Sennheiser G2 SKM100 - LOW BAT

[chrislea_10outof10]

Hello hive mind,

 

We had a customer swing by today with a G2 SKM100 transmitter. A fresh set of new batteries, turn it on and it says LOW BAT, obviously I know this means low battery, but after 4 sets of batteries I can only assume there is an issue with the transmitter its self. After leaving a set of batteries in the transmitter for an hour or so, I went to touch it and the PCB and metal chassis it is bolted to is really hot.

 

Now, Sennheiser no longer offer repair or support for this series, so can anyone here recommend a repair centre who can still fix these? Admittedly parts maybe inaccessible. The customer is a church, the poor little old lady who came to see us said they don't have a lot of Money and would rather get it repaired. Is it even worth repairing? I did suggest upgrading (they have a G1 too!) but she doesn't think its viable.

 

Thanks in advance :)

[sandall]

Now, Sennheiser no longer offer repair or support for this series,......

That's a bit of a downer :(. My local disco hire shop has a Mr Fix-it who can replace G1 switches, but this sounds a bit more serious. As mentioned in an adjacent thread, used SKMs come up on the usual auction site at (sometimes) sensible prices, but it sounds like you'd be the one stuck with sorting it.

[HilcrRWise]

Hi Chris

Was this issue ever resolved? We have an SKM300 transmitter that has started exhibiting the same issue.

Thanks

 

[kgallen]

The schematics for the G2 SKxxx (the beltpack version) are here:

 

https://www.manualsl...-Sk-100-G2.html

 

I believe the SKM (handheld) is essentially the same for the control and RF parts of the circuit.

 

From page 19/27, mainboard circuit diagram, it looks like the battery voltage sense (per thresholds on page 14/27 section 5.3), is handled pretty much fully by an ADC (ADC5, pin 46) inside the 68HC908 microcontroller with the exception of a simple low-pass filter of R336 and C224 on the ADC input. It might not of course be the battery sense circuit at fault, but with the report of a "hot" chassis there could be a fault in the "main circuit" which is drawing excessive current and thus dropping the supply voltage where the battery low signalling kicks in. Per page 12/27 section 2.2, the "in operation" current consumption should be <=170mA with a 2.4V battery supply. Can you check this?

 

If you have more test facilities, section 2.1 of this table suggests a bench power supply with a current limit of 300mA - and such a supply going into current limit when supplying the SKM would suggest a step-by-step investigation of which of the internal supplies are drawing excessive current. These units have a number of internal supplies - the raw battery supply is not used as such. There is a boost circuit and a number of internal regulator circuits to derive all of the required internal supplies - which is why the G2 (and later models) can use a such a low voltage primary supply from a pair of 1.5V AA cells. The G1 of course uses the 9V PP3 battery so has a very different internal power supply structure.

 

OK maybe I got a bit technical there, but it might help someone out!

 

Kevin

Edited July 5, 2019 by kgallen