Huawei R4830 Repair - Page 1
Forum section: electronics / repair

author: AutoSpark
author_rank: Newbie
timestamp: February 28, 2025, 09:30:58 pm
content: Hi guys. I'm new here and don't really know very much about electronics, so be gentleI bought a charger for my electric unicycle from aliexpress. It's based on a modified Huawei R4830, which I think was originally a charger for UPS batteries in cell phone towers. Anyway, one of the FETs in the charger shorted and I need to try and repair it. I'm looking for a little advice about mounting the new transistor. The original one is bonded to the heatsink with a white rubbery stuff that feels kinda like the silicone you would seal a bath tub with (photo attached). I'm guessing that transistor was fitted as part of the modification. On the opposite side of the unit there are other power components that are bonded in a much neater fashion using a grey coloured stuff that is hard like epoxy. I'm guessing those are original components fitted by Huawei. What I'm wanting to know is how should I mount the new transistor? Should I just bond it back on in a similar way to how it was done originally as shown in the photo (only hopefully not quite as messy). Or should I drill the heatsink and bolt the transistor to it? Can someone tell me what the stuff is the transistor is bonded with. Surely it's not really bath tub silicone. If I was to drill the heatsink, should I remove the thick thermal pad and replace it with a thin silicone one? Or would it be OK to leave the thick pad in place. That looks like it has been bonded to the heatsink with the hard grey epoxy type stuff. It might be a PITA to remove it.Thanks in advance for any advice you can offer.

author: benj38
author_rank: Frequent Contributor
timestamp: March 01, 2025, 08:31:54 am
content: What is behind the transistor (i.e., between it and the heatsink)? It is hard to see from the image if the white stuff is only on top of the transistor, or also behind it, and your description is a bit contradictory at times. Test if there is electrical contact between the transistor's metal tab and the heatsink.If the white stuff is only on top, it is just a lazy way to keep the transistor from moving away from the heatsink, and it can be simply RTV silicone. Note, however, that "bathtub" silicone has acetic acid in it, and should not be used inside electronic equipment as the acid vapors may corrode things. You will need "electronic grade" silicone instead.If the stuff is also between the transistor and the heatsink then it should be special heat conducting epoxy (this is certainly what the hard grey stuff is), though I have seen Chinese stuff where it was just some garbage material. You should not try to reuse this glue (i.e., remount on top of it), you must remove such glue to ensure good heat conduction between the new transistor and the heatsink.If I were you, I would in any case drill and install the transistor using a screw and nut. If the transistor should be electrically isolated from the heatsink I would use a "SIL PAD" (make sure that you also have a small plastic sleeve preventing the screw from touching the front and sides of the hole in the transistor's metal tab); if it is not electrically isolated, I would just use a little bit of heatsink compound.

author: AutoSpark
author_rank: Newbie
timestamp: March 02, 2025, 11:05:24 am
content: Hello and thanks for taking the time to respond.The white stuff seems to be what is bonding the transistor to the thermal pad. If you look to the left in the photo you can see some of the stuff spreading out from behind the neighboring transistor.I just measured the resistance between the heatsink and the terminals of the transistor. It shows an open circuit.I'll take your advice and secure the new transistor (and it's neighbor) with a screw and nut. The white thermal pad that the transistor is currently mounted to seems to be some sort of ceramic material. Do you think it would be possible to just drill through that and mount the new transistor to it? Or do you think the ceramic will just crack if I try?

author: benj38
author_rank: Frequent Contributor
timestamp: March 02, 2025, 01:07:29 pm
content: Quote from: AutoSpark on March 02, 2025, 11:05:24 amOK, I now understand better what is in the image.Quote from: AutoSpark on March 02, 2025, 11:05:24 amIt will crack for sure. Forget about my advice about drilling, I did not know it was ceramic.You will have to clamp or glue it then (or use a special drill bit for ceramics, but why bother?). I guess the easiest would be to glue it. Look for "heatsink glue" or "thermal glue", or "thermally conductive glue".