MRC Sound Decoder

[John]

I removed this MRC sound decoder from a locomotive I picked up a while back .. can't find a lot of information, but something is out of whack ..  I haven't had a chance to test it yet, but is this blob of solder normal on the "top side" -- it appears to be a pass through to the "motor contacts on the bottom ... also seems to have a resistor - but my vom doesnt measure anything ..s o it may be burned out

[peteski]

Images you posted are rather tiny so I can't get a good look at that component. If it is between the motor contacts than it is likely a MLCC  (Multi-Layer Ceramic Capacitor) for reducing the Radio Frequency Interference generated by the motor's arcing brushes.

[John]

Hopefully better

[Tad_T]

I have a new in the package MRC 1636 decoder which I got in a Box o’ Stuff.

[John]

Thanks Tad .. now i can try to figure out if this is any good ...

[John]

Thanks Tad .. now i can try to figure out if this is any good ...

Well, I let the smoke out when I attached the motor .. it ran for about 3 seconds .. but I still have the speaker -

The frame is already milled out - so will just put a better decoder into the engine

[peteski]

Yes, as suspected that is capacitor I mentioned earlier.

From my experience with those, I consider them low-end decoders.  The ones I worked with use an old school analog audio amp chip (LM386) designed probably 40 years ago.  It is very inefficient compared to the Class-D (digital) amps used on most other decoders.  Not that this is a deal breaker, overall I find those decoders rather basic.  One I troubleshoot actually had a design flaw in one of the headlight circuits. IIRC, one of the headlight current limiting resistor had a trace bypassing it. I had to cut that trace to fix the problem. I don't remember the details. Would have to review my documentation. I guess, IMO the only thing these have going for them is the easy fit into  the models.

[John]

Thanks for the additional information.  Like I said, no big deal, it was more of a challenge than necessity.  I’ll put in something more current

[$2 Bill]

4 Ref...
Tiny MLCC can have large to huge value vs size.
If C4 above is across Motor's Brushes to "kill" EMI/RFI noise... It often replaced a Ceramic "coin" format typically 0.001 to 0.1 uF at 25-50v.

While Motor etc may use 12-18 DCV, the "noise" and back feed power can be much higher "AC" or DC Volts so anything like "Flyback Diodes" on Relay Coils needs 25v or higher in that locations. Some may remember GM AC Clutch Coil with a Diode at the Plug... Because when the 12V stops, The mag field collapse and generate > Negative 600V and without that 1N4007 1000v or relative Diode fries many other things.

Also, One of MLCC "Failure Modes" can happen even when Not in use for weeks to months just because so much stress internally when made and more stress from soldering to whatever using "Proper" soldering and above isn't such at C4. Worse many RoHS Lead Free Solder needs Higher Temps that puts even higher stress on parts. So this part was likely a "Ticking Bomb" waiting to fail. ⚠️ The new decoder from MRC and other sources may have same problems more so if installed poorly. The "Joke" on Snail Mail was often Do Not Fold etc... Just flexing boards can damage surface mounts parts and fail fast when used.

C9 & C10 may also be EMI/RFI control too. (Those maybe going to "ground" from each Brush.) All three could be a cheap Snubber to kill back feed from motor when power stop suddenly. "Good" Snubber would be C4 and 100-500 ohm in series.

[Tad_T]

Well, I let the smoke out when I attached the motor .. it ran for about 3 seconds .. but I still have the speaker -

The frame is already milled out - so will just put a better decoder into the engine

Sorry to hear that.

At least there is a silver lining.

[daniel_leavitt2000]

These things were the absolute worst. I tried a few when I first started getting into DCC to see what the sound would be like. I quickly found out that function remapping is not possible. No additional functions are provided for additional lighting. Motor control is probably the worst I've seen, with no ability to adjust the "kick" like NCE or TCS decoders. Weren't the speakers a really odd ohm rating, which made them incompatible with the aftermarket speakers we use on other decoders? I think I still have a few in my parts box.

I'm kinda bummed because American decoder companies have fallen so far behind the competition. TCS is still there and innovating, but NCE and Digitrax have been rolling on with their old line for decades. I rather like NCE, and they CAN be really dialed in to work well. But no sound systems. No advanced lighting effects. Nothing for E24 and their Next-18 is too wide for many installations.