| Electronics > Repair |
| HP 3458A - Fails Slave Test - Convergence |
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| Ade911:
--- Quote from: Dr. Frank on July 02, 2023, 09:32:25 pm --- --- Quote from: Ade911 on May 05, 2023, 11:47:16 am --- Monitoring TP11 on A1 I can see a signal jumping from zero to around 7 volts, the screen also shows its sampling, then it beeps and stops , or some times it will sample , a relay will click and TP11 will jump to 10V before failing again. Ive checked the Convergence circuit with my very limited understanding of the circuit. :palm: FETs are on and off , ive cycled functions to see if the FET will change state just to see if its working. As the test sometimes goes further than other times I took the decision to replace U7,8,9,20,17, 14,11 [I know some chips don't need changing but desperation cut in] |O --- End quote --- @ TP11 / A1 you measure behind the input multiplexer, which applies DCV input, then zero for the AZ cycle. Therefore it's completely normal, that you see varying voltages on that pin. If you apply 10V to the input jacks in the 10V range, you should see 10V / 0V , with about 200ms time for each cycle. If you switch off AZ, then the 10V should be visible continuously @ TP11. Please try that out --- End quote --- when I apply +10V to the input with Auto Zero ON or OFF i only see a constant +10V on A1 TP11, its not dropping to zero volts. |
| Ade911:
--- Quote from: Dr. Frank on July 02, 2023, 09:40:15 pm --- --- Quote from: Ade911 on May 08, 2023, 01:18:11 pm ---update So I guess the dallas chips are from 1995 week 38 / 47 after watching videos about them, ill order replacements and hope you can clarify how i can download and replace. I also found an old communication from TiN re the TARM command, OMW , after doing this the meter came to life and did a few samples before stopping again [but it is repeatable]. With nothing connected im getting various voltages between +5.xxx and +10.xxx , If you do an autozero i then get +10.xxxxv. --- End quote --- What PCB / TP are you talking about? Where do you measure these signals? --- End quote --- This doesn't work now but when the meter would not sample, if i selected TARM i could at that time get a voltage at A1 TP100. it varied , this is why i looked at the circuit around A1 Q106, Q107 and Q108. Monitoring At TP100 without TARM enabled , the voltage would over 1-2 minuets rise to 10 Volts. After replacing A1 Q106 as it had a dent in the can ? , no change which was expected. I then removed A1 Q107 and A1 Q108, checking I found low resistance between S and G of Q107. As both devices were out the board I replaced with 112 devices which others had suggested. After powering back up the voltage drift disappeared, A1 TP100 is now stable. |
| Ade911:
--- Quote from: Dr. Frank on July 02, 2023, 09:41:43 pm --- --- Quote from: Ade911 on May 08, 2023, 07:45:28 am ---I checked U181 and this looks fine, All signals are fine except pin 3 where it toggles only to 7v during a DCV cal then fails / stops sampling, or 10v when it does a little more sampling and the output jumps to 5v [plus relay clicks on]. U142 , again I believe is ok, the output is toggling between 0 and 5v, however if I scope the input pin 3 the sampling fails [TP140] , its as if the system doesn't like the extra load, which is strange as its only a normal scope probe. --- End quote --- How did you confirm / measure, that the EL2018 are working ok? Did you scope on their outputs? --- End quote --- when the unit was reading some random voltage , yes I scoped the input and out put of EL2018 , as I also mentioned later on I removed and tested on bread board A3 U111, A3 U112 and A3 U140. All three devices did work but U140 exceeded the current drawn as stated on the manufacturers datasheet. I managed to purchase replacement devices for A3 U111 and U140 but need to use U112 as I cant purchase a replacement even though its still running very hot [greater than finger hot] |
| Ade911:
--- Quote from: Dr. Frank on July 02, 2023, 09:49:29 pm --- --- Quote from: Ade911 on May 08, 2023, 04:57:50 pm ---Todays update only samples if i manually trigger TARM command, no idea why. when i initially started to look inside i found the voltage on U100 pin 6 is slowly drifting up , its still doing it now. Unfortunately I damaged Q106 but was able to replace so didnt think any more about it, but today I find myself again back here as the signal DC_AD is slowly rising and is reflected on the screen [least I know the DAC is ok] --- End quote --- Have you applied a fixed input voltage (only at the input jacks, of course!) for that test? Use TP100, instead of A1U100 - pin6. switch AZ off for that test. If you apply fixed 10V in 10V range, then you should see that 10V at TP100. apply 1V in 1V range, you should see 10V as well, repeat that for 100mV in 100mV range, please --- Quote from: Ade911 on May 08, 2023, 04:57:50 pm ---screen readings DCV volts ? +0.689 max then freezes - error message time out read A/D 108 [but it cant be the DAC] ACV mv samples for ever 1.945mv AC ACV 000.081 and rising VAC OHMS +0.VLD GOHM OHMS +OVL.D MOHM stops sampling OHMS +OV.LD KOHM OHMS +OV.LD OHM DCI nA 000.173 DCI uA 0.000159 stops sampling DCI mA 0.0000248 DCI A 0.0000135 ACI uA 000.0434 ACI mA 0.000286 ACI A 0.000268 FREQ 50.xxx HZ Data error matches system asking for DCV and OHMS calibration Any support most welcome please Tx Ade --- End quote --- Please explain, what are you doing here, I do not know (or can't guess) what you mean with screen display, which sources you apply, and what mode / range you select in each case --- End quote --- Sorry Frank, I dont recall , I believe I was checking all functions on the front panel and reading the main value on the front screen. Now nothing so I can not check sorry. |
| Ade911:
--- Quote from: Dr. Frank on July 02, 2023, 10:02:43 pm --- --- Quote from: Ade911 on May 24, 2023, 07:37:11 pm --- For some strange reason when you inject 10V and manually cycle through the X1, X10 , initially when you selected mv the meter wouldn't do anything now it states overload and samples , no idea why. Monitoring A1 TP100 X1 10V I/P 10V TP100 X10 10V I/P 1V TP100 x100 10V I/P 0.1V TP100 Below X1 [mV ?] 10V I/P Overload [ sampling] TP100 = 17.39V I thought the input to the A3 would never exceed 10V , is this incorrect. Ive also noticed that if you remove the 10V I/P and set to X1 TP100 starts to drive up towards 17.39V, is this normal. Also removed Jumper P100 and there is no input via P102 and still does it. Any idea where to check next please, I hope that because the voltage from A1 if above 10V the A-D will not sample and there is some limit circuit stopping 17V damaging the device. Trying to be logical and methodical in finding what is wrong. --- End quote --- TP100 (w/o AZ) is the input signal times the range amplification, i.e. x1, for 10V range, x10 for 1V range and x100 for the 100mV range. Where did you inject the 10V? At the input jacks only, hopefully. If you overload the amplifier, then of course you get +/-17V something out of U100. The input to the ADC can of course be +/-12V for full scale. --- End quote --- I only inject via the front panel , never in circuit as I dont know it that well. At present I get what you have stated above , good to see +17 V is normal as well , I thought this may kill the A/D. Also no voltage drift since changing the FETs on A1 Q106,7,8 |
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