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Mc240 hum problem

K

KarlFrey

New Member
Hello I am a new member and I’ve been very grateful to the many posts all of you have on here that have helped me over the years.

I have a MC 240, two of them actually, both bridged mono. One of them has hum, but only when the selector switch is in mono. Becomes less in twin mode and still less in stereo mode. Touching the chassis reduces it as well. When turned down it’s silent. The pot seems to be scratchy, but it’s an Alps pot and when taken apart looks and measures perfect. It dramatically changes volume near the end of the taper.

This is all in the context of a recap with Russian PIO K40y-9 caps. They were replacing some Jensen caps I put in 20 years ago some of which were leaking. None of these Russian caps showed leakage. No resistors are out of spec. I also cleaned the stereo-twin-mono switch.

Thanks in advance. Any help would be appreciated

regards,
Karl
 
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The first thing I’d try if it has the original non-polarized wall-plug, is to unplug the amp from power - rotate the wall plug 180* - plug it back in.
 
I tried that. It didn’t do anything. I once again tried to verify what I was saying about how it seems to be a little quieter on the twin setting and quieter still on the stereo setting and that is the case, but there still is a little bit of hum even in those positions that goes away when I touch the chassis. Must be a grounding problem right?

in addition to film caps getting changed out I also changed the voltage doubler cans, and used those four prong Mundorf cans, those ones that have 2 input, output, pos, neg(a little confusing to me), but I installed them the same in both amps. I soldered one of the ground pins on one of the cans directly to the chassis and even used some steel wool to rough up the solder point before soldering.

I even swapped tubes though I didn’t think it made sense that it would only affect the mono setting. It had no effect.

Karl
 
In my experience that selector switch is a signal 'weak link' in the 240's. Whenever (not if) one of my 240'2 start to hum I get a little aggressive with that switch to clean the contact points and it eliminates the problem (until next time). I've been tempted to hardwire that switch to the position I use 100% of the time but that strikes me as sacrilege so I haven't done so yet. Hope that helps.

Jamesspud
 
JamesSpud said:
In my experience that selector switch is a signal 'weak link' in the 240's. Whenever (not if) one of my 240'2 start to hum I get a little aggressive with that switch to clean the contact points and it eliminates the problem (until next time). I've been tempted to hardwire that switch to the position I use 100% of the time but that strikes me as sacrilege so I haven't done so yet. Hope that helps.

Jamesspud
Click to expand...
The front end of this model has too much "stuff" for its own good, and the swedged-in sockets and modern grippy cable ends don't help matters.
 
It really does seem like the switch, or I thought maybe the pot, but tried swapping that and nothing. I tried deoxing it, but didn't go so far to take it out, take wires off it and thoroughly clean it. I hardwired the speaker outs because I bridged it mono. Probably no reason I couldn't just hard wire it. I actually have 3 of these. One was my father's. That one I don't mess with. It's all original except for what audio classics repaired. These other 2 have been violated quite a bit, but they really sound good. Except the damn hum on only one of them.

Thanks much for the reply.

Regards,
Karl
 
It's hard to diagnose an amplifier that has been modified, were it mine I'd return it to original as the first step.

You say it is wired "bridged mono", which means inverting one of the signals and a significant modification to the signal path. This is where I would look first.
 
The other point is that while Sat's definition of "Bridged" operation is technically correct, the term is often misused today to simply mean paralleling the two channels of a stereo amplifier. This approach requires connecting an 8Ω load to two paralleled 16Ω outputs, for twice the power output of a single channel at 8Ω.

The original concept of bridging is as Sat described, with the signal to one channel inverted, and then an (say 8Ω) load connected between the 8Ω terminals of the two amplifiers. Such a concept actually quadruples the power output if the amplifiers are willing (i.e., the power supply is willing), but the concept is typically more valid for use with SS amplifiers. Using this approach with the MC240, the best approach would be to provide the inverted signal to one channel, connect the two Common outputs together, and then connect an 8Ω load between the two 4Ω output terminals. This approach again doubles the power output of a single channel, offering no benefit over the parallel approach described above, other than offering other load matching options that the parallel arrangement won't accommodate. The downside is the possible signal degradation that the phase inverter circuit might impose.

