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2465 Sweep Problem


Craig
 

Had a challenging 2465 failure that may interest the group. This was on a 2465 I picked up on ebay a couple of weeks ago (you may remember the one with the bent knobs). Other than several bent potentiometer shafts the instrument was in good physical condition and very clean internally.


After repairing the pots and recapping the LV power supply (thank you maxx6668 for the spot-on list of caps to buy), the instrument turned on normally and did not display any error codes. Upon further checking, I found that the sweep cal was off by about 33% (100uS markers read 67uS). This cal error was very consistent across all horizontal time settings.


When I attempted to run the cal 01program, the marker alignments in both step 2 and step 3 did not have enough adjustment range to complete the alignment. Out of range messages were displayed on the CRT when the end of the adjustment range was reached.


After swapping the A and B sweep hybrids, many hours of taking measurements, and a lot of head scratching the problem turned out to be a .47 uF cap, C712 in the A sweep circuit. It was leaky to the tune of ~8K ohm. What surprised me most was that after the cap fixed the problem, the voltages on U910B changed only slightly. I suppose this must be because U910B is in the heart of the current mirror and the small current leakage through C712 had a large effect on the currents in the circuit but in this case the voltages just didn't show much.



I appreciate the knowledge I have picked up from this group, perhaps this will help someone else with a similar issue.


Craig


 

Thank you Craig, very interesting !

Matthias

Am 01.09.2014 04:04, schrieb craigyard99@... [TekScopes]:



After swapping the A and B sweep hybrids, many hours of taking
measurements, and a lot of head scratching the problem turned out to
be a .47 uF cap, C712 in the A sweep circuit. It was leaky to the tune
of ~8K ohm.

I appreciate the knowledge I have picked up from this group, perhaps
this will help someone else with a similar issue.


Craig


 

I think you are right about the voltage levels. If U910B is correcting the Vbe
of the integration current source which is typical in these designs, then the
voltage change is going to be small to produce a sweep error of only 33%.

The TL072 is not what I would consider a precision operational amplifier but it
does have low input bias current and the precision level needed over temperature
is only within line 1%.

On 31 Aug 2014 19:04:44 -0700, you wrote:

After swapping the A and B sweep hybrids, many hours of taking measurements, and a lot of head scratching the problem turned out to be a .47 uF cap, C712 in the A sweep circuit. It was leaky to the tune of ~8K ohm. What surprised me most was that after the cap fixed the problem, the voltages on U910B changed only slightly. I suppose this must be because U910B is in the heart of the current mirror and the small current leakage through C712 had a large effect on the currents in the circuit but in this case the voltages just didn't show much.