HP 3455A Repair help - Page 1
Forum section: electronics / repair

author: Thirtykay
author_rank: Contributor
timestamp: May 16, 2018, 04:40:53 pm
content: Hello,i recently picked up a HP 3455A Multimeter for 25 bucks, it was sold as non-working.i have the following problems : the unit turns on but the display stays blank and all the led button's lights up.So i've gone through the manual troubleshooting section and chose to follow the service group A.everything seems to be ok until the "c" step of the 8-A-5 Outguard troubleshooting section.The U26 ( 74LS74 D-Type Flip flop ) pin's 9 pulse is ok, U5 ( 74LS05 inverter circuit) receive it, but the output of each inverters seems to be completely noisy.Is that a normal behavior ? or the IC is faulty ?Inverters InputAnd the OutputThanks,

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2018, 05:54:25 pm
content: The 74LS05 has open collector outputs. So the rise time can depend on the external pull-up resistor. So by it's own the waveform might be OK, but unusual as some pulses are so short that they hardly reach a good logic high level.If the shown input and output should belong together, there is something wrong. This could be the chip, but could be a supply problem too.

author: Thirtykay
author_rank: Contributor
timestamp: May 16, 2018, 06:26:33 pm
content: Thank you for your answer.I suspect the chip because like you said the input and output should belong together.But now that you said it could be a power supply problem, you might be right.The previous owner told me that it was working like a charm before he put it to storage. Could it be a capacitor who suffered from humidity stress or something else ?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2018, 07:09:36 pm
content: It is possible to have electrolytic capacitors that went dry or even leaking. O it is a good idea to check the supplies for voltage and ripple. However the error does not look that much like a supply problem. Depending on what else is connected the waveform might be close to normal, as the open collector chips are often used for some extra functions (e.g. wired AND) and usually not with just a simple pul-up to +5 V.It is also possible to have EMI filter caps that now have an increased danger of blowing up if humidity entered through cracks.According to service manual the signal should be more like a stable clock with full amplitude. So the large number of pulses is OK, but they should be more regular.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 16, 2018, 07:59:59 pm
content: Definitely check the supplies first. That seems to be a popular failure with these meters, though in general they're extremely reliable. You'll chase all sorts of phantom problems until the supplies are good.

author: Thirtykay
author_rank: Contributor
timestamp: May 17, 2018, 04:38:51 pm
content: After some checking on the power supply, it appears that i have a serious problem, let me explain:The Outguard board power looks good:9V : 9.47V5V : 5.1V12V: 12.2VBut on the Inguard board the power is totally absent from the board:30V : 0V-24V : 0V29.3V : 0V9.38V: 0V15V : 0V-15V : 0V-2V : 0V5V : 0VNevertheless i still have both AC output coming from the transformer and going the inguard board.The AC input at the first full bridge rectifier is about 60VAC @ 50Hz , the bridge is composed of 1N4004 diode and the output is about 1.16V.The AC input at the second full bridge rectifier is about 32VAC @ 50Hz, the bridge is composed of 1N4004 diode and the output is about 250mVWhich let me think that both bridge are dead. what do you think guys ?Thanks,

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 17, 2018, 10:02:20 pm
content: It is possible. But with that low a reading, the first question I'd ask, is did you turn the meter to DC Voltage range to check the output. Those voltages look like AC ripple voltages on the bridge outputs? It's just a double check.

author: Thirtykay
author_rank: Contributor
timestamp: May 18, 2018, 05:07:25 am
content: I tried both AC&DC output stays the same, the output look like a kind of sawtooth.

author: Thirtykay
author_rank: Contributor
timestamp: May 18, 2018, 08:22:30 am
content: After checking the diode, they all look good, 530 mV across them with the diode tester. I've checked the capacitor sitting next to them and they seems to not charge at all, so that might be the issue.

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 18, 2018, 08:28:23 am
content: Yeah, I was about to say to check caps on the powerline, as a sawtooth suggests the bridges are working, but then getting shorted as the voltage climbs. Cap might meter as ok, but short once it reaches a certain voltage. I'd replace it and see what happens.

author: Thirtykay
author_rank: Contributor
timestamp: May 18, 2018, 11:21:18 am
content: So i swaped caps (1000µF 25V) and..miracle +30V, -24V, -15V and +5V came back online.I will try to swap the other one just in case... however the +15V and +9V are still absent, lets go for some more troubleshoot.

author: Thirtykay
author_rank: Contributor
timestamp: May 25, 2018, 12:58:45 pm
content: back at the thing one week after... sorry no time for it the last week.I might have found why i dont have any +9VDC or +15VDC, this is because the 7815 seems to be dead. Check the posted schematics, and tell me what you thinks guys.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 25, 2018, 01:47:52 pm
content: A dead 7815 is possible. Another possibility would be a short - like a dead tantalum capacitor at the 15 V. If the 7815 is getting hot, it is more like a short.

author: Thirtykay
author_rank: Contributor
timestamp: May 25, 2018, 01:49:34 pm
content: will check but i dont think it was terribly hot, looking forward this. thanks for your answer.

author: Thirtykay
author_rank: Contributor
timestamp: May 28, 2018, 11:40:19 am
content: It's alive, i switched the old 7815 with a new one and it's now working !Thank you for the help.

author: Thirtykay
author_rank: Contributor
timestamp: June 08, 2018, 01:08:57 pm
content: After the repair, i've noticed ( by playing with meter's function) that by pressing auto-test button, auto-cal test #5 and #4 are failing, but where it get interesting is that after 10 minutes of operation #5 failure has disappears and 5 minutes later #4 failure goes off too, that's very strange, it's like the meters is warming up...I've gone through the manual but so far i've not tested anything but i've looked at the components that are maybe faulty and look for an economic way to replace them... not a easy job, since every component like MC1436CG or HA2-2526 are not produced anymore and they cost more than 50 euros each...that's not worth it for an 15 euros meters...Anyway, i just want a feedback on what that could cause a such weird behavior.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2018, 02:01:09 pm
content: It is normal for a precision meter to need a 20 min to 1 hour warmup period before getting full performance. It could be some offset that is borderline in the cold state and gets better than warm. So the first point would be to check what the errors #4 and #5 are. They seem to be related to the 100 V calibration. It could be as simple as adjusting the right pot to bring an offset back to the allowed range.So I would not worry that much about these messages.

author: Ray Gianelli
author_rank: Contributor
timestamp: June 08, 2018, 02:05:52 pm
content: Agreed. The manual states that for the meter to reach its rated accuracy it needs to be on for at least one hour.

author: Thirtykay
author_rank: Contributor
timestamp: June 08, 2018, 02:14:46 pm
content: Wow, didn't know about that, thank you but can you give the page number ? didn't see anything about that.Edit: Nevermind i've found.

author: Ray Gianelli
author_rank: Contributor
timestamp: June 08, 2018, 02:49:27 pm
content: Page 3-1, Section 3-4Never mind, I see you found it. Note to self: read entire post before replying!

author: Thirtykay
author_rank: Contributor
timestamp: June 08, 2018, 02:51:05 pm
content: Yeah, so everything is alright, topic marked as solved. Thank you.