modified: Monday 25 March 2019
Repair: Allied Telesis AT-8000GS 24port gigabit switches
I recently bought a pair of used AT-800GS switches off eBay:
The units were slated as fully working, but unfortunately I found the fans in both had seized:
Crispy! It looks like the lubricant turned reactive (acidic?). All of the laminated steelwork and bearings had been eaten, so it wasn’t a worthwhile option to try and refurbish these fans. I’ve been known to strip fans and relubricate them to get some extra life (especially when they turn noisy), but we’re looking at a situation that’s far from that simple.
The units appeared to function without these fans, but I had no trust in their longevity. Sitting on my airy bench with no load is one thing, sitting in a stack switching gigabits of traffic is another.
The eBay seller was kind enough to send me a couple of replacement fans, but they were in almost similar condition, so I started hunting some alternatives.
A friend informed me that this model of CeraDyna cooler was used on a lot of VGA cards back in the day and was renowned for early failure. Luckily the footprint of the fans and heatsink is a common size: 40mm fan with 60mm peg spacing.
eBay has lots of really cheap fan+heatsink combos in this size (keywords: “vga cooler”) going for a few dollars:
I found that this is the cheapest way to get the heatsinks — buying them separately actually costs more.
The seller chucked the fans in a plastic shipping bag with no padding. One of them arrived broken:
These fans are of very dubious quality to begin with. Never trust unbranded sleeve bearing fans being sold for less than $2, that’s like trusting “free” ATX supplies that come in cases. I need to make up some “Gigglepower” stickers.
The heatsinks themselves come with annoyingly placed labels and don’t have the golden anodisation milled off the thermal contact surface:
These heatsinks are much larger and beefier than what they’re replacing, and they’re cheap, so I’m not complaining.
For replacement fans I ordered some 40mm units from the well known brands of Sunon and Delta. The sunon is particularly interesting motor-wise: it only seems to have a single coil:
I’m not sure how that works. Multiple taps? That bent sheet steel on top looks specially shaped.
Getting to the heatsink pegs for removal required completely disassembling the switches and taking the motherboards out. They are an inconvenient fit in this model and require bending the case slightly.
Removing the heatsinks themselves required a notable amount brute force. The old thermal paste appeared to be (or have become) a hard adhesive. It took lots of physical abrasion with alcohol to remove the yellow plague and I lost the main MCU markings (black writing) in this process:
Pasting and installing the new heatsinks was relatively simple. Don’t stare too closely at the mismatched pegs, I might have lost one under the lounge:
Plugging the new fans into the motherboard wasn’t straightforward. Most computer fans come with 2.54mm (0.1”) fan headers. The headers on this motherboard are 1.25mm pitch and appear to be Molex Picoblade, something that’s hard for me to cheaply and easily get small quantities of crimps and casings for.
I instead opted to butcher the original (dead) fan cables and make some adaptions. On the left are some soldered and heatshrunk splices, with black tape acting as a thick anchor point for the clear heatshrink. This was visually satisfying, but means that if I have to replace the fan again I’ll have to cut all the connections. On the right is an unmodified fan using a small picoblade<->2.54mm adapter cable, something I think is a better idea in the long term.
Caution to future cable tanglers: the 3 fan wires are no in standard order on these boards, make sure to re-arrange them appropriately before plugging a standard PC fan in.
Everything now works and the sound levels are manageable. The replacement heatsinks+fans are much taller than the originals, but they still have a workable airgap above them in the 1RU cases. I’m quite happy now and I’ll have to see how things go.
These units use RJ45 (ethernet cable) for their console. I didn’t get any cables with the units and authentic ones are going for >$100 on eBay. I couldn’t find any mentions of the pinout on any documentation for the switches and the various Allied Telesis pinouts I found on the web were all conflicting.
In the end I put a few bits of information together to reverse engineer the pinout myself:
- Most management interfaces are RS-232. The minimum you need for this is 3 wires (GND, RX, TX)
- The chip on the motherboard next to the console connector has a publicly available datasheet. It’s a level shifter with a known pinout.
- Most things are 9600 or 115200 baud with 1bit parity.
- GND pins are easy to determine: they’re at the same voltage as the case.
- RX and TX pins tend to be held “high” by pull-up resistors.
- It’s generally safe to trial-and-error RX and TX combinations if you put something like a 1K resistor in series to avoid shorts.
A bit of gruntwork later and I had fully working console access:
Gladly these units had the default passwords set, so I didn’t have to delve into a deeper world of repair.
For now however they sit in hibernation, until I have some more time to fix my lab again: