Re: 7904 readout on the wrong place - vertical signal squeezed


David C. Partridge <david.partridge@...>
 

Checking the vertical plates are connected inside the CRT is easy - check
ohms from vertical plate input pin on CRT to the corresponding pin for the
terminator. Also of course check that the terminator is correctly
connected.

Dave

-----Original Message-----
From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com] On Behalf
Of David C. Partridge
Sent: 20 January 2010 14:32
To: TekScopes@yahoogroups.com
Subject: RE: [TekScopes] 7904 readout on the wrong place - vertical signal
squeezed

Before you condemn the CRT (or the vertical output amp) check the values on
the large emitter resistors R724, R725. They should be 500 ohms (early
7904) or 332R in the later (post B260000) version. These tend to run
*very* hot, and are often WAY off nominal value.

Certainly I would agree it is a good idea to check the connections to the
vertical plates.

If they are out of specification, I typically replace each of them (in an
early 7904) with two 1K0 2W resistors in parallel and make sure they are
mounted well off the board.

HtH
Dave

-----Original Message-----
From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com] On Behalf
Of Jerry Massengale
Sent: 20 January 2010 13:32
To: TekScopes@yahoogroups.com
Subject: Re: [TekScopes] 7904 readout on the wrong place - vertical signal
squeezed



Hi,

Sounds like the horizontal + pin on the neck of the crt is open.

jerry
--- On Wed, 1/20/10, kupkagerhard <ghm1@gmx.de> wrote:



From: kupkagerhard <ghm1@gmx.de>
Subject: [TekScopes] 7904 readout on the wrong place - vertical
signal squeezed
To: TekScopes@yahoogroups.com
Date: Wednesday, January 20, 2010, 7:07 AM



In my 7904 the readout is placed 1 division below the middle of the
screen. Additionally the vertical signal can't be moved to upper half of the
screen. When I move the vertical signal from bottom of the screen upwards,
the signal will be squeezed vertically in the left part of the screen. The
output signal of the vertical main amplifier doesn't show any queezing. The
output of the horizontal main amplifier shows no distortion on the sawtooth
signal









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