EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: DavidG on May 10, 2016, 05:40:23 pm
-
I'm in the process of attempting to repair an HP 8161A Programmable Pulse Generator that suffered corrosion damage from a leaky battery. Yesterday, while I was repairing the area around the battery, I noticed an unknown component that was damaged. It's a disk of some kind and two soldered-on leads. The disk had separated from one of the leads and was only barely holding on to the other.
I've attached pictures of the part and the front and back of the board immediately surrounding it. (Sorry, for now, I don't have better pictures.) The part seems to be part of the circuit connected to the backup battery (which was a 2-cell NiCd pack for 2.4V). I suspect it's a capacitor, varistor, or thermistor, but I need to know for sure. What's throwing me off is that it isn't dipped. I find this very peculiar - it makes me wonder if it might be a component I'm not yet familiar with.
Does anyone know for sure what it is? Thanks.
-
Might have been a cap to store a charge until the batteries are replaced when removed. Check the schematic ...
-
Hi There,
If it is in series with the battery and the battery is being charged by a constant voltage, it is most likely a PTC to prevent excess charge or discharge current. For your pictures, it looks like it is in series with the negative battery terminal. The switch-over circuit is probably on the high side,right? In that case putting a 50-100mA PTC of the same size will be OK.
If it is in parallel with the battery, it may be a MOV to prevent over voltage, to large of a battery pack, but that is unlikely.
Disk capacitors are generally not of the high capacitance type, so I do not think it is a buffer to sustain backup during battery replacement. Is the inside of the disk solid or is it multilayered? If multilayerd it may yet be a capacitor as suggested, maybe even a gold cap or super cap.
Good hunting.
-
I snipped you an area around the battery from the schematic for this unit. Check it out ...
Edit - cancel that - that schematic is not for that unit, not sure why the site had it listed as such. I'll try again. :-\
-
I agree with EPTech, looks to be in series with the battery so it's likely a PTC or Polyfuse. NiCd batteries can provide enough current to vaporize traces and explode shorted components, so a PTC makes sense here as a current limiting device. Google images for "PTC polyfuse" and you will see the similarity, especially the leads that cross the device at an angle, that's something I've seen in MOVs and PTCs but little else.
It may be tricky to choose an appropriate replacement without the original schematic or parts list. Maybe measure the maximum current consumed from the batteries (or pushed into them during charging) and choose a polyfuse rated at about 2x that current.
-
Thanks, everyone. This weekend, I'll try to map out the area of the circuit around that part to try to confirm EPTech's theory, because it makes quite a lot of sense to me.
-
Sorry I didn't see this before.
I have a HP 8160A, which seems to have the same CPU board. Also have the service manual. Here's photos of that part intact, the schematic, and the parts list entry for it.
Unfortunately the manufacturer code 28480 means HP themselves. But at least the part's values are given.
And thanks for reminding me this thing has a NiCad battery in it. Mine has been sitting on the shelf for a long time, with the battery completely dead. Would have started leaking soon no doubt. Removed now.
Someday I'd like to make something constructed entirely of nickel cadmium batteries, tantalum capacitors and bad electrolytic caps, and give it to my worst enemy.
Edit to add: And those stinky 'mains rated' polyester caps, that make the most vile stench when they (always) eventually fail. That I'm now sitting in a room with that smell.
I'd taken my 8160A apart to take that pic. Desoldered the old battery. Noticed there's one EPROM, desoldered and read that, socketed it (not going to bother with the other 3 mask ROMs, unless I find there is no online copy of the images. )
Put it back together without the covers and powered it up, thinking I'd have another quick look for the fault. (I didn't have a manual originally.)
Ah, but I should have taken the power supply apart first to check for those stupid caps. Took about 3 seconds to fail, with a big cloud of vile smoke. Now I have a large fan blowing air through the workshop, in one door and out the other, to try and clear it. An hour later and it still stinks. Also it's a cold evening.
There are two more of those caps in mains voltage parts of this supply. Hard to get at... so there goes this evening.
Edit: And 4 more on another board, also with mains voltage across them. So 7 in total.
-
Ah, but I should have taken the power supply apart first to check for those stupid caps. Took about 3 seconds to fail, with a big cloud of vile smoke. Now I have a large fan blowing air through the workshop, in one door and out the other, to try and clear it. An hour later and it still stinks. Also it's a cold evening.
Before I clicked on the pic I thought "I wonder if it was made by Rifa". ::)
-
DavidG, you'd better check the power supply in yours too.
After replacing all 7 of the RIFI polyester foil caps in the high voltage parts of the power supply, I was about to put it back together. Looking at the 'caps and inductors' board of the power supply, thought 'wait a minute, those are tantalum wet foil'. Which I've recently learned are very unreliable when old. See https://www.eevblog.com/forum/repair/grc-106-military-radio-repair-source-odd-capacitor/ (https://www.eevblog.com/forum/repair/grc-106-military-radio-repair-source-odd-capacitor/) (I ended up replacing *all* the aluminium-can tantalum caps in that.)
So I checked them all. And all 10 are hopeless and need replacing.
There are 2 x 650uF 20, and 8 x 180uF 30V
Actual measured values:
650uF are 190uF, 168uF.
180uF are 16, 112, 83, 4, 65, 119, 114, 10uF
Wow, these things are terrible. This type of cap is now added to my 'replace on sight' list.
Pics are the board (from power supply of HP 8160A) and mug shots of the guilty caps.
This type of capacitor can be recognized by the lead coming out of the PLUS end (with the teflon/plastic seal) being made of tantalum (a little purplish in color) then being spot welded to an extension wire that can be soldered.