Lathem LTR6-384/etc mstr.

The Lathem LTR6-384, like the other LTRx-128 and LTRx-384 models, is a digital electronic master clock based on an RCA CDP1802 8-bit CMOS microprocessor clocked at 2.097152MHz. The power/output board contains eight relays, which allows the unit to operate six program circuits and one clock system.


Note that most of the information presented here is also applicable to other LTRx-384 series and LTR2/4/6-128 models, which all share the same basic “small-case” configuration—except the number of program circuits for the LTR4-384 and LTR2-384 models, having four and two program circuits, respectively; and, additionally, the ability to select alternate program schedules, which the LTR6-128, LTR4-128, and LTR2-128 models lack. Other model extensions contain the same circuit boards: an LTR4-384-S is simply an LTR4-384 with a surface-mount backbox, for example. The identical-looking models with names that start DWA rather than LTR are simply bell-ringers without the ability to control secondary clocks. Other companies also re-branded these units. I have also seen units containing similar internal parts under the Dukane name in a rack-mount case.

program circuits

Each of the six program circuits can be set to latch for a duration of 01 to 99 seconds beginning at the start of any minute of any day of the week (the duration is global to all events per circuit). The LTR2/4/6-384 models provide seven 64-event program schedules, of which each pair can be activated together for a possible maximum total of 128 simultaneously active events. (The LTR2/4/6-128 models only have one schedule with 128 events.) An event can be dual-scheduled for AM and PM to save space. The events are automatically sorted to chronological order (listed starting at midnight, with dual-AM/PM at the end) within each schedule, and schedule selection is entirely manual. There is no native way to “turn off” the program schedule, other than disabling the zones individually or selecting the seventh schedule that has been left empty.
If the duration of a circuit is set to 60 seconds or higher and an event is programmed to occur each minute consecutively, then the circuit remains latched, which allows the program circuits to act as “Control circuits” to a certain extent (although I used external latching relays to fake the operation of the Control circuits in the LTR8-128, when I had to).
A seeming bug that I noticed with my LTR6-384 is that it will not reliably execute one-second events—it skips (ignores) those every so often. (In the set of latching relays to fake control circuits mentioned above: three relays were wired to latch and another pair of relays driven together unlatched all three, thereby making four program zones into three control circuits. In this manner, the unlatch zone duration had to be shorter than the latch zones, so that a zone could be unlatched while maintaining a latched state on another. The functional form used a duration of four seconds for the latches and two seconds for the unlatch; less extravagant durations of two and one second, respectively, occasionally failed to unlatch. Zone 4 was used to unlatch, being an easily-memorable reference to Japanese culture. I remember, by chance, watching and listening to it not do anything in at least one instance. Another zone was a five-second signal, and the sixth zone was set for a 14-second duration with a dual-AM/PM event at 5:58 entered in all seven schedules to correct Simplex clocks, at the time. Anyone else care to repeat the experiment? I would be interested to know if they all have a problem with one-second events, and how deep the problem is.) As far as I know, it has never skipped an event having a duration of two, four, five, or fourteen seconds.

clock systems

The Lathem LTR-series is like “the universal remote control of system clocks” (like a universal remote control for a television set), so they control just about anything (with the right EPROM version) but don’t always do so perfectly (so, exactly like a universal remote control for a television!).

Standard GRC synchronous (system code 03)

One of the first things I noticed about my LTR6-384 when I got it is that it runs the GRC 12-hour correction properly from 5:12 to 5:28. The manual, on the other hand, indicates that it should use the same 5:15 to 5:30 scheme as the LTR8-128. I always assumed that I must just have a newer unit with a newer EPROM version written after the manual was last updated. (I would be interested to know if older units do this the other way.)
The manual system advance (manual circuits, zone 7) runs a regular 29-second hourly correction during the minute after the advance is initiated. This is not quite long enough for most clocks that are indicating less than about ten minutes after the hour. The automated daylight-savings +1hr adjustment runs an additional hourly correction starting at 3:00.30am, which is too early after the regular hourly correction for most GRC reset mechanisms to make it out of the dead-space—it is at least a minute too soon to work reliably, and is therefore useless for this system type.
The second bug (for lack of a better term) that I have found in my LTR6-384 is that it randomly releases the hourly correction exactly ten seconds late about once every couple weeks (though I probably only caught it about half the time). This doesn’t effect the time display or programmed events in any way. I think it is very odd. (I would be interested to know if other small-case Lathem masters do this, and if it does something similar under other system programs.)

Standard AR3 and AR2 minute-impulse (system code 17)

It actually runs properly the AR3 and AR2 systems in their native forms (unless you want to get really technical). There are just enough terminals provided to connect both AR3 and AR2 simultaneously. See wiring diagram below.

