one of my macs an mc2300 serial 3yxxx has a fan occasionally come on where as the other two amps doing the same duty do not . The fans are quiet but when they stop, a tiny pop sound like a popcorn kernel bouncing off the cone occurs . I am not a tech . I am about 4 hours from the west coast of Canada . There isnt anyone i know of that i would allow to touch these within driving distance. Are there any obvious things i can look into ? I’m not a tech but I am talented in most other ways. Thanks
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The service manuals in the Akdatabase and hifiengine site do not show the many changes and upgrades Mac made to these amp over the decades.
I would start with contact Mac parts and order a schematic so you have a roadmap in hand to sort through this.
The fan coming on "early" can happen for a few reasons, one really bad.
If the thermal sensor is more sensitive, has a lower threshold by a few degrees, it will switch on one fan earlier......not a big deal.
If you speaker has a electrolytic cap in it's woofer circuit.....ie a Zobel network and it gets leaky it can draw more current from one amp causing it to run hotter than the other.....you can swap the one speaker from one amp to the other to test....might hurt your back but not not smp critical.
Then the big one, Mac had problems with the predriver transistors for over a decade with these amps. In the late 70s they came up with the 3rd version and had a policy of updating anyone one of these that we took into service or came into a amp clinic.
Since most of these amps were not subjected to pro type abuse and were to darn heavy for owners to bring to a clinic, many never got upgraded.
As the PNP predriver transistor was a critical component in establishing the NPN driver to " act" like it was a PNP device and was therfore a really critical part of the Szikali or coumpound part of the output stage when it would go wonky the amp would overheat and usually takeout a number of output transistors. Sometimes all of them.....
This repair gets expensive.
So we always ask owners to remove the top cover and get us a picture of the driver cards......if the predriver heatsinks are round flying saucer looking the upgrade was done in the past.....if not it needs to be done.
There are in between things that are now constantly cropping up on theses 50 year old amps.
The axial caps on the driver and input boards are regularly wonky......I just replace them.
As mentioned earlier these amp's have so much reserve they will often pay just fine with 2,3, or more individual output transistor open, whether they have failed of the .33 or .56 wirewound has gone open...only a bench stress test will show it.
There are more things that a proper test bench test could show but the first two things you can try yourself by swapping speaker wires, speakers and amp duties.
Oh and stock up on the NSAIDs, people forget how heavy these amps are.......
I would start with contact Mac parts and order a schematic so you have a roadmap in hand to sort through this.
The fan coming on "early" can happen for a few reasons, one really bad.
If the thermal sensor is more sensitive, has a lower threshold by a few degrees, it will switch on one fan earlier......not a big deal.
If you speaker has a electrolytic cap in it's woofer circuit.....ie a Zobel network and it gets leaky it can draw more current from one amp causing it to run hotter than the other.....you can swap the one speaker from one amp to the other to test....might hurt your back but not not smp critical.
Then the big one, Mac had problems with the predriver transistors for over a decade with these amps. In the late 70s they came up with the 3rd version and had a policy of updating anyone one of these that we took into service or came into a amp clinic.
Since most of these amps were not subjected to pro type abuse and were to darn heavy for owners to bring to a clinic, many never got upgraded.
As the PNP predriver transistor was a critical component in establishing the NPN driver to " act" like it was a PNP device and was therfore a really critical part of the Szikali or coumpound part of the output stage when it would go wonky the amp would overheat and usually takeout a number of output transistors. Sometimes all of them.....
This repair gets expensive.
So we always ask owners to remove the top cover and get us a picture of the driver cards......if the predriver heatsinks are round flying saucer looking the upgrade was done in the past.....if not it needs to be done.
There are in between things that are now constantly cropping up on theses 50 year old amps.
The axial caps on the driver and input boards are regularly wonky......I just replace them.
As mentioned earlier these amp's have so much reserve they will often pay just fine with 2,3, or more individual output transistor open, whether they have failed of the .33 or .56 wirewound has gone open...only a bench stress test will show it.
There are more things that a proper test bench test could show but the first two things you can try yourself by swapping speaker wires, speakers and amp duties.
Oh and stock up on the NSAIDs, people forget how heavy these amps are.......
I do not like how the bias diode is mounted under the far left saucer heatsinked transistor. If have seen some done this way, IT IS NOT how DOB taught me as it does not give the diode intimate contact with the heatsinked device.
I can not see how the board on the LH side is done.
Obviously you should plan on replacing the axial caps.
Take the two screws out, pull the driver cards and repost pics of the boards.
I can not see how the board on the LH side is done.
Obviously you should plan on replacing the axial caps.
Take the two screws out, pull the driver cards and repost pics of the boards.
Obviously someone has been in here before.
I would get a digikey order together and order replacement Vishay axial caps for the four pictured as well as those on the input board.
In the past the sentry monitor transistors have been replaced( the two small ones below the predrivers) as has the differential pair. ( the two small ones to the right).
What parts did the use?
The class A drivers have also been swapped out with some not recommended TO220 type devices. The originals were TO66 case devices.
What parts are these?
Lastly I would carefully pull off the saucer heatsinks and confirm which parts were used there. Take a picture as I am curious about the bias resistor mounting for both cards. The large brown ceramic caps just above the heatsinks were supposed to be replaced with a smaller value and size when doing the upgrade if I remember correctly. Those you have on your boards just seem to be too large to my tired old memory.I will have to look up the correct upgraded value.
Just because, you might want to check the other two amps or at least triple up on the cap order to save on shipping.
I would get a digikey order together and order replacement Vishay axial caps for the four pictured as well as those on the input board.
In the past the sentry monitor transistors have been replaced( the two small ones below the predrivers) as has the differential pair. ( the two small ones to the right).
