Electro-Harmonix Micro Pog

I think this is the last entry in the EHX POG series that I haven't repaired. As usual, this came from eBay and doesn't work. No signs of life.

On first inspection, it looks like the diode in the switching power supply is trying to escape the PCB. I don't think that overheating could cause this without scorching the board, someone probably attempted to desolder this. My multimeter confirmed that this diode was a short circuit, so that's probably the fault. The diode is an SS14, which is also the same part used for input polarity protection. The switching IC is a CS51413 buck regulator.

I removed the diode and still measured a short across it's pads, so the switching chip is probably bad. That came off as well.

 

I ordered a replacement CS51413 but actually received a CS51414, which was annoying. However, the CS51313 and CS51414 are very similar, the '313 has an external sync pin where the '514 has an external bias pin. The Micro POG doesn't actually route pins 4 & 5 anywhere, so either chip can be used. I suspect that both are the actually the same die, with different pads broken out to external pins.

After replacing the controller IC and the switching diode, I still had a short from output voltage to ground. The output capacitor (C3) is also connected across these nodes, so I removed it and then the pedal worked. C3 is filtering the output of the 3.3V switching supply and doesn't seem to be always necessary. I don't know the value of C3, but the datasheet recommends 100uF. A 100uF ceramic capacitor in this package is actually quite expensive, so I used a 47uF.

The rest of the pedal is very similar to other Electro-Harmonix XO series units. There's a PIC18F2431 microcontroller, a DSP56364AF100 DSP and a PCM3052A audio codec.

When I had this working the "Octave Up" pot felt a bit weird. The shaft had actually broken away form the pot and had been re-inserted. I replaced it with a new 5Kohm part from Smallbear which is an almost perfect match.

 

That's it. Maybe someday I'll look at the HOG series as well.