Replies

Ok, laugh if you want. Yes, I know I could buy a new one for $20, but the experience gained from fixing it is priceless,IMHO.

My old 8-port 10baseT hub which has a utilization indicator bargraph started crapping out. It was/is a problem in the power supply (bad filter caps and possibly a bad bridge rectifier which may have ruined the caps), and I will describe how I troubleshot this problem.

I first noticed it when FTP'ing files--when I put a lot of traffic through it, random LEDs in the bargraph indicator would go on and off, the link LEDs all turned off, and the blinking light which indicates that the CPU is functioning stopped blinking.

The hub has a light sequence it goes through when power is first applied. It stopped doing that shortly after I noticed that it wasn't working right.

Upon further investigation, I determined that the voltage out of the 5-volt regulator was low--about 4.5 volts, should be much closer to 5.

I tried powering it from a 12VDC power supply and it worked fine, doing the light sequence. (Normally it uses an 8VAC power supply). This confirmed that the problem was in the power supply section.

I checked the bridge rectifier, and although it was getting 8.5 volts in, only 6 were coming out. I thought that was an indication of a bridge rectifier failure so I desoldered it and replaced it with a new one ($1.39 from Radio Shack).

With the new bridge rectifier installed, it still didn't work. Further investigation was needed and was done.

I had initially thought that the filter capacitors (two 1000uF 16V electrolytics) had gone bad.

The normal way to check this is to stick a scope on the DC line and check for 60Hz "ripple". I have a scope, but I really didn't want to dig it out of the basement so I had another idea.

I thought I'd connect a normal telephone to the DC supply and listen for 60Hz hum in the handset.

This worked great. With the 8VAC power supply connected, there was a LOT of hum on the input to the 5VDC regulator. There was slightly less hum on the output. I was pretty sure this confirmed that the filter caps were bad, but I wanted to do a bit more testing.

I connected the 12VDC power supply to the hub, and did the same test. A little hum on the input to the regulator. No hum on the output. It started to look like the filter caps were really bad.

One more test: I decided to go dig through my box of parts and find a capacitor. The largest I found was 470uF, but I thought this would work to confirm the problem.

I connected the 470uF capacitor between the input to the voltage regulator and ground. When I powered up the hub, it did the normal light sequence.

From what my testing indicates, it should work fine if I replace both 1000uF filter capacitors in the power supply section.

Unfortunately, Radio Shack only sells 35V 1000uF capacitors which are too big to fit into the space that the 16V 1000uF capacitors take, so I'll have to stop by a real parts store and get them.

By the way: I have no formal electronics training (other than a course in high school, which taught me nothing I didn't already know..it was an easy "A" though). It is and has been an interest of mine for quite a few years.