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2465 Recap - Trace Returns, But Jumping Everywhere

Don N3DEB
 

I posted last year regarding a 2465 (with GPIB option) that died after swapping out a hybrid to diagnose another scope.The donor was working fine until the hybrid was returned and the scope turned on. Trace was gone, turning up the grid bias gave the typical rows of dots verifying a heartbeat. The symptom was very much like the other scope's Z-Axis hybrid failure, which threw me off initially.

Power supply voltages were within tolerance, some ripple was on the high side but did not seem to be enough to kill the trace. Wrong.

Just recapped the power supply boards using the parts list floating around group, trace back sharp and bright but with issues. I logged every step, measuring and keeping old caps. C1113 and C1116 were the obvious failures. A Hakko FR-301 desoldering tool drops the parts out quickly and clean - amazing.

My theory is that because this repair scope was seldom used, power cycling cause a coincidental power supply fail that boosted ripple. This killed the trace and probably killed U2435 on the A5 Control board.

Some of the most conclusive troubleshooting results:

1) Trace jumps above and below visible screen area with sweep starting right of left screen edge.
2) Position controls have very little or no affect on trace, on any channel. Ch3 & Ch4 are somewhat more stable than Ch1 & Ch2.
3) Shorting vertical output at Delay Line test point brings trace back to center, suggesting the issue is before Vertical Output Amp.
4) Calibrator signal viewed on test scope via Ch2 Out of repair scope is nearly perfect on a test scope, with just a little flicker in sync with the repair scope trace jumps.
5) +5v, -5v power to U2435 all good but most voltage checks on pins do not match the test 2465 scope.
6) Dly Ref 0 at A1J118 Pin 2 - STUCK at +1.17v, does not vary as it should by changing REF Delay Position pot.
7) DAC Current Reference TP503 +1.367v, TP504 -1.253v. Good.
8) A1C853, A1C854 lifted leg, both correct capacitance. Dissipation factor similar to new cap. (DE-5000 meter)
9) U800 pin 18 sweeps look reasonable with DMM, -1.25v to +1.25v.

Conclusion - replace U2435?

Thanks for the help,
Don

DaveH52
 

U2435? maybe. What is pin 8 doing? It should be a steady logic low, and U2656A pin 4 should be a steady logic high.

Don N3DEB
 

On Sat, Apr 4, 2020 at 07:55 PM, DaveH52 wrote:


steady logic high
Thank you for the reply.

The logic probe that I have must be disturbing U2435, as it causes it to reset on both the repair scope and a good 2465 scope. I will locate another logic probe.

Meantime, I viewed these pins signals on both the repair scope and another 2465 good scope as a comparison. I see about -14v on U2435 pin 8, +5 on U2656A pin 4. The signal traces on both scopes appears steady and equal in appearance.

Don N3DEB
 

Measured with DMM U2435 pin 8 is +.0005 on repair scope and good scope. Both chips low, high correspondingly as expected.

Don N3DEB
 

Measured again U2435 Pin 8 & U2656 Pin 4 with new logic probe and a working oscilloscope. Very educational adventure.

U2435 Pin 8
Logic Probe (CMOS setting, neg to ground, pos to U2435 Pin 4 +15v supply). Solid LOW
Oscilloscope to Pin 8. Solid -13.4V, no pulses

U2656 Pin 4
Logic Probe (TTL setting, neg to ground, pos to U2656 Pin 16 +5v supply). Solid HIGH
Oscilloscope to Pin 4. Solid +5v, no pulses.

These matched the measurements on the same test points on another working 2465. So these are looking OK I assume.

Manual says if no control from all channel positioning pots look to Display Sequencer and Vertical Switch. I have also swapped most of the socketed hybrids & ICs on the main board with known good units, without change.

I also measured ripple again with a scope, even though I rebuilt the LVPS. All within 120Hz spec or very close.

Any suggestions for next path?

Don N3DEB
 

BREAKTHROUGH!

I am so happy, isolated this problem to the GPIB board. Beautiful sharp traces now when bypassing the GPIB, wow!

I did not know that the front controls route through that board, and this caused lots of trouble that is not found by the typical manual troubleshooting methods. I spent a lot of time on the Vertical Switch and Display Sequencer, experience gained was time well wasted.

Seems the PS failure took out something on this GPIB board.

The front panel pot lines resistance measured normally for this scope through the GPIB board had very high fixed resistances to ground such as 5M, 1.2M, 1.2M, etc. Proper are variable resistance of the pots of 45 ohms to 500 ohms.

Anyone have schematic and board layout information to help me troubleshoot that board? I have Artek purchased manual but does not cover this option. Ser# is B024509

Or clues on what to check?

Don N3DEB
 

Found a service manual.

If anyone has some thoughts let me know.

Thanks

Andy Warner
 

I recently had a similar problem, post recap.
My problem was that channel 2 was never on the screen.

People (myself included) were surprised that the gpib card would misbehave,
I tried reseating all the ribbon cables, but it did not help.

Since I will never use gpib on this scope, I just removed it and made a
blanking plate where the connector lived.

On Sun, Apr 19, 2020 at 00:09 Don N3DEB via groups.io <dbawatsonville=
yahoo.com@groups.io> wrote:

Found a service manual.

If anyone has some thoughts let me know.

Thanks



--
Andy