HP/Agilent 4284A/4285 electrolitic capacitor defect & A2/A3 board schematic - Page 1
Forum section: electronics / repair

author: scroky
author_rank: Contributor
timestamp: October 11, 2024, 03:31:09 pm
content: Hi,I have a problem with my LCR meter HP4284A:Due to some faulty electrolytic capacitors, some burns occurred on the A2 and A3 boards.Unfortunately, the HP service manual is not complete with diagrams in all its parts (I guess due to confidentiality constraints), so now I cannot complete the repairs if I do not find the value of the R432 resistor on the A2 "modulator" board.I would like to thank anyone who can help me with the diagram of the A2 and A3 boards or simply with the value of the R432 component on the A2 board?ThanksAlessandro

author: picburner
author_rank: Frequent Contributor
timestamp: October 11, 2024, 04:10:17 pm
content: If no one tells you the value of the components you have to find the CLIP manual, there you can find the schematics.I have the one from the 4285A, I checked but unfortunately the A2 board is different.

author: scroky
author_rank: Contributor
timestamp: October 11, 2024, 04:25:03 pm
content: hello, thank you so much for your quick reply. But what is the CLIP manual? Sorry I'm ignorant, but where could I find it? thanks

author: picburner
author_rank: Frequent Contributor
timestamp: October 11, 2024, 04:34:05 pm
content: CLIP means Component Level Information Packet.Basically it is the service manual that also provides you with the schematics.I bought mine from Artek Manuals.

author: scroky
author_rank: Contributor
timestamp: October 11, 2024, 04:43:33 pm
content: I searched around... also onhttps://artekmanuals.com/manuals/hp-manuals/but unfortunately for the 4284A it seems it has not been made public... I will search around better.If I find it I will post it for the benefit of all.Thanks again for the helpBest regardsAlessandro

author: picburner
author_rank: Frequent Contributor
timestamp: October 11, 2024, 05:05:02 pm
content: Yes, I searched me too but Artek Manuals only has the CLIP for the 4285A, the one I have.I searched around a bit but the one for the 4284A is not found, at least not that easily.Happy hunting.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: October 15, 2024, 12:37:01 pm
content: You can try to figure out something from this ebay image,https://www.ebay.com/itm/126654785564it looks like correct board. In few weeks my 4284A will arrive, I can post better photos by then.

author: scroky
author_rank: Contributor
timestamp: October 25, 2024, 09:14:55 am
content: Hi ZhuraYuk,yes the card is this one you found, if you could recover the value it would be fantastic. thanks Alessandro

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: November 19, 2024, 03:19:05 pm
content: scroky can you figure it out ? I see red brown green gold brown, but I am not sure even with naked eye that it is 2.1Mohms

author: picburner
author_rank: Frequent Contributor
timestamp: November 19, 2024, 05:36:26 pm
content: It's a challenge to figure out the colors from those photos but it looks like a 21.5 ohm 1% resistor.

author: scroky
author_rank: Contributor
timestamp: November 19, 2024, 05:55:37 pm
content: Hi ZhuraYuk,thank you very much for your picture, now i can proceed to the repair of the instrument. Best RegardsAlessandro

author: scroky
author_rank: Contributor
timestamp: November 20, 2024, 11:39:58 am
content: Hi everyone,Thank you again for the photo and advices.Since the resistor burned out in a few seconds because a downstream electrolytic capacitor was shorted, I don't think it's a 2.21 MOhm resistor, but more likely a 21.5 Ohm resistor.In fact, I think the maximum voltage on those boards is 24V: the calculation p=V^2/R gives 2.7 x 10^(-4) W, less than a milliwatt.Looking at the colors, the value 2.21 MOhm seems correct, but I have a doubt: can you please measure the resistor in question?Attached is a photo of the same board in which I highlighted the component with the illegible name that seems to have heated up a lot. The case is that of a TO220 power transistor. Can you please read what is written on that component? Notice that there are two TO220 components placed one behind the other very close, the first (the unknown and highlighted one) hides the second.can you help me once again, please?ThanksAlessandro

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: November 20, 2024, 04:30:06 pm
content: The TO220 reads 7912CT NLFI 012. I did not desoldered resistor, in scheme it measures as 21 Ohm for DC current and 19 Ohm measuring with LCR meter.

author: scroky
author_rank: Contributor
timestamp: November 20, 2024, 05:12:40 pm
content: Hi ZhuraYuk,from the new photo and your measurements. I would say that there are no more doubts:RED, BROWN, GREEN, GOLD (x0.1) and BROWN (1%) so: 21.5 ohm (1%)Thanks for all.Best RegardsAlessandro

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: July 07, 2025, 05:22:55 pm
content: So, I did this device PCBs review in the hope that it might help someone fix it