Modifying the Marantz CD67MKII

[ApD]

I have owned a Marantz CD67MKII CD player for some years and have been quite happy with it. Over time, however, its sound quality gradually faded away. This was especially noticeable when I hooked it up to an external DAC I recently built with the CS8420/CS43122 chips. This motivated me to look 'under the hood' and what I found was not pleasing...

The 'mod' involves the analog circuitry follwing the DAC outputs. My objecives were to 'clean up' the signal path and to increase dynamics, without deviating substantially from the original schematic, so as to use the existing PCB, preserving the original layout where possible. Typical steps like adding small caps to all power supply 'reservoir' electrolytics, etc. are assumed by default and will not be discussed.

As the mod involves replacement of most parts, special care should be exercised when desoldering/removing them so as not to damage the pads/traces on the PCB. The original components were machine-mounted on the PCB and have their legs bent under the pads. A sharp watchmaker's flat-blade screwdriver or similar tool for straightening them up before removal is recommended. A good (no kick-back) vacuum pump and a temperature-controlled soldering iron are essential. All original resistors are with 300 mil hole-spacing. I did not purchase components specially for the mod, but used 400-mil types from my 'stock', mounting them vertically. This is ok. It is, however, very important to match resistor values with a DMM. Failing that, a DC offset of a few millivolts would be present on the outputs, as all decoupling capacitors in the original circuit have been removed and substituted with shunts. Good resistor matching will result in zero offset: R1=R2; R3=R4; R5=R6; R7=R8. The capacitors I used are leaded 63V, NP0-type ceramics, which work fine for me. If you believe in special 'audio' caps, please use whatever types you think are best. Values should again be matched: C3=C4; C5=C6. C7: add 200pF in parallel to 1nF. A deviation of 5% from schematic values is ok.

The original part annotations have been substituted in the attached schematic. The CD cover is closed and I don't feel like opening it up again to revise my annotation, sorry! If you feel that this is a challenge, please don't start on your own and ask a friend to help you with the mod.

Some general notes:

The SM5872B DAC: outputs are quasi-differential, unipolar, pulse-width modulated signals for each channel. Strangely, the two sides of each output have a slight imbalance in their idle state pulse width, leading to a DC offset of around 20-30 mV when DC-coupling is used with the first OP-AMP. Therefore, two tantalum caps have been added in series to the input resistors R1, R2. Assemble them before insertion into the pcb. The value of these caps can be up to 47uF, however the circuit works fine with 33uF. Decreasing the value would lead to a roll-off for the lowest audio frequencies. Be sure to connect them in the right direction, with the positive pins facing the DAC outputs.

HDAM: no comment! Just remove the two resistors on each channel which provide power to the circuit and bypass the whole HDAM with a short length of cable, preferably shielded. The shield should be connected to ground at one end only, near the output RCA jack. In my opinion, the HDAM contributes to noise and distortion only.

OP-AMP selection: I installed two DIP8 sockets and auditioned a few types. The 5532 (from JRC) sounded best for me. Most dual op-amps are pin-compatible, so feel free to experiment for 'best results'. Some types exagerrated the midrange, others sounded 'brittle' in the trebles. The two pull-up resistors Rb1, Rb2 force the chip's outputs into single-ended class A operation, which I strongly recommend you use too. As there are no holes on the pcb for them, they have to be mounted on the bottom side of the pcb, directly on the chip's (socket's) pins. Current draw is approx. 4.5 mA from 12 volts.

Muting transistors: after the mod was done, I left mine intact (not shown in the schematic for clarity). If you believe they degrade the sound, remove them at your discretion. Some strange noises will be audible occasionaly, especially when powering down the unit. It is important to replace the series resistors (R9, R10) in the signal path following the op-amp outputs and also the parallel cap (C9) with the correct values.

Some HF noise is still present at the outputs after the mod, probably attributable to Marantz's way of routing grounds. It's level is not excessive, however, and it is way above the audio spectrum, so I did not try to eliminate it completely. Most amps will kill it at their inputs anyway. This is a 'mod' after all, not a complete make-over, which I would probably start with a completely different DAC..

Now my CD67MKII is again making me happy with it's sound. The whole exercise takes up about a saturday afternoon and costs next to nothing. Have fun with yours!

Cheers!

p.s. I suppose this 'mod' would be applicable to most of Marantz's models based on the same DAC/HDAM combination.

The schematic: