Monday, 15 May 2017

Electro-Harmonix Pog2

Much like the Nano Pog repair, this one is also written up after the fact and I have probably forgotten a lot of important details. I sold this one on, so this is mostly based on blurry pictures I took.

I got this with one of the slide pots broken off, and dead on powering up. The picture below is out of order and was taken after it was repaired.

All good.

Inside, the problem was immediately obvious. FB3 near the DC jack is missing but the pads have visible solder on them. It's very common for high-speed digital pedals to have ferrite beads installed on the 9V input and ground lines, to keep high-frequency noise out of the power wiring and comply with EMI/EMC regulations.

FB3 is gone.
My guess is that someone broke off one of the slider pots and tried to remove the board for a repair. It looks like an attempt was made to desolder the 9V jack and FB3 was lost in the process. I added an 0805 ferrite bead and the Pog powered up again, everything working.

 The architecture seems to be similar to the Nano Pog, with a larger Analog Devices ADSP-BF531 DSP, and additional controls for filters, volume swells and presets.

After desoldering the broken pot.

Replacement slider pots are available from Small Bear, replacements should be 10 kOhm. They also sell the rubber slider tips.

Update: I came across a similar repair at this blog that states the pots are 5k. My order confirmation email says 10k, I'm certain I matched the new part to the old one. It's likely that these are just used as voltage dividers and both values may be used in different batches or versions.

Compared to the Nano, I much prefer this version. The additional octaves are nice but attack/filter controls really take this from being a novelty to something pretty inspiring. Fun!

Monday, 1 May 2017

Rozz R8 Repair

Another analog delay with no delayed signal. I had never heard of Rozz (not Ross), apparently it was a brand applied by a Japanese OEM that released pedals under many different names. Effects database found quite a few variants, it seems that the Ampeg rebrand is sought after nowadays.

Freestompboxes has a schematic for the Guyatone variant. It looks like it was drawn using the official Boss DM-2 schematic as a starting point.

The narrow English font looks very Japanese.

Inside is a Mitsubishi MN3005 BBD and MN3101 clock generator, which is nice. The only other IC is an NE570 compander, all buffering and filtering is done with discrete amplifiers. Otherwise it's similar to the DM-2/AD80 style delays of the time, except that there is no "cancel" trim pot to dial out any clock noise, and the wet and dry signals are passively mixed with no output buffers. This might have been a lower-cost design.

PCB is the usual single-sided type used by Boss in the 80s.

Looking around the board I could see clock signals at the 3101 and BBD, but I was confused as they seemed to disappear when I flipped the board over to see the traces on the solder side. Pushing down on the MN3005 would give a momentary burst of delayed noise, so the delay circuit appeared to work, only with a broken connection somewhere. I moved all the wiring from lap-soldered pads to run through the PCB in case a bad joint was causing the problem. No dice.

Eventually I realised that the trimpot for setting clock rates was broken, it only made contact when the pedal was upside down! After desoldering it literally fell part, the wiper only contacts the conductive track when held in place by gravity.

Original timing trimpot.

With a 100k replacement pot the delayed signal was back. The available delay times were not quite right, it would happily go to ~1/2 second at one end of the pot but the other was not much shorter. Fast short delays were not possible. Clock bleed-through was also really bad, it would whistle at the clock frequency even in bypass mode.

100k temporary replacement.

After finding a 50k replacement things were a lot better. Clock rates were manageable from ~8 kHz up to 15 kz or so, allowing long and short delays. Clock noise dropped a lot but is still audible at long delay times - I think this will need a trimpot added to the BBD outputs to dial this out.

With 50k trimmer. Much better.

This sounds very nice, it does DM-2 style delays on the verge of oscillation very well. With some minor mods to remove any clock sounds I think these would be really nice analog delays.