Testing Capacitors From a Dead Modem

A couple weeks back I started having problems with my ADSL2+ internet connection.  A lot of dropouts and slow downloads, and at times speeds lower than dial-up.  I knew what my connection was capable of, before the problems started I was getting around 14 Mb/s downloads and after it was around 3 Mb/s at best, so I knew that the distance from the phone exchange had nothing to do with my issues.

After several phone calls to my provider, TPG, and going though their frustrating isolation tests and checking cables they confirmed that there was fault on the line and they'd send a technician out to investigate.  While waiting, I decided to replace the modem.  It was old and I wanted to make sure that the problem wasn't on my end.  After removing the old modem and setting up the new one, the connection was still slow, so I decided to reconnect the old one until the issue was sorted.  After turning it on all I got was flashing lights.  I'd had this problem before, leave the modem on for a couple of minutes, power cycle it, and it would start working.  This time, no luck, it just wouldn't start.  I put the new modem back in and threw the old one in the corner for the time being.  I suspected the capacitors in it were dead.

Old NetComm NB6Plus4W modem

Old NetComm NB6Plus4W modem

Old NetComm NB6Plus4W modem

After the technician came out and fixed the line fault, I still didn't have a decent connection.  One more phone call to TPG and things were back to normal.  The last phone call involved a troubleshooting step that required me to plug my phone line into the network port to reset my firmware.  Yeah, it sounded dodgy to me too, there can be over 100 V on the phone line when ringing, but after two weeks of almost no internet connection I was willing to sacrifice a modem on the off chance it'd work.  I figured the designers of these things anticipate that at some point someone would accidentally plug the phone line into the network port and there would be appropriate protection in place.  Surprisingly this worked.  It doesn't sit well with me though.

I still don't know what the real problem with the connection was, it's unusual for two things to fail at once.  If I had to guess, I'd say that the suspected dead capacitors in the modem created noise on the line that caused it to switch to a low speed profile.  As for TPG finding a fault and getting it fixed, if you look long enough you'll find something wrong and something to fix, but to be honest I don't really know, I'm just happy things are working again.  That old modem was bugging me though, I needed to know what went wrong.  Time for a tear down.

Let's go through and see what does what on the PCB.

1. Power supply.  It takes a 12V AC input, rectifies and filters it to about 15.5 Volts DC.  Two MP1410 step down converters provide 1.8 and 3.3 Volt rails
2. Ethernet sockets and associated isolation transformers.  I suppose plugging in a phone cable wouldn't actually hurt the device
3. USB input.  You can see the traces coming out of it and heading to the main processor
4. Phone line input and transformer
5. Broadcom BCM5325EKQMG Ethernet switch
6. 7805 5 Volt regulator and Broadcom 6301KSG ADSL line driver
7. Elpida DS1216AGTA 128 Mbit SDRAM - working memory
8. Broadcom BCM6348KPBG single chip ADSL2+ controller
9. Samsung k8d3216UBC 32 Mbit Flash Memory - non volatile memory to hold software
10. Broadcom BCM4318KFBG 802.11 b/g transceiver
11. Skyworks SE2521A60 Wireless LAN front end.  Power amplifier and associated RF functions

Modem PCB Top Side

What's underneath? Nothing much, just some passives and transistors by the look of it.  At the top of the board you can see where heat from the main processor on the other side has discoloured the PCB over time.

Modem PCB Bottom Side

My initial assessment of why the modem stopped working was that the electrolytic capacitors had dried out and weren't doing their job any more.  I based this diagnosis on what I knew about the modem.  It was at least 4 years old and had operated pretty much non stop since then.  With a maximum power draw of 12W and only passive cooling, it did get quite warm.  These are perfect conditions for capacitor failure either by an increase in their ESR or by a decrease in value.  I also suspected bad caps when I tried to start it and the lights on the front kept flashing in a manner that indicated that it was powering up but a power glitch caused it to reset and start again.  From a cursory visual inspection it seemed this was the case.  Everywhere I looked I saw bulging capacitors, a sure sign of a capacitor on it's last legs.  One of them was positioned right against the heat sink of a voltage regulator.  I suppose that was to keep it warm if it got cold.  Facepalm.

Bulging Capacitor

Bulging Capacitor

Bulging Capacitor

Bulging Capacitor

Bulging Capacitor

After seeing the state of the capacitors I decided to test them to see what kind of condition they were really in.  I removed 5 to test their capacitance and ESR values.  I don't have any equipment designed to specifically measure capacitor parameters so I thought I'd see what I could do with what was lying around.

