Capacitor Replacement Project for the MCI JH-636


Here at the University of Hartford, we have a MCI JH-636. It is a recording console built back in 1982. This console features VCA automated faders and a built in patch bay. It has 30 mono channels and 4 stereo channels. Each channel is a module that can be removed separately to be worked on. The faders can be removed as well to work on the electronics inside. This console is great for the technician because the parts in the channels are discrete and not surface mounted. The pots are sealed, however, making them hard to clean.

Because this console is old and runs really hot, the caps in the audio path tend to get fried. Most of them are going or already gone. The best way to fix this is to replace all of them. At last count, there are 4112 electrolytic caps in this console. The originals are rated for 85 celsius. In order for the new caps to deal with the heat, we replace them with 105c caps. This will insure a longer life console. The caps in the signal path are also dual polarity. Most electrolytic caps are single polarity meaning current flows in one direction through the cap. These are good for most uses, but an audio signal flows in both directions. So, by providing a dual direction cap, you're ensuring the full wave of the audio is passing through. This gives you a full waveform making the resulting signal a lot less noisy. Replacing the caps is usually the first thing to do when revitalizing old electronic equipment. The next would be to replace the dreaded red IC sockets that plague a lot of MCI equipment. These sockets don't have a good hold on the ICs and many of them can fall out during use. This console, thankfully, had newer sockets, so that wasn't a problem on this one. Once these cap and socket replacements are done, most problems will be fixed. Then you can go through each channel and fix whatever is left.

Here is the process I went through to do these caps. This is a single mono channel and fader from the console. I also replaced the caps in the stereo and master sections as well. You can click on each picture to enlarge.

This is the MCI JH-636 console. Each channel pulls out of the console frame. These are the automated fader modules. The fader module has the physical fader and electronics. The sliders in the original fader module has plastic pieces holding up the rails. This plastic has worn away after many years. Here you can see the broken plastic piece out of the module. We had replacement pieces made from aluminum.
The aluminum pieces get replaced and the module goes back together. This is the fader module without the physical fader. This is before the cap replacement All the old electrolytics come out. The new electrolytics go in. Everything goes back together and here is the completed module. With the aluminum rail supports.
This is the channel out of the console. There is a mic pre module and an EQ module that detaches from the channel. The mic pre has one screw and just pulls off. There aren't that many caps on the mic pre. Old ones come out. New ones go in. The EQ module has 2 screws and pulls out. You have to take the knobs off, though. It's hard to get the plastic inserts out. Use a nylon pry tool or a fingernail. Just be careful, they break easy.
The old caps come out. The new ones go in. Remember that the card slips in where the connector is. The caps have to be pushed in all the way for them to clear. This is the channel with the eq and mic pre removed. The assign to tape buttons module has 3 screws and a post that has to be removed. There's a module on the right that doesn't have to come out. There's only one cap under it and you can do it if your careful. All the electolytic caps come out. There's a lot of them. And make sure you know where they go back. :-) All the caps go in and everything goes back together. Here's the completed channel and fader module.