However, the MC240 (and at least the original MC275) do not employ a grounded Common output, which presents a third possibility. These particular amplifiers could have their channels combined by applying a common phase signal to both amplifiers as with the parallel arrangement, but then the outputs are wired in series, although with a somewhat different arrangement to that of the inverter arrangement: For example, the Common output connection of one channel could be connected to the 4Ω output of the other channel, and then an 8Ω load connected between the remaining Common output of the one channel, and the remaining 4Ω output of the other channel. This too doubles the power output of a single channel, offering the additional load options that the inverted signal approach offers, but without any potential signal degradation from an inverter circuit, since none is required in this scenario.

So with the MC240, there are 3 ways to interconnect the two channels for the doubling of power output depending on the loading requirements you have. But the important point relative to your thread, is that in doing so, you are also interconnecting the unique hum components of the two channels, so that no doubt, different positions of the input switch may in fact change the hum character at the output depending on how the channels have been combined. As a result, it may not be the input switch causing the hum issues at all -- or, it could be, but the only way you'll know for sure is to disconnect the two channels so that they can once again operate as a stereo pair, and then assess the hum from each channel individually for a true assessment of any hum contributed by the input switch when the channels are combined.

Dave
 
Thanks much Dave. You guys obviously have extensive knowledge about amplifier design. My knowledge base is very small. I simply tied the two eight ohm speaker outs together and the commons together for a 4ohm speaker load. I did that over 20 years ago and it worked very well for a long time. I guess I have to try separating them again, though in truth it’s quiet when I have something plugged in the input.

To the other suggestions I did try bypassing the switch and it didn’t change anything.
 
One other thing I noticed. When I’m testing voltages, bringing my pos probe near the input section becomes more noisy the closer I get. If I touch the .1 µF input capacitors with the probe it gets very noisy.
 
It was common knowledge “SOP” back in the days when this was new, but not anymore.

One side of the plug is hot, the other neutral (which is connected in the house to ground) of your outlet. Inside the amp there is a cap-coupled relationship between the grounds and one side of the power plug, and it is best to have that side be plugged into the neutral side of the outlet. Also, your preamp (depending on what you’re using) might also be built this way and therefore sometimes you have one plugged in one way, the other unit the other way, and this causes conflicts.

On my amplifiers I have installed polarized plugs (the wider blade is neutral) so that I always plug them in the way that the fuse is on the hot side and the chassis is coupled to the neutral side. On yours you can use a paint marker or nail-polish to mark on the plug so that you know which side to plug into the neutral (NEC “always” uses white as the wire color for neutral, so a white mark for instance on the neutral side, or maybe red nail polish on the hot side will help you to always plug it in the right way(.

Also note that there is a smooth and ribbed side of the cord, you can open the bottom to see which wire goes to the fuse and plug that side into the hot conductor, which is likely how you have it plugged in now).
 
KarlFrey said:
Hello I am a new member and I’ve been very grateful to the many posts all of you have on here that have helped me over the years.

I have a MC 240, two of them actually, both bridged mono. One of them has hum, but only when the selector switch is in mono. Becomes less in twin mode and still less in stereo mode. Touching the chassis reduces it as well. When turned down it’s silent. The pot seems to be scratchy, but it’s an Alps pot and when taken apart looks and measures perfect. It dramatically changes volume near the end of the taper.

This is all in the context of a recap with Russian PIO K40y-9 caps. They were replacing some Jensen caps I put in 20 years ago some of which were leaking. None of these Russian caps showed leakage. No resistors are out of spec. I also cleaned the stereo-twin-mono switch.

Thanks in advance. Any help would be appreciated

regards,
Karl
Click to expand...
you may have a bad ground connection. Can you determine if the hum is 60hrz or 120 hrz?

that would give more of clue as to possible problems.
 
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