Cutting Head gets really Hot

[Jccc]

Hello, i noticed my cuttinghead starts to get really hot after just a few minutes of cutting. The level i am sending into it is not distorted when played back on recordings.
Its actually really low!

Ive turned down the cutting level to the point where the recordings are really low to the point where surface noise is almost the same level as the music.

I have a 1 amp fuse inline with the head to protect it.

The amp i am using is a QSC RMX 1450.

The cutting head is a Neumann MS 52 H.

The head i believe is wound for 16 ohms.

And the taps i am using coming out the amp are 8 ohms.

Do you think that might be causing the problem?

Any help or suggestionswould be awesome.

[markrob]

Hi,

How hot are we talking here? Can you hold you finger on it without pulling away within a second or so? If so, then its probably not too hot (< 50C I would guess). I don't see that the amp you specified has any impedance matching setting (it is solid state after all) other than allowing mono bridge mode. In that mode, its rated at 900 Watts into 8 ohms. At 16 ohms, you could expect at least 450 Watts out. That's plenty of reserve power. A 1 amp fuse would be on the verge of popping at about 16 Watts (I^2 x R). That might be on the high side as far as protection as I would think head should be able to handle about 10 watts or so on a continuous basis. It might be a good idea to reduce this to 1/2 A just for safeties sake. I don't know the specs on your head. Have you found any info on that? Are you cutting or embossing? If the levels are really low, you might need to have the head serviced (e.g. re-magnetized). Have you measured the DC resistance of the coil?

Mark

[Jccc]

Thanks for the reply Markrob.

The temprature of the head after 20 minutes of cutting is pretty hot. To the point where i cant leave my finger on there.


At one point before It started acting this way i accidently send a really strong signal into it and quickly the signal.

After that incident it has been acting this way. With the level of the cuts below normal.

The sound is really clear its just seems below what it used to cut.

I posted a sheet that came with the MS 52h in the schematics section.
I dont quite understand what it means but do see that it says 16 ohms.

http://www.lathetrolls.com/viewtopic.php?f=9&t=7104

I was checking the magnet on the head and it still seems strong.
The type of recordings i am doing is cutting not embossing.

[markrob]

Hi,

That sounds too hot to me. Have you monitored the average rms ac voltage your are driving the head with?

If I'm reading correctly, the head has a sensitivity of 1.71 cm/sec per Volt of drive. The frequency is not given, but I assume it in the midrange about 1khz. So to reach 5 cm/sec cutting level, you would need drive the head with about 3 Vrms. Into 16 ohms, that's only about 1/2 watt of power. It would be good to check this by driving the head at this level and doing a cut to see if you get close to the 5cm/sec velocity. You can read the voltage right at the playback pickup terminals. Use the spec for the pickup to get in the ballpark. With a stereo pickup, the velocity will be .707 times the true lateral velocity on each channel (3.54 cm/sec when cut at 5cm/sec lateral). Or, compare to a test record with a known velocity level band. If its way low, the head may need servicing.

Did you measure the DC resistance of the voice coil? It should be under 16 ohms (maybe as low as .

Mark

[markrob]

Hi Again,

I did some searching and found some specs on the head:

MS-52H.jpg

Maybe after all these years the damping grease has dried out. That would reduce the sensitivity quite a bit.

Mark

[Gus]

Hi!
did you try to calibrate the armature?

[Jccc]

Hello! Thank you for all the help and replies.

I checked the resistance of the head and it measures around 4.9 to 5 ohms

I also checked the armature. And realined it and made sure it was straight and used a piece of paper to make a gap on each side.

I checked the grease thats inside thats used for damping and it seems quite gooey.
Maybe i could try taking it all out and putting new grease inside.
The thing is i dont know what i would use to replace it.

[markrob]

Hi,

Was the head always like this or did it just start acting this way?

Mark

[Jccc]

The head was acting good until i made a mistake & did not check the levels of a song i was cutting and sent it a very loud signal. I did not know it was loud until about 3 minutes into the cut.
Thats when i felt the head and it was hot.

Ever since then the head gets hot when i am cutting normal level recordings.

Maybe im just doing something wrong or this Amplifier is just too powerful for it?

Either way, it cuts records but the signal is lower then before and after 10 minutes or so it starts to heat up.

I will try those test that Markrob posted when i get home.

[markrob]

Hi,

Based on what you are saying, I'm thinking the head has a developed short due to overheating. The DC resistance seems a bit low for a 16 ohm head. Maybe somebody here has the same model and can measure. Can you measure to see if there is a short from the metal case of the head to the drive coil? If a short developed, the effective coil length might be much less than spec and therefore low level cuts. I'm thinking you would need to get this re-wound. I think Todd is in your area. He should be able to do this for you.

Mark

[dubcutter89]

Hi!

To me this also sounds like a shorted coil - sorry...
Mine measures around 38 Ohms, and somewhere in the literature I've read 40(?) Ohm DC resistance.
The 16Ohm information is to the old V69 amp which it was used with back then...

The moving iron Neumanns are super simple - one round coil, that's it.
Rewinding is a piece of cake!

Cheers
Lukas

[markrob]

Hi,

40 ohms DCR seems way too high if the head is 16 ohm impedance. Were these heads available with different windings? Is there any other documentation from Humane on these heads?

Mark

[Jccc]

Ok i was checking the dc resistance with 2 different multimeters and i am getting different readings?

image.jpeg

image.jpeg

[markrob]

Hi,

On the analog meter, did you short the two probes and adjust the zero? Is it possible your probes are bad? 125 ohms seems way out of bounds. I'd tend to go with the digital meter. Also, since the digital meter is reading a pretty low value, did you short the two probes and subtract out the probe resistance?

Mark

[Jccc]

Whoops! I forgot about adjusting the probes to zero on the analogue meter.
After adjusting it. It reads around 4 ohms

there probably is a short in the head

[markrob]

Hi,

Did you try checking from the metal case to the drive coil?

Mark

[Jccc]

By checking do you mean, touching one probe to the metal casing and another probe to one of the leads to see if it beeps?

If so it does not beep.

I also checked by touching both probes to the leads coming out the head and it beeps.

[markrob]

Hi,

At least you know its not shorted to the case.

Mark

[dubcutter89]

Hmm,
probably take off the case and look inside?
Does it smell cooked?

Here's the info on the MS52:

[Jccc]

i removed the metal casing and looked inside and everything looks alright from what i see.

I do not smell any burned up smell. i do notice that the plastic material that is around the coil looks a little warped. Maybe from the head heating up.

i can run sound into it and i can hear it. its just after a few minutes of cutting it gets really warm and if i keep cutting then it gets pretty hot.