Rockmate Donca Matic repair


A friend of mine found a Rockmate Donca matic at a fleemarket. Once he got home he plugged it into a 9V adapter and heard a popping sound. Now he is wondering if I could repair it for him.

All of the electronic components, with their corresponding values, are marked out on the circuit board. Having built some guitar pedals and random noise machines in the past I am pretty confident that I could replace whatever needs replacing.

However, I have some questions before I start. Specifically regarding which steps to take and in what order. I’m guessing that I need to clean the circuit board, desolder the capacitors and replace them and then what? I watched this video on washing dirty circuit boards. He mentions replacing the capacitors which makes sense. Do any other components age badly? I’m guessing the diods and resistors last forever but I might be wrong.

Thank you in advance for any pointers you can provide.

You are liable to get a lot of different opinions. None of them will be wrong.

Most parts can drift in values, but don’t go swapping all of them out: You will most likely be creating a lot more problems. It’s better to fix it so that it works again, and then find out how it sounds. You do not want to be trying to figure out what mistake you may have made. And ALWAYS take detailed notes and take pictures as an aid to putting it back the way you found it.

Some of the really old carbon resistors from the 1940’s can drift too far. I’ve not heard of any issues with diodes. A resistor has one simple property: Resistance. A diode has a voltage drop. If you suspect a problem with either, you can measure them with a meter to see if they’re still in range. What you have here should be fine despite age.

Electrolytic capacitors are the most notorious since they can dry out over time; you will typically see them bulging or leaking. Capacitors have multiple properties: capacitance and internal resistance. A bulging capacitor will act more like a resistor. They may test fine on a basic capacitance meter, but will leak when placed in a circuit.

I would not “wash” the circuit board unless it is absolutely needed for repair - tap water is loaded with minerals that will remain on the board. It works fine for Synthchaser, but I think he’s creating extra work. If it’s dusty, just use a can of compressed air. You can use a brush and some denatured alcohol to clean the solder side from any flux residue.

Ok, that sounds reasonable. I’ll start with a visual inspection, clean it lightly using compressed air and possibly some denatured alcohol if needed, then only replace the components that are leaking or otherwise broken.

I did do a quick google search on how to test if capacitors still work as intended. It seemed like you really have to remove them from the circuit board to do that. Is this true?

Also, when I’ve successfully replaced busted components and move on to testing if it works is there any “safer” way of doing that than just plugging in the 9V adapter? Like could I start with a lower voltage or something and work my way up to 9V. That doesn’t really make any sense to me but I might as well be transparent with how limited my knowledge is.

Thanks for a great first response by the way!

I’ve been looking into ESR meters that are supposed to be able to read in-circuit, but I have my doubts that you’d get a good measurement: Just too many other components that can affect the reading. You’re not measuring just the capacitor - you’re measuring the circuit.

I’m not sure if a lower voltage will help. If you had a variable supply, you could probably dial it in at a low voltage and measure the current draw as you raised it. I think I’d double check his power supply to see that it isn’t putting out too high a voltage.

You’re absolutely right to have doubts about getting accurate measurements with an ESR meter when testing capacitors in-circuit. The presence of other components can indeed affect the reading, making it difficult to isolate just the capacitor’s characteristics. A variable power supply could definitely help in this situation by allowing you to control the voltage and observe the current draw as you mentioned. It’s also a good idea to verify the output voltage of the power supply to ensure it’s within the appropriate range for the components you’re testing. This approach might provide more reliable insights into the health of the capacitors and the circuit as a whole.