I have been wanting to add individual outputs to the TR-505 for quite some time and after I installed the HKA Rom Expansion I did so. I found this page and followed the instructions on it to build in the individual outs. After some testing I soon found out that the mod did not fully meet my expectations. The culprit of the remaining original mods is that for the most important sounds, the volume envelope is bypassed, thereby introducing digital grit into the sound decay. Continue reading
I´ve had a broken Alesis Midiverb II lying around for quite some time now. Just recently I remembered that I´ve always liked Patch #28, even before I dove into ambient and drone. So I thought now was finally the time to open it up and give it a go, especially since not to long ago I aquired a Midiverb 4 from an old friend and thus had a working 9V AC/AC adaptor again which the Midiverb II also requires.
You´d plug in the power adaptor and nothing would happen, neither visually nor sonically. The unit was basically dead, or so it seemed …
It´s always a good start to make sure that all the supply voltages are there and within range. The Midiverbs rectify internally so I started at the power adaptor plug and made my way through the PSU unit. Power on all paths was present but at very low ratings. I unplugged it again and did some continuity measurements. I found a short circuit between the output and ground of the 7805 regulator. I took it out and found that it measured okay out of circuit so it itself couldn´t be the problem. I still had to replace it because the input leg simply broke with no real force applied,
So there was a short circuit on the 5V supply line. I measured continuity across all capacitors and marked al those that seemed short which I then soldered out. Basically I removed all ripple filter caps of the digital ICs. Subsequently I no longer measured a short circuit across the 7805. It turned out that two of the filter caps had shortened. I replaced those with 470pF that I had left over from the Opra 6 repair and put all the other working ones back in.
That was all it needed. Plugged in the adaptor and there it was up and running again.
I noticed that the unit emitted a slight humn and a significant amount of noise when cranked up fully. I then remebered that the device has always been a little noisy. I realized that it might be a good idea to check whether the analog circuit could be improved upon. All opamps where LF347 and a little research revealed that the TL074 is compatibel and seemed to have better ratings. So I decided to replace all LF347 with TL074. I used sockets just in case that I where proven wrong and had to reverse it, which didn´t happen. I am quite stisfied with the improvement and really happy to have the unit back in working condition.
If all of a sudden you are getting an error message on your Computer stating that your USB device failed then this is most likely due to worn out capacitors of the power supply.
I successfully got my device working again by replacing the two 1000uF electrolytics. The original ones where causing ripples on the supply voltage which lead to instable USB signals.