Author Topic: Help needed HP3456A self test [partly fixed]  (Read 5307 times)

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Offline enut11

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Help needed HP3456A self test [partly fixed]
« on: May 05, 2016, 10:21:30 am »
Hi All. Have done a search of the forum but nothing concrete showed up. Just taken delivery of HP3456A from eBay (series # 2201A). Listed as working but found it would not read volts or ohms (nothing else tested). It was a "no returns" purchase so not expecting anything from the seller.
On power-up, after the HP intro, boots up with all "-" for the digits. Does not respond to inputs.
The self Test button stops at 3.0000. That's it. Have not been able to find what this (error?) means.
Will pull it apart soon to look for obvious faults such as leaking electros and start measuring voltages around power supply.
Any hints on how to progress this repair will be appreciated.
enut11
« Last Edit: June 02, 2016, 11:27:36 am by enut11 »
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Online VK5RC

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Offline lowimpedance

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Re: Help needed HP3456A self test
« Reply #2 on: May 05, 2016, 01:52:13 pm »
Dodgy power supply electro's cause all sorts of errors with these old timers, before casting a wider net give the power supply a recap. And note the caps usually dry out without generally any signs of leakage or bulging so removal and test is the best way.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline Bill158

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Re: Help needed HP3456A self test
« Reply #3 on: May 05, 2016, 03:00:27 pm »
Hi All. Have done a search of the forum but nothing concrete showed up. Just taken delivery of HP3456A from eBay (series # 2201A). Listed as working but found it would not read volts or ohms (nothing else tested). It was a "no returns" purchase so not expecting anything from the seller.
On power-up, after the HP intro, boots up with all "-" for the digits. Does not respond to inputs.
The self Test button stops at 3.0000. That's it. Have not been able to find what this (error?) means.
Will pull it apart soon to look for obvious faults such as leaking electros and start measuring voltages around power supply.
Any hints on how to progress this repair will be appreciated.
enut11

Self test errors are described on page 8-58 in the manual I am looking at.  My experience, with one exception, has always been the power supply.  Voltage not being with in specs or power line ripple high due to dried out electrolytic caps.  Use an oscilloscope to check ripple on each of the power supply voltages.  These meters a notorious for failed electrolytics.  All of the power supply voltages should have ripple below 5 mv p-p with the exception of the +33 which can be much higher because of the way HP generates this voltage.
Good Luck
Bill
 

Offline guido

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Re: Help needed HP3456A self test
« Reply #4 on: May 05, 2016, 03:57:55 pm »
Caps is indeed the first thing to replace (done for both my machines). PROMs could also be bad (had that on one of my machines, replaced with eproms).
 

Offline enut11

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Re: Help needed HP3456A self test
« Reply #5 on: May 06, 2016, 04:10:25 am »
Thanks to VK5RC, Lowimpedance, Bill158 and Guido.
All your suggestions are useful and will be investigated in due course, especially as the 5V rail is low and measuring 4v with decided overheating evidence on the PCB near the regulator.

A friend suggested that I re-seat the circuit boards. On close inspection I found 2 loose boards! Obviously as a result of shipping from USA to Australia coupled with inadequate packing protection. I specifically asked the sender to package it well. :palm:

I am glad to report that the 3456A now boots up and responds to volts and ohm measurements.

Now I need to investigate the cause of the low 5V rail and the reason for overheating the PCB. The voltage across C5 (4000uF input to the 5V reg), is around 10v which seems reasonable to me.
enut11
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Offline lowimpedance

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Re: Help needed HP3456A self test
« Reply #6 on: May 06, 2016, 05:25:46 am »
Even so the ripple could be excessive and this cap is one of the main culprits of 'funny issues'.
These are great meters even if they chew up lots of bench space  :D, you will enjoy being an owner.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 
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Online VK5RC

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Re: Help needed HP3456A self test
« Reply #7 on: May 06, 2016, 09:51:56 am »
i am not sure if you are aware , for high quality AC readings, esp if your unit came from USA i.e. 60Hz, there is a crystal that needs to be changed to get the correct AC readings for 50Hz mains supply , there is a topic about this here https://www.eevblog.com/forum/projects/modyfing-50hz-to-60hz-hp-3456a/
I have one coming from KRYSTALLY, Joseph is the main contact and has been helpful, you need a 4.875MHz crystal but the one with the THICKER pins i.e. the HC50/U type (not HC49U). Good to hear nothing too serious. :-+
Whoah! Watch where that landed we might need it later.
 
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Online VK5RC

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Re: Help needed HP3456A self test
« Reply #8 on: May 06, 2016, 10:16:41 am »
PS If you have any buttons that don't click or indent ( as was the case with mine)  there is another link about that as well
https://www.eevblog.com/forum/repair/hp-5335a-non-working-enter-key/  and there is a link to a video  http://www.rbarrios.com/projects/hpswitch/

I was missing some of the little metal strips out of the base of the switches, some were floating around the inside of the front of the 3456A!!  FFS!

