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December 15, 2018, 12:00:07 PM
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News: In order for you to be able to post a new thread in the "For Sale" board, you need at least 10 posts in the rest of the forum, however you'll be able to reply to any topic in the forum even if you dont have the 10 post count. Any posts in the "For Sale" board will not appear in your total post counter.

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+  MCI Console and Tape Machine Forum
|-+  Recent Posts
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 on: October 26, 2018, 06:08:17 AM 
Started by TonesOnTail - Last post by Fenris
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.

 on: October 25, 2018, 05:10:47 PM 
Started by TonesOnTail - Last post by W1DAN
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!


 on: October 21, 2018, 01:26:16 AM 
Started by TonesOnTail - Last post by Fenris
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.

 on: October 20, 2018, 01:28:17 PM 
Started by Berwyn636 - Last post by Berwyn636
Anyone know how many 652s were made/sold?

 on: September 28, 2018, 12:22:31 PM 
Started by Berwyn636 - Last post by Westrek
Check the soldering on the male molex pins on the motherboard underneath the problem channel(s) that lead to the patch bay. EQ patch I think it's marked. That will break the signal flow.

 on: August 30, 2018, 03:18:20 PM 
Started by Berwyn636 - Last post by Rob
There is a story of a 500 series that had its frame bent while moving it round a corner before installing.   This meant that the middle channels did not engage with the motherboard connections.    I believe the remedy was jacking it up in the middle with a  bottle jack to gently bend the frame up so connecton was made for these channels ...........

 on: August 26, 2018, 05:42:43 PM 
Started by Berwyn636 - Last post by brianroth
With the desk I remember with intermittent channels (they were all in low numbered positions), I removed a bunch of the higher numbered modules so I could get a strong light and my head down into the frame to observe what was going on. 

The visual inspection revealed that the mother board connectors were not mating well with the channel strips.  It took some shimming to move the motherboard into a good position for reliable mating with the modules.


 on: August 26, 2018, 01:43:50 PM 
Started by Berwyn636 - Last post by Berwyn636
they appear random then sometimes they are next to one another

 on: August 24, 2018, 12:43:11 PM 
Started by Berwyn636 - Last post by brianroth
Are the intermittent channels randomly located across the desk, or are they in the vicinity of each other?


 on: August 24, 2018, 03:07:31 AM 
Started by Berwyn636 - Last post by Berwyn636
just when you push down on some channels they go in and out so I go through and wiggle an push on the strips til they seem like they're staying live then bolt em down but i still get intermittent dropouts depending on vibrations and what not.

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