suggestions for MC225 power supply rebuild.
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dshoaf
That high voltage buzz
The small power supply card has the:
- 2 diodes for the high voltage section. They are side by side and usually look like little top-hats. The filter cap is there in front of the choke and right behind the power supply board.
- The selenium rectifier is the square thing on the back side of the card. It has to go. This is the bias circuitry diode seen in the schematic. Its resistors need to be checked for being in spec and the filter cap needs to be replaced. Check the Hints and Kinks stickey for more details on the rebiasing technique for this circuit once you rebuild it. You'll need a variac, too.
- Lastly, the power supply rebuild is not complete without swapping out the 2 voltage doubler caps that are mounted to the metal plate running down the middle of the amp.
I guarantee that you'll hear a much improved difference once those items are replaced and set up properly. I do replace the thermistor since the original one is usually quite brittle and will likely fall apart as you mess around inside.
Hope that helps.
Cheers,
David
- 2 diodes for the high voltage section. They are side by side and usually look like little top-hats. The filter cap is there in front of the choke and right behind the power supply board.
- The selenium rectifier is the square thing on the back side of the card. It has to go. This is the bias circuitry diode seen in the schematic. Its resistors need to be checked for being in spec and the filter cap needs to be replaced. Check the Hints and Kinks stickey for more details on the rebiasing technique for this circuit once you rebuild it. You'll need a variac, too.
- Lastly, the power supply rebuild is not complete without swapping out the 2 voltage doubler caps that are mounted to the metal plate running down the middle of the amp.
I guarantee that you'll hear a much improved difference once those items are replaced and set up properly. I do replace the thermistor since the original one is usually quite brittle and will likely fall apart as you mess around inside.
Hope that helps.
Cheers,
David
Russellc
Addicted Member
The caps are already replaced. what usually used to replace the selenium rectifier part number wise? Same question concerning the little "top hat" items? Does the thermisitor live on, or does it need replaced.
I'll check the sticky.
Thanks, Russellc
dshoaf
That high voltage buzz
OK, didn't understand that the power supply caps had been done. 1N4007s are fine for the high voltage supply. I used the same for the bias supply. You can use the low-noise/Hexfred devices but I'm not seeing the reasons to so this.
Do pay attention to the rebiasing procedure. I went for a few months when I rebuilt my first MC-225 early last decade without doing this and wondered why the amp sounded thin and anemic. All the sonics returned with a vengeance once bias voltages were set properly.
Cheers,
David
Do pay attention to the rebiasing procedure. I went for a few months when I rebuilt my first MC-225 early last decade without doing this and wondered why the amp sounded thin and anemic. All the sonics returned with a vengeance once bias voltages were set properly.
Cheers,
David
Russellc
Addicted Member
Hey, thanks a lot appreciate the advice. The MC 225 is a very desirable amp and worth seeking out.
Russellc
Russellc
Addicted Member
One more question, any suggestions for the thermistor replacement?
thanks again for the time and advice!:yes:
Russellc
Russellc
Addicted Member
Russellc
Addicted Member
Ok, here's some pics, first the thermistor, then some interior shots showing the re cap. There are still a few small electros on the board and the small power supply board also has one. There are still some bumble bee caps in here, I think the 2 over the (Newly installed) pots are .01 If I recall right.
I added the 220 uf Cerafine to stiffen the power supply a bit.
I added the 220 uf Cerafine to stiffen the power supply a bit.
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Russellc
Addicted Member
Couple more:This power supply probably needs a little explaining. The two blue Sprauges mounted next to each other are 200 uf 250 volt caps, wired so they are standing on top of each other equaling 100 uf at 500 volts. This makes up the first capsection, the second is a 220 uf Cerafine, and the final section is the smaller blue cap, another sprauge 20uf at 450 volts.
Russellc
Russellc
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Russellc
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Please feel free to suggest parts that need to go, other than those already spoken of. My thermistor doesnt "look" to be in to bad of condition, but I have taken your advice not to mess with it! (other than to replace it once I learn what modern part number to do so with.)
Thanks again, hope the pics help,
Russellc
Thanks again, hope the pics help,
Russellc
Russellc
Addicted Member
That's what I figured, any suggestions for caps to do this with? Seems like most Mcintosh parts are more in the "Industrial strength" category rather than the boutique category. I figure the amp is worth what ever is best for it, staying in the Mcintosh tradition, but not yielding reduced sound quality.
Any suggestions greatly appreciated!:yes:
Russellc
Russellc
Addicted Member
Not to dis Terry in any way, but seems to me like Russellc is certainly more than capable of swapping in new caps, etc. ...de wickt time
I have read good things (but not tried) Auricaps as a 'reasonably priced' boutique cap ... then there are the Mundorfs, etc.. Too many choices ...
I'm sure you know all this ...
I wonder if some of the smaller signal path electrolytics (Assuming there are some?) can at least be replaced with film/foil caps? I would think these would sound much better. Looks like some caps (white with yellow end caps) have already been upgraded. I am not at all familiar with the circuit in this baby (but one of these years (after wife's graduate school is finished) would like to get one to play with!).
