McIntosh C38 issues

I have a client's C38 in for service.

Some liquid dripped into the unit, leaving a crusty residue, shorting out input IC1 causing it to fail.

Installed replacement IC from Mac and now the unit will pass thru signals, with some strange start up issues.

Plug unit in, turn on, stays on input cd1.... No sound.

I could manipulate the switching IC on the switch board, bend the board a bit, and unit would function and pass signal.

Soldered IC directly to board and input would no longer function at all, manipulation had no effect.

Put call ino Landon, and left unit plugged in but off. On a lark, before he returned my call, I turned the unit on and it worked with no issues....except

If unit is unplugged it will not work until unit has been plugged in for at least 10 minutes.....5 is not enough. If not working and left on, it will not recover proper functions, it must be turned off and plugged in.

The zone B, record input selector has never faltered, once IC 1 was replaced.

The schematic for this section is 5 feet wide.....and while consulting with Landon (he has never seen this issue before) we wonder if it might be a be a problem with IC7, the timing circuit to lockout the front panel controls when a remote home function is called for.....

It is bewildering....so I toss this out for any ideas....

C_dk, I'm not following the diagnostic procedures too well. Got a schematic to post - I know it will be cut up but better than nothing.

Also, if there was liquid damage, could it have migrated? Seems like the mystery of control circuits are a bit more complicated that plain ol' audio signal chasing.

Cheers,

David

I have the schematic. The leftovers from the drip were isolated to the IC leads.

The input board is stacked on top of the control circuit board, can't see how it would have seeped thru, but you never know.

It almost seems like a cap needs to "charge up" so the control circuit can "remember" it's place and then switch to other inputs.......

Generally, for the control circuits I have to deal with these days (Software Defined Ham Radios), when there are control circuit problems, suspect power supply problems first. Some voltage rail is getting pulled down (bad caps) or there's noise on the rail(s) that propagate signals to other circuits (open bypass caps, poor grounding, cruddy connectors).

Cheers,

David

Ghost In The Machine. Since it seems to be from the same era as the MHT100 - not too far off - does it have some sort of back-up power supply board that holds settings? As you know, this is a major issue with the MHT series that exhibit the same type of problem.

More information this morning. It will start operating after being plugged in both in standby as well as turned on. I had not checked the on status since soldering in the indicator IC.

Still takes 10 minutes or so before it will start working......

My C100 was doing some similar weird stuff before I sent it back to Sound Ideas to fix it. Must be a thing with units from this era.

Could be, but I really need to find the solution. Can not return it to client like it is.

Will leave the unit unplugged overnight and see how it acts tomorrow.

I wish I could give you more insight, I know how frustrated you must be especially when you're typically the one with the answers. In the case of my C100, they swear up and down and left and right that it was because someone put a modem cable in the box instead of the correct unit to unit interconnect cable.

Some of the strange behaviors included just not turning on at all, then (after I disconnected said interconnect, left it unplugged from the wall power for a while and then came back it) "tapping" or even pressing lightly on the top panel of the control unit causing the volume to erratically move up and down, and erratic behavior from the balance control.

Until I plug it in and play with it more I can't confirm that it has been fixed (I have it back at home now, but I haven't had time to unbox it and test). In other words, if they had done something special to mine to repair it, I'd let you know the details. And you better believe if it's still not working it's going right back.

I timed it today. After being plugged in overnight, unplugged, then plugged in again, it took 3 minutes to obtain listen control, record has never had a issue.

It is unplugged tonight, we will have to see how it reacts tomarrow.

I have discovered a 10 mfd cap on one 5 volt supply which is only for the listen input....#47 I wonder if it is leaky?

I am always learning new things trying to keep these old relics working at their best.....Monday I had to learn how to correct the color of the MC2125 project meters now that they have LEDs. Thank goodness for shop neighbors who are old school photographers that used colored gels....

It is possible that of through vias in board got corroded. Hard to find though. You may try freezing spray when unit works to identify problem area of the board.

If a C38 is an "old relic" I have a collection of dinosaurs!

Yes you are right, I seriously doubt that a C38 will hold a place of historical significance. Not compared to the tube preamp reissues released in the same time frame.

At some point will need to find a way to make it sound as good as the updated C35.....

I see this is quite a old discussion, but I'm having a startup problem with my C38. Were you able to solve the issue?

Welcome to AK.......

Yes, their cat threw up into the preamp.....I had to lift a number the control IC sockets off the circuit board to clean off the residue that had seeped under the IC sockets causing them to short out.

If you live in a humid, coastal area I would suspect similar issues are possible as they are with the MR80 tuners which have been reacting badly to deposited salts from ocean air.

Thanks!
Thank You for the quick response. We will take a look.