NAD T512 Service

This player had a number of faults when I got it. It seemed to me that many had tried to diagnose the problem but were unable to. This is a very complex DVD player, and if there is a fault, then you had better call a NASA engineer, because the "average" engineer will have difficulties.

One of the problems with this player is the tray motor drive belt getting hard and then slipping. It is very strange, because drive belts normally become soft and stretchy, but this one was suffering from rigour mortis... They must be using a different rubber formula these days... A slipping belt causes two problems. The loading tray will not open when the "Open" button is pressed. This can be annoying, if you need to change the DVD. The second problem with a slipping belt is that the CD does not properly load inside the tray when the tray is in. Typically, the spindle motor is on a small platform, which rises to engage into the CD. When the belt slips, then the internal platform does not raise the motor high enough to engage with the CD. If this does not happen, then the CD will slip when it is spinning and the servo will give the "DISC ERROR" message.

If you have a CD player where the tray does not open, but you can hear the motor, whirring then replacing the drive belt will often solve the problem. In the following sections of this multi-page article, you can see how to replace the loading tray drive belt.

DISC ERROR

The problem on this player was that it was giving "DISC ERROR" for every disc inserted. I tried audio discs, DVDs, CD-R, and CD-RW, and nothing worked. There was clearly a problem. It was detecting the disc in the tray, so clearly the laser was working, otherwise it would have given the "NO DISC" error message. The disc was registered, and the system was trying to read it by spinning the spindle to the required rpm. The sled was already in the correct position, which is the innermost track near the centre. The problem was that it would spin for a while, and then stop and eject the disc by opening the tray. It just did not like any discs. Perhaps it wanted something else.

At first, this seemed like a classic worn out motor problem. A worn out motor will not be able to reach the correct rpm, and if the servo cannot maintain a constant speed, then it gives up. When the motor brushes wear out, the speed becomes erratic, and servos are very sensitive to this sort of issues.

The original motor was a CL928U; however, I could not find any information about it. Measuring the voltage at the motor terminals indicated that it was reaching a maximum of 6.3 V, so I figured that a replacement 5.9 V or 6.0 V DVD spindle motor should fix the problem.

I managed to find a 5.9 V DVD Motor (RF-300C) on eBay for two pounds and decided to get that. Unfortunately, that did not fix the problem either so I decided to leave it for a while until I could find a solution.

I once read an old Fiat manual and the diagnostic flow chart went like this; does car start? If not, then check air pressure in tyres... Does car start now? If no, then replace engine...

The service manual for this player has a lovely diagnostic flow chart for "Disc Error". The standard procedure is to check the CD to see if it is OK. Check the servo, and if that is OK, then replace the main board.

I ran through the diagnostic procedure for checking the servo systems, and everything was fine there as well. The diagnostic centres around IC201, which is the HDC25D811 DVD servo chip. The voltages at the various pins were fine, crystal was fine, and everything checked out OK.

My gut instinct was that the motor was trying to spin up to the required rpm, but could not sustain it for whatever reason and was crashing back to a halt. I decided to focus my investigations on the motor driver chip ICM1, which is the KA3032 5-channel motor driver chip by Fairchild Semiconductors. This chip has an internal voltage regulator circuit centred on Pins 39, 40, 41. An internal OP Amp block provides a regulated reference voltage on Pin 40. The regulated voltage enters Pin 39, which is marked PVCC2. This voltage powers the five internal operational amplifier drivers to drive the focus, tracking, spindle, and sled. However, on this design, Pin 39 receives 8 V directly from the main power supply circuit and does not use the internal voltage regulator block.

I checked for the 8 V power supply, on Pin 39, and was getting around 5.5 V. This was a little out of spec so I decided to check IC105 (KA78R08), which is a four legged voltage regulator IC. Pin 1 -- starting from the LHS -- is the input voltage, and I was getting around 14 V there. Pin 2 is the regulated output, and I was getting 5.5 V, Pin 3 is the earth, which was at zero, and Pin 4 is the High / Low control signal, which was 5 V.

The 5.5 V output from the regulator did not seem right, so I replaced IC105. The new chip then provided 8 V on the output Pin 2. Moreover, it also cured the "Disc Error" problem, as the DVD was able to spin up to the maximum required rpm.

This Article Continues...