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MCI JH 618, Rise from the dead!

PostPosted: Thu Sep 07, 2017 3:00 pm
by Ryebot
Hello Everyone!

I recently acquired MCI JH 618 SN 0308 built May 1981. It has been dead for a while, and I'm really hoping to resurrect it. The Audio Power Supply is fried, but the peripheral supply works! (The lights are on, but nobody's home...)

I have a quote from GRS in Hemet, CA for a brand new 18V (and phantom) supply. However, the main and most confusing item is that the -18V audio power rail is shorted to ground and +18V rail all over. I cannot find out where this is happening. It certainly won't help to have 8-16 Amps of fresh clean power running through that!

So before I get a new PSU, all of the channel modules have been pulled, and the Master section motherboard has been disconnected and removed from the rest of console (all power, ground and jumper connections de-soldered.) Before I pull the entire harness out from channels 1-18, and beyond... does anyone have an idea where I could check that I might be missing? I am an amateur tech at best, I know enough to be dangerous, and stupid... But I'm very willing to learn, and certainly get a lot of enjoyment out of it.
The previous owner(s) did quite a hack job all over... If anyone is interested I have many hilarious pictures to share.

It's not all bad, however.

SUPPOSEDLY, all of the channel modules are in working order. HA!

Other highlights include 2 dbx goldcans in the master section, one John Hardy preamp, and 2 Vari-Q channels. There is also a mystery non-factory group return(?) module fashioned for the lower right of the board, under the patchbay.

No automation and VU meters make this console a bit easier to deal with, but still a much bigger project than I've ever attempted! Hopefully some of you out there know the right questions to ask, and can guide me... I'm grateful for your wisdom! I'm a long time lurker, first time poster.

Many Thanks in advance!

Sincerely,

Ryebot

Re: MCI JH 618, Rise from the dead!

PostPosted: Thu Sep 07, 2017 4:21 pm
by Ryebot
Some pics of the Mystery Module...

Returns.jpg
Mystery Module.jpg

Re: MCI JH 618, Rise from the dead!

PostPosted: Thu Oct 05, 2017 9:49 pm
by AnalogStudio7
I admire your enthusiasm! Sounds like quite a project.

Have you been able to narrow down the power rail faults inside the console?

Cheers!

Greg

Re: MCI JH 618, Rise from the dead!

PostPosted: Fri Oct 06, 2017 9:25 am
by Jim Williams
Once all the modules are pulled, use a continuity beep setting on a DVM and check the power rails for continuity. Then check the rails to rails and to ground for any short. Then proceed to each module, check the diode power inputs for any shorts to ground or other rails.

Once the frame and bussboard check out, apply power to the frame. Then check the buss board for proper voltages. This all assumes the power supply is operating properly, if not, that is the first item to repair.

The 600 is a well designed product but is riddled with crappy bipolar coupling caps and filters on the hi frequencies. Once those are removed and opamps replaced with modern low noise, low offset types, that console comes alive. The difference is like after you land in a jet and your ears pop.

Re: MCI JH 618, Rise from the dead!

PostPosted: Sun Oct 08, 2017 10:54 am
by Ryebot
Thank you for your replies!

I've been in touch with MCI guru Steve Sadler and have been going through this beast slowly but surely. I determined that the continuity between + or - 18V rails and ground was the 9700 uf caps in the chassis charging and discharging. I replaced them just to be safe.

Meanwhile David at Studio Electronics is repairing the Audio PSU for me and has found a host of problems, some of which I had seen, namely on the 18V regulator board... Incorrect component values, blown transistors, and uninsulated crowbar triacs that are causing a dead short between + and - 18V... nasty business when a previous owner repairs a PSU without knowing what they are doing... That was the condition I received it in...

Meanwhile, the mother board checks out with no shorts, as long as nothing is connected! As soon as I begin to connect faders (my fader packs are non-automated) I get continuity across +24V and ground. This would be the Mute switches and indicator lamps per channel. Each fader has a 300 Ohm resistor past the lamp and ALL of them are burnt. This is odd because the 24V peripheral supply seems to check out.

The meter bridge also has some blown components, and a charred molex connector (also 24V and ground) that need to be addressed in the weeks ahead.

I shall keep plugging away. I hope to have the Audio PSU back in my hands this week, and I will begin to check channel modules... I am hopeful to replace any non-socketed chips with sockets, so I can compare the original design's sound with the upgraded tone that Jim Williams suggests, by swapping chips, although I believe there's a bit more to it than just the IC's.

Thanks for your interest and advice! I'll keep y'all posted!

Best,

Ryebot