[AMRadio] Re: GB> Thank You! --WAS:Matchbox Melts Down IC-735 ??

James M. Walker chejmw at acsu.buffalo.edu
Fri Jun 24 00:29:42 EDT 2005

Ah a few additional comments and corrections please.

Merz Donald S wrote:
> Comments about the user's use of the Matchbox are summarized as:
> 1. It's a balanced tuner. Running it with an unbalanced output from a transceiver could be unpredictable.

The input to the MatchBox can be 50 or 75 ohm input determined by the
tape point
on the input coil, however direction is important here, The input to the
is from an unbalanced driver (the transceiver) The output of the MB is
either a Balanced feedline to the antenna, or an unbalanced feedline
to the antenna, The purpose of the MB is to present 50 ohms impedance to
source (transceiver).

> 2. It has a T/R relay that defaults to R. Was the T/R relay properly energized?

The relay defaults to receive true, however the receiver contacts are
NOT for
a 50 ohm receiver input, it is more on the order of 100 to 300 ohms and
available at the terminals on the strip not the coax connector , which
normally connected to the transmitter. Left side rear as you face the

> 3. Icom's literature says that the IC-735 does have thermal overload protection against high SWR. Why didn't it work?

Not known, but there could be several reasons! The user needs to clarify

> 4. The meltdown could have occurred due to a violent mismatch into which the operator let the TX transmit too long.

Again, if the operator was trying to transmit into the coax connector
the unit (MB) in receive mode there would be no load for the transmitter
this case, and reading his front panel meter on his transceiver would
indicated that. Sounds like a severe case of "SOP READ YOUR MANUAL"
on there. You can't be responsible if the new owner wasn't familiar with
his equipment.

> So I guess I still have a decision to make. I am not sure what I will do yet.

Input, need more INPUT!
with three MatchBox units 2 1KW models and 1 250 watt model, all with
switching functional.

More information about the AMRadio mailing list

This page last updated 17 Dec 2017.