How many different OM-4 circuits boards were there?
Going back over my old notes, I see that, if you include the OM-4t's circuit, there were actually 5 main circuits for the OM-4 (Mods 1, 2, 3, 4, & 6).contributed by Clint Rumbo
The original "Mod-1" or "Old" type circuit had several kludges -- discreet components and jumpers added on after manufacture -- that were incorporated within the "Mod-2" or "New" circuit. At the same time that the Mod-2 main circuit was developed, the LCD Driver circuit was also updated to the Type 2 LCD-D, which was used on all later OM-4's. It appears that the "Mod-2" main circuit is by far the most common version.
For a brief time, a "Mod-3" main circuit was supplied by the factory, but I believe it was only available as a complete assembly with the entire front casting (prism, mount, dial, mirror, etc.). They are so rare I doubt they were even used in line production -- I suspect they were only for repairs.
The Mod-4 circuit did away with the RS (or "reset") circuit entirely. I don't recall seeing these in production models either, but they were the "standard" replacement for the last several years of (factory) repair. I suspect that the Mod-4 does function similarly to the Mod-6, but I've honestly never really paid any attention. The "upgrade" instructions for installing a M6 (OM-4t) circuit into an OM-4 note that the "M6 circuit board is loaded with IC No. 4. If an OM-4 is loaded with IC No. 3 or older, there will be changes in specifications when it is being replaced with the M6 circuit board. In this case, give the OM-4 owner a copy of the instruction sheet at the end of this booklet." This implies that the OM-4 _may_ already have "IC No. 4" in a Mod-4 circuit board, in which case there wouldn't be any "changes" to pass on to the user.
Once the Mod-4 circuits were used up, the factory announced the method of adapting the main circuit from the OM-4t (aka, the "Mod-6" circuit -- don't know what happened to the "5", if there ever was one!) for the OM-4. The following "improvements" were supposed to be passed on to the user in certain cases (see above) when the original circuit in an OM-4 was replaced with a new "advanced version" (i.e., the OM-4t circuit):
- Where the battery check used to stay on until switched off (or the batteries died), the new circuit will shut off in 30 seconds.
- With the original circuit, the Finder LED lights whenever the flash ready light is on. With the new circuit, the Finder LED "clears" itself (shuts off?) after 120 seconds even if the flash ready light is on.
- In Auto mode, the original circuit will only fire a T-series flash -- the new circuit will fire any flash if the shutter speed falls below 1/60th second.
- In the original circuit, the finder must be ON for the memory to be cleared -- with the new circuit, the memory can be cleared even if the display is off.
That's all I've got in my (old, yellowed, frayed) notes.