#143544  by softmachine72
 Thu Dec 25, 2014 9:56 am
what is the sentry mod? does it remove the protection circuit ? what are the benefits ?
 #143546  by TI4-1009
 Thu Dec 25, 2014 10:05 am
I'll bet any minute now hippieguy will supply you with at least 3 links to searches where this has been discussed to death. :-)
 #143551  by hippieguy1954
 Thu Dec 25, 2014 1:17 pm
Thanks for using the search. I won't list the threads here. I'm glad you read thru them. This one is especially good and read Brad's WARNING about NOT shorting your speakers: forum/viewtopic.php?f=309&t=8253&start=15

It's easy. You just have to lift one leg on two resistors and two diodes on the output card/cards.

I took a photo to share below.

At the top of the board you can see where I lifted a leg on two resistors. R27 and R29.

Toward the middle lower right on the board you can see where I lifted one leg of two side by side diodes located just below the two orange ceramic caps between the big square heat sinked transistors. D5 and D7.
If you do both boards, you do the same to the other one.
The service manual. Scroll to page 4 to see the layout.
http://akdatabase.org/AKview/albums/use ... ervice.pdf

Personally, I don't think this is a make or brake mod in as that the McIntosh will not clip until very high volume is achieved. I do, however, feel it is part of the equation.
Image

Special thanks to Mike Wald :smile:
 #143552  by softmachine72
 Thu Dec 25, 2014 2:02 pm
Thanks for a simple answer. BTW, I did dn. load the service manual. Are the 150mf and 30mf electrolytic caps original? I will be glad to answer any questions that are more in my wheel house. :hail:
 #143553  by hippieguy1954
 Thu Dec 25, 2014 2:49 pm
softmachine72 wrote:Thanks for a simple answer. BTW, I did dn. load the service manual. Are the 150mf and 30mf electrolytic caps original? I will be glad to answer any questions that are more in my wheel house. :hail:
You're welcome!

Yes, they are the original caps. That pic was taken to have as a reference. But, as I said in your other thread, I changed all electrolytic caps on both output cards and the input board.

Also, any resistors that have drifted out of spec. I didn't find any out of spec on the input board, but found a few on the output cards.

The two large "can" caps should be checked for any leaking. Other than that, they should be ok.

There is also a multi cap "can" on the input board that should be replaced for a complete service.
It can be had here: http://www.hayseedhamfest.com/

This is most of what a tech would do if you sent it for a complete service.
WARNING: FOLLOW PROPER PROCEDURE FOR DISCHARGING CAPACITORS OR ELECTRIC SHOCK CAUSING INJURY OR DEATH CAN OCCUR
 #143554  by Jon S.
 Thu Dec 25, 2014 4:18 pm
According to McIntosh themselves, "SENTRY MONITOR CIRCUIT - McIntosh designed and patented circuit that activates in case of accidental “shorts" in speaker wires."

My McI sounds great as is I'm leaving this useful circuit the heck alone!
The McIntosh Sentry Monitor

Although the autoformer provided an efficient match between the power transistor output and a variety of load impedances, a short circuit at the amplifier output or a load that was much lower than the selected autoformer tap could cause excessive current to flow in the output transistors. To complement the new transistor amplifiers, the McIntosh Sentry Monitor circuit was developed which prevented destructive current levels from occurring under any conditions. This circuit sensed the dynamic operating time, voltage and current of each amplifier output stage and controlled the current flow, confining it to non-destructive limits. The arrangement assured complete circuit reliability for all load conditions. The Sentry Monitor did not limit the rated power output available from the amplifier in any way. McIntosh power amplifiers continue to use the Sentry Monitor circuitry.
http://www.roger-russell.com/mcintosh1. ... trymonitor
 #143555  by Searing75
 Thu Dec 25, 2014 5:23 pm
The transformers in these puppies are hard to replace! Also, to get anything out of the mod, you would need to be playing at very high levels!

I chose to not do it to my MC50, but am curious if I would notice or feel a difference?
 #143556  by hippieguy1954
 Thu Dec 25, 2014 6:21 pm
Like I said, this mod is not a deal breaker unless you are playing at huge volume. It's also VERY, VERY important that you DO NOT short your speakers.
AGAIN, read Brad's warning here: forum/viewtopic.php?f=309&t=8253&start=15

Also, unless you have experience or feel confident desoldering and resoldering components on a 30 or so yr old circuit board, I DO NOT recommend doing this yourself.
Especially you, Jon S ... :lol: ..Just bustin your balls, man :smile:

Yes, at VERY high volume I can feel and hear a difference.
 #143557  by hippieguy1954
 Thu Dec 25, 2014 6:35 pm
Check out my recent jammindead soundcloud. We are playing at huge volume. Most of my playing (excuse all the screw ups) is with no effects except the obvious, but the clean with no effects gets very gritty when I raise the guitar volume knob and push everything.
 #143559  by Searing75
 Thu Dec 25, 2014 7:43 pm
Hippie guy. I can't see the other resistor on the upper part of the board that you lifted a leg of? I see the middle resistor in the grouping of three, but where is the other?
 #143560  by hippieguy1954
 Thu Dec 25, 2014 9:27 pm
Searing75 wrote:Hippie guy. I can't see the other resistor on the upper part of the board that you lifted a leg of? I see the middle resistor in the grouping of three, but where is the other?
Try making sure you are viewing 100% and not 1250% or more. If that doesn't work, it is R27 in the service manual.
http://akdatabase.org/AKview/albums/use ... ervice.pdf
Scroll to page 4 to see the layout for the left channel output circuit board. It's R27. Same on right channel only different "R" number name for right channel.
 #143561  by hippieguy1954
 Thu Dec 25, 2014 10:00 pm
Note: When recapping, these are polarity sensitive. The plus + and negative - must be observed and connected appropriately. :smile: