Diagnosing high current draw/battery drain on PM2 Rev 2.3

[flummy]

Hello everybody,

and thank you in advance for any help. I just recently was lucky to get a model C for an affordable price. So I'm a rather new member to the community of c2 owners.

After changing batterys the third time in three days I realized that something has to be wrong with my PM2. Because I'm a big fan of repairing stuff instead of throwing it away, I started the journey to track down this issue.

The PCB showed minor signs of battery leakage so as a first step I performed the "regular cleaning" cylce using vinegar, distilled water and IPA.

The naked PCB (Rev 2.3, the one without MAX1044 for the display) without display mounted constantly draws ~133mA. I cannot see any obvious damage to the board and all the suspicious tracks I measuerd are conductive.

The +3D point against GND battery terminal yields only 2.87V, so I further investigated MAX859. I checked its data sheet and noticed that it has a maximum rating of 125mA, so it's obviously operating at its limits (it's also getting quite warm...).

So I believe there must be a short circuit somewhere behind MAX859, but this is where I got stuck. When the circuit is off I measure 4.1kohms against GND of the battery terminals, which is too much resistance for the high current draw. So I guess the short circuit is "switched" somehow, when the board is powered.

Does anybody have a hint on how I could proceed on fixing this issue or where I should have a deeper look (other than buying a new one... ).

Best regards from Germany

[Citroen]

(other than buying a new one... ).

You know the real answer.

The PM2 was standard between 1995 (30 years ago) and 2003 (21 years ago), there's probably no value left in repairing it. Buy a new PM5 for €225.00 and get the benefits of Ergdata on your phone.

[Carl Watts]

You have possibly the best monitor C2 ever made in terms of reliability, as you have found out its just battery leakage that kills them or it would still be going.

They are a pretty hard repair job for a one off and if the battery leakage is so bad its wicked up the keypad its pretty much toast.

The Rev 2.3 draws 1.2mA in Run and 0.060mA in Standby. The batteries should last about 12 months with decent Alkaline cells.

Most likely you have an open circuit track around the MAX859. Common places of failure is the vias between the sides.

Do you have +3V and the +1.25V reference from the MAX859 on the two 100 Ohm resistors to the right of the lower battery terminal ?

Check the yellow tantalums its common for one to crack in half with a bad leak.

Trying to recall if I have ever seen that sort of drain, nothing springs to mind unless the repaired track under the right hand side battery terminal shorts to the battery contact above it.

It was common for the chip 0.1uF mono decoupling caps to develop high resistance but this would just be a few extra mA.

Even had the micros fail but they still work but draw too much current.

When it powers up, does the display still come on ?

[flummy]

You know the real answer.

maybe it's rather a question of principles for me than an economical decision...

Thank you Carl for taking the time and for sharing your expertise!

The display turns on regularly, also the high current draw does not affect operation. That's why I noticed the issue only after a few days.

The +3V and the +1.25V reference from the MAX859 is present on the two 100 Ohm resistors.

Also the yellow caps do not have any signs of damage to me. I also haven't repaired any tracks up to now, so at least this is no reason for shorts.

I can't quite get how the open vias/open tracks could cause the high current draw. I'd rather expect it to fail in operation somehow in this case? I still have to have a deeper look at this.

The failing micros seem to be a possibility. Is there a way to diagnose this further without desoldering them? Compared to MAX859 at least they do not seem to get very hot...

How would you proceed?

Thank you again for your assistance on this!

[Tsnor]

definitely fix the pm2. (and inspect and oil the chain on the model c, maybe even clean the flywheel area).

Then strongly consider putting the pm2 on the trophy shelf and getting a Raspberry Pi and loading https://github.com/laberning/openrowingmonitor or getting a pm5.

The C2 log integration is well worth the price. It's nice to be able to see HR on the display too. IMO Model C rows the same as a model D. IMO PM5 is distinctly better than a pm2, ymmv.

[Carl Watts]

Something is not right around the MAX859, have you tried removing it and then trying to power up the PM2 ?

Either the MAX is at fault or there is a possible earthing problem for it. Check the two Schottky diodes to see if they have a low Voltage drop on diode test and check they have a high resistance when you reverse the leads.

The MAX859 which is the adjustable version can be replaced with the MAX858 if you short one of the voltage setting resistors in the divider on the output and remove the other and the whole PM2 will work on 3.3V.

Pull the MAX859 and see if the current crashes, it pretty much has to if its getting hot. There is a track under it and a via that commonly goes open circuit. With the part removed you can check the continuity on every pin.

[flummy]

Hi folks,

sorry for the delay, it took me some time now...

definitely fix the pm2. (and inspect and oil the chain on the model c, maybe even clean the flywheel area).

Then strongly consider putting the pm2 on the trophy shelf and getting a Raspberry Pi and loading https://github.com/laberning/openrowingmonitor or getting a pm5.

The C2 log integration is well worth the price. It's nice to be able to see HR on the display too. IMO Model C rows the same as a model D. IMO PM5 is distinctly better than a pm2, ymmv.

Thank you for all those hints, especially the openrowingmonitor looks pretty interesting. I'll definitely take a closer look at it when I have some spare time or the PM2 shall move to the trophy shelf.

I already oiled the chain (little bit of rust, but seems to be alright), and put more tension on the chain pulley. I'm not quite sure about this one, the chain still wobbles a bit, but maybe I'll just have to use it for some time to get a feeling if this is ok.

Something is not right around the MAX859, have you tried removing it and then trying to power up the PM2 ?

Either the MAX is at fault or there is a possible earthing problem for it. Check the two Schottky diodes to see if they have a low Voltage drop on diode test and check they have a high resistance when you reverse the leads.

The MAX859 which is the adjustable version can be replaced with the MAX858 if you short one of the voltage setting resistors in the divider on the output and remove the other and the whole PM2 will work on 3.3V.

Pull the MAX859 and see if the current crashes, it pretty much has to if its getting hot. There is a track under it and a via that commonly goes open circuit. With the part removed you can check the continuity on every pin.

Thank you very much for sharing your expertise on this. Today I think I finally was able to fix it. I checked the diodes and the area around the MAX859, everything seemed to be fine.

I think I wouldn't have removed MAX859 if you wouldn't have told me, so thank you for this. After removing no damage below the IC and also no high current without it. So I searched around where to buy the MAX859 for a reasonable price (I've got 2 more left, if anybody near Germany needs one...). After replacing it with a new one, I measured regular current draw (if i remember correctly 2mA in operation and 90uA in Standby). So let's see how long the batteries will last.

Thank you again for this warm welcome and for sharing knowledge!

[Carl Watts]

Only ever use Alkaline batteries or better in your PM2.

For those in Germany you make the best batteries now, Varta are great you can buy them in New Zealand in packs of 30 for only NZD$17. This essentially would give you a lifetime supply of batteries for your PM2 as a set should last at least 12 to 18 months.

The very best battery to use is the likes of the AA Lithium Energizer, this technology simply never leaks and a pair of these probably last 3 to 5 years on your PM2 if the run current is 1.2mA and the standby is 60uA @ 3.2Vdc as measured on a Fluke 8840A.

[gvcormac]

Have you considered wiring in an AC power adapter? If you're like me you have a drawer full of these and probably one is 3V or thereabouts.

Alternatively, maybe a toggle switch to cut the battery power when not in use.

For diagnosis, 100ma @ 3v might generate enough heat that you could isolate the problem with an infrared camera or mayb just an laser thermometer.