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Another C-28 S.O.S.

Ricktptman

Ricktptman

High Fidelity Enthusiast
Subscriber
I thought I had this preamp settled down and it was behaving fairly "rehabilitated" until very recently. I'm fairly certain it's the volume pot. I have another one and installed it and it works "kinda sorta". Very odd things happening with the channel balances. I I think I see ground wires run inside the signal wires. I didn't connect them. Was I supposed to? It turned out to be a bigger pain than it appears.

Not sure what to do next.

The weird thing is the problem channel (Left) is the channel that works fine now. The level issues appear to be in the Right channel. The original pot would have a constant level in the left channel with no ability to control it and the right channel would work fine. Is this the same "we hate this replacement part" thing I read about? The pot came from ePay and looks like it's reasonably well made. But as for its impedance or other characteristics, no stats were included with it. I could use a little help, please. I'm in the SE U.S., btw...thanks...
 
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Addendum: The headphone output has lost the right channel as well, but that was a previous symptom. I wanted to address this one first. I've read that many folks have to recap the headphone outs...that's a battle for another day...it started acting up about four months ago. I usually tapped the amplifier's (MC2155) headphone outs and they work fine.
 
DOB called them drains so that is what I call them, that is the wire that runs with the foil shield of the various signal cables.

Unfortunately it was found in the late 70s by engineering that the drain wire and the ferrule crimps that attached the short leads that connect to the PC board grounds would corrode between the 3 metals and become intermittent. It therefore became SOP to solder those connections when ever a C28 came in for service or were brought into a amp clinic. If these are not reflowed who kn\ow how the unit will react......just look at the schematic and follow the ground connections from board to board.

Any and all C28s or MA6100s should have all these reflowed and the value for this time consuming effort priced in to the units selling price.

Slit/ reflow, lift the ground off the PC and add new heatshrink tubing adds time. DOB would often just tape them and have us add the heatshrink later to keep the bench moving along.
 
Well, Dave O'Brien would know best, that's for sure. Do you solder them to the same connection as the core signal wire? I haven't changed anything on the PC board end. But I didn't "reflow" anything anywhere, so there are no "drains". They're all lifted. Not good. I like the preamp and it's been worth the trouble...so far...but that seems like a quirky way to do things. I'll have to probably run new external wires to all of them. There's not much "overage" in terms of wire. That's going to be ugly. Literally. Any additional advice? Thanks, c_dk for the response. Maybe I should sell it as is and try again. But it's so close to being "right". Annoying... : - ) This is definitely a "First World" problem, so that can't be all that awful...Happy New Year to everyone and wish me luck... : - )
 
You need to study the schematic......the drain connections from board to board are shown as well as the ones only attached at the " input" side.


The culprits are under the heat shrink, brass ferrule crimped to the shield wire and the short lead to the circuit board has 3 different metals forming a corrosion cell.

Solder those connections with the ferrule in place and replace the heat shrink......you really only have to do those that are connected on both ends as shown via the schematic.
 
Okay, I just got done taking a second look. I think I actually got those wires. (The red and the brown ones?) They're connected and connected correctly. It's these little bare insulation wires that run the length of the wire with no colored insulation of any kind that were puzzling me. I'm pretty sure I have a Service Manual for it, but I didn't pull it out because I thought this would be fairly straightforward. I think there were about 10 connections on the pot. The pot does reduce and increase volume (slightly) with rotation (non-linearly-it gets louder, softer and then louder again at the end of its travel) and the signal is otherwise fine, is completely separated and reflects the settings for Stereo/R/L/Reverse, etc. I can turn the volume up and down significantly with the output pots on the top. if I open the pot all the way it sounds REALLY good. Better than the first one, maybe. I'll post back what I find. Thanks again c_dk...
 
