Can you identify this resistor?
December 2, 2019 1:16 PM   Subscribe

Here are pics: resistor, power board. I'm trying to resurrect my iMac G4 (iMac G4/800 17-Inch Flat Panel - M8812LL/A - PowerMac4,5 - M6498 - 1936), with help. We've gone through all the easy and obvious fixes on iFixit et al. We've got down to the power board and found a burned-out resistor. Can anyone help identify this? Size is about 20 x 6mm. Colour bands indicate is either 12ohms or 1k, no certainty. Advice from various forums has not been definitive.
posted by glasseyes to Technology (21 answers total) 2 users marked this as a favorite
It's 1K, +/- 5% :

Unless you have some non-mainstream reason to think it's 12 ohms?
posted by amtho at 1:28 PM on December 2 [2 favorites]

I have no knowledge of macs, but that is a switching power supply. In addition to the burned resistor (I'd call it 1.0K, 2w) there are likely shorted diodes or transistors.
It might be better to search for a used or reconditioned board.
posted by H21 at 1:28 PM on December 2 [4 favorites]

H21: wondering how you knew it was 2w? Asking for my own edification :)
posted by amtho at 1:31 PM on December 2 [1 favorite]

1k or 10k: the third band (from the left) is/was either red or orange. So again not definitive.

The bigger problem is why it burned out. It sits between what must be one of the DC pins of a rectifier (probably +, but that's actually not really relevant) and transistor Q1. As that one looks a tick unhealthy too it may well have been the cause of that resistor letting out its magic smoke.

We do have that shaving mirror Mac in our museum, and I can take a peek inside it the next time I'm there. Possibly this Friday, else Tuesday 17th.
posted by Stoneshop at 1:35 PM on December 2 [2 favorites]

Amtho: size. And eyeballs trained on determining electronic parts. Which I guess H21 has a pair of, too.
posted by Stoneshop at 1:38 PM on December 2 [1 favorite]

amtho: it appears to be a carbon film resistor. 20x6 mm is about the size of a 2w carbon film resistor. Really, from experience, it looks like a 2 watt.
posted by H21 at 1:40 PM on December 2 [1 favorite]

I guess the third band could be orange?

The only way I'd be able to tell the color for sure would be to find other similar resistors that had a slightly oranger or redder band to compare.
posted by amtho at 1:57 PM on December 2 [1 favorite]

All of you are amazing. Stoneshop, it was quite a lot of unplugging wires and hoiking bits of boards about to get to that resistor, but if you're ok to do that, that would be wonderful. I think there were 3 models of the G4 - mine is the earliest I think - and I suppose that's why when I look at spare G4 power supplies for sale online there are quite noticeable differences.

My technician (firstborn son) did say that black tub-looking thing looked a bit puffy but on further inspection seemed to be sound.

In a museum eh. This world! Aiye re.
posted by glasseyes at 2:15 PM on December 2

Does it test open with an ohmmeter? If you're judging burnt out from just the appearance, it may be fine, since as others said it's a high-power resistor and may run pretty warm normally in this circuit.
posted by fritley at 2:16 PM on December 2

The bigger problem is why it burned out.
Maybe I did stupid things to it the first time it wouldn't turn on?

Yes he went through all the test points as described in the repair manual, took the thing apart testing all the way and found power was going into that unit (ticking) but not getting to the resistor. I realise this sounds reckless but it's because I'm not describing it right rather than that he did anything stupid.
posted by glasseyes at 2:22 PM on December 2

In a museum eh. This world! Aiye re.

Well, we're not the Living Computer Museum, nor The National Museum of Computing nor the Heinz Nixdorf Computer Museum but we do have a large pile of not quite scrap fairly decent collection of somewhat aging stuff, of which that Mac is probably one of the newest.
posted by Stoneshop at 2:33 PM on December 2 [2 favorites]

Maybe I did stupid things to it the first time it wouldn't turn on?

As a friend of mine has stated: there's two kinds of components: ones that are broken, and ones that aren't yet.

It's quite likely the transistor went to its Silicon Burial Grounds because that's what they do sooner or later, and as a result took the resistor with it. Especially when that transistor is only marginally sufficiently cooled and operating close to its maximum specs it will do so sooner rather than later, and a lot of switching power supplies will self-destruct if one of a few critical components gets to operate outside its nominal envelope. In this case there's one clearly visible victim; there may well be others.

I'm often somewhat amazed that components can run as hot as they do for as long as they do, and not even noticeably degrade. Like, a 100W laptop power supply just slightly larger than a packet of cigarettes, no active cooling and hotter than you can comfortably hold in your hand; still they work alright for years.
posted by Stoneshop at 2:57 PM on December 2 [2 favorites]

Maybe I should have taken that tip about replacing the thermal paste seriously :-(
posted by glasseyes at 3:02 PM on December 2

Is that whiteish smear on Q1 a bit of thermal paste? If so, what is it supposed to make contact with?
posted by Stoneshop at 3:07 PM on December 2

Dunno, think it's adhesive, fixing it in place. We didn't really touch the power board except to take out the resistor. You'll see there's two more places with the same substance
posted by glasseyes at 3:25 PM on December 2

There's not all that much detail in the repair manual, the advice is to replace with a new component - not so easy 15 years on. The power boards I've seen for sale haven't been the same as my one
posted by glasseyes at 3:30 PM on December 2

Putting thermal paste on top of Q1 and having it press against a bit of metal (of the system chassis) would have been a rather odd way of adding cooling to it, but it wouldn't be something that Apple wouldn't do in its urge to squeeze more components into a volume they can't actually physically fit into.

Anyway, I'll have a look soon.

By the way, are those newer power boards the same shape, and with similar wiring coming off them? Do you have a link for one of those?
posted by Stoneshop at 1:48 AM on December 3

Since I last looked, one's come up that looks the same here. So at least that will be an option I guess. But it would be very satisfying to spend a sixth of the money and also solve a conundrum. The others I found are the same shape with slightly different configuration. Here's a different one. And another one. Here's a link to the service manual page about applying thermal paste. I could mail you a copy if it's useful?

Thanks so much for pursuing this.
posted by glasseyes at 3:37 AM on December 3

The thermal paste warning refers to the heatpipe-to-case transfer; the other end of the heatpipe will be sitting on the processor (and possibly the video controller). Whenever one of those contact areas is disturbed it's preferable to clean it and reapply fresh paste, although when you don't then even with gear the age of your Mac the processor will start throttling instead of frying itself; you'll just notice a slowdown. If you're in the process of troubleshooting a different problem it's no big deal to not refresh the paste every turn, and only do it properly the first time around and at the end.

Lack of proper thermal coupling hasn't been the reason for the resistor expiring anyway.
posted by Stoneshop at 7:14 AM on December 3

The Mac in our museum has the newer power supply boards, unfortunately, but from the circuit on its power input board I would make the somewhat informed suggestion that the resistor on yours once was 10k (2W).

Also, even if that's wrong and it actually was an 1k resistor, putting a 10k in is less likely to blow things up rather than starting with 1k when it should have been 10k.

The thermal paste warning is about these contact areas where the heatpipes from the processor mate to the metal chassis inside the bowl:
posted by Stoneshop at 7:00 AM on December 7

Thank you so much Stoneshop. 100 internet brownie points for you
posted by glasseyes at 4:22 PM on December 8

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