AKAI GX-95: Quartz Locked Tape Speed Control
This unit comes from the factory with an adjustable FG servo tape speed control. While it is pretty much bulletproof, quartz locked tape speed control is still a desirable feature. Since the system control printed circuit board in this cassette deck has provisions for the quartz locked servo tape speed control circuit, the conversion from FG servo to quartz servo is straightforward.
First, I completely removed the system control PCB from the machine. The difficulty at this step was to desolder all command and control wire ribbons. But with some care and a good vacuum pump, I removed every connection point in less than ten minutes. The FG servo control circuit is located on the left side of the unpopulated quartz servo section.
Then, I removed all FG servo related components and populated the quartz servo circuit section with the expected parts.
I have one mention, though. The solder side of these AKAI PCBs is coated with some kind of flux lacquer that emits some stinky fumes while heated. First of all, I used a combination of isopropyl alcohol and high purity acetone to clean the lacquer from the circuit board zones that I worked on. This operation is not mandatory, but I like to work in a clean way.
Here is a close range picture of the quartz servo tape speed circuit. I have used high quality components such as precision metal film resistors, quality film capacitors, C0G MLCC parts, and Texas Instruments integrated circuits. The quartz locked control IC is made by Toshiba and I bought it from eBay. I got the BA15218N high slew rate and low noise operational amplifier from the same source.
Finally, I reinstalled the system control circuit board back in its place. Those familiar with this cassette deck will quickly observe that I installed a small heatsink on the LB1649 dual bidirectional motor control integrated circuit. This part gets pretty hot during normal operation and although I'm sure it works as expected, adding a small heatsink could not possibly harm at all.
I tested the whole system and it worked perfectly. Tape speed is a bit faster than expected due to the unavailability of a 4.07010 MHz quartz crystal. The closest match I could find is a 4.096 MHz quartz crystal which gives a +10 Hz tape speed deviation.
For the record, I have found a Canadian company that can make quartz crystals to user specifications. I will try to contact them and see whether I can obtain a 4.07010 MHz part.
A particularity of the quartz servo circuit board is that upon pressing the PLAY button twice, the tape speed is halved and the PLAY triangle symbol flashes on the vacuum fluorescent display. This is an interesting behavior. I haven't tested if I can make recordings at half tape speed. Probably not, but I will still verify.
That's it for now. The conversion was a success and everything works as expected. In fact, as I write these lines, I am listening to a 90 minutes SONY UX chrome class audio cassette with some '90s disco selections. I previously recorded this tape with the Revox B215 machine. There is no audible wow & flutter at all.
For scientific documentation purposes, I measured all electrolytic capacitors that I extracted from the FG servo circuit. I used the HP 4276A LCZ Meter to collect the data in the following table.
|Nippon Chemi-Con||Electrolytic||Radial||100 uF / 10 V||91.9 uF||1.70 Ω|
|Nippon Chemi-Con||Electrolytic||Radial||22 uF / 10 V||21.67 uF||6.14 Ω|
|Nippon Chemi-Con||Electrolytic||Radial||22 uF / 10 V||20.20 uF||7.26 Ω|
|Nippon Chemi-Con||Electrolytic||Radial||22 uF / 10 V||19.56 uF||7.46 Ω|
|Nippon Chemi-Con||Electrolytic||Radial||1 uF / 50 V||1.03 uF||40.02 Ω|
|Nippon Chemi-Con||Electrolytic||Radial||470 nF / 50 V||479 nF||140 Ω|
|Nippon Chemi-Con||Electrolytic||Radial||220 nF / 50 V||230 nF||361 Ω|
Immediately we observe that the capacitances are well within their specified range. However, the ESR is on the high side, especially for the small (nF range) electrolytic capacitors. As a matter of fact, I took the measurements at 120 Hz. At higher (kHz range) frequencies, the ESR drops considerably. But anyway, the high ESR is a drawback of small capacity electrolytic capacitors.