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Russellc
Addicted Member
Yes, i bought this as a "only one channel works doorstop" condition and brought it back to life. then a few years ago it started making some noises, stability issues. Recaping, reflowing a bunch of solder joints, and tightening a ground point etc brought it back, now I just need to switch out these bumble bees, perhaps the thermistor as well. ( although the existing one "looks" fine. Its been re tubed, new RCAs installed and the volume pots replaced.
I now need, besides the bumble bee swapout, to replace the selenium bias with a 1N4007, replace the bias cap on that board, and the 2 cathode bypass caps and set the bias. I have a variac, that will allow setting the plate voltage at 390, and letting it stabilize. Then replace R 46 with a 33K resistor, or there abouts until the bias voltage is at -21 volts.
Doing the works not a problem, I just dont know what modern part these thermistors are. Once I know the value, I can find the parts. The sticky that converts Bumble bees is a little confusing, but I have located them on the schematic.
I'll likely use orange drops or sprauge poly prop for the .1 uf on the pots,
and either the sprauge poly prop (730's) to replace the bumble bees, maybe a nice paper and foil or PIO type.
The 100 uf 100 volt sprauge Atom will be used for the cathode by pass. A similar electro of around 22 uf and 450 volts or so will go in the bias circuit.
I'll eventually be sending Terry my MR 67, its been all recapped, but needs a little bit more tuner related servicing that I cant do.
Russellc
I now need, besides the bumble bee swapout, to replace the selenium bias with a 1N4007, replace the bias cap on that board, and the 2 cathode bypass caps and set the bias. I have a variac, that will allow setting the plate voltage at 390, and letting it stabilize. Then replace R 46 with a 33K resistor, or there abouts until the bias voltage is at -21 volts.
Doing the works not a problem, I just dont know what modern part these thermistors are. Once I know the value, I can find the parts. The sticky that converts Bumble bees is a little confusing, but I have located them on the schematic.
I'll likely use orange drops or sprauge poly prop for the .1 uf on the pots,
and either the sprauge poly prop (730's) to replace the bumble bees, maybe a nice paper and foil or PIO type.
The 100 uf 100 volt sprauge Atom will be used for the cathode by pass. A similar electro of around 22 uf and 450 volts or so will go in the bias circuit.
I'll eventually be sending Terry my MR 67, its been all recapped, but needs a little bit more tuner related servicing that I cant do.
Russellc
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I don't know for sure in the 225, but I would recommend a little more 'boutique' caps (yes, PIO or paper/foil!) if these are in the signal path. Not that Orange Drops don't sound OK (I used many of them in a Dyna SCA-35 rebuild a long time ago), but when I put in some RelCap RTXs (interstage) and Hovland Musicaps (output coupling caps) in their places, things sounded WAY better ... of course, that's when ODs were about $1.60 each and Hovlands, etc. were only about $7. These days ...? might be prohibitively expensive.
There used to be an 'axiom' from rec.audio.tubes to use 'highest res' (film/foil or teflon) caps for interstage, then 'softer' ones (like PIOs) toward the outputs ... seemed to work for me! altho I didn't have the patience to swap things around etc.. Don't know if this is the 'recommendation' for MC225 rebuilds ...? If so, please comment for my future use!
You might know all this and have reasons to not do this (and certainly there may be/are better choices now), but thought I would just butt in and give my (humble, really!) opinion anyway!
This stuff is really fun! I look forward to (MAYBE) getting a 225 or something one of these years (after wife gets out of graduate school and I can spend a bit of moolah on non-essentials again!).
Until then, I have to live vicariously thru other AKers upgrades and rebuilds!
Have fun ... - Rick.
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dshoaf
That high voltage buzz
Illinois Capacitor products are excellent for the interstage coupling caps and are quite reasonable over the Hovland/Mundorf type caps. For whatever reason, tube amp caps have achieved the same level of mystique as tube brands, wines and coffees. They all have as many opinions as there are posters, judging from the posts I see on many audio sites.
For me, I'm up for a solid capacitor of proper value that will rend the amp reliable so I don't have to worry about it becoming unstable.
I rebuilt my 2 MC-225s back in about 2004/2005 when Auricaps were still reasonable and used Solens for the values I couldn't get at the time in Auricaps. I wouldn't do that again today. My last big-time rebuild of a set of MC-75s used Illinois Caps and Orange Drops for interstage work. I just didn't hear any difference in them and the MC-225s once they were both fully rebuilt.
Hope that helps.
Cheers,
David
For me, I'm up for a solid capacitor of proper value that will rend the amp reliable so I don't have to worry about it becoming unstable.
I rebuilt my 2 MC-225s back in about 2004/2005 when Auricaps were still reasonable and used Solens for the values I couldn't get at the time in Auricaps. I wouldn't do that again today. My last big-time rebuild of a set of MC-75s used Illinois Caps and Orange Drops for interstage work. I just didn't hear any difference in them and the MC-225s once they were both fully rebuilt.
Hope that helps.
Cheers,
David