Hello Guys,
Used to have a thread over at mackie forum with discussing the possibility of modding the 1640i. I finally went ahead and did it. It was only when I decided to post my findings when I found that the mackie forum is no more.
So, here's what happened.
With my tech's instruction, I actually did the mod myself. Took about 2 weeks. Yes, most of the components are surface mounted. No, they are not impossible to work with. They actually are quite easy to deal with given some practice.
So here's what was done so far:
1. Opamp replacement. The board uses 4580 and 2068 generously. I am not quite sure if there are cheaper/worse opamps out there than these. So these were replaced. There're about 170 of these little guys.
1. Coupling capacitor replacement. With the power supply providing +-14.3v, all coupling caps were changed from originally polarized caps to bipolar caps as it should. There's about 400 of these.
A few things to note.
Be careful about desoldering the opamps. The proper way to do this is to mount excessive amount of solder on the legs pf the opamp, and then you heat them up with your iron by moving from side to side, about a sec on each side, repeat until the opamp chip comes off naturally. DO NOT FORCE IT! Force it off and you will break the traces on the circuit board, thus forcing you to embark on a crash course on rebuilding broken circuits. Trust me it's not as fun as it sounds.
Some of the opamps sit on the side of the board where all components are surface mounted with a dap of glue under them.Be gentle to desolder those. Use the same methods mentioned above. It takes a while, but they do melt under heat. Don't force it.
Be careful when working on desoldering opamps where it's crowded on the board. Your could have just easily flick off a surface mount resistor or a cap. They are not hard to remount, but they do break fairly easily. If they do(i.e one of the contacts fell off), just mark the location on the board, and replace them at the end.
Some of the opamps sits underneath the jacks on the back panel. You will need to remove some of the jacks to get to them. The xlrs are pretty easy to do, the trs on the other hand are very tough to remove. A desoldering station comes in handy.
Watch your costs. At 170 opamps needed, any decent opamps can cost several hundred dollars in components alone. The same goes for the caps. I for one do not wish to spend the same or more than what it would cost me to purchase a new 1640I.
Don't bother choosing replacement caps at the same capacitance as the original. The board is very crowded. Anything bigger than original won't fit. Go smaller. It's fine. I think I went from 100uf 25v polarized to 22uf 25v bipolar. The lower value netted me a loss of something like 0.2db at 20Hz according to simulation, but I was able to get the caps fit, just barely. I think the trade off was well worth it.
Lastly, the power supply does suck. At all channels muted, main fader at bottom, we still get huge spikes at 50Hz and all its harmonics. We will look into this at the next stage of mod.
