MAC 4200 - A couple questions about the speaker protection circuit

hirscwi · Sep 7, 2014

I have a MAC 4200 that has been having a problem with fading and drop-out on the left channel. I cleaned the speaker relay and used DEOXIT on the selector switch. During this process, I determined that a wire had come loose from the main connector to the speaker protection circuit. It's the grey wire shown in the 1st and 2nd photo. I don't have a service manual so I'm not sure the connection point - there are several open slots on the connector.

Also the speaker protection relay appears to be a non-standard - 3rd photo. It is just glued on its side to the PCB - which hardly seems an approach that McIntosh would have used. Possibly a repair made at some point in the past? Again, not having the service manual puts me at a disadvantage. Should this be replaced? If so, does anyone have the McIntosh part number?

Any help will be greatly appreciated.

c_dk · Sep 9, 2014

I pulled the schematic....the output signal processing board part of the schematic is only up to serial number DP1525. I believe it does show the relay double sided foam taped to the board.

As for the wire, where does it come from?.....then we can tell you for sure where it goes.

You will have to check with Mac to see if they have the schematic.

hirscwi · Sep 10, 2014

c_dk said:
I pulled the schematic....the output signal processing board part of the schematic is only up to serial number DP1525. I believe it does show the relay double sided foam taped to the board.

As for the wire, where does it come from?.....then we can tell you for sure where it goes.

You will have to check with Mac to see if they have the schematic.

I spoke with Mcintosh and they sent me an email copy of the service manual. The wire in question originates from the regulated p/s board. Unfortunately the service manual shows a different regulated power supply board than I have. The board in my unit is 046141 and the board in the service manual is 046559. My serial number is DP1380.

My guess is that the output processing board (or part of it)formerly resided on the regulated p/s board (at least that's my guess). So I am struggling to find the correct connection point. According to the schematic I have for the output processing board (which should be correct because it is for DP1001 to DP1525) several pins in the connector are unused (2, 5, 6, 7). The other connections on my unit are intact. So I wonder if the wire is a vestige? There is another wire connection point on the regulated p/s board that is not used, which also supports my theory. The wire point is running at about 21V and I can't find anything on the output processing board that needs that voltage.

With all that in mind, I fired it up using a dim bulb tester and everything seems to work properly. I wish I was closer to your shop and I would stop by for a visit.

Any other thoughts are much appreciated.

c_dk · Sep 10, 2014

What pin does it go back to at the regulated board?

hirscwi · Sep 11, 2014

c_dk said:
What pin does it go back to at the regulated board?

I'm attaching a couple of photos that show the location of the wire. It has been removed for now, but was connected at the point marked with an arrow and a "*". This point doesn't have a number on it - or at least I can't find it because my board doesn't match the board in the service manual.

Thanks.

Classic_CJ · Sep 18, 2014

*** My guess is that the output processing board (or part of it)formerly resided on the regulated p/s board (at least that's my guess). So I am struggling to find the correct connection point. According to the schematic I have for the output processing board (which should be correct because it is for DP1001 to DP1525) several pins in the connector are unused (2, 5, 6, 7). The other connections on my unit are intact. So I wonder if the wire is a vestige? There is another wire connection point on the regulated p/s board that is not used, which also supports my theory. The wire point is running at about 21V and I can't find anything on the output processing board that needs that voltage. ***

I would say you have it about right, except vice-versa-- the *later* versions moved part of the output/headphone processing to the regulated board. In the earlier version, there was a headphone relay on the output processing board, and a connection for 12 volts that doesn't feed anything else besides the supply for this relay. Check your schematic-- I believe it connects to pin four.

Try the headphone jack(s) and see if they work. If not, then I'd guess that the detached wire in question went to this circuit. Your voltage reading may be higher than 12 volts if the supply for this relay is unregulated. When the relay is activated (by a transistor switch-- see the schematic) the voltage may drop to around 12 volts.

Unfortunately I can't offer much more than this, as my schematic shows only the later version of the regulated PC board, and not the earlier one you have, probably just a goof when they made up the service manual. (Yes, that happens!)

You can also check with Mac service to see if they have a schematic of your earlier regulated PC board-- have the board number available when you call.

hirscwi · Sep 18, 2014

Well, your suggestion started me thinking. I have been testing with the headphone jack - and everything worked fine. So I just now tried the speakers and they are not working. So this wire must be power to the speaker protection on the output processing board. Any suggestions where it connects? It appears that the schematic I have is correct for the output control, even though it's wrong for the regulated p/s.

Thanks for your continued support.

Classic_CJ said:
*** My guess is that the output processing board (or part of it)formerly resided on the regulated p/s board (at least that's my guess). So I am struggling to find the correct connection point. According to the schematic I have for the output processing board (which should be correct because it is for DP1001 to DP1525) several pins in the connector are unused (2, 5, 6, 7). The other connections on my unit are intact. So I wonder if the wire is a vestige? There is another wire connection point on the regulated p/s board that is not used, which also supports my theory. The wire point is running at about 21V and I can't find anything on the output processing board that needs that voltage. ***

I would say you have it about right, except vice-versa-- the *later* versions moved part of the output/headphone processing to the regulated board. In the earlier version, there was a headphone relay on the output processing board, and a connection for 12 volts that doesn't feed anything else besides the supply for this relay. Check your schematic-- I believe it connects to pin four.

Try the headphone jack(s) and see if they work. If not, then I'd guess that the detached wire in question went to this circuit. Your voltage reading may be higher than 12 volts if the supply for this relay is unregulated. When the relay is activated (by a transistor switch-- see the schematic) the voltage may drop to around 12 volts.

Unfortunately I can't offer much more than this, as my schematic shows only the later version of the regulated PC board, and not the earlier one you have, probably just a goof when they made up the service manual. (Yes, that happens!)

You can also check with Mac service to see if they have a schematic of your earlier regulated PC board-- have the board number available when you call.

MAC 4200 - A couple questions about the speaker protection circuit

hirscwi

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c_dk

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hirscwi

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c_dk

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hirscwi

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Classic_CJ

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hirscwi

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