Merit Mod Xfmr

[K9PNP]

Anybody have the connection matrix for a Merit A-3106 125 W mod xfmr??

Have finally gotten the HV and modulator power supply done for my 1954 [roughly] 813 rig I'm slowly building to go with my homebrew receiver.  Had this mod xfmr in stock, but have no idea where the paperwork went that shows how to connect the 6 input and 6 output connections to match a specific impedance.  As with the receiver, this will be a 'use what you got if at all possible' rig.

Thanks

[KA2DZT]

I have the same Merit mod xfmr.  I also have no info.  Just use an AC voltage on the windings and figure out the turns ratios.  You only need to know the turns ratios, impedances are the square of the turns ratio.

Try to use the whole winding if you can, especially on the primary.  Most all these 12 terminal mod xfmrs are about the same.  There will be a step-up from the entire winding on one side to the other.  The taps on each of the four windings are not in the center of the winding, they're offset.

Find whatever your class C plate load will be, then using the turns ratio you can find what mod plate load will be reflected back to the modulators.

First start by putting a low (10-20 volts) AC voltage on one winding (terminal 1+3) forget the taps for now.  Now measure the voltage on one winding on the other side. May be a step-up or down.  Now add in the other winding on that side to determine the phasing.  Usually the middle numbers are the center, meaning 3+4 on one side and 9+10 on the other.  But, check it out.  If the windings are in phase the voltage will double, if out of phase the voltage will drop to zero.

Once you found the correct phasing on one side then put the AC voltage on the entire winding.  Once again if you see no voltage on the other side you have the two windings (with the 10-20 volts) out of phase.  If in phase you'll see a voltage on the other side.  These new readings will be half of what they were before because you have put the same 10-20 volts on the whole winding (1+6 with 3+4 jumped) instead of just half.

You, at this point, should be able to tell which side of the xfmr is the step-up or step-down.  I think, the taps favor the inner sides of each winding, but I'm not 100% sure.  On the primary side (like I mentioned above) try to work with the whole winding, meaning don't use the taps.  Normally you'll put the secondary windings in series and there you may not use the total of the two windings in series.

You need to figure which side of the xfmr will be the primary.  Step-up to the PA or step-down to the PA.

Also use an ohmmeter to find the terminal numbers that go with each winding.  The numbers I used above probably are right, but check them out.

The two windings on each side are the same.  But, when you use an ohmmeter on each winding of a given side you'll find that the resistances are not exactly the same.  This is normal, as the windings are over each other.  Because of the ever increasing diameter of the windings on the core, it takes more copper to complete each turn.  More copper yields more resistance for each turn.

This will give you enough to work with, lets us know what you figured out.

Fred

[K9PNP]

Thanks Fred.  Thought I would give it a shot to see if anybody had the info before I dug into doing it the hard way.  Guess I'll have something else to do now to go through this.  When I eventually get it figured out, will let you know.

The xfmr is really similar in specs and physically to the Stancor A-3894 and its info is posted here on the site.  So, think I will go initially on the premise that they are grossly similar and see if the testing agrees.

[Opcom]

It's probably between the 1952 and 1966 catalogs. Closer to 1966, listing the A-3107 and A-3105. Wish I had more Merit catalogs online.

[KA2DZT]

It's probably between the 1952 and 1966 catalogs. Closer to 1966, listing the A-3107 and A-3105. Wish I had more Merit catalogs online.

I have Merit catalogs but I never could find the impedance charts for the mod xfmrs.  Somebody must have one somewhere.

Fred

[K9PNP]

It's listed in the top section on page 8.  Just no impedance interconnect matrix.

[K9PNP]

Been a while since I have done this, but here is the resulting data.  Have not checked the taps; plan to do that when I have a chance.  Was going to scan the paper I worked this up on, but scanner refuses to work now.

Primaries are:  1-2-3 and 4-5-6.  For series connect 3 to 4.

Secondaries are:  7-8-9 and 10-11-12.  For series connect 9 to 10.  For parallel connect 7 to 10 and 9 to 12.

With 10.0 V across full primary [1 and 6] with 3 and 4 connected:  series connected full secondary is 11.8 V.

Using E(sec) = [Nsec/Npri] X Epri;  Nsec/Npri = 1.18    {Turns ratio}

Using Zpri = [Npri/Nsec] squared X Zsec;  Zpri = 0.718 Zsec.

For parallel connected secondary, turn ratio figures out to be 1.754 using above formulas with the measured 5.7 V across the paralleled secondary.

This give Zpri = 0.325 Zsec.

When I get a chance to check the taps, will post that info.