I recently (spring 2003) bought a nearly working Akai GX-630DB reel to reel deck on eBay and am very happily tinkering with it. Back in about 1975 I bought the best quality on a budget that I could manage: an Akai 400DS MkII, a kit for the Wireless World (UK) magazine Dolby unit, a Rotel RA312 amp, Trio KP1022 deck, Shure M75 ED II cartridge and some Yamaha HP1 headphones. Over the next couple of years I happily recorded around 80 LPs on Ampex 20/20 (consumer packaged 407 I believe), TDK Audua and Maxell UD XL.
The 4000DS saw heavy use, suffered badly from a dusty atmosphere (the friction take-up spool drive got stiffer and stiffer until it slowed playback down, particularly at the end of a reel), and the playback head wore out. I remember replacing the playback head once. By the time the second one was coming to the end of its life, cassette decks were looking possible with gimmicks like auto bias and I made the switch. I was never as happy with cassettes - the sound wasn't quite as good, and they felt too Heath Robinson (Rube Goldberg). More recently I have bought most of the same recordings on CD, and listen to them as 128kbps or better MP3s on computer and Archos hard disk MP3 player.
In late 2002, I wondered vaguely whether I could transfer a couple of older tape recordings that I hadn't found on CD to PC, so I made a Google search for Akai reel to reels. I found and was amazed at the selection available on eBay. All the excitement of HiFi hunting from 1975 came back to me and soon I was salivating over Akai, Pioneer and Revox machines. Looking around, I found Steven Bender and Frank Oomens excellent reference pages, and decided that by going for an Akai GX unit I stood a good chance of avoiding head wear.
Other goals for reliability and maintainability were no belts, no irreplaceable ICs, no auto-reverse heads on solenoids, and I was looking for a Dolby model. After about 3 months I bought a GX-630DB with low and distorted playback volume for $125.00, a pretty good bargain. $50 for shipping, $45 for Rick Stout's (StereoManuals.com) excellent Akai GX-630DB Full Doc Package (User Manual, Sales Brochure and Service Manual), and $75 for an MRL reference level tape built the price up a bit I suppose. Then there was the Teac NAB adapters, a 10.5" take-up spool and a couple of 10.5" tapes that I didn't really need, but I sure wanted to see what the machine looks like 'fully loaded' as it were.
Sure enough, when the 630DB arrived, I found the mechanism in great condition, but playback sound distorted, weak and irregular. By injecting music from my MP3 player at various points in the playback amp (through a resistor) I reckoned the problem was with the second and third transistors. I desoldered them and was surprised how low their hfe (gain) was. I tried replacing them with PN2222s (far from ideal replacements, but I had them lying around), and the sound was markedly improved. For good measure I decided to replace all the transistors and electrolytic capacitors. From an old Towers International transistor selector, I reckoned the 2SC458LGs could be replaced by the more modern BC184Ls (9 cents each) and the 2SC945Ls by BC182Ls (also 9c). That's just over a dollar to replace all the transistors in the playback amps. The capacitors were much the same price. All replaced, it sounds great, except that the bass is too strong.
Reading through the MRI reference tape documentation, since the bulk of the tape I'm using is really Ampex 406/407, I chose the 250 nWb/m reference tape. Now, reading about tape reference levels, I guess that as a commercial machine, the 630DB should be set up for 185nWb/m, so I followed MRI's substitution chart and have set the PB adjustments to output 1.09 v rms from the 1khz 0DB test tone, with the meters adjusted to show 3dB. I'm pretty happy with the results, except that the response isn't as flat as I was hoping for, especially at the low end. RMS output voltage is 4.8dB higher at 63hz than at 1khz. Not surprisingly it sounds boomy and confused in the low end. (All tests with Dolby off).
I understand that equalization reduces the volume of higher frequencies to avoid (I think) saturating the tape, so the signal on tape can be expected to be bass heavy, and I assume it is the job of the PB amp to compensate. I see the CR network in the feedback loop from TR2's collector to TR1's emitter and I'm wondering if the resistors or capacitors could be failing - though I thought ceramic capacitors and resistors were highly reliable. I checked the head, and it's marked P4-202, which is the right head for this machine, and I pulled out the 68p capacitor in series with the head and one of the feedback capacitors, and both measured the correct capacitance. I've tested with the Dolby unit bypassed with the same results.
So that's where I am - having a lot of fun working on it, enjoying the problem solving, learning about flux, equalization, VU, dB, dBv, dBm, rediscovering the pleasure of soldering, and listening to some tapes I haven't heard for 25 years. - David C. Brown