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My first instinct was correct. The solder job's not as neat now thanks to checking it in order to get an answer to that question, but no, this potentiometer isn't doing what it's supposed to do. But it is consistently delivering matched (and unmolested) audio to both channels consistently. A sideways win. Of sorts. Maybe. The audio is fine in the first few degrees of rotation and "wide open". The On/Off function works perfectly, so that's a step forward if little consolation. I have a pretty high-quality external remote volume control. Look like it's time to solve the problem the common sense way. I don't have to worry about power strips or whether or not it's going to be the same preamp (in terms of output) that I turned off earlier. I have the replaced pot. I guess at this point I could just send the potentiometer out to have it rebuilt. That seems to be the next, least awful solution. I don't hear good things about the replacement Mac potentiometers either...which is a bit concerning...
 
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Just 10 days ago a PEC sourced pot from MAC parts was reported installed into a MAC 1900 with good results.

Yes it is a generic replacement, yes a tech can engineer out the needed mods to place the loudness tap at the correct ratio, so successful repairs are made all the time.

It is also true that complainers get more "air" time than happy clients.
 
Well, that's a better solution than selling it I guess. I like the preamp, but it just got to the point where it was becoming fundamentally unreliable. After getting that ePay pot all the way in, its pedigree began to show. It doesn't tolerate being tightened. (It apparently pulls the shaft out of position.) The Left channel is working more or less correctly, only with some very strange gain differences from the original. Once in and all the panels/screws tightened, it's the Right Channel that's doing odd things. There is a point at almost "wide open" where the balance and tone are "right". (But it changes in its rotation, especially in the Right Channel.) The remote option doesn't work. Maybe it's an impedance mismatch. As you've pointed out, those wires don't like being installed, uninstalled and reinstalled. That's going to be tricky. I only meant to do this once. The original pot was flakier than I knew apparently. I kept cajoling it into working. This is verging on escalating out of my comfort zone. It "works" and it sounds "okay", but I'm gain riding the source components so the C-28 is basically a source switcher. I can only adjust the volume using the amp and the sources within a very narrow range with that volume pot. I've replaced volume pots before in a variety of devices and haven't had these surprises. I'm usually (to have not done it for a living) pretty lucky because I'm super cautious, but I think I've just made a pretty expensive mistake. (At a time when I can't really afford to make one, lol.) I always say pessimism isn't a survival positive attribute. As I said, the preamp was approaching being unusable, so something had to be attempted. I usually don't opt for shipping gear around for obvious reasons, and I'm not crazy about being without it for months or longer. But if I can't get a Mac pot or get it installed without wire issues, it might be better to tap out and seek out a pro. It takes a lot to get me discouraged, but sometimes discretion is the better part of valor. It is at least usable now, but it's not "right". I don't want to leave it that way and I appreciate the advice. I'll call Mac Tomorrow and see if I can get a replacement from them and post back. Happy New Year! : - )
 
Chronic complainers who don't try to mitigate for improving their own circumstances aren't helpful to anyone including themselves. But that descriptive doesn't really fit most of the persons whose accounts I read here about these types of things. At least those that I've found, and I probably haven't read them all. Most just want helpful advice and I don't really think that falls under the rubric of character flaw. On the other hand, if Mac's still at least TRYING to keep replacement parts for this preamp, that's more than my other cherished audio collecting nameplates have done with their Golden Age gear. But on a "third hand", I will say that their gear isn't as fraught with age-related pitfalls. I own three Mac components and they are all similarly "cranky" in terms of being in need of ongoing maintenance. Two out of the three were "attic finds" and have settled down to be fairly reliable. (Unless I count the hyper-fragile power meters in my 2155. They're fine and I've replaced the lamps with LED's, but they make Eggs look like Brass Balls by comparison.) I expect to change things like idlers, belts and the (very) occasional out of spec cap, but as beautiful as it (my C-28) is (and as lovely as it sounds) I wish it wasn't as expensive to maintain as it's turning out to be. I guess these things are the "British Sports Car" of the Audio World. A certain amount of Masochism is required if you decide to own one...lol...
 