What parts did the use?
The class A drivers have also been swapped out with some not recommended TO220 type devices. The originals were TO66 case devices.
What parts are these?
Lastly I would carefully pull off the saucer heatsinks and confirm which parts were used there. Take a picture as I am curious about the bias resistor mounting for both cards. The large brown ceramic caps just above the heatsinks were supposed to be replaced with a smaller value and size when doing the upgrade if I remember correctly. Those you have on your boards just seem to be too large to my tired old memory.I will have to look up the correct upgraded value.
Just because, you might want to check the other two amps or at least triple up on the cap order to save on shipping.
Ok for the MC2300 you kept the original .0012 mfd caps, you replaced them with pico farad caps for the Mc2100 and Mc2105.
I happen to like the vishays over the Nichicon axials.
While you have it apart don't forget to check the input board.
We will have to see about the choices made about the replaced transistors.
I happen to like the vishays over the Nichicon axials.
While you have it apart don't forget to check the input board.
We will have to see about the choices made about the replaced transistors.
I must be getting tired, it is the diode under the predriver in the right hand pictured part that is not installed as DOB taught me, I guess I meant to say I could not see how the LH one was mounted.
This is why I asked you to pull off the saucer heatsinks, to see how the diode was mounted and see what parts they used.
This is why I asked you to pull off the saucer heatsinks, to see how the diode was mounted and see what parts they used.
The two small (to92 case) on the left are correct subs for the originals
The one with the diode underneath looks like it is a 2n5681, that is not what Mac parts would have shipped, the other I can not see what it is.
A stab in the dark.... I wonder if the to220 is a tip41.... Certainly not what would have been sent by MAC parts.
Lastly the two lower ones, what are they?
The red jumper wire, is that burnt or just black marker?
The one with the diode underneath looks like it is a 2n5681, that is not what Mac parts would have shipped, the other I can not see what it is.
A stab in the dark.... I wonder if the to220 is a tip41.... Certainly not what would have been sent by MAC parts.
Lastly the two lower ones, what are they?
The red jumper wire, is that burnt or just black marker?
The parts my cross shows that Mac would ship for the predrivers were 2n5320 and 2n5321.
Central semi makes them now. They also make the 2n5681 and 79. You can google them and compare the data sheets if you wish, they are close but different.
I just do not like to screw around with these beasts...you know when the show needed to go on techs on the road would have to do what they had to do, but this is now different.
I would take the time to rebuild this amp correctly as much as a pain as it will be.
Get the right parts that Mac engineering told us to use.
I would pull every heatsink one at a time and remove each transistor and do a resistance and diode check on each one. I would also expect to find one or two to be wonky so I would plan on buying 2 dozen mj15003gs.....replace them all, just because. If you had a test bench and could stress it maybe you could determine it would be OK....but....
I think when you pull out that "hot" heat sink you will find it has a bad transistor on it, or the bias diode is not tracking correctly and letting it over heat. Or the improper Class A driver is oscillating or,or,or.....
Continue to explore, check out the hot heatsink first. And post pics.
My schematics and such are buried in temp storage so I hope Patrice might see this and help me with my memory. I know that Mac engineering has found a good NTE sourced sub for the orginal TO66.... I raked Landon over the coals over their choice and he defended it vigoriously.....just have to find the number.
Big Lake adventure tomorrow so limited connections but you will need to order some parts next Tuesday anyway so be patient and double check your work.
Almost time to root for big blue....
Central semi makes them now. They also make the 2n5681 and 79. You can google them and compare the data sheets if you wish, they are close but different.
I just do not like to screw around with these beasts...you know when the show needed to go on techs on the road would have to do what they had to do, but this is now different.
I would take the time to rebuild this amp correctly as much as a pain as it will be.
Get the right parts that Mac engineering told us to use.
I would pull every heatsink one at a time and remove each transistor and do a resistance and diode check on each one. I would also expect to find one or two to be wonky so I would plan on buying 2 dozen mj15003gs.....replace them all, just because. If you had a test bench and could stress it maybe you could determine it would be OK....but....
I think when you pull out that "hot" heat sink you will find it has a bad transistor on it, or the bias diode is not tracking correctly and letting it over heat. Or the improper Class A driver is oscillating or,or,or.....
Continue to explore, check out the hot heatsink first. And post pics.
My schematics and such are buried in temp storage so I hope Patrice might see this and help me with my memory. I know that Mac engineering has found a good NTE sourced sub for the orginal TO66.... I raked Landon over the coals over their choice and he defended it vigoriously.....just have to find the number.
Big Lake adventure tomorrow so limited connections but you will need to order some parts next Tuesday anyway so be patient and double check your work.
Almost time to root for big blue....
The part Mac would have shipped in 1975 would have been a Bc416c most likely, it was the more current sub for a MPS5087 but all these parts were and have been obsolete for decades......I think they were all replaced by the 2sa970 which is now obsolete for all practicle purposes now.
Because you replaced some caps...
Where did you get this amp? Was it a road warrior? How about the other two?
I would check the hot heatsink transistors while you have it open.
I turned it back on. The only thing that came up to heat right away was the tip41 . The fourth heat sink isnt getting warm i reached my finger in and felt the resistors on warm up . After 10 minutes i shut it off unplugged it and felt around for more signs of heat . The ceramic disks started to get warm . The transitors on the sinks were only slightly warm . I couldnt reach the bottom ones finger not long enough.
This is one of the two sequential number units . All in beautiful cosmetic condition. These two came from an old feller in toronto area somewhere.
This is one of the two sequential number units . All in beautiful cosmetic condition. These two came from an old feller in toronto area somewhere.