I noticed that HF loss over time when I was in my late teens, my hearing was obviously better then too.
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Old 18th Dec 2023, 6:01 pm   #27
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Default Re: AKAI GXC-46D Service Manual

Distortion from the GXC-46D at 1 Khz, at Dolby Level.
I will cal later.
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Old 18th Dec 2023, 6:26 pm   #28
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Default Re: AKAI GXC-46D Service Manual

I have no distortion meter, so this will serve as an approximation.

Total Harmonic Distortion calculated at 1Khz at REC-PB at Dolby Level from TDK D C46: I sought just the first two harmonics as the rest add very little to the calcs.

The ratio of ...
  • V2k:V1k ~ 10^(-45/20) or about 0.00564 and
  • V3k:V1k ~ 10^(-49/20) or about 0.00355.

So T.H.D = Sqrt( (V2k/V1k)^2 + (V3k/V1k)^2 ) ~ 0.0066 or 0.66%.

Allowing for small errors that's not a bad figure!

Of note: the input amp easily saturates when input > 7dB (0VU), and the harmonics from the Fourier Transform dominate. One can see this too on the oscilloscope when clipping occurs.
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Old 18th Dec 2023, 6:30 pm   #29
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Default Re: AKAI GXC-46D Service Manual

Ignore the 'meter levels' in the above picture.
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