Merit Modulation Iron

[W9ZSL]

  I scored a mint Merit A-4005 modulation transformer off eBag.  It has 12 terminals and is rated at about 60 watts.  The tag says it is 2-20K.  I want to build a proto-type 150 watt CW/120 watt AM rig based on the 1957 ARRL handbook classic parallel 807 amp in advance of the 300 watt rig I've been dreaming about here all these years. I have several choices for the modulator though 807s modulating 807s is too cool to ignore. 

My first question would be does anyone have the pin outs for this iron in their files?  I went to Bunker and nothing.  Knowing the impedance, what is the best way to translate the measured resistance to the proper ratios to determine which terminals are which?  Mike-W9ZSL

[KE6DF]

I have a 3110 Merit mod trans. It's also 60 watts.

I think it is a 60 Watt multi-match version of mine.

I'm just guessing here, but it probably has two primary windings and two secondary windings.

Both of these have a tap.

So that would add up to 3 times 4 or twelve pins.

So, a good guess would be:

Pins 1 and 3 are one winding of the primary with pin 2 as a tap.

Pins 4 and 6 are a second winding of the primary with pin 5 as a tap.

Simlar for pins 7 through 12 -- the two secondary windings.

It's possible pin 4 goes with the first primary winding and pin 3 with the second.

For some reason UTC does that on their audio trans -- just to confuse people I think.  

When I say "primary" and "secondary" in the above disucssion I'm using the terms loosely since most multi-match transformers work either forward or backwards.

As for ratios, once you determine the windings with an Ohm meter you can determing the ratio by putting a low ac voltage from a filament transformer into one side and measuring the output.

The two secondary coils can be connected in series or parallel to get more options.

I'd operate with the above assumptions as a starting point and investigate from there.

Dave

[KA2DZT]

I have the 125 watt version of the Merit mod iron.  I don't ever remember seeing any data charts for the Merit xfmrs.  Probably something very similar to the Stancors.

Most all the multi-match xfmrs are made up of four windings, each with a tap.  The taps are not at center, they're off center.  Usually the total number of turns on one side is not the same as the other side.  There is usually a step-up from one side to the other.  Each side of the xfmr is wound with two the-same-number-of-turns windings.

It is always better to use the whole winding on each side of the xfmr.  Just a matter of which side will get the higher number of turns, primary or secondary (step-up or step-down).  You may find that not using the whole secondary may work better.  You have to experiment a little when you get the rig built.

Using a low voltage xfmr, you can apply voltage to one winding and then measure the voltage on the other side.  You can use this method to find the correct phasing for each of the four windings.

Usually 3 and 4 are tied together on one side to get the center terminal and I would think 9 and 10 are tied together for the other side.  But you'll have to check this with the voltage measurement testing.

Fred