HP 3435A -17v rail - Page 1
Forum section: electronics / testgear

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 02:14:02 pm
content: Hello all - I am working on another HP 3435A multi-meter. This one has an issue with the -7v rail. It is at -5.5v. The +7v rail is fine and is adjustable per the manual. Current draw from both + and - rails is ok per the manual. I have a parts 3435 and have swapped in the -7v regulator, checked related components, etc. The -2v bus (fed from the -7v rail) is also low.More bits of info:+15 VDC off of C403 is at 17VDC-15 VDC off C405 is at -12VDCChecked rectifier diodesSwapped power transformer with parts unitAll caps in the PS section have been checked (pulled and tested)U9 and Q403 have been swapped from parts unitAt the junction of R411 and R412, I get .89 VDC (should be 0?)

author: tom66
author_rank: Super Contributor
timestamp: August 20, 2015, 02:17:13 pm
content: What do you get on the base of Q403? Opamp U9 could still be defective (seems old opamps are prone to drifting badly)

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 02:25:10 pm
content: I will check that. Seems it was -5.5 also, but will validate.

author: mij59
author_rank: Frequent Contributor
timestamp: August 20, 2015, 02:46:10 pm
content: Why is the unregulated negative rail so low ?Did you check the ripple voltage of the unregulated rails ?How did your check the rectifier ?, there could be a problem with the diodes CR406 and or CR405 under load.

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 02:55:22 pm
content: I have not checked ripple, but did pull all bridge diodes (CR403 through CR406) and they tested ok. And just to test if it was load related, I replaced them all with 1N4007's. The reason I swapped the transformer with one from the parts meter was because I could not find a loading problem (after the regulators). I get 19mA on the -7v rail and 22mA on +7.I should mention that the VDisp which should be 6.5 vdc is 6.2 vdc.And - line voltage in to the meter is 124vac, and line voltage selection jumpers are in proper places and read 2.7 ohms each as expected.

author: mij59
author_rank: Frequent Contributor
timestamp: August 20, 2015, 04:38:47 pm
content: Check the ripple voltage of the unregulated rail, could be a case of bad caps, thinking C405 or c409.Power switch section D OK ?

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 04:45:23 pm
content: I will pull them both again. I tested for value the first time, but can dig out my leakage tester.Power switch, section D is okay - zero ohms when closed. And when open allows test of current as AC power is always applied.Also - I should mention, this came with the charger option which I've removed. I've jumpered JM401 accordingly.

author: mij59
author_rank: Frequent Contributor
timestamp: August 20, 2015, 04:48:41 pm
content: Quote from: mbear2k on August 20, 2015, 04:45:23 pmYou can check the ripple voltage in-circuit with the multi meter set to ac volts.

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 04:53:12 pm
content: Will do - that's one thing I have not done yet.

author: jlmoon
author_rank: Unknown
timestamp: August 20, 2015, 05:00:31 pm
content: C405 high esr or open, C409 the same? I believe you should have around -17 unregulated as you do on the positive rail after the bridge. Is switch D in position as illustrated in schematic? (No - supply load present)Where does CR407 goto?

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 05:04:52 pm
content: I'll have to double check the voltages with the switch off (as presented in the diagram). I am sure I checked both positions, as in the off position I also checked current readings for each rail. But good point!

author: jlmoon
author_rank: Unknown
timestamp: August 20, 2015, 05:15:57 pm
content: One more question, how are you checking those caps?

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 05:18:51 pm
content: I pulled one leg of each and used did a simple test with a Fluke 87 cap function. I can test with a Sencore cap tester for leakage, ESR and all that - but will measure ripple first as suggested. Do some diagnosis instead of parts swapping!

author: mbear2k
author_rank: Regular Contributor
timestamp: August 20, 2015, 08:39:00 pm
content: I found the issue... the -15v bus had nearly 6 vac on it! And... C405 was bad. Popped in a replacement 220uf cap and off it ran. All voltages look good and initial test of all functions look good.Thanks for the prompting!Mark

author: mij59
author_rank: Frequent Contributor
timestamp: August 21, 2015, 03:12:41 am
content: Very nice, love happy endings.Consider replacing the other filter caps as well

author: PedroDaGr8
author_rank: Super Contributor
timestamp: August 21, 2015, 09:46:24 am
content: Quote from: mbear2k on August 20, 2015, 05:18:51 pmCapacitance function on a multimeter is basically useless for testing capacitor heath. By the time you see any sort of drift, often that cap was dead a long long time ago.

author: jlmoon
author_rank: Unknown
timestamp: August 25, 2015, 04:19:21 pm
content: Great job mbear. Glad you found the issue.