Sony Stereo Amp Question

Thread Starter


Joined Mar 2, 2021
I have a Sony Stereo System, more specifically a Sony PMC DR50L, which is somewhat old. However, it's not old enough to be fully analog which means that the control is digital. For some reason that I can't really troubleshoot, the buttons no longer work consistently, sometimes doing nothing, others doing something completely different from their intended function and rarely doing what they are meant to do. As it is, I can sometimes use it by plugging my phone in the Line In and pressing the button for that channel until I luck out and then not touching the system anymore while I listen to music.

A while back I found its service manual online ( and set out to design a circuit which would replace the control/preamp/mixing unit with a really simple circuit which would just do Line In to the speakers input with some basic volume control and the ability to power the device ON and OFF.

I've mostly figured out how to do the path from Line In to the speakers by studying the schematics, figuring out the preamp steps that it is doing and changing some things to remove the unnecessary complexity given that I'm doing something a lot simpler.

Now I'm going through the power delivery part. The way this system works is that the main speaker, in this case the left one, has the transformer in it and then it feeds power to the main unit through a connector. This is on page 37 of the pdf, identified with a (G). The connector has 7 pins, 6 of which are power related and then a Standby signal which is used to turn on and off the power amplifier (

After this very long introduction, my question is the following: as far as I can track in the schematics, the standby signal is only related to plugging in the headphones(there is another path for it but then it leads into a box concerning only the DR70L, which is a model above mine). This would suggest that the rest of the time the amplifier is on, which sounds a bit weird to me because that would make for significant power waste?

The On/Off button in the main unit is represented in the schematic in page 34 of the pdf, 55 of the page numbering, and its marked as OPERATE. It goes on to the control chip and I can only deduce that being pressed changes the state of the signal POWER CON OUT on the same control chip which goes on to control a transistor switch in page 31 of the pdf, 47 of the numbering. What it seems to me that transistor is doing, with my limited knowledge, is "turning off" those voltage regulators which feed the preamp circuitry in the main unit, among others, but it doesn't cut the power supply to the power amp which is the element I would suppose is dissipating the most power. Is this a correct understanding of how that part of the circuit is working?

Another question I have then is, for designing this replacement board, does it make sense to tie the state of the standby signal to the operating state of the device? That is, if the board is off should I turn off the power amplifier as well or is there a reason as to why Sony didn't do that or am I getting it wrong? Another thing that confuses me is what the Mute Pin in the power amplifier does, since this is not described in the datasheet.

Any help would be fantastic.
Lots of thanks!


Joined Jun 16, 2018
Check the connections from the board that has the switches on it to the main CPU board. Also, check the 5V supply for AC ripple or low value. As stated by Delta Prime, also verify that the switches (tact switches) are good. A defective that appears to be shorted on occasion could cause this as well.