click-click from HP3456A

[macona]

Was given this 3456a today. Turns on, displays the model and then the hpib address and then just ------. Pushing test does not return a code and I hear click-click over and over. Checked voltages and I think they are fine. 33 is 45v but the manual says it is unregulated 33-45. Same double click happens when I press reset.

Checked connections too. nothing seems to be amiss and it does not look like anyone has been poking around.

Any ideas?

[Vgkid]

On my unit it was ripple on the power supply. I believe it is a common fault.

[macona]

I'll throw a scope on it tomorrow. Thanks.

[mikeg]

A complete recap of the supplies should fix it. I had to do this to mine.

[pqass]

Make sure you have the GUARD switch (on the front panel) pressed-in, nothing connected to the posts, then press the Test button.
Cleanup, restora­tion, and cal­i­bra­tion here.
Service manual here.

[macona]

Ordered caps, we will see what happens.

The guard switch was in, I did see that in the manual.

[macona]

Caps arrived and replaced everything and no change in function, to lack of. Reseated everything, no difference. This does not have any socketed ICs so nothing there.

Could this be the ROM issue?

[srb1954]

Caps arrived and replaced everything and no change in function, to lack of. Reseated everything, no difference. This does not have any socketed ICs so nothing there.

Could this be the ROM issue?

If your unit has Mostek ROMs it is quite possible that they have faulty bits as it is a known problem with these devices.

A quick check can be performed on the processor operation by probing the address pins starting from the MSB downwards. If each address line has a constant square wave, doubling in frequency for each lower address, then the microprocessor has crashed, possibly due to faulty ROM code.

[pqass]

Reseated everything, no difference.

Did you lift/re-seat the board-to-board gold fingers?

Could this be the ROM issue?

Possibly.

Looking at the schematic, I see that it uses ICM7218s (U29,U30) which has an 8-byte memory and handles display multiplexing automatically.  So it is possible that the CPU crashed/stuck after booting.  After pressing the LINE button, you should first see "HP3456", then the GPIB address like "Add003  0", then six dashes "------", then the readings are shown.   

See this BAMA link for ROM pin info and hi-res service manual.

See ROM mod and image here.

You could check for clock or other activity on the CPU (see datasheet).

[macona]

It displays everything till the ------ and then it is just stuck. click click.... click click. If I hit the auto zero button it stops clicking and just sits there. The display and LEDs do respond to button presses so I dont think the CPU is locked up. Pressing test turns on all leds and it just starts clicking faster. Im having a hard time locating the relay that is clicking.

So it may be something with the boot sequence where it zeros itself?

[macona]

Looking through the manual page 8-A-17 it says that if the test button is pressed and it just displays all segments and does not fail test 3 that I check U17 pin 19 for toggling, which is does in time with the relay click. Then check pin 10 of U13, if it is not clicking to replace U13. No activity on U13 pin 10. Its just a generic 74LS02 which I don't have so ill have to get one a see what that does.

[macona]

Found some 74HCT02s and replaced it and no change. Noticed I can manually select the test and tests 1 through 3 pass. When I try to run test 4 it starts doing the clicking thing. Test 4 seems to be X1 gain setup. So I guess I need to start from there.

[pqass]

With no leads on the posts...
After pushing RESET, I hear a click-click, then readings are shown.
After pushing TEST, I hear click-pause-click-click-pause-click-pause-click-click-pause...repeat  while +1.8.8.8.8.8.8.+8. slow flashes.

I think the click is just a normal range change.

[macona]

I think it is constantly resetting itself because of an issue in the outgaurd or inguard circuit. Im following the manual to narrow it down. It brings up a "signature analyzer" which I have never heard of or seen before, pretty much just for servicing HP equipment from the looks of it. Found that there is a sigrok hp signature analyzer module that will work for the little saleae logic analyzer a boss gave me years ago. So ill give that a try.

[GigaJoe]

self test has 12 steps ..  each test can be trigger manually , see page 8-58
are you able to trigger let say test 1 ?  if it passed it would be +1 indication for 1 to 3 ; 4 to 12 AD output

[macona]

1 to 3 pass, everything breaks down after that.

[GigaJoe]

I would start looking at input analog switching \ amplifying opamp , is something on display for test 4 only ?