On Wed, 26 Mar 1997 21:17:18 -0600 John Bradley <jmb@eden.com> writes:
>I obtained a Dentron MLA-2500B shortly before ARRL SS last fall, and
>proceeded to renovate it into operating condition (replaced filter
>caps and
>equalizing resistors, blew out dust etc.). When I finally brought it
>up,
>my TS-450S exciter was unable to drive it properly on any band. The
>SWR
>indicated on the transceiver meter was not extraordinary (<3), but the
>power was [apparently] being cut back to about 20 W. by the protective
>circuitry. The built-in antenna tuner was unable to find any suitable
>match.
The TS-450 has a reputation for being very finicky about VSWR plus a ATU
with a less than ideal matching range.
>At this point, I examined the input circuits (both schematic and
>actual)
>and noticed two things:
>
>1. The MLA-2500B doesn't have a tuned input circuit; it relies
>on the
>[usually] low input impedance of the 2 8875's in conjunction with a
>100 ohm
>resistor across the input.
Many have disconnected this resistor in an attempt to obtain more output.
In the initial design it was only meant to be removed when using very low
power exciters. This is not covered in the "manual". It MUST be
installed for 100W nominal drive exciters.
The MLA is capable of 1500W + with good tubes but 1200W or so would
greatly extend tube life since they are over $500 each now.
Perhaps a improved input circuit patterned after the various 2 8874
Alpha's would help the TS-450 along a bit better. I have experienced no
difficulty when using a TS-940 as an exciter.
Good Luck and I hope that your tubes are not already toasted.
73...Carl KM1H
>2. The 100 ohm resistor was not connected.
>
>I concluded that the resistor had been disconnected when the mod to
>add 10
>meters was made. I also concluded that it shouldn't have been
>disconnected, and I reconnected it. Because of the part location, I
>connected it by running a wire from the input relay output terminal to
>the
>resistor (about 3 in.). Had it been easier to get to, I would have
>run the
>coax to the resistor and then on to the tubes.
>
>So, here's the problem. The amplifier seems to work (sort of). Power
>output is about 350 W. CW, 600 W. SSB. This is low (I think), but I
>don't
>really know what to expect from this amp. The real problem is that I
>still
>have an input SWR on 10 meters that is beyond the TS-450S tuner's
>ability
>to fix. 15 meters is much better, but still not <2.
>
>This leads to my questions:
>
>1. Was I correct in concluding that the 100 ohm resistor should
>be in the
>input circuit?
>
>2. Do you think my problem with 10 and 15 meters is caused by
>excessive
>inductance of my 3 in. wire from the input signal to the 100 ohm
>resistor?
>If not, any ideas?
>
>3. What kind of power should I be getting out of this
>amplifier? The power
>in is between 80 - 100 W. (depending on band).
>
>4. Is there any simple modification to the input network that
>would improve
>the match with a driver like the TS-450?
>
>Any other advice regarding this amplifier would be welcome as well.
>Thanks
>for any help on this; and thanks to everyone contributing to the
>reflector,
>it makes for enjoyable reading.
>
>73--
>John (KK5MH)
>
>
>
>--
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>
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