muting transistors / Rotel RCD-975

[androll]

I would like to remove muting transistors in my Rotel 975....

I need more information about it...

Androll

[Fin]

Have a look at this thread.

http://www.diyaudio.com/forums/showt...442#post368442

[focal7C]

The above thread link is for the RCD855 model with different schematics. Are you able to point them out in this attached picture?

There three 3/4 moon transistors in sockets T301 (next to two capacitor) plus T302 and T303 (next to each other) along outside edge of PCB (on right side of CD player). Are these the muting transistors?

If I understand correctly they are just un-soldered with flux? No further bridge wire soldering of the empty circuit board holes is required?

Thanks in advance, Roger

[Mooly]

Is that a relay near the sockets (RY301) ?
Looks like it uses a relay for muting rather than transistors

Edit... maybe there are different versions of this player. The two unused transistor locations T306 and 307 near the relay look as though they could be mute transistors (if fitted) and used instead of a relay. The transistors you circled appear to be in the wrong physical location to be anything to do with muting, more likely regulators in view of the diode and caps nearby.

As ever examination of the circuit is essential before attempting any mods... a little knowledge can be a dangerous thing

[focal7C]

Hi Moly,

thanks for your prompt reply.

The parts list says: RY301 LY00000459 RELAY RSA-12. See PCB connection schematics enclosed. Is that the muting component? Can it be unsoldered without other changes?

Also, the 3/4 moon transistors that appear to be closely connected to the relay are:
T302,T303 TR70000194 TRANSISTOR BC547B
T301 TR70000200 TRANSISTOR BC557B

While I'm at it, can you advice on another forum's post suggesting "test removing small value film or ceramic capacitor from output to ground. The purpose if that capacitor is to reduce risk of interference from other devices and mobile phones. This forms a lowpass for output signal". Which will affect bass. "Try removing one channel capacity and go lower capacitance if bass holds up fine".

Assume these would be the C356 and C357 parts: CAP Polystyrene 160V 1000PF?

Regards, Roger

[focal7C]

The following may also assist.

All the service manual downloads seem to be of the same poor photocopy/scan version. However think I managed to find and indicate C356, C357 on attached diagram (1) and T301, T302, T303 plus RY301 on attached diagram (2).

(A comment on the schematic state that T306 and T307 are not used in the RCD-975.)

[Mooly]

A bit hard to make out clearly but the relay appears to work by shorting the signal to ground to mute the audio rather than simply being in series with the audio.
I would have expected nothing less tbh. So what that means, is that the relay has to all intents and purposes zero effect on the audio... as audio doesn't pass through it at all in play mode.

The cap you have marked is strange. Fitting a cap to prevent stray RF is normally done at the input to an audio stage i.e. in the amp or preamp.

I wouldn't be 100% sure that it's purpose is to prevent interference as used here. The output of the DAC is coupled directly to the differential amp (the opamp) with no other hf filtering at all which is very very unusual. The filter is to reduce the amplitude of hf from the DAC. Is it a TDA1305... which is a bitstream DAC? All D/A convertors produce this hf. The early players without oversampling used a brickwall filter with a steep cut off response, oversampling and bitstream produced hf artifacts that were much higher in frequency and consequently much easier to filter out "gently" often with a simple filter... such as this.

I think you will find the cap is best left exactly as is, removing it will probably cause a large increase in ultrasonic signal along with the audio. If you have a scope it would be interesting to see what hf is present.

Edit... that cap doesn't alter the bass in any way. Increasing it's value would roll of the treble earlier and omiting it as mentioned will extend the treble (or rather hf response) allowing ultasonic hash through.

[focal7C]

"The TDA1305T features a unique combination of bitstream and continuous calibration techniques."

TDA1305T datasheet, Pinout ,application circuits TDA1305T Stereo 1fs Data Input Up-sampling Filter With Bitstream Continuous Dual DAC (BCC-DAC2)

[Mooly]

It's a good DAC... capable of excellent performance.
I'm sure that cap is an essential part of the filter arrangement, it's just done in a slightly different way to what you normally see. It also may explain why the opamp has the extra transistors added as normally they would not be required for an opamp based line out stage. The cap and resistor filter add a significant load at hf, hence the transistors.

Here's another implementation of the TDA1305 this time with a more conventional filter wrapped around an opamp stage.

So I would definitely leave the cap in place.

[focal7C]

Started a new thread with question about other specific mod recommendations.

Modification suggestions for Rotel RCD-975