SET AR3 and AR2 wiring diagram for small-case LTR masters
SET AR3 wiring diagram for small-case LTR masters

On the other hand, the manual page containing the wiring diagram for the AR3 system is one of the screwiest things I have ever seen. Even if they really did think these use two voltages, they still labeled the outputs backwards. In short: it’s just plain wrong. So, I also edited the AR3 page to show that system type by itself; see wiring diagram above.

Standard AR2A minute-impulse (system code 04)

Operates the same as equivalent program in LTR8-128; same wiring diagrams as above.

impulse note

A non-corrective minute-impulse system may also be driven by either of the impulse programs listed above, by simply wiring the minute contact and a set of reset contacts in parallel (or one after the other), thereby providing all sixty minute pulses per hour (with one having an odd duration).

SET AR3 and straight impulse wiring diagram for small-case LTR masters

In the case of a 24v parallel system, there are just enough terminals provided to also run an AR system (one variety of wiring only) alongside the straight impulse system, as shown above. All can be driven simultaneously with the addition of appropriate external relays. The automated daylight-savings adjustment programs should also work properly for non-corrected impulse systems.

IBM/Simplex/etc synchronous (system code 01)

One would think it would run its own company’s clocks properly, right? It does; but I take issue with the normally-open-wired relay that is forced to stay latched constantly under this program, which wastes energy (money). If I were to actually run this type of system with this unit, I would just wire the secondaries normally (with the drive motor permanently connected to power) and unplug that relay. In my opinion, the automated daylight-savings adjustment is really only useful for impulse systems (without 12-hour correction), anyway—and the timebase is never going to be accurate enough to not have to set it at least that often, I’m sure.

resource library

Full copies of the manuals for all models are freely available on the Lathem website. (I intend to upload these documents if the company ever stops offering them. If you find these to be absent over there, please tell me about it.)
Be warned: the wiring diagrams in those manuals are rather confusing (or just confused, maybe). Mainly, the components (that the LTR2/4/6-128 manual and the schematic on the inside of the door identifies as MOVs) that show up in every diagram are seemingly meaningless—as the contacts are already protected by MOVs on the power/relay board (the red things next to each relay in the example below, they are rather more commonly blue in color). Additional MOVs and such in those positions would, in theory, provide greater transient surge suppression (may also be provided by plugging entire system supply into a surge-suppressing power strip), but that is entirely unnecessary for any practical purpose. Reading “between the lines” is often necessary.
Additionally, the pages including my modified wiring diagrams for the small-case models are shown and linked below.

page a-7 - modified manual supplement for small-case LTR masterspage a-16 v1 - modified manual supplement for small-case LTR masterspage a-16 v2 - modified manual supplement for small-case LTR masters

Or, the file linked here (pdf, 03 pages, 715kb) contains all the above manual pages, packaged in a conveniently printable format that may be used as a supplement to the original manual. The wiring diagram pages in the LTR2/4/6-128 and LTR2/4/6-384 manuals are numbered equivalently, so these pages may be added to either version of manual.
(Note that my additions to the manual, as with most of the content of this site, will likely be more useful/interesting to a system clock collector than to anyone charged with using this type of equipment in-the-wild. Use at your own risk, of course, in any event.)


These pictures are of my LTR6-384 with EPROM version F-9D (shown sitting on a Johnson plastic GRC-shaped movement dustcover). This seems to be the second-highest model of Lathem master clock ever made, before the LTRx-512 series was introduced circa-2000. I used this example in constant service for almost two years. It is currently relegated to testing and emergency backup purposes.

LTR6-384 relay/power board

This is the relay/power board. It has eight relays, same as the current LTR8-512 model. The difference is that these are hard-programmed to operate one system and six program circuits. (In my opinion: this is basically equivalent to the current highest model, and the LTR8-128 is still the highest model Lathem ever made.)

LTR6-384 inside of front panel

This is the inside of the front panel. I never bothered to replace the backup battery, so it’s probably original. (The unit works perfectly well externally connected to a physically larger battery.)

LTR6-384 front panel without acrylic

And this is the front panel assembled without the red acrylic piece, exposing the computer in operation, prepared for timebase adjustment. Note the same type of timebase crystal and variable capacitor as the LTR8-128 contained in the top-left corner of the board. (See: LTR8-128 timebase accuracy section.)

LTR6-384 from the inside

LTR6-384 inside of front panel

This is the inside of the front panel with the battery and the voltage regulator cover removed. The voltage regulator is an LM317K, just like the LTR8-128, also.

LTR6-384 front of computer board

This is the front…

LTR6-384 back of computer board

…and back of the computer board, showing a bit more detail than before.

LTR6-384 relay/power board

This is the relay and power supply board, with the cool blue relays removed. You may notice that there is a lot of “10-94” and “11-94” and such handwritten in various places. I always assumed those were original factory inspection dates (as in, it was made in late-1994).

LTR6-384 parts pile

And this is what it looks like when you pile all the parts together. It is actually rather simple, as far as things that can be disassembled goes—and it takes about ten minutes to put back together.