The shield/drain crimp is something that shouldn't have happened. Regardless, it happened and now should be a once and done repair. The amount of humidity and general environment of its last 50 years will determine its severity. While you're in there I believe that the input/output jacks should be better grounded to the chassis.

The volume pot is another issue. Some have had regular use with frequent adjustments and simply worn out, others like mine are still happy as the volume pot seldom is turned and I use a smart-power strip to turn everything on so again no need to constantly turn the volume down to the off position. Having a separate power switch would have helped a LOT, but these things do wear out, look at how few Kenwood C2 preamps still have a functioning balance pot and they're half the age of the C28, .. and now often do you turn a balance pot?

I believe that your C28 will be reliable once a half-century of use and possibly neglect from an un-known past has been rectified. Kind of like a nice air-cooled 911, if you never adjusted the valves or changed the oil, ... an otherwise dependable GT is now a mess. A saying among some of us is that there's nothing more expensive than a cheap Mercedes, ... I think that the same can be said of a cheap McIntosh or other vintage high-end audio component. Now that it's old it needs restoration to be dependable (or even perform as it should).

If I were to dispense one piece of advice for the owner of any McIntosh piece with a volume/power switch? Lube it, and put the system on a smart-power strip or other device so that you don't use the pot much.
 
This volume pot issue and the Headphone output are the only two issues it has, and I got several "mostly" good years out of it before they started becoming recurrent. It wasn't abused. The volume pot problem wasn't presenting at all when I bought it. The headphone output was kind of schmutzy from the get-go. One side's out now, but it's not a problem just to use the headphone out on the MC2155. I think DISUSE was the most harmful thing to befall this preamp. There wasn't a lot of rust (another annoyance that other Manufacturers seem to have avoided) and it otherwise generally sounds fine. I just heard back from Mac and the pot's available. So here we go again...wish me luck...
 
Luck wished, ... good luck at that!

Many of the Mc pieces of this era had the film-pots fail, it was probably just not robust enough to make the mileage and the film wears out. I do agree that non-use can cause problems, especially when a long-stored piece comes out and the owner dusts it off / plugs it in to see if it works.

Let us know how the Mc replacement pot works. If you're happy with it I will likely order one as a spare, ... never know when but it probably is when not if.
 
There are 10 connections to the volume control....2 under a insulated metal cap for power, 2 to reference ground at high level section and the balance pot, 2 input from the balance pot wiper, 2 from the volume wiper to the hi level and lastly 2 from the loudness tap (single opposite the 3). Originally this was tapped at 60/40, the replacement is at 50/50.

This loudness tap effects the taper of the control and the amount of loudness compensation added when you switch the loudness in.

When I install these I add resistance to the input (and/or output) sides to balance out the 50/50 to 60/40 and bring the two tracks up to 1 meg.

It is a pain, sure it was easier 30 years ago when Mac parts stocked 4 replacement pots but at least they offer something with a loudness tap and the power switch.

It does cost more time to do it right.

Is the ebay purchased control a PEC sourced control? At one time a picture was posted that showed it to be a PEC control that was the same a sourced from MAC parts.

Has the OP attached and repaired the ground drains?

Any C28 or MA6100 should be purchased only when the status of this needed restoration has been done and the price adjusted accordingly. If these are not done all kinds of strange behavior can be expected.
 
