# Repairing the CMI Sounder throttle



## Knightshadow (Dec 27, 2007)

Hi everyone, I just wanted to share my experiences with an older throttle from the late 80's that has had some failure issues over the years. 

A bit of Information: The throttle is a CMI sounder or Chicago Model international unit that has the capability of running G gauge equipment. This is accomplished by by a scaled output that is controlled by a separate knob on the unit. The total output is 40VA which is quite enough for my needs. 

The internal circuit is rather simple: Power is routed through a stepdown transformer and then through a bridge rectifier. DC power is then applied to a single power transistor at the emitter. Control of the transistor is by an LM358 op amp which is controlled by a voltage divider created by the main speed control and the scale pot which acts like a trimmer pot. It is all rather simple. 

Failure area: I have had to repair a couple of these units and in every case it has been the LM358 IC that has failed. These devices can be found at places like Digikey for around a dollar each and sometimes as low as 25 cents on other sites. 

Repairing the unit is a simple matter of removing the old IC and replacing it with a new one. Some soldering skills are required and I reccomend that if you remove the IC, you install an IC socket just in case it goes out again. This will make swapping out the IC that much easier if needed.

As a semi retired electronics engineer, I look where ever I can to find ways to save money for model railroading. Though I know there are some great new throttles out there, the price for a new one is prohibitive, especially when compared to the extremely low cost of the components needed to repair this unit. Repairing the unit saves me funds that can be used to buy more track and build more rolling stock. 

Happy Modeling!
Richard Stewart


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## Cataptrra (Mar 16, 2015)

Thanks for posting this! I also own this same throttle and really like it {although I'd rather be using my 10-Amp Ultima power packs}. 

So now I know what to look for if and when it may fail. 

May have to go visit my local electronics surplus outlet and see if I can find a few of these chips to have on hand when or if my CMI Sounder ever fails!

BTW: Did you need a special tool to take your throttle case apart? Looks like mine has some type of Allen screws holding it together.


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## Dan Pierce (Jan 2, 2008)

Some units have torx screws and to make matters worse, some have a pin in the middle.
I find that a small screwdriver can remove these and then replace with a regular screw.
I do have torx, allen (metric and US) tips for universal screwdrivers.
Stay away from dollar store units as these are not high grade tempered steel.
Get good sets from sears/Lowes/Home Depot, etc.


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## toddalin (Jan 4, 2008)

I love to see posts like this.


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## Cataptrra (Mar 16, 2015)

Dan Pierce said:


> Some units have torx screws and to make matters worse, some have a pin in the middle.
> I find that a small screwdriver can remove these and then replace with a regular screw.
> I do have torx, allen (metric and US) tips for universal screwdrivers.
> Stay away from dollar store units as these are not high grade tempered steel.
> Get good sets from sears/Lowes/Home Depot, etc.


I also have a wide assortment of driver tips, none are $1 variety, all are of good quality and been using them for years with no problems. Just got to find the right one for the job. I've come across so many different styles of screws holding things together, sometimes I've found some will even go so far as to have 4 screws, but 2 of the 4 are still different from all the rest, requiring 3 separate type tips to take the unit apart. That just drives me crazy!

All this comes from my former employment in the electronic-mechanical assembly, rework, touch-up and repair, technician, and prototype engineering labs.

Don't and can't do any of this today like I did many years ago, now being medically disabled, along with being legally blind, so soldering small things can be quite a big challenge for me now, but I still try. Just isn't up to par of the military specs I used to be able to work to!

So I have a lot of tools of the electronics and mechanical trade for that kind of work.


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