Ampex MM1100 Hum

I'm having a problem with a hum coming from my Ampex MM1100/16track.
The 60hz hum is approximate, but I think pretty close to what I'm hearing.

Here's a real quick (1 minute) test to describe what's happening:

With no tape on the heads and with it powered on of course, I took the arm (under the supply reel) and switched it all the way right so the reels don't spin.  I then took all the channel toggles and switched them to ready.  In Normal Mode, I here the hum and here popping when switching the sel sync/repro toggle.  In Input mode, with all the toggles still in ready mode, there is no popping or humming at all.  With all the channel toggles switched to safe, there is hum in the input and normal mode, but no popping when switching and also hum when switching the sel sync/repro toggle and popping as well.

All of the electrolytics on the channel cards have been replaced, but that's it.

And I thought that the problem was isolated to a few channels, but when switching channels from ready to safe mode and back, the problem got gradually worse and better. So I don't think it's a problem with a specific channel. 

One other thing, when the hum is there, it gets at least 2-3 time louder when I engage the supply reel by taking the arm and releasing it to the left and the reels spinning away.

With the tape on the heads normally, the situation was the same.

Thanks for any help.  I also sent this over to the ampex list for some help as well.
Nate

when was the last time the PSU was recapped?

Ummmmm... -does the head guard make it any quieter?

when was the last time the PSU was recapped?

Click to expand...

If it's quiet in input, your PSU should be in the clear. -In addition, the hum would be 120Hz, not 60Hz.

Airtech, -how long have you been using analog tape machines...?

Keith

I'm not an expert on the machine, but I'm pretty confident on how to set it up and use it and also
general maintenance, etc...

I've been using this machine for about 5 years(none before).  This is a new hum that wasn't there a few months ago.
I use the machine for my own projects and have been using it for the last few months as a mixing tool
from my pro tools sessions, just routing the mix through the electronics.

All my tracks were recorded to it originally with no hum.  So not sure what happened.

60hz was only a guess as I took my signal generator and tried to match the tone.

The power supply has never been recapped.



Nate

Hi Nate,

have you confirmed correct voltages on your power supplies? the 15/27 and 39 volt test points on the back at the bottom of the machine. If the reels are operating correctly, I'm assuming the MDA is OK.

Also, I'm a little confused when you say the "arm under the supply reel". Do you mean the end of tape switch located by the take up reel?

byron

I'm a novice here so forgive my layman terminology.

The "arm" is if you are standing at the front of the machine, the reel on the top right (takeup reel) has an arm that
extends from underneath the reel that when I load the tape on, I take it to the right and the reels stop.
Then I swing it to the left and it rest along the tape.  I called it an arm b/c I wasn't sure the technical
name for it.  Maybe it's call the "end of tape arm."

I'll look for the voltage points.  I've got the manual and will take a look through. 

Thanks,
Nate

Yeah, the voltages are good, 39/27/15 at the 3 points.

Maybe a capacitor going bad? Is there a big run or start cap for those motors? Not real sure here.  Something's spilling over into the audio path though on all channels.

Maybe the coupling capictors on the card cages.  I changed those when I recapped my 440.

Thanks again,
Nate

So the hum I'm pretty sure is 60hz.  I recorded into pro tools and used a frequency tool, not sure how precise, but
it peaked at 60 hz.  I also took off a random xlr out on the back of the ampex, plugged in my meter and took some readings.

Without the hum, -67db/0.32mvAC - Input Monitor
With the hum, -41db/6.6mvAC - Normal Monitor
All channels in Ready Mode for both.

Well, if you don't get a better response from someone more knowledgeable........

what I might do is try to isolate the issue, first by removing the MDA, you should still be able to pass signal. If the problem goes away then I might suspect C6/C7.

I might try isolating the electronic assemblies by removing the bias/record/repro cards from all 4 channels of one assembly at a time in an attempt isolate the hum.

Sorry I don't have a more enlightened idea but I would think if caps were failing in the power supplies that your voltages would be off. Of course, if you can test the caps, removing the power supplies is very easy.

how many tracks is your mm1100?

byron

If the hum shows a peak at 60Hz and not 120Hz then the problem is probably not the caps in the power supply. My guess is that a ground connection someplace in the machine has gotten loose or its solder connection has gone wonky. I'd start, I think, by looking at all of the connections in the power supplies carefully, including connections to chassis.

Peace,
Paul

The machine is a 2inch/16track.

I'll take a look around at all the grounds I can find and see if they are connected well.

Thanks again,
Nate

Well, I'm out of luck trying to find the hum.  Unplugged all the xlr's going in and out to isolate the machine from everthing else and that didn't helped.  Tried different electrical circuits thinking the electrical ground might be an issue, but that didn't help.  I took off the side doors and took off the back bottom panel to access the power supply, but couldn't find any grounding points or for that matter, any connections that seemed loose or disconnected.  I'll keep looking around some more.

Nate