Ampex MM1100 - 60cycle hum

[dawsonaudio]

I'm having an issue with my MM1100.

Its been a few months since using the machine. I've had it on for the last week or so running tape every day for a little bit. I'm noticing a 60 cycle hum when tape is threaded on and the end of tape arm is engaged against the tape in the "ready to play/record" position. When I swing the end of tape arm to the right/off the tape, the hum goes away. Seems like the motors are causing the hum...I've checked around inside the machine and couldn't figure out the source.

Any ideas?

Thanks,
Nate

 

[Ike Zimbel]

Been ages since I've seen one of these machines...is there a head shield, and if so, is it maybe not in place when you hear the hum? Another possibility is that something that is normally grounded has lost its ground connection, possibly due to one or more screws loosening up or falling out altogether. Try pushing the head block against the top plate when you hear the hum and see if it changes. ALSO: Make sure the AC wiring is still correct and safe!!! FIRST!!!!

 

[radardoug]

Are you hearing the hum in reproduce through your monitors? Do you have the head shield up?

 

[dawsonaudio]

Yeah there's a head shield. Flipping that reduces/ eliminates other stray noises, but doesn't change the 60hz hum present.

I checked the input wiring and made sure all of the a/c connections were tight in the machine. I didn't checked the power supply because I'm pretty sure that would present a different frequency like 120hz. I tested all of the voltages/dc and they are spot on.

I'll check the head block and make sure it's secured down. I'll also do some more snooping around for potential loose ground lugs/screw connections.

Thanks for the advice.

 

[dawsonaudio]

Here's a clip clip of channels 1-8 of the Ampex MM1100 from a pro tools session. I recorded them to a mono audio track. The deck is in repro/normal and the end of tape arm engaged onto the tape. The head shield is up.

First couple seconds are the deck stopped, then playback of the 8 tracks, then tape stopped for a few more seconds. I should say that I did increase the gain of all 8 tracks in pro tools by 12db so it would be more audible playing of the internet.

Also the noise isn't extremely audible in a mix with just one track of audio. It's the build up of all the tracks when it's noticeable. Some channels have more hum than other it seems, 3 and 4, 7 and 8. These channels are in the card cages just below the take-up reel so not sure if that has anything to do with the 60 cycle hum?

There is also some other noise going on at 180hz and 300hz...but that is much quieter and not too noticeable.

Audio clip from tape deck

 

[radardoug]

So the noise is 7 dB below the level when tape is playing. You need to do actual signal to noise measurements. Bear in mind its tape, signal to noise will be around 60 dB best case. Also, what noise level happens in sync and input.

 

[dawsonaudio]

Did a quick alignment of the first 8 channels of my deck. I've got some rmgi 911 threaded. Repro is on average around -55 db. Only option at the moment to read the output of the deck is in Pro Tools on the various analyzer/scope plugins. The normal tape noise is hovering around -58 db and the 60hz is +3db louder at around -55db.

I forgot to align the sel sync so don't have those numbers, but the noise is hovering around -49 db.

Switching to input mode drops the noise down to -78 db.

Hope I'm measuring correctly. I should also say that I'm not feeding any signal into the tape. Only connections are the output feeding into convertor.

 

[radardoug]

So what flux level are you recording at? Sounds like you are in the ball park for signal to noise. On the recording you supplied, the off tape S/n was 7 dB less than the stopped noise level, so tape noise is more than the hum. Try disconnecting all inputs and outputs to the machine. Then using only one channel connect XLR 2 and 3 to your converter, and leave pin 1 disconnected. Is the hum gone?

 

[dawsonaudio]

Flux level...I'm recording at +6 on 456 tape. My alignment tape is an MRL NAB 355 nWb/m tape.

Hum still present with pin 1/shield disconnected.

 

[Ike Zimbel]

You said: "I checked the input wiring and made sure all of the a/c connections were tight in the machine. I didn't checked the power supply because I'm pretty sure that would present a different frequency like 120hz. I tested all of the voltages/dc and they are spot on."
That's mostly true, but...if the 0v connection to electrical/chassis ground in the power supply circuit is failing I believe you would see that at 60Hz.

 

[radardoug]

So you dont have a ground loop. Thats good. To do s/n test, record some tone at the 3% distortion point on 1 track, thats about 10 dB over your VU zero. Put that in a file, then record without input for a bit. Machine in repro the whole time. Then stop the tape and put some of that in the file.
Then put the file on here.

 

[dawsonaudio]

Here is the audio clip.

 

[radardoug]

i cant seem to do anything with those files. Also, the reason for the 1K is to provide the zero reference for the other signals, so ideally its all in one file.

 

[dawsonaudio]

Just re-uploaded one file. Should play now. Sorry about that.

 

[radardoug]

OK, so, s/n to tape running is -30, and to tape stopped is -48. So your hum level is well down on the tape noise. But There is a lot of noise on that tape by itself. I suspect your machine is not correctly lined up. Also, the tone does not look like it is at the 3% point, its much lower, because the waveform is too clean. When you put your test tape on, where is the reference tone on the VU meters?

 

[dawsonaudio]

I set vu meters to 0 when doing the repro alignment.

Yes , channel 1 on that roll of tape has some extra noise. I’ll move to a better channel next go.

 

[dawsonaudio]

Re-aligned my tape machine. Set tone/signal to 3.88vac, which I think is +10 db over 0vu. I should say that I'm using my pro tools signal generator and a volt meter to check reference tone and level. Don't have my outboard tone generator at the moment. Also, recorded to a better track on the tape I've got at the moment.

Thanks for your assistance here.

 

[radardoug]

That tape you are using is pretty bad.

See the above from Audacity. The highlighted section is amplified 27 dB. Your signal to noise is set by the amount of noise coming off the tape. As you can see the background noise level is being affected by the large noise spikes. If this is old 456, you need to throw it away. Also note there are various ways to set bias. The conventional way is by x dB overbias, but sometimes its better to tweak bias to get rid of 'bias rocks' which this tape has heaps of. To do this, record 30 Hz, and listen to the tape output. Adjust bias to minimise the noise spikes. But in my opinion, that tape is shot. Your overall s/n to the amplifier noise is still 47 dB. On the 1200 there was a lot of mods done and one of those was to reduce repro noise. Its a SPARS paper. May or may not apply to the 1100.

 

[radardoug]

Just thinking about it, on the original file you supplied there is a bit at the very front where the tone actually drops from a peak. Have you got some compression going on on this system? If so, there is your missing signal to noise.

 

[pucho812]

my advice is to get new tape.
How old is the mrl? How is it stored?
A 355 nanowebber is a +6/185 alignment tape. So any repro, sel-sync playback you want of a +6 would have you do vu meters to 0VU.
From there on the record side you should have a 1.23vac sine wave. 1.23v = +4dBu = 0VU
I suspect your alignment is off.
I also suspect you need new recording tape.
Of the 2 available brands both have their issues but they are not bad. I do prefer atr brand tape.