Hi everyone
Ok I finally had some time to look at my misbehaving mic pre, and
now I'm even more confused.
So after digging in a bit deeper this is what I've discovered.
Fist problem distorted noise with no input (peak light flashing)
replacing the TL074 seemed to make a slight improvement. But the
signal was still distorted but not as bad. So I reflowed some
solder, I replaced the 100 ohm resistors feeding the chip, and the
ones feeding the transistors, no change. Then I swapped the
transistors from a spare channel no change. This was about the
time I realized that the gain pot was not affecting the signal. It
seemed to have fixed gain (It may have been this way I don't think
swapping the transistors did this). By turning the pot its full
range it only mad a slight change in sound the last 10% of its
travel the sound got darker and that was all that i could hear
changing. All of my testing has been with me talking into a 57 so
not very scientific, and I may not be hearing something more obvious.
Looking at this circuit is confusing me so I think I need someone
with some real knowledge to help guide away from replacing every
component in the mic pre.
I have attached a copy of the schematic.
Sorry for the long post
thanks,
Jared
(612)802-3150
-----Original Message-----
>From: Ike Zimbel <[email protected]>
>Sent: Jan 26, 2010 2:55 PM
>To: [email protected]
>Subject: Re: NEOTEK: Mic pre transistors
>
>Hi Matt,
> All of the documentation that I have seen says "factory
selected"
>on the 2SB737's, which are the only one of these transistors that
I have
>seen (or at least noticed) in a Neotek mic pre. Incidentally, I have
>never seen one fail in a mic pre yet. Jared, I agree with Matt
that you
>should check all of the other components 1st, both for soldering
issues
>and value. Have the caps been done?
>best,
>Ike
>
>On 1/26/2010 12:36 PM, Matt Syson wrote:
>> Hi Jared
>> All or some of these transistors are approaching obsolete but
should
>> be obtainable by post from the internet.
>> I would suggest getting a bag of them (they are not expensive)
then
>> you can match them using a multimeter for similar Hfe.
>> (note, don't hold them for long as the finger temperature
affects gain!).
>> You could simply fit one and it will work but distortion would be
>> reduced by matching. I don't suppose Neotek matched them
originally
>> though.
>> It may be worth running the multimeter over the whole circuit
>> comparing it to others (DC conditions) as there may be
something else
>> amiss.
>> Matt S
>>
>> Jared wrote:
>>> Hello everyone
>>>
>>> I'm having a problem with a series 1 mic pre (just the mic
pre, line
>>> in is fine). I'm getting a lot of noise and extremely distorted
>>> signal. Everything looks fine visually. I swapped the chip and
that
>>> didn't fix it. My next thought was that it could be caused by
a bad
>>> transistor on the front end of the circuit. So I did a little
looking
>>> and now I'm a bit confused.
>>> It seems that I need either of these transistors: 2SAB737,
2SA991, or
>>> 2SA1084
>>>
>>> I'm sure their are others but this is what is in my console
and the
>>> 2SA1084 was written in some of the documentation that I have.
Does
>>> anyone have a lead on where these can be sourced or other
replacement
>>> options? Also what is the best way to match them? Can it be a
simple
>>> DMM hFE test?
>>>
>>> Also maybe I'm going down the wrong path, any other thoughts
about
>>> what could be causing this?
>>>
>>> Sorry for the long post,
>>>
>>> Thanks,
>>> Jared
>>> ------
>>>
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>>> Thanks.
>>>
>> ------
>>
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>> an email to mailto:[email protected].
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>> The Neotek list is lovingly hosted by Catapult Web. Remember
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>> message or reply sent to the list will be automatically
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>> all subscribers.
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>> Vive Le Neotek!
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>> Thanks.
>>
>>
>
>--
>Ike Zimbel
>Zimbel Audio Productions Ltd.
>Toronto
>416-720-0887
>
>"Studio House Calls". Authorized Warranty service for Allen&
Heath, Clear-Com, Drawmer, Soundcraft, SPL and Yamaha. Repairs and
upgrades to analog audio equipment including: AMEK, dbx, Neve,
Neotek, MCI, Soundcraft and more. Ask about our RF Frequency co-
ordination services!
>www.zimbelaudio.com
>
>------
>
>View the list online at: http://www.mail-archive.com/
[email protected]
>
>You can unsubscribe at anytime by sending an email to
mailto:[email protected] with "unsubscribe neotek" in the
message body. If you have any difficulties, or questions --please
send an email to mailto:[email protected].
>
>The Neotek list is lovingly hosted by Catapult Web. Remember that
any message or reply sent to the list will be automatically
forwarded to all subscribers.
>
>Vive Le Neotek!
>
>Thanks.