I tried all sorts of materials to replace the 0.05mm spring steel strip (I measured one) even sacrificing a cheap steel tape measure, the best was some clock spring I got from eBay http://www.ebay.com.au/itm/200935501408?_trksid=p2060353.m2749.l2649&var=500166826284&ssPageName=STRK%3AMEBIDX%3AIT

If you can't guess I am a bit of an HP and renovation fan.
Whoah! Watch where that landed we might need it later.
 
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Offline enut11

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Re: Help needed HP3456A self test
« Reply #9 on: May 07, 2016, 10:29:44 am »
Thanks VK5RC
It never ceases to amaze me the amount and quality of help from forum members. Thanks all. Much appreciated.

Happy to report more success with the repair of my HP3456A. After isolating 5v feeds to other boards I found no difference in the 4v originating on the PS PCB.
I then replaced the stock(?) regulator with a 7805 and now have the full 5v on the rail. I confirmed the dodgy unit was outputting 4v using my benchtop power supply. It has some strange part numbers on the case - see pic.

Trying to work out why it went low in voltage. I found the original reg was not actually touching the heatsink due to a bent clamp. Perhaps it overheated and drifted down to 4v??

The 7805 replacement was bolted on to make sure. Both regulator and PCB are now both cooler to the touch.

Next steps are to address the other potential issues raised by forum members.
enut11
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Offline enut11

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Re: Help needed HP3456A self test
« Reply #10 on: May 07, 2016, 11:08:02 am »
PS If you have any buttons that don't click or indent ( as was the case with mine)  there is another link about that as well
https://www.eevblog.com/forum/repair/hp-5335a-non-working-enter-key/  and there is a link to a video  http://www.rbarrios.com/projects/hpswitch/

I was missing some of the little metal strips out of the base of the switches, some were floating around the inside of the front of the 3456A!!  FFS!

I tried all sorts of materials to replace the 0.05mm spring steel strip (I measured one) even sacrificing a cheap steel tape measure, the best was some clock spring I got from eBay http://www.ebay.com.au/itm/200935501408?_trksid=p2060353.m2749.l2649&var=500166826284&ssPageName=STRK%3AMEBIDX%3AIT

If you can't guess I am a bit of an HP and renovation fan.

Hi VK5RC
All the buttons I have tried so far seem to work. They are heavy to press and make a loud clacking noise. No sure if this is normal.
enut11
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Re: Help needed HP3456A self test
« Reply #11 on: May 07, 2016, 12:05:25 pm »
@enut11 Sounds Normal to me, I have quite a few old HP beasts of that vintage and they all do the same,  if you want to get really enthusiastic you can bend the spring a bit (as in the video) which makes it a bit smoother but I wouldn't bother, I only did mine which had no 'detent' feel at all, but they still worked as buttons.
I am only passing on mostly others' experience; I have learnt so much from others here and I have had a lot of fun in the meantime. :-+
There is a sticky topic in Repair in which a lot of these techniques are collated, some other issues may be of use, perhaps. https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/
Whoah! Watch where that landed we might need it later.
 
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Offline MadTux

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Re: Help needed HP3456A self test
« Reply #12 on: May 07, 2016, 12:43:00 pm »
Hi VK5RC
All the buttons I have tried so far seem to work. They are heavy to press and make a loud clacking noise. No sure if this is normal.
enut11

I usually put a drop of oil into the knob mechanism and onto the spring, if buttons behave like that.
 Buttons become much softer and less scary to operate afterwards.

Also reduces the risk of spring failures, because buttons become hard and clicky if the spring doesn't bend like it is designed to do. And if the spring doesn't bend, it is much more likely to snap out of that tiny top groove in the button => loose spring somewhere inside the instrument that cand make all kinds of trouble :(
 
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Online Vgkid

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Re: Help needed HP3456A self test
« Reply #13 on: May 07, 2016, 03:56:43 pm »
@ Enut11: Those are HP part numbers on that 7805.
If you own any North Hills Electronics gear, message me. L&N Fan
 
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Offline enut11

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Re: Help needed HP3456A self test
« Reply #14 on: May 08, 2016, 09:25:01 am »
Hi VK5RC
All the buttons I have tried so far seem to work. They are heavy to press and make a loud clacking noise. No sure if this is normal.
enut11

I usually put a drop of oil into the knob mechanism and onto the spring, if buttons behave like that.
 Buttons become much softer and less scary to operate afterwards.

