Hare Turnout Decoder Repair

I have a few DCC Specialties Hare decoders that failed on me, and what's funny is they all failed with the same symptom, and the same repair brought them back in service. So in all cases, the Tortoise machines had at one time been working fine, then they just stopped throwing. The LED on the board indicated they were receiving a DCC signal OK, and the panel indicator would switch, but the Tortoise would not move.

I'm not an electronics engineer, just your basic technician type of repair guy, and I can noodle my way around a multimeter OK. I figure that a new Hare costs $30, and SMT devices are super cheap these days, so I'll give it a shot.

So I took my meter out and measured the Tortoise motor voltage, and it was almost 0V. Reading Positive or Negative 15-30mV. Huh, if I toggle the Hare, the polarity switches at the Tortoise motor, but is still almost 0V.

Next I measured a working Tortoise/Hare switch, and I measured either + or -10.5V at the Tortoise motor. So I left my meter on pin 1 of the Hare J5 connector (the Tortoise motor outputs are J4 or J5 pins 1 and 8), and moved it to every pin on the PIC, (PIC16F636) and could not find the 10.5V. This told me the PIC does not drive the Tortoise motor directly.

I moved over to U5, the LM358 dual op amp and measured that 10.5V across the output pins 1 and 7. This told me the LM358 must be getting input from the PIC and it's outputs are driving the Tortoise.

Next I put my meter across pins 2 and 3 inputs from the PIC, and measured +2.5V, then toggled the working hare, and measured -2.5V. I did the same with pins 5 and 6 and had the same result.

So now that I know how a good working Hare measures, I measured my bad ones. I had 3 bad ones so far, and they all responded the same, the PIC toggles the polarity of the inputs to the LM358, but the outputs do not work. Those op amps seem to fail easily. These are all old Hare's, version 1 that I have had for about 15 years.


I ordered a tape of 50 pieces of LM358 SOT-8 dual op amps from Amazon for $8, and they arrived yesterday, so this morning I got out my heat gun, set it to 325C, and easily slid the bad chip off the first board. If I didn't have a heat gun with small nozzle, I could have just used my flush cutters to cut all the legs off the chip.

Next I brushed on a bit of flux, then used some solder wick to clean up the solder pads on the Hare.

I brushed on some more flux, placed the new chip, and easily soldered it on. There is plenty of clear space around this chip making for an easy repair.

I hooked it up to DCC power pins 1 and 2 of J1, placed a bi-color LED with 1K resistor across pins 6 and 8 of J1, and connected the tortoise motor, then toggled it just fine. The LED changed from yellow to green, and the tortoise motor threw from one side to the other. Repair worked just fine. I repaired a second Hare that was no longer mounted, and it too tested good.

The third failed Hare was still installed on the module, so I decided to not even test, but rather just go ahead and replace the LM358. It was a tight fit with less than 1/2" of space between the Hare and the adjacent Tortoise machine, but the heat gun tip slid in just fine, and took that baby out quick.

Popped in the replacement chip, and soldered it in easily enough, but it did not test working at first, so I brushed on a bit of flux, and took the heat gun to it and pressed down on the chip. That re-flowed the solder just fine, and the Tortoise threw fine afterwards.

So now I just got a mess of Hare's left to test, and I have a mess of LM358 chips to fix any more bad ones.

 

Yeah, but to be fair, the heatgun I used is part of my reflow workstation, and I can control both air flow and heat temperature. I found using 325C at 50% airflow melts and flows the solder fast. Chips can take a much higher temperature than people think. I worked 40 years in chip factories, and silicon starts off at 1100C for initial oxide, and ends at 700C for glass passivation, so the silicon can survive, it's the packaging that's the weak spot. It's better to have a higher heat for less time when doing this kind of repair work.