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Good evening all, I am sharing my experience in repairing my Logicon thus far. May this help anyone interested in making a similar repair. In particular, I haven't tested my logicon yet as the repair and write up ate up my evening. Testing will soon follow.
1. The discrepancy: When the fan speed switch is moved from off to MIN thru MAX, the fan motor makes a series of rapid clicking sounds and the fan will not operate. If the switch is manipulated, the fan will come on and the clicking stops.
2. Probable Fault: The switch is not making proper contact with the rest of the circuit. Probable broken or cold solder joints.
3. a. Once removed, unscrew the 4 screws that hold the faceplate to the logicon body.
3. b. In the top and bottom center of the logicon, tabs hold the faceplate to the body. Carefully use a flathead screwdriver to unlock the 2 tabs on the top and 2 on the bottom of the logicon. Move each tab to the left or right to unlock, respectively.
*CAUTION* Damage to tabs may occur if too much force is used to unlock each tab.
3. c. Separate the faceplace from the logicon body.
3. d. Disconnect the 2 faceplate wire connectors from the logicon body. Use a small screwdriver or non-marring (plastic) tool to pry up each side of the connector.
3. e. Separate the faceplate from logicon body.
4. Unscrew 2 phillips screws from the left & right sides of the faceplate then remove paper backing.
5. Inspect the solder runs and joints for greyish color and/or broken joints. I found 2 bad solder joints on the rheostats (slider switches) along with some suspect solder joints.
Suspect solder joints. The 2nd from top joint is not supposed to be connected to anything.
Greyish and flattened solder joint on the bottom right.
Another greyish and flattened solder joint on bottom right.
*NOTE* One side of each rheostat has 4 pins. Only 3 of the 4 pins are electrically connected. It is normal to NOT see solder on the 2nd pin from the top (or bottom) of each rheostat.
6. Use a combination of #2 phillips and PH00X40 (jeweler's phillips) screwdrivers to remove 8 screws; 4 jeweler's type along the bottom edge and 3 #2 type screws along the top + 1 #2 screw on the left 1/3rd side of the faceplate.
7. Desolder the bad joints with a solder sucker, desolder iron (one of the items you CAN still get at Radio Shack), or desolder braid; ensure that all old solder is removed.
8. Add solder flux to the eyelet and pin of each rheostat. Think, “Mary had a little lamb” with flux – every where that flux went, the solder was sure to flow.
9. Apply solder to each joint, ensuring solder fills in the eyelet of each joint.
*CAUTION* Do not let solder flow down to the bottom of each pin. A glob of solder could form between the board and the rheostat resulting in a short between the surrounding pins.
*NOTE* If this occurs, desolder the rheostat then remove from circuit board to remove excess solder in Step 7. Fit rheostat into circuit board then proceed with Steps 8 and 9.
10. Use a cotton swab with isopropyl alcohol to clean each joint and remove the excess flux.
11. Inspect each joint to ensure no solder globs short out any other pins.
New solder to joints -1
New solder to joints -2
New solder to joints -3
12. Installation in reverse order to reassemble faceplate.
Other Troubleshooting Tips:
I had to desolder the fan speed rheostat because of solder globs. While removed, I tested it with the following results (results verified on known good faceplate):
1. The rheostat is upside down in the photo and the schematic is seen on the circuit board. Measurements were taken from the 2 pins highlighted by the yellow circles.
2. This photo shows rheostat slider to full right and DMM reads 1.1 kOhms.
3. This photo shows rheostat slider at mid-range and DMM reads 561 Ohms.
4. This photo shows rheostat slider at 1-click from full left and DMM reads 104 Ohms.
5. This photo shows rheostat slider at full left and DMM reads open / “infinity” ohms.*
*See next post for photo.
6. The temperature rheostat reads as such (no photos):
6. a. Cool (full left) = 5 kOhms (my actual was 4.97 kOhms)
6. b. Warm (mid-range) = 2.5 kOhms (actual)
6. c. Hot (full right) = 0.02 kOhms or 28 Ohms (actual)
The test of all my work comes tomorrow morning. Feel free to comment or follow for future reference/sticky purposes.
Today was test day for the repaired logicon. It appears that my soldering skills proved successful! IT WORKS, IT WORKS! Thankfully, the rapid clicking sounds of the fan relay are over.
And cheers to you and all that good stuff, and the community would like to take the time to thank you for putting forth all the effort to illustrate some of the problems plaguing aging RX7s. You'll be a marked solderer for those in need of your help.
Thanks, satch! My soldering skills are a little rusty. I need a more steady hand for it too!
So today, I buttoned up the Vert after repairing the logicon. Since I had the instrument panel garnish out, it was also a good time to fix the vents too. I mainly focused on the pegs with angled retaining clips. One peg on each vent broke; fortunately it could be fixed with some Permatex Plastic Weld (a binary compound much like epoxy). Since it was mixed, I reinforced the other 2 pegs. I hope this is the last time they need to be fixed.
So if anyone else is interested in a 1-stop shop for logicon and related repair tips, here are links to these posts for reference:
09-05-16, 11:28 PM
