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Re: HP 3455a DVM repair

Bob Albert
 

Should I replace U2 or maybe Q7 and Q8?? With what?? Those have proprietary HP numbers I am sure.

But maybe a bit more troubleshooting to make sure.? No sense in replacing all three parts when perhaps they are all good.? It's not simple to remove parts without damage to the board, I think it's double sided and takes a lot of heat.

Bob

On Thursday, April 2, 2020, 10:22:43 PM PDT, Orin Eman <orin.eman@...> wrote:


R12?? Both sides.

Looking like it's U2 at the moment..., with Q7, Q8 next in line.? If the main amplifier is working correctly, either side of the diodes should be pretty much the same.

On Thu, Apr 2, 2020 at 5:09 PM Bob Albert via <bob91343=[email protected]> wrote:
24.50 and 24.04 on each side of the diodes.

On Thursday, April 2, 2020, 04:51:49 PM PDT, Orin Eman <orin.eman@...> wrote:


So, it's looking like the main amplifier.? We can look at U2's inputs by measuring both sides of CR4 or CR5 (they are in parallel).

They are above and to the right of TP5, close to J1;
from below the right end of J1, L to R with J1 uppermost, the components are: CR4, CR5, U2, R11, C4, R12, R13

Then look at U2's output by measuring both sides of R12. ?(One side of R13 goes to TP4, the other to R12.)


On Thu, Apr 2, 2020 at 4:04 PM Bob Albert via <bob91343=[email protected]> wrote:
U5 pin 2, 13, 14 read -22, -14, -22 V respectively

On Thursday, April 2, 2020, 03:55:41 PM PDT, Orin Eman <orin.eman@...> wrote:


Two should be -24V, the other should (I think) be close to TP3.? I suspect you'll find one a diode drop from TP4.
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