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Old 27th March 2011, 03:46 PM   #11
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You beat me to it.
That would be a better approach than hacking an exteremly rare vintage piece. jer
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Old 28th March 2011, 09:59 AM   #12
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The thing always was hissy; it's just that, when it came out (in '66, I believe; the year I first saw a moog synthesizer) everything hissed: Fender Rhodes, clavinet, tape machines, guitar amps, perhaps not the Hammond organ, but that rattled. So it went unnoticed. The basic idea of running that many magnetic playback heads in series, and amplifying the lot at once almost guarantees that.

Unfortunately I gave all the schematics away when we sold the studio instrument; and it was a Novatron, anyway, but I think any appreciable improvement in noise was due to the removal of the capstan speed servo information from the audio electronics. The Mellotron preamp picked up radio signals, and they made extra noise; ours was screened with aluminium foil, over just about all its surface (with a grounding wire from the body to the removable cover.

It would be possible, as the mellotron has no envelopes or anything. to build a mechanical detecor if any key were depressed, and mute the output otherwise. Or even put a good, rapid noise gate or dynamic noise filter on the output, thus leaving the instrument unmodded. Does the mechanical noise of the motor turning and the tape slithering back when you release it not bother you?

If you absolutely had to record your own Mellotron tracks there was a kit that stepped the tape down from three eighths to quarter inch, and you could record two tracks (starting at precisely the same moment, not that easy back in analogue days) on a standard quarter inch transport, at 19 cm/sec (seven and a half inches per second), then cut to length, mar start, install tape rack in instrument, mark start point with grease pencil, adjust start point to exactly over the heads for each note…
Modern samplers are slightly easier to use, as well as not dropping out of tune if you play more than a three note chord.
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Old 28th March 2011, 05:16 PM   #13
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I've thought about the idea of just replacing the transistors- but the tinker side of me is thinking about fets or opamps, something that might be a bit better sounding. I'd make a tube circuit for it (Streetly has one) but I don't need this thing getting any heavier than it already is- I actually play gigs with it.

FWIW I have the new speed controller. You can pretty well sit on the keyboard without it slowing down.

The noise I have is irregular so its obvious that a transistor has become noisy. I suspect that the input has an impedance mismatch on account of the series heads. So I have thought about an FET input...

Last edited by atmasphere; 28th March 2011 at 05:20 PM.
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Old 28th March 2011, 05:38 PM   #14
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Have you tried using cold spray to maybe find the culprit?

When a noisey transistor gets froze, usually, it will become quiet and then sometimes crackle and pop, then gets noisey again as it warms up .

Sometimes it will actualy fix the transitor for a while, but it will eventualy become noisey again. jer

Last edited by geraldfryjr; 28th March 2011 at 05:46 PM.
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Old 28th March 2011, 05:46 PM   #15
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Its not responsive to temperature.
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Old 28th March 2011, 06:07 PM   #16
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Cold spray Is in the order of -40c.
I hate intermitent issues as they are the worst diagnose.

Another trick is to take a small plastic rod or toothpick (something rigid and doesn't conduct) and gently poke and scrape at the boards and parts on the board.

This will help to find a bad solder connection or some times a intermitent part.
I have found bad caps this way especialy electrolytics,they can get noisey aswell. jer

Last edited by geraldfryjr; 28th March 2011 at 06:17 PM.
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Old 28th March 2011, 08:16 PM   #17
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This issue is not intermittent. Its noisy all the time. I've already replaced most of the resistors with precision MF types, and I even tested the transistors by heating them with a solder iron (works as well as freezing them, IMO). The power supplies are rebuilt. All the electrolytics in the circuit are new.
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Old 28th March 2011, 10:11 PM   #18
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I guess the thing to do is to use a scope and start at a tape head and work your way back through the stages and try to find at which point the noise starts to get considerably higher and try a new lower noise transistor at that point on one preamp board and compare it to a unmodified preamp section.

I realize how fustrating it is without a schematic.

I know that this is a long shot but can you get a preamp board out and take some quality closeups of it to post?

Have tried contacting streetly or one of those other guys that claim that they do work on the mellotron?

I searched the best I could do but I will keep trying for you.

I am pretty good at reading boards. jer

P.S. can you post the transistor numbers?
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Old 28th March 2011, 10:37 PM   #19
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I'm pretty sure I can sub the transistors easily enough. Of course one plan was just to go through the equalizer board and replace the transistors en masse. What is happening here is that in addition to wanting to fix it, I also want to make it better- and I don't think that is so unreasonable since transistors have come a long way since the Mellotron was first built. But I am to update it with newer parts (FETs for example) I am going to need the schematic.

I can trace it off the board and was hoping that I would not have to do that but it looks like that may be the best shot.

Streetly does have support for the older machines despite not showing it on their website. They are the ones I got the new speed controller from.
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Old 29th March 2011, 01:23 AM   #20
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How many tape head channels are there?
Because I could use the schematic from either my tascam 38 or MSR-16 and make a pattern of a board for you.
And all you would have to do is print it off on a laser printer iron it to some pcb material etch it, drill it, populate it and wire it in. jer
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