Also reduces the risk of spring failures, because buttons become hard and clicky if the spring doesn't bend like it is designed to do. And if the spring doesn't bend, it is much more likely to snap out of that tiny top groove in the button => loose spring somewhere inside the instrument that cand make all kinds of trouble :(

Hi Madtux. I presume I have to dismantle the entire front panel in order to oil the button springs. That looks like a difficult task. I have to think about that one.
enut11
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Offline MadTux

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Re: Help needed HP3456A self test
« Reply #15 on: May 08, 2016, 09:52:34 pm »
If I remember correctly, it actually isn't that scary to take apart the 3456A front panel.
The acrylic LED light guides (status indication) are supported by a second plate, so that you can unscrew the button/LED PCB quite easily, without disturbing all (30+) those light guides. Would be a real pain to assemble otherwise :)
« Last Edit: May 08, 2016, 09:54:40 pm by MadTux »
 
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Offline enut11

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Re: Help needed HP3456A self test [partly fixed]
« Reply #16 on: June 05, 2016, 05:39:23 am »
Removed the front panel after finding that last "secret" screw. I then removed each button and spring, bent the spring slightly and reassembled. After a while I found that I could remove the spring without removing the button. Buttons are much smoother now, maybe even the way they were as new.
Enut11
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Offline Nuno_pt

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Re: Help needed HP3456A self test [partly fixed]
« Reply #17 on: June 05, 2016, 11:15:31 am »
Hi guys,

Sorry to hack the tread, but I been offer from an friend of mine, for few euros, an 3456 with error 4 OL in all the ranges, and another with intermittent readings with no relation.
I've look at the manual and troubleshooting list for error 4, they ask for an Signature analyzer, witch I don't have, will this be worth the trouble to dive in to this and look at it with an DMM only?
Or should I stay away from it?
Nuno
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Online VK5RC

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Re: Help needed HP3456A self test [partly fixed]
« Reply #18 on: June 05, 2016, 11:31:53 am »
@enut11 good to hear all has gone well, my new crystal has arrived (has 60Hz one in it at present) the one for 50Hz ACV measurement has arrived from Krystally.
@Nuno PSU esp caps appear to be a common-ish fault but I recall reading another fault ????balancing a series of FETs or something like that - sounded a real PITA, if all ranges effected, sounds a bit PSU-ish to me. Check out Volt-Nuts site and Yahoo Hewlett-Packard site for further info.
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Offline Nuno_pt

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Re: Help needed HP3456A self test [partly fixed]
« Reply #19 on: June 05, 2016, 11:43:16 am »
VK5RC, thanks from what I've been reading also lead me to PSU, but I don't feel comfortable that the steps require an signature analyzer.
I'll look on the voltnuts and HP yahoo.

Let's see if I grab anyone of this, for now the intermittent readings is in front. 
Nuno
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Offline TiN

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Re: Help needed HP3456A self test [partly fixed]
« Reply #20 on: June 05, 2016, 12:30:47 pm »
Firmware/SRAM could also be the issue, as was revealed by some other 3456A owners. I had this covered before, in this repair worklog, including some other tweaks and mods. I personally don't have any experience 3456A, but it's safe bet to go usual steps for troubleshooting, starting power rails, replacing all old capacitors, reseating and checking interface cables, reseating IC's in sockets, checking any visual damage anywhere..
« Last Edit: June 05, 2016, 02:16:47 pm by TiN »
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Offline Nuno_pt

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Re: Help needed HP3456A self test [partly fixed]
« Reply #21 on: June 05, 2016, 12:43:03 pm »
Thanks TiN, I already have gone look at your website. :-)

I'll see what bargain I can come up with, I think it should be worth 10-20€. :-)
Nuno
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Offline enut11

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Re: Help needed HP3456A self test [partly fixed]
« Reply #22 on: June 06, 2016, 11:00:38 pm »
@enut11 good to hear all has gone well, my new crystal has arrived (has 60Hz one in it at present) the one for 50Hz ACV measurement has arrived from Krystally.

@VK5RC. I have ordered a 50Hz crystal from Krystaly. It would be interesting to know if you see any difference in 6 digit sensitive measurements with the new crystal. For example, is there any improvement in the LSD on low DCv or mid ohms?
enut11
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Online VK5RC

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Re: Help needed HP3456A self test [partly fixed]
« Reply #23 on: June 06, 2016, 11:30:24 pm »
It might take me a quite few days to test it,  my only AC V reference  source is a DMM-check,  I am not sure how good that is in terms of AC accuracy.
(If all else fails,  I'll read the manual!) HiHi
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Online VK5RC

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Re: Help needed HP3456A self test [partly fixed]
« Reply #24 on: June 11, 2016, 02:55:48 am »
HI unut11
I warmed up my 3456A then compared my DMM check to my out of cal 34465
34465 consistently gets         0.000mV                4.99889V AV
3456A                               with shorting bar           DMM Check
 with "wrong AC freq x-tal        0.000254 V               4.99975 V
 with correct AC freq X-tal        0.000324 V               4.99972 V

The leads were a set of home brew Kelvin leads, so the shields are only connected at the meter.
I suspect to really pick the difference I would need to go through a full calibration, not sure I am enough of a volt nut for that!
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