UPDATE: The power supply rebuild was successful - dim bulb as expected, no magic smoke. Voltages are all within spec except the 90 volt rail, which is a bit high at 106 VDC. Should I be concerned about this? Mac states a 10% tolerance in their specs, this is a bit over that. Would the fact that the driver boards are out of the amp have any bearing on this? Many thanks again for any thoughts!
MC-2105 refurb
- Thread starter teehas53
- Start date
Maybe you have a high AC outlet voltage? Compared to what Mac's nominal voltages were tested at?
More fun emerges… I’m moving on to the input board next, finding a couple of unexpected surprises.
I’m the original owner of this amp so you’d think I’d know its history. But all the electrolytics on the input board are Nichicons, which probably would not have been original to a ‘74 McIntosh amp (please correct me if I’m wrong about that.) So my memory must not be reliable because apparently this board was recapped at some point and I just don’t remember it. Sorry for the digression.
What’s also odd is the two big-ish (reservoir?) caps on the board should be 640µF for an amp of my vintage but it currently has 470µF parts in it. Also unexpected is the related dropping (?) resistors (R9 & R10) that are supposed to be 3.3k are 2.2k, and they appear to be original. I’m inclined to replace all this with the factory spec’d values but I’m not enough of an engineer to know if that’s a good idea, or if McIntosh maybe did this for a reason. I know there were many undocumented changes over time, maybe this is one of those? Or maybe the tech who recapped it used what he had that felt “close enough” to him at the time? I would have used the very reputable local guy whom I wouldn’t expect shortcuts from, so it’s a bit of a mystery.
Bottom line though, should I revert to as close to the factory spec as possible? Everything else matches the schematic and even the old carbon comp resistors mostly test in spec (still!)
I’m the original owner of this amp so you’d think I’d know its history. But all the electrolytics on the input board are Nichicons, which probably would not have been original to a ‘74 McIntosh amp (please correct me if I’m wrong about that.) So my memory must not be reliable because apparently this board was recapped at some point and I just don’t remember it. Sorry for the digression.
What’s also odd is the two big-ish (reservoir?) caps on the board should be 640µF for an amp of my vintage but it currently has 470µF parts in it. Also unexpected is the related dropping (?) resistors (R9 & R10) that are supposed to be 3.3k are 2.2k, and they appear to be original. I’m inclined to replace all this with the factory spec’d values but I’m not enough of an engineer to know if that’s a good idea, or if McIntosh maybe did this for a reason. I know there were many undocumented changes over time, maybe this is one of those? Or maybe the tech who recapped it used what he had that felt “close enough” to him at the time? I would have used the very reputable local guy whom I wouldn’t expect shortcuts from, so it’s a bit of a mystery.
Bottom line though, should I revert to as close to the factory spec as possible? Everything else matches the schematic and even the old carbon comp resistors mostly test in spec (still!)
Joth
Member
I had the same thoughts on my refurb, original amp, also 1974, same pale blue nichicons and yes they are original and you can confirm this with many photos online of the same exact caps, and you will see them with the 470uf value.
I can't comment on the resistor value difference as I left mine original and they escaped scrutiny.
I did use film caps instead of the electrolytic 10uf.
I can't comment on the resistor value difference as I left mine original and they escaped scrutiny.
I did use film caps instead of the electrolytic 10uf.