SRM 7900s Zero offset increases over time...

[WeightySteve]

Have sent my cranks to SRM 7900s to SRM UK (who might send them off to Germany), but just looking for any info that might help us/them isolate/fix my cranks if someone's had a similar issue (surely I can't be the only one in the world who's ever seen this or had this fault?)...

Start of a ride Zero offset (z.o.) will be normal. (Mid 500's, stable). After an amount of time (say 30 mins / 45 mins ) the z.o. will starting increase.

(Temps stay the same throughout the ride - its an indoor ride on rollers in a air-conditioned room, and the same ride on some Wired 7800 SRM's the z.o. stays completely constant on them)

Test z.o after 10 mins , it has not changed, after 20mins, still the same and stable.

But, after 30 mins, it's up to 600-700 (and not stable, fluctuating by around 30-60hz).

5 mins later , 800-900

5 mins after that 1000-1100

After a couple of 75 min rides, it had increased to over 1800hz.

2 rides in a row (24hrs apart) went like this.

3rd ride (ride 48 hrs after ride 2) exact same route and temps, z.o. stayed pretty constant for 45 mins, but then increased gradually for the next 30mins until by the end of 75mins had risen, but to only 800-900hz.

So its not an exactly reproducible scenario, but obviously there is a problem.

End of each ride, leave the bike alone for 10/20 mins, z.o. is still high (sometimes even higher than the last test at the end of the ride).

Leave the bike 24/48 hours, z.o. has dropped back to its original level of 500ish and is stable again.

I've tried a few static tests, starting with normal stable z.o. and jumping on pedals with the brakes on to send the z.o. reading into the 2000hz+, it returns to normal after pressure is released from the cranks. So any 5-10 minute test would not reveal any issues, you gotta ride them for 30/60mins+.

Any helps VERY much appreciated...

[Geoff]

I have had that happen twice on different systems. In each case, it was the battery dying on me...

[WeightySteve]

Hey Geoff, Thanks for the reply.

But... This just started happening the on the very first day after I got my cranks back from SRM UK. And the reason I'd sent them in in the first place? A battery change and slope calibration.

I will direct them to this thread too, but if they put a new battery, any clues what else could cause it?

My thoughts are residual strain left in the gauges after prologned riding time, but how could that fault develop during a battery change? But I know nothing really, just what I see happening.

[sungod]

if srm uk assumed the original cause was battery, then perhaps it was not extensively tested after the battery was replaced, just calibrated, quick check and ship back to you

now it seems clear that it is not the battery

the strain gauges used need sensitive analogue circuitry, perhaps there is a component that has degraded and is causing the drift

if the bulk of circuitry is powered down when idle then the fault could be dependent on power on duration...

for instance charge leaks and build up causing the observed drift, then if left idle (power off) for many hours it dissipates and all seems ok until the next time power is applied, then the process starts again

to fix this would mean swapping the circuit board or component-level isolation and repair (i'd think uneconomic on such a low-value board unless they have 'spare' time to do it)

at least if it goes back to srm germany they will be able fully investigate

[WeightySteve]

Originally, it was only sent to SRM for a battery change and slope cal. So they weren't actually looking for any problems when they received them the first time.

This problem only seems to have happened since I got them back after the battery.

(Not that I'm blaming SRM UK for breaking my cranks! It might have been a fault just about to happen, and just co-incidence/knocks during postage or taking them apart and disconnecting/reconnecting the power supply for the replacement has pushed it over the edge).

But yes they are going to Germany. So now the waiting game begins...

[$mokeyJoe]

I have some wired and wireless srm's and are always serviced in Germany. I also had once a drift like you describe and yes, it was a battery problem. If this is not the case on yours, i'm very confident that SRM will find that problem as they've always been very helpful at servicing my powermeters. I would wait a bit and don't panic too early. If it's their fault, i'm sure they'll rectify it without a cost. If it's your only powermeter, then you're forced to train on bpm's at the moment...

[WeightySteve]

About 12 months ago my missus struggled to cope with the idea of me wanting 2 sets of power cranks (one for each bike). She got over it

My next task is making her come to terms with the fact that second set needs "upgrading" (I avoid the word "replacing") to wireless.

For now though my Wired 7800's will keep me right.

PC8 might even be out by the time I get my 7900s back!

[WeightySteve]

Follow up...

Needed new circuit board. £500 ish to fix.

[pritchet74]

Ouch. Thanks for letting us know what the problem was.