Hello, I have a D6C which plays very fast and the motor never stops. I replaced the CX20084 chip and the play speed become normal, but the motor was still running when stopped. I checked the pin 15 was about 5.8V and pin 7 was about 0V when stopped and about 6V when running. I thought may be the Q601 was bad, so I replaced it with a new 2SB1013, but then the CX20084 chip was dead again. It now runs very fast and the motor never stops. Is there something wrong with the auto off circuit? Which made CX20084 went bad again? Please help me. Thank you.
The first thing that I would consider is where did you source your replacement chip from, as there are many counterfeit CX20084 available from China. Genuine CX20084 chips are very hard or even impossible to find as of 2021. Most of the remaining stocks were depleted last year, as there seems that powering this unit with reverse polarity/higher voltage is extremely common. It's unfortunate, because the IC is pretty complex (it probably has a couple of dozen transistors), so building it with discrete components will result in a much larger footprint, not to mention there would be chanllanges in soldering such a module to the original pads. So I'm afraid that if the chip you bought is not genuine, the chances of fixing your D6C are slim. Online reviews of the Chinese counterfeit chips show that 2-3 out of 5 do not work. Not to mention the longevity: even if one of these chips is working now, you can't have any guarantee that it will work in 10 years. That's why it's so important to only use this unit on re-chargable batteries, if you have one in working condition. Because I'm afraid most of them won't be repairable in the future, unless someone makes a reliable aftermarket IC (which is very unlikely) or if someone makes a discrete version (also pretty unlikely). The original Q601 is 2SB1063 (not 2SB1013), so that could also play a role if its hFE is too low and hence it will have higher base current at the same collector current, burning the output of the IC. Q601 should be checked before replacing: meter in diode mode, should measure ~0.6V from collector to base, also ~0.6V from emitter to base.
Thank you for your reply. The chip I had was from a working DD3. I have ordered another one from a seller in Japan. I hope this one will work. I will order some SB1063 as well. The things bothered me are that the signal from the auto off circuit is either 0V when off and about 6.3V when on instead of 1.2V and 4.4V. Pin 5 of CX20084 shows 1.77V instead of 1.65V. Is there something wrong with the auto off circuit? And could it damage the CX20084 chip? Thank you.
Pin 7 is connected to a pull-up resistor (R605 - 10K) to the positive rail. It is a shutdown pin to the internal voltage regulator of the IC (makred stabilizer). That shutdown pin is driven by Q702 of the auto-off circuit: however, I don't see how that circuit going bad would cause the internal regulator to fail. What power supply are you using ? 6.3V on pin 7 indicates you're using a power supply with voltage output higher than 6V. The higher voltage on 6V rail can explain why pin 5 in also reading a little higher (since it's a regulated voltage coming out of the internal regulator).
My first thought when I saw your message was "what sort of power supply are you using?". I agree with @Valentin that using rechargeable batteries is the safest way to power a D6C during normal use. When working on one, you need to be using a bench power supply with current monitoring and limiting. Most of the plug top power supplies sold today are switch mode supplies which will destroy a CX20084 due to the high common mode AC signal (nearly 300V Pk to Pk in the UK) on the output. A customer in the United States had problems with CX20084s dying and he had been using genuine Sony 6V linear power supplies. However, when he measured their outputs, they ranged from 6.5V to 7.6V. Even a fresh set of AA alkaline batteries can deliver 6.3V. The CX20084 is extremely vulnerable as it is connected directly to the incoming power supply, either batteries or external supply, with no protection at all. Worse still, it is connected all the time, even if the D6C is switched off. If you plug in an incorrect power supply and realise your mistake before pressing play, FF or REW, it is still too late. The CX20084 will have been damaged. Although I have never seen a datasheet for the CX20084 (and I am never likely to see one either), I think that 6V is at, or beyond, the upper limit for the allowable supply voltage for this chip. These chips are used on other walkmans, such as the WM-DC2, where they are powered from 3V, and are generally very reliable as long as they are not subjected to a reverse polarity power supply. There is the question of why Sony went to the expense and trouble of re-designing the main circuit board for the D6C in 2001, to use the older design CX069A instead of the CX20084, at a time when the D6C was nearing the end of its production life? A very high failure rate on production test or during the warranty period would have been a good reason to do so. @Phoenix Chan, regarding your D6C, if the motor is running fast and never stops, this is normally a sign of a dead CX20084. The fact that you are getting 6.3V on pin 7 suggests that your power supply voltage is high. The "high" signal to pin 7 is derived from the switched 6V power line, via a 10k pull up resistor. It is normally pulled down to about 4.4V, when running, by the circuit in the CX20084. This does not appear to be happening with your chip. This line is pulled low by Q702 switching on when the end of the tape is reached in FF or REW. Pin 5 is normally 1.65V, which is generated by an internal regulator in the CX20084. The fact that it is high suggests a problem with the chip.
I used 4 fresh AA alkaline batteries which delivered 6.3V. Never thought that extra 0.3v could damage it.
I think some CX20084s are more tolerent of higher voltages than others. I have heard of them being damaged by using alkaline batteries.
