Here's a guide to cleaning the mode sensor (or mode switch) contacts in the HV-MX100. Mine had all of a sudden started to misbehave, loading a tape and then shutting off, then trying to eject the cartridge without properly unloading the tape, or shutting itself off after 5 seconds... After I finally figured out this fix, it's now working like it should.
Background: A VCR uses a "mode sensor" or "mode switch" so that the electronics can know the physical state of the tape and the loading mechanism. In this VCR, the mode sensor is partly electrical - with some contacts connected to the shaft of the loading motor - and partly optical, with LED-driven sensors that detect the end of the tape and such. The problem was with the electrical portion.
The culprit: Lubricating gel within the cap that holds those electrical contracts had migrated onto the circuit board that the contacts trace. The contacts stopped making consistent electrical contract. The VCR's microprocessor saw states, or state transitions, that should be impossible, and shut the VCR down for protection.
Here's a photo, after I'd just started cleaning the circuit board traces (I only cleaned the outer ring, top-right in this photo). While it looks like the copper traces have been heavily dented, what you're seeing is actually the gel that's all over the traces and that the moving electrical contacts have tried to push out of the way:
A tissue is enough to clean the circuit board, but it's also good to clean the contact fingers and get rid of some of the extra gel in the cap. Contact cleaner or circuit board cleaner or an alcohol wipe are better, to get all of the residue off.
Here's a photo of the contacts in their carrier within the cap. The blue color is only due to the lighting:
And here's a photo of the cleaned board with the cleaned contacts sitting on it. The shaft is keyed and the carrier fits on it in only one position:
Here are the steps to remove and clean that board:
- Unplug the VCR.
- You will not need to do any soldering. It's just mechanical disassembly, but you need to do it fairly precisely. The only tools you'll need are two Phillips screwdrivers (#2 and #1). A small long-nose pliers is also helpful for reattaching cables.
- Before you start, remember to keep track of where each screw came from. The screws are not all the same!
- I presume you've already removed the case cover.
- Remove the two knobs from the front panel (one's inside the other) by pulling carefully.
- Remove the front panel - it unclips on all sides. There are no screws.
- There's a metal frame cross piece toward the back that holds in the vertical board that's behind the video mechanism. Before you remove it, unhook the flat cable that comes over it from the back board.
- The remove the frame cross piece; there are 3 screws, one of which is practically underneath the frame and you'll want to take care not to lose it inside the unit.
- The next piece to come out is that board that's right behind the video mechanism. There are 2 more cables to unhook; one goes to the erase head (unhook it at the erase head and unthread it), the other presses into a board in the video mechanism right in front of this board we are removing. Then remove the 2 screws that hold this board in.
- Now, there are 5 more screws that hold the video transport mechanism in. One is in the rear, two are in the front in wells in the black plastic, and two go in from the underside of the base. Remove them, and carefully pull the entire video mechanism out to the front. Note how the mechanism is seated at the back, held in under a couple of notches.
- Carefully turn the video mechanism upside down. The board you are interested in is the largest board on the bottom, shown here in close up. The contacts and gel are right under the circular cap you see here:
- There are 4 cables to remove from this board. Two are on top - a flat cable that goes, I think, to the motor assembly (pull up the latch on the connector before you pull the cable out), and a 2-conductor cable that clips into this board (gently wiggle the connector to clear the bit of metal frame right near it).
- 2 flat cables connect to this board and go to adjacent smaller boards; you can pull them out (at either end) after you remove the 3 screws that hold the board in. (These are the screws that need the #1 Phillips screwdriver). Here's what the other (top) side of the board looks like; you can see one of the flat cables, and one of the small clips that holds in the black cap poking through:
- Finally, you can unclip that black cap - there are 3 clips to wiggle on the other side of the circuit board - and take it apart and clean it. You don't need to mark the position because the cap is symmetrical, and the carrier with the contacts will only go in in one position on the shaft. (Do not force it on!!)
- Now, clean the board, clean the contacts, clean some of the gel out of the cap, and carefully put everything back together! Do not plug it in until all of the internal cables are back in place. You can leave the front panel and cover off, but everything else must be put back and screwed down.
You will want to try out the VCR with the cover off so you can see what it's doing, but unless it's pretty dim, you can expect it to immediately eject every tape you put in. Do not despair; this is happening because the optical portion of the sensor system is being fooled by the light coming in from the top. Do notice that the VCR is no longer turning itself off after 5 seconds (I hope!!).
Now, put the cover on, or shield the mechanism from light, and you should see it working like it should. If so, be happy and put the rest of the VCR together.
Here's a mechanical diagram of the video mechanism that may be helpful. (I had to break it into 2 pieces because of the size limit here, and they won't line up.) The mechanism is TN-6500-232, and the board we are working on is near the bottom and labelled "BASE C.B.":