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Now that information is really helpful. It's good to know what does what. I had the AC side figured out, but the signal and loudness contour taps were not obvious, so I appreciate that information. I said there were 10 connections earlier, but I didn't know which did what. The C-28 pricing was such that I felt it was affordable and as yet, I don't regret it, but if I do, I'm not likely to throw good money after bad. This pretty much pro-inflationary recurrence in your postings says little about how to economically address my issue. It DID work when I received it, but as I said earlier, I think the main issue was it might have spent years in storage unused. Please know I appreciate your advice, but the proselytizing for spending as much as possible isn't as helpful as actual aid in how to OTHERWISE move forward. I likely will install the Mac sourced pot and hope for the best. Just sayin'. I think I might have a source for someone local to help me with it, but if it's an "I'll be happy to take a look at it two years from now" prospect then I'll probably reassess. That is if this Mac potentiometer doesn't work significantly better than the online part did. I hope it will. I read that the C-32 has ribbon cable issues so I'm not certain that's the next best option if I decide to move the C-28 along. I'm researching to find something with better legs for the long haul, but yes, with a MINIMUM of additional outlay. I'm going to try using ground isolation on that remote volume control and see if it helps.
 
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The only proselytizing I am trying to get across is that these 40+ year old units need a certain amount of service to get them back up to their original performance.

The typical c28 needs 4-7 hours of tech time at the professional service rates your local area demands. A purchaser should know this up front and look for proof of the work being done or discount the offer appropriately.

A proper restoration of these units should progress in a systematic nature, jumping all over the place is not productive.

Shield/drain/ ground issues must be dealt with first because without a reference ground anything can happen.

Volume controls can be measured (out of circuit) to check their taper and channel balance.

The most common failure I have seen is large portions of the carbon track flaking off as shown in the pictures I posted elsewhere. I assume this was caused by improper solvents used for cleaning.

As usual buyer beware.
 
A couple of follow-up findings that I hope will help the "DIY'er's" here.
1. The Mac pot didn't solve the one remaining (not counting the Headphone Right Channel being out) problem, but it did improve the performance dramatically.
2. The ePay pot is actually not too bad by way of comparison.
3. The Mac pot is by far the largest of the three, and it causes the most problems with installation. I had to add wire to three connections, which I hate doing on principle. (Further, the protrusion that should line up with the hole in the chassis does nothing of the sort. I had to bend it flat.)
4. The Mac pot shipped without a hex nut. Fortunately, the ePay pot's nut worked just fine. The original nut and plate are MUCH smaller and I have it in storage with the original pot. For the ask, that's really inexcusable.
5. The Mac pot doesn't sound significantly better than the ePay pot at its ideal rotation point, but the Mac pots performance (this was a surprise) was much more consistent throughout its rotation. Which flies in the face of what I've read about it. But again, that's compared to the ePay pot, not the original, which worked just enough for me to get a baseline.
6. If I have to rip out the signal wires and rewire it (it'll likely be necessary) I'd like to know what to do with those ground wires. (Those are not the Red and Brown signal wires. Everything is connected as I found it, but that doesn't mean it was right in the first place-someone had been inside it when I bought it.) If they're supposed to go somewhere on the "pot" end, I'm pretty certain they weren't connected when I first dismounted the original pot.
7. The upshot is it's slightly better than it was before. But still not "100%". It sounds great at minimum gain and Maximum gain-like before, just with fewer changes in tone throughout the rotation.
8. Just to reiterate, I solved the remote volume control issue, so at worst, it's far more reliable (and the channels match very closely) so I'm in better shape than I was when I purchased it. I now know that it was an original pot and not a replacement. Recapping the headphone amp board should put it back to full functionality. (Again, not counting this current drain mystery ground connection issue.) The Power Supply caps are cool to the touch and there is no hum, so I'm not certain how profitable it's going to be to tackle that. In various threads about recapping the Power Supply, nobody's posted "after" photographs. I would probably elect to just put the caps on the underside of the amp. The question is how (exactly) to tie the caps together. (The schematics are pretty sketchy on details.)


I'm going to move on to recapping the headphone amp. I renewed the solder joints and it's not a broken connection. (I did find several intermittents in this pot adventure, though.)
Any helpful hints (about any of the two remaining issues) would be very much appreciated. Thanks to those who've already posted It did help for me to stop and think outside of my own narrative. AK's good for that...

Best to everyone...
 
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