Modifying tuner module in Sony CMT-CP500MD Micro HiFi

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Modifying tuner module in Sony CMT-CP500MD Micro HiFi

Post by KPC850 » 05 Dec 2016 10:28

Modifying the tuner module in Sony CMT-CP500MD Micro HiFi Micro System

I recently happened to obtain a Sony CMT-CP500MD Micro System (with (CD, MD, tape and RDS FM/MW Tuner) that was over a decade old (useful to me to play Minidiscs as I had no other MD player at the moment) but I did some investigations to improve the tuner and I found I could (a) improve its AM/MW audio quality by disabling its 'AM Hi-cut' by just cutting a surface mount capacitor and (B) disable muting during tuning by installing a resistor.

Dismantle radio so as to gain access to screened tuner module (directly behind antenna socket) and remove the top cover from the screened tuner module (just pops off ).
Of interest is the LA1837 IC which is 'a single-chip AM/FM IF and MPX' from Sanyo. (specs are available online)

The 2 separate modifications:

(1) To Improve AM(MW) fidelity (treble). Remove the capacitor which goes to Ground on track originating from the IC's Pin 24 (AM detector output) . (The capacitor in question I later measured to be about 44nF (probably is 47nF a standard value) .
Rather than remove the capacitor, I carefully filed the edge of the capacitor solder to put it out of circuit. That way I could temporarily bridge it with a tool to put it back in circuit to hear the difference in audio quality between having it in/out. (obviously be very careful not to accidentally cut any nearby tracks/components)

This 'treble cut/high-cut' of AM audio post demodulation is not unique to this radio. The manufacturer will probably argue it was done to reduce noise, but the disadvantage of the muffled AM audio (even on the strongest of signals) which the general public is unlikely to listen to, far outweighs any advantage.

(2) Remove muting during tuning: Its deeply annoying (especially for DXers) that the audio totally mutes out during manual tuning, with on this receiver a pause of about 1 second before audio returns! , making it tiresome to find stations that are not found during search tuning.
To remove the muting install a 4.7kOhm resistor between pin 13 and GND (pin 13 is the 'AM and FM IF buffer output to counter, AND output control switch (mute switch)' ).
PLEASE NOTE tuning muting is now totally removed - i.e. no muting during the auto/scan tuning mode as well as manual tuning mode). Whilst I can't imagine anyone desires muting of radios during manual tuning mode (manufacturers please take note), some users might object to muting not happening during auto/seek tuning mode)

The resistor value is not critical - (and its possible you may have to try a different resistor value to what I found works) but note:

Too high a resistance - tuning muting still happens or low distorted sound is heard in place of muting
Too low a resistance : tuning muting is gone BUT auto/seek tuning no longer works
Ensure the legs of the resistor do not touch anything else

This mod also has the added benefit that it removes the rather annoying and needless 1 second pause in audio that happens when you switch between FM stereo and mono (as you would if stereo was noisy especially during speech programmes).

Obviously only attempt the above if experienced in repair etc. The above info might be of use to modify similar receivers,

Picture of tuner module with both modifications available here:

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