FM-100 Mod For Power on Reset Problem

 

 

The Problem:

The Ramsey FM-100 would not consistently operate at startup. The LED numeric display would not light up. Sometimes power cycling the unit would cure the condition but not always.

Unit History:

I built the unit from a kit many years ago. The unit has worked great except for the occasional startup problem. I'm going to describe what I did to cure the problem below. Please drop me an email if this procedure helps your FM-100!

Caution!:

Read and understand the High Voltage dangers present in your FM-100. The users manual can be downloaded from the  Ramsey web site. I take no responsibility for anything you do to your FM-100 or yourself.

 
1.  I could not find the schematic at the manufacturer web site or on the web! After some searching I located the manual in my library.
2. Pin 1 of the U4 microprocessor is the clock input. During normal conditions there is a reasonable square wave present. During the fault condition, no clock is present. I moved on to investigate the clock source.

3. U2 is a MC145170 now made by Lansdale Semiconductor.  I suspected there was a problem with this chip. Pins 1 and 2 always had a good clock signal during the fault condition. After reading the data sheet I learned that pin 3 can be disabled but should be enable during the power-on reset. I suspected the POR was not working.

4. Two notes in the datasheet stressed how critical it is to have the chip inputs in a know state during power on. The second note gave the reason why the chip was not resetting in our case. "The on-chip power-on reset is only activated when the supply ramps up from a voltage below approximately 1.0V"

5. Time to measure the +5V rail going to U2!

A scope was connected to the output of VR3. My scope is a little out of calibration but very usable for this measurement.

I discovered that after the power switch was turned off, the 5V rail took about 25 seconds to reach 0V. (The noise in the left half of the screen shot is the 91.5 MHz carrier picked up by the scope)

 
6. The cure.

C50 on the output of VR3 was taking too long to discharge. Adding a small load to it might help. I grabbed my analog computer and dialed up 100 Ohms.

I power cycled the FM-100 and noted a drastic improvement in the discharge time!

 

 

6. The Scope time base was readjusted to accurately measure the new discharge time.

The 5V rail now discharges in under 1 second.

 

7. The fix.
  • Unplug the unit from
    the mains.
  • Install a 1/2 W 100 Ohm
     resistor. Soldered it
    from 5V to ground
    at VR3.

Calculations:

5V/100 Ohms = 50mA P = 0.25 W

 

8. A quick test of the unit shows that it now turns on all the time. Click the picture or text to watch the video.

 

Video

 
9. Conclusion:

Loading the 5V power rail improves the Power-on Reset feature in U2 ( MC145170).

 

 Please drop me an email if this procedure helps your FM-100!