Getting My OLD Pact Chronograph Working Well

[Milanodan]

Decided to try my old Pact "Precision" chronograph one more time before I bought a different brand. It's been erratic lately, only "seeing" about 25% of the shots.

Started by calling Pact. Guy who answered the phone said he could help with tech. questions. Asked him if there were any checks I could do at home to figure out what was wrong. Told me nothing can be done on the chrono. itself except to send it in, where they check it on a "$10,000 machine".

Then he told me I could check the sensors with a VOM. Ah, should I expect milli or microvolt readings? Nope, they work via resistance. OK, how many ohms should I expect. "I don't know--they work on changes in resistance." OK, does the resistance get higher or lower as the light increases? "I don't know".

Armed with all this new info (right), I started checking resistances vs amount of light. After several screwy readings, I discovered the NEGATIVE lead of the meter must be connected to the TIP of the phone plug. I found infinite resistance at low light, decreasing rapidly as the light was increased. Under a 9 AM pure blue sky day here in Arizona, I got ~63,000 ohms--without anything over the sensors/Skyscreens.

After taping a very thin piece of translucent plastic shopping bag ~6" above the sensor, the reading dropped to ~28,000 ohms, verifying that a blue sky is actually fairly dark.

Fired a few shots from my low power air rifle over the sensors with the plastic above them. Got readings every time as long as the pellet passed over the sensor by at least ~4-5". Got no readings if the pellet was too close to the sensor--was probably off to one side a bit.

Lesson learned--keep the shots high and use thin white/translucent plastic over the sensors.

[scrapcan]

good info. I have a pact model 1 that says int ehmanual to use the light diffusers on the sensors. Makes more sense now.

[GeneT]

Oehler swears by orange sky screens. I haven't tried them with pact sensors, but the sensors work the same for both (are compatible) so they may be a significant improvement. The Oehler screens seem to catch everything.

GsT

[scrapcan]

GeneT,

Good info thanks for sharing

[Whitespider]

Milanodan,
Very effective, and durable, light diffusers can be made from wide strips cut from plastic milk jugs. Use flat black side panels (something stiff and painted flat black) to hold the diffusers above the “eyes”. The wider this stuff is the better because it blocks more direct light from reaching the sensor. Placing the milk jug diffusers about 12-15 inches above the sensors should work just fine and give you a bit more longitude/latitude for bullet path.

[Rocky Raab]

Thanks, whitespider. I discovered that trick 25 years ago with a very early Chrony unit, and have been passing it along ever since. Often to disbelieving readers.

I start with a gallon jug and cut a six-inch wide band (basically just remove the tapered top and the bottom), then cut lengthwise at one corner. Punch holes to hold a thin bamboo skewer instead of the wire supports. Don't worry about the loose "diamond" or "tent" shape the jug will have, it may even be better than a flat-topped screen.

[lwknight]

True , the blue sky does not make much light for a chronograph sensor. The sky just looks bright because there is so much of it. It helps so very much to set up the chrono at a time and in a way that the diffusers are well lit up.