Using the information found on Geoff Graham's Measuing ESR page I thought I could put together a quick and easy way to test capacitor ESR.  All that was needed was to apply a quick voltage pulse to a capacitor under test through a resistor.  Any ESR will cause the voltage across the capacitor to immediately rise but will be too fast to charge the capacitor.  The size of the voltage step can be used to calculate the capacitor's ESR. This is when I found out my function generator is pretty much useless.  It can't do pulses, only square waves with duty cycles above 50%.  So I scrapped that idea and built a tester myself with an old development board and a few components out of my junk box.

An AVR development board was used to create the waveforms required for testing.  Its output was connected to a logic inverter on some perf board.  The inverter gives a better rise time, giving a nice sharp transition to drive the capacitor under test.  A buffer would be better but I didn't have one, so I used the inverter and then inverted the pulse in software.  One more stage is needed though, the logic inverter doesn't have a large enough drive current to charge the capacitor fast enough.  To fix that a transistor push-pull stage was put on the output to increase the current capacity.

Micro-controller Driven Test Rig

Before testing the ESR of the capacitors I thought I'd see if their values had degraded much.  I started by testing a 22uF tantalum control capacitor that I was reasonably sure was in good working order.  For this test I set the development board to generate a square wave with a period of 500 ms.  By taking measurements from the charge discharge curve of the capacitor through a 100 ohm resistor I could determine the capacitor's value.

22uF Tantalum Control Capacitor Under Test

RC Step Response 22 uF capacitor, 100 ohm resistor

RC Step Response 22 uF capacitor, 100 ohm resistor

The voltage across a capacitor in an RC circuit when driven with a step response can be described by the following equation.

As the resistor and capcitor are driven from a push pull BJT output stage the capacitor will never get to 0 volts.  This doesn't matter though, you just have to consider the change in voltage of the step and the change in voltage of the capacitor. With a bit of rearrangement it can be shown that the time taken for the capacitor voltage to rise 50% of the step voltage is equal to.

We know the resistor value, we can measure the time, this means we can calculate the capacitance.

From the scope screen shot above it can be seen that the capacitor takes about 1.5 ms to reach the 50% voltage point when charged through a 100 ohm resistor.  This gives a capacitance of about 21.6 uF which is in agreeance with the components markings.  So I know my test rig gives usable results. By no means are they accurate or precise, but they give me an idea of what's happening.  Now to test the five capacitors from the modem.  I've shown screen shots of one of the tests below.

2200uF Electrolytic Capacitor Under Test

RC Step Response 2200 uF capacitor, 100 ohm resistor

RC Step Response 2200 uF capacitor, 100 ohm resistor

RC Step Response 2200 uF capacitor, 100 ohm resistor

RC Step Response 2200 uF capacitor, 100 ohm resistor

RC Step Response 2200 uF capacitor, 100 ohm resistor

Capacitor Number	Capacitor Value (uF)	Temperature Rating (C)	Step Size (V)	50% rise time (us)	Calculated Capacitance (uF)
1	2200	105	4.1	275	3.96
2	1000	105	3.7	600	8.65
3	1000	85	3.8	200	2.88
4	1000	85	3.8	350	5.04
5	2200	105	3.8	2000	28.8

The results weren't great.  None of the capacitors really followed a standard exponential RC charge discharge curve.  No matter how much I turned the timebase down I couldn't see a step caused by ESR. It was more of an asymptotic curve.  I think there's more going on here than just an increase in ESR or a drop in value.  Although I don't think it's entirely valid, the previous formula was applied to calculate the capacitance.

Maybe an LCR meter could give me a better idea, but one thing is certain, capacitors that big shouldn't charge that fast.  Although the I think the calculated capacitance is probably wrong because the situation is more complex than I first thought, they definitely shouldn't be that low.

I could have tried replacing the capacitors, but decent quality parts would be pricey and I was in the market for a modem with more flexibility.  The modem I bought to replace this one is only a stop gap.  In time I'd like to get a modem from a company like Billion with a built in VPN.  This would allow me to use a mobile device more securely when I'm on the go.

What's clear is that the capacitors in the modem have definitely failed.  It does seem to be a common problem with this model.  This would cause the voltage rails to become unstable, causing soft failures at first followed by more obvious symptoms like complete failure of the device.  This was most likely caused by spending extended periods at high temperatures.  Although some of the capacitors probably had too low of a temperature rating for a passively cooled device with no heatsinks that operated in a city where 40 degree days aren't uncommon, they did last a significant amount of time, and any attempt to improve the lifetime of the modem with better components, active cooling, or more heatsink would have increased its price tag.