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).
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):
1. Where the battery check used to stay on until switched off (or the
batteries died), the new circuit will shut off in 30 seconds.
2. 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.
3. 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.
4. 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.
Clint
John Hermanson wrote:
> Those may have type 2 and 3 circuit. Type 4 being a T circuit. Clint can
> jump in and clarify how many intermediate circuits there were.
>
> John Hermanson
> ___________________________________
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> ___________________________________
> ----- Original Message -----
> From: "Walt Wayman" <HIWAYMAN@xxxxxxxxxxxxxx>
> To: <olympus@xxxxxxxxxxxxxxx>
> Sent: Saturday, October 06, 2001 5:14 PM
> Subject: Re: [OM] Om 4 circuit's
>
> John Hermanson wrote:
>
> >It is possible that any 4 could have any one of the 4 different circuits,
> >the last circuit being a 4T type. I don't know if the 2nd and 3rd type
> >could be identified by the self timer turning off in 30 seconds. Maybe
> >Clint knows.
>
> I have three 4s. Two of them, Nos. 1021xxx and 1023xxx, drain a set of
> batteries, even when not in use, in about three or four months. The other
> 4, No. 1108xxx, runs almost as long on a set of batteries as my 4Ti --
> 10-12 months. None of the 4s turn off after 30 seconds, which indicates to
> me there are some that will fail this test, yet probably have one of the
> newer circuits.
>
> Walt Wayman
>
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