Hi Jared,
I had a channel with a very similar problem that drove me nuts for a
while. Try replacing both of the diodes and see what happens. I didn't
analyze the circuit to determine the failure mode of one of them failing,
but it fixed the problem. Give it a shot and let me know. Good luck!
Best, Jim
At 08:11 PM 2/5/2010, you wrote:
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
>>> ------
>>>
>>> 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.
>>>
>> ------
>>
>> 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.
>>
>>
>
>--
>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.
------
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.
