Icom IC-746 Faq page 46

Switching Rates Explored
Closer examination of the maximum switching rates reveals a possible problem. Suppose,
for example, I transmit at only 1 word per minute. We average 5 characters per word, 3 dit-
dahs per character, and 2 switches per dit-dah. So, CW at 1 wpm will generate 30 relay
operations per minute. That's the limit of what the relay can handle at full load. 10 wpm will
generate 300 op/min, which is the no-load limit for the relay. 30-40 wpm (what my local
CW contesters do) pushes us to about 1,000 op/min; which is well beyond the relay's rating.
The max internal keyer speed of 67 wpm (as per ARRL review) would yield a brutal 2,010
op/sec.
This could account for the varying results encountered in the field. We'd all assumed till now
that the problem was simply one of current or voltage overload.
those operating QSK CW are overstressing the part, no matter what the load. Pushing the
part that far beyond its operational limits would certainly make it more failure prone, which
could explain some of the failures that have happened within the supposed current
limitations.
Expected Lifetime?
Consider this: CW at 20 wpm would generate about 600 op/min, which would reach the
expected mechanical life of the part after less than 3,000 hours of transmitting. But wait; if
we apply that same rate to the expected electrical life, we get less than 30 hours of
transmitting time.
In contrast, AM/FM/SSB/RTTY operation would likely require 200,000 to 2,000 hours
(respectively) of operating time to hit either expected lifetime.
The Schematics
The schematic on page 11-4 of the Service Manual shows the SEND relay as an AHY103,
not the OMR-109F listed in the Parts List on page 6-12.
(this is listed in the "Manual Discrepancy" section of the FAQ)
Currently, we believe that the parts list is correct and that the schematic is wrong. However,
this might help explain why Icom Tech Support has quoted different specs at different times.
Mysterious Successes
OK. To be totally fair, there are some amps out there that seem to work OK without an
isolation switcher. Alpha Power, for example, uses an IC-746 daily to test their Alpha 99
series of amplifiers. They report no problems. Off course, their 25 Vdc exceeds Icom's
specifications but are within the relay specifications. Go figure.
Connection Confusion
So far, we've concentrated upon the SEND jack on the back of the rig. There are also
HSEND and VSEND signals available via the ACC connectors. What makes matters worse
is when some folks mistake these signals for the relay driven SEND line, when in fact they
IC-746 FAQ
Page 46 of 113
2003-02-08
Now we can see that