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intensity failure

Jerry Massengale <jmassen418@...>
 

Hi,
 
My current bench scope is a 7704. This morning while using it the A and B intensity quit working. The beam went to max intensity and neither intensity control works. The readout intensity control does work. This makes U170 of the logic board a major suspect, I printed out a schematic of the logic page and the logic board layout. My logic board is different from the manual. My unit is B192250. The manual does not appear to have a date. Is there a newer version? If so, where can I get it. My 7704 has a blue phosfer and I kinda like it. Does anyone have an extra z-logic ic? I am trying to find TP175.
 
jerry
Craig Sawyers <c.sawyers@...>
 

Ah – I had this precise problem while using a sampling set up.  A tubular tant in the sampling head when short.  What happened was that the scope went to max intensity and the power supply did not shut down.

 

Check the power lines for shorts before troubleshooting the z-axis.

 

Craig
David DiGiacomo
 

On Tue, Aug 31, 2010 at 1:22 PM, Craig Sawyers
<c.sawyers@...> wrote:
Ah – I had this precise problem while using a sampling set up.  A tubular tant in the sampling head when short.  What happened was that the scope went to max intensity and the power supply did not shut down.



Check the power lines for shorts before troubleshooting the z-axis.
And then check the Z-axis DC restore circuit before looking at the logic board.
Dave Baxter <dave@...>
 

Re: intensity failure
Posted by: "David DiGiacomo" daviddigiacomo@...
david_digiacomo
Tue Aug 31, 2010 3:28 pm (PDT)


On Tue, Aug 31, 2010 at 1:22 PM, Craig Sawyers
<c.sawyers@...> wrote:
Ah - I had this precise problem while using a sampling set up.
A tubular tant in the sampling head > when short.
What happened was that the scope went to max intensity and the power
supply did not shut down.



Check the power lines for shorts before troubleshooting the z-axis.
And then check the Z-axis DC restore circuit before looking at the
logic board.

Is it just the intensity flared right up, or did the display (incl'
readout) shrink to about half size too? If so, I had that problem on
my 7704, ended up replacing the chain of (I think) 30Meg resistors that
load the Cathode HV, so the thing can regulate correctly. One had
failed open, and as they run hot(!) I changed all of them for higher
wattage types, that oddly are smaller than the originals.. Normal
service was resumed...

Regards.

Dave B.
Jerry Massengale <jmassen418@...>
 

Hi,
 
The trace is normal length, also the readout intensity works fine, the problem must be before the z-axis board. I have ordered a replacement logic board and have a donor 7704A in the short term for a logic chip. I just need time to pull the power supply back for access.
 
jerry


--- On Wed, 9/1/10, Dave Baxter wrote:

From: Dave Baxter
Subject: [TekScopes] Re: intensity failure
To: TekScopes@...
Date: Wednesday, September 1, 2010, 3:45 AM

 
> Re: intensity failure
> Posted by: "David DiGiacomo" daviddigiacomo@...
david_digiacomo
> Tue Aug 31, 2010 3:28 pm (PDT)
>
>
> On Tue, Aug 31, 2010 at 1:22 PM, Craig Sawyers
> <c.sawyers@...> wrote:
> Ah - I had this precise problem while using a sampling set up.
> A tubular tant in the sampling head > when short.
> What happened was that the scope went to max intensity and the power
supply did not shut down.
> >
> >
> >
> > Check the power lines for shorts before troubleshooting the z-axis.
>
> And then check the Z-axis DC restore circuit before looking at the
logic board.

Is it just the intensity flared right up, or did the display (incl'
readout) shrink to about half size too? If so, I had that problem on
my 7704, ended up replacing the chain of (I think) 30Meg resistors that
load the Cathode HV, so the thing can regulate correctly. One had
failed open, and as they run hot(!) I changed all of them for higher
wattage types, that oddly are smaller than the originals.. Normal
service was resumed...

Regards.

Dave B.
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