Monday, 5 November 2007

To hell and back

Last christmas, I broke my '09, but this very great day, I finally got it working again...

To make a short story long: While trying to figuring out what was wrong with the hihat and crash cymbal sample circuits, and after borrowing an oscilloscope, I managed to put PCB001 on top of my keys, thus short-circuiting something and blowing the fuses. I probably broke some components as well, because as I replaced the fuses (with two pieces of wire, smart eh?), some of the opamps got really hot. I tried to replace the hot opamps (I had no quick way of figuring out if they were broken or not, and I didn't have enough opamps to replace all of them. This didn't work very well, as other opamps quickly overheaded as well. In a stroke of genius, I decided to disconnect PCB002 as well as all the potmeters, to prevent damage to those as well. This, I've understood retrospectively, only made things worse, and ended with the 7915 voltage regulator exploding. This was the 22nd of December, and got me rather depressed.

So, for the rest of the year I left the machine untouched. The first week of 2007 I decided to try to fix it in a more scientific way. First, I built a astable multivibrator, to test the Opamps. I found 12 defect opamps on PCB001 and none on PCB002, leading me to believe that PCB002 was left untouched by the forkup.

Next, I tried to find a way to test the transistors, but after realising the risk of ruining the PCB while unsoldering three legs at the same time, I ended up just clipping away the transistors and replacing them with new ones instead. So, I replaced all 549 and 559 transistors on PCB001, as well as testing the rest of the transistors. Next, I measured all resistors and diodes, unsoldering one leg if the measurements were odd. I ended up replacing 5 diodes in the process. I also checked that none of the capacitors had failed short circuiting anything. Finally, I checked the voltage regulators, rectifier and replaced all logic ICs and opamps.

To prevent new disasters, I then mounted the pcbs, connectors and front panel to a piece of MDF.

Thursday last week I finally worked up enough courage and confidence to power up the drum machine again. And it worked! (Or, at first nothing happened, but I quickly realised that all level pots were at 0).

So began the task of fixing the hihat and crash cymbal. Using the oscilloscope, I figured out that the hihat seemed to be working, except it didn't let any sound through to the output. After measuring (and listening - I built a small head phone amp and connected it to various parts of the hihat circuit) my way through the circuit, I decided to replace three transistors. Spot on - it fixed the hihats :-)

Finally, the cymbal. This is where I expected some real problems. As mentioned previously, I managed to break the PCB while replacing an IC socket. I started out measuring the various connections between parts. I found several errors, only to discover that two of the gates in Trevor's schema are wrongly labeled (U61D should be U66A, and U66A is really U66D).

The whole circuit checked out OK, but upon closer inspection, the connection between the potmeter and the PCB seemed a little fishy. I checked the crash tune connector, and guess what - the connection crimp was so flat it didn't touch the PCB part! Once that was fixed, everything worked as it should.

So, there you go - after a few sleepless weeks ( ;-) ) everything works as planned - the sound circuits are finished except for the head phone amp - and I am a happy little vegemite again :-D

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