Hi, I bought one of these systems (see the "Infamous Marantz Mini Stereo Boombox CH-53/CP-53" thread in the chat area) at a flea market; it doesn't power on, and I found two problems. Someone might be able to help with this. A resistor is burnt out in the amp, the SC-53. This is in series with the power input, I don't know exactly what its purpose is, and since it's burnt I can't find out what value it should be. Has anyone opened a CP-53, or has one they would be willing to open and take a photo? I'll give more details if anyone does want to do this, I know these are nearly irreplaceable. The real problem seems to be the Marantz power brick should be 13.5V but is putting out almost 19V, and that might be what caused the resistor to overload and burn. If you have one of these systems, a simple check you might want to do is unplug one speaker (to free up a power wire end) and measure the voltage with the system on. If it's over maybe 14V or 14.5V, that might be stressing your system. Here's a photo of the corner of the CP-53 for those who are curious, with the burnt resistor and scorched capacitor. It will be really nice to get this going again. Andy
What pieces did you get? Can you post your power brick information label? I just pulled a random one apart, I have a few, it has the same issue! I know I have a working one around here.
The brick is model DA-30, with a bottom-side switch for 110V or 220V (I'm in Canada, so it's set to 110V). On the back, the number 830603 is etched or stamped into the plastic - probably the manufacture date. The other parts I have are the amp SC-53 and tuner ST-53, plus the AM/FM antenna, a pair of speakers and all the cables. I just opened the brick now and was thinking that since the voltage had failed I would find some kind of voltage regulator with a problem, but it's just a hefty transformer, two rectifying diodes, and a 4700 uF cap. Hard to understand why it's putting out 18V instead of 13.5V or even how that could be fixed. I'll likely just buy a modern 13.5V 1.5A supply to replace it.
This was the next phase of work on the amp - deriving the schematic for it, to figure out what R251 does and what value it should be. I think it's only to limit peak current if the amp is on at the moment the power cable is plugged in, so Q104 doesn't get damaged. Can't figure out any other reason for it.
Hello Andy, Well most of those Power bricks when not having the load on it will put out more voltage because a lot the older ones just have a transformer and some diodes and a Electrolytic filter capacitor with no regulations . the regulator were in the stereo unit them self .
Hello Andy , There a lot items that were made over the years that used unregulated wallworts or powerbricks and Q105 and Q106 are used as voltage regulators and those resistors are biasing the transtors to ajust for the right voltage regulation .
Eventually I'd like to find what controls those two resistors but the parts become very dense on the board, hard to see reference designations and part numbers. I do think it's just the zener that sets the voltage and these are somehow involved in turning everything on or off. If I acquire a dead SC-53 in the next year or two I can remove parts as I go through the board and maybe build up the entire schematic.
Has anyone ever figured out a way for the speakers of this system to not turn themselves off on low volume? Perhaps there's one component inside, in the "adequate volume detector circuit" that has to be disconnected, grounded, or pulled high so they always stay on. I find they turn off too easily at moderate-low volume. Next step is to open a speaker.
Sorry I've been busy doing a million things, there is a photo of an open speaker in the other thread. I can find a working set and see if I have the same issues.
I've opened mine and started to examine it, thanks for offering though. Fortunately these are all one-sided PWBs so I can follow the circuit; in a few days I'll post part of that schematic and what I think auto-switches off the speaker amp.
It's working now - these little MS-53 stereos sound extraordinarily good, for how tiny they are! And even though one of the specs for the amp unit inside the speakers is "less than 10% distortion" Here's the schematic for the circuit board in the speaker. There's a simple modification to disable the feature that auto-shuts off the speaker when the volume is low, and it sometimes turns off even though you really do want to listen quietly. I've added a page for the MS-53 on my website, www.andy-fraser.ca
