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7L14 Bad first mixer?

Dallas Smith
 

Thanks for the response Henad,

I had one of these before it had a catastrophic over voltage which fried many components, don't ask.
The reason I think the first mixer is bad is it has the same display conditions as my old 7L14 with bad mixer.
The greater sign by the -30 ( < -30) means that the variable gain is turned out of the cal position
to some increase of gain. and the noise floor is to high, should be 20 dB lower than shown.
Neat trick, turn the variable gain all the way up, position the vertical position so the -30 reference is at the top graticule line.
You are right, the storage is off, the 'A' and 'B' lights are off. and no video filter selected.
I have repaired first mixers successfully before but the current price now on the action is more than I'm willing to spend.

Dallas

Nenad Filipovic
 

Hi Dallas,

The greater sign by the -30 ( < -30) means that the variable gain is
turned out of the cal position
to some increase of gain. and the noise floor is to high, should be 20
dB lower than shown.

You're welcome and yes, you're right. I tried now the same settings on my
7L18, may not be fully comparable but noise level is indeed much better.

The reason I think the first mixer is bad is it has the same display
conditions as my old 7L14 with bad mixer.

I don't have much experience with SA mixers, always thought these are
simple devices that either work or not. Is it really possible to "singe" it,
so that it still works, but with lower sensitivity? I mean is it common to
get only partial damage?

The price on the auction is already stellar so I'll pass for sure as well.

Best Regards, Nenad

Roy Thistle
 

Hi all:

It's prob. well known, people usually damage the front end of a SA, by applying too much D.C. (Statistically, that results in opens or shorts [There is a low prob. of other modes of failure])

That said, over the years, I have had a chance to buy Tek SA plugins, for 7000 frames; but, never bought one. The ones I looked at were cooking away, in the frames, producing a lot of heat.

I don't know (And, I would like to know.) whether that is a common failing? of the Tek SA plugin designs (producing too much heat?)

If there is a lot of heat, and supposing one is talking about a Schottky diode ring mixer (for example)...there are papers that suggest "heat", over long term causes the diode's parameters to change.

So, if in fact Tek SA plugins operate normally producing lots of heat, and a particular SA plugin has been in service a long time...I would think that the mixer could degrade.

Best Regards.

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Pjotr Tolstiakov <ilmuerte@...>
Sent: December 20, 2017 12:25:54 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7L14 Bad first mixer?

Hi Dallas,

The greater sign by the -30 ( < -30) means that the variable gain is
turned out of the cal position
to some increase of gain. and the noise floor is to high, should be 20
dB lower than shown.

You're welcome and yes, you're right. I tried now the same settings on my
7L18, may not be fully comparable but noise level is indeed much better.

The reason I think the first mixer is bad is it has the same display
conditions as my old 7L14 with bad mixer.

I don't have much experience with SA mixers, always thought these are
simple devices that either work or not. Is it really possible to "singe" it,
so that it still works, but with lower sensitivity? I mean is it common to
get only partial damage?

The price on the auction is already stellar so I'll pass for sure as well.

Best Regards, Nenad

Dallas Smith
 

Roy,

It certainly gets hot but so does my 7D20. My failed mixer was degraded diodes.
I believe there is a DC blocking module before the mixer if my memory is right.
This is the best site on the subject. http://www.firstpr.com.au/tequip/7L14/
Was working good until I fried my 7L14 by plugging in a connector in the wrong position and backwards, oops!
Two connectors had the same number of pins and color.
Still looking for one.

Dallas