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+  MCI Console and Tape Machine Forum
|-+  MCI Tape Machines
| |-+  JH-110 (Moderator: brianroth)
| | |-+  110B/C Record Headroom
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Author Topic: 110B/C Record Headroom  (Read 341 times)
TonesOnTail
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« on: May 25, 2018, 02:22:34 PM »

Another long shot, I know.... Does anyone else remember the topic on the forum pre data loss on component changes to increase record headroom? I can't remember the forum member or details, but would be interested in revisiting this if someone does.

Thanks
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'75 MCI JH16-24 w/ JH114 xport ----- '84 Sony/MCI JH110C-HP 1/4" 2trk ----- '82 MCI JH636-VU-AF
brianroth
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« Reply #1 on: May 27, 2018, 11:30:37 AM »

I recall the discussion, but not the specifics.  There was a lot of "back and forth".

Bri

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W1DAN
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« Reply #2 on: May 30, 2018, 02:07:16 PM »

Hi:

I remember something about it. The person posting stated he was able to get more signal on tape by reducing a series resistor feeding the record head. I do not know what tape he was using, or the reference flux.

As the JH110B/C has good record power to feed hotter +9dB tapes such as Ampex 499, I am not sure this modification is necessary.

Dan
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Fenris
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« Reply #3 on: October 21, 2018, 01:26:16 AM »

The audio electronics of the JH24 and the JH110B/C are largely copied from the Studer A800. For some reason lost to history, MCI doubled the value of the two series resistors after the record amp. This results in half the voltage and 6 dB less output.

The JH110C has two resistors in series, one on the record card and one on the motherboard. I bypassed the resistor on the record card, R46, and left the resistor on the motherboard in place.

On the stock machine, 1" 8-track, aligned for Quantegy 499 at +9/185, 15 ips. IEQ equalization, the record amp has approximately 6 dB of headroom. The mod increases it to approximately 12 dB. You should check the output of the record amp with an oscilloscope before and after the mod, determine the clipping point, and make sure you get the same results.

Wider trackwidths such as 1/2" 2-track are more efficient and the headroom is much greater.

Based on information from John French, the record head can handle the extra currrent with a comfortable margin. However, nobody really knows the specs for original MCI heads or whether they all use the same wire gauge. This mod should only be done if you're a qualified tech, you're not happy with the performance of the machine, and you're willing to risk the possibility of burning out the record head.

Some people will dispute this information. Anyone who is interested can do the relevant measurements on their own machine in about five minutes.
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W1DAN
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« Reply #4 on: October 25, 2018, 05:10:47 PM »

Hi Fenris:

Good info, thanks. I am not concerned about record headroom as I use my JH110B as a player. But for education, I am following your efforts.  My only concern would be the possibility of opening the record head winding with too much current, which you stated that Mr. French says you are safe. So much the better!

I would assume that the Woelke JH110B and C record heads have the same construction (I bet they are the same part number), and thus the same DC resistance and AC inductance? With the Studer, do their heads have similar inductance and resistance values as the Woelke ones?

When you now set your new standard operating record level, do you set to a particular third harmonic distortion level on tape? Is tape headroom affected with the higher recorded flux level?

I have seen a patent where someone takes a current sample from the record path and sends that back to the driving amp for negative feedback to help linearity a bit. They key here is your bias signal has to pass through the amp.

Maximum flux ahead!

Thanks,
Dan
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Fenris
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« Reply #5 on: October 26, 2018, 06:08:17 AM »

After doing the mod I recalibrated the EQ, bias, and record level so it's still recording at +9/185. The purpose of the mod is to eliminate clipping on the transients. The rest of the circuit has plenty of headroom.

I don't know the specs of Studer record heads, but the impedance is swamped by the loading resistors anyway. The efficiency (relationship between voltage drive and nW/m) is more relevant. It's probably about the same as MCI heads, but I don't have any data.

I imagine the headroom was less of an issue in the 80's when most machines were 24-track, +9 tape didn't exist, and most people used NR and conservative recording levels.
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