McMurdo-Silver Masterpiece VI T10 Interstage Transformer Missing

[Felix-A. Gravel]

Hi!

My McMurdo-Silver Masterpiece 6 is currently being professionally restored and the project is going well except for one pretty big road block. The T10 interstage transformer in my unit is missing, and it is quite unique in design. Yes, we have found a few transformers that will *work*, however there is always a big compromise in performance. On top of that, this radio being a very high-end model with a complete known pedigree from the day it left the factory (I am its 2nd owner!), I believe it deserves to have the correct part installed.

I am at my wit's end trying to source an original replacement, so I thought I should post here.

If an original unit cannot be found, we would need to know the ratio of the original transformer, which I personally am not qualified to reverse-engineer. We do know the DC resistance readings of each winding (thanks, Alex!), but my restorer says it would be unwise and problematic to extrapolate the ratio from only this data.

I would be eternally grateful for any help on this matter, as this radio is very dear to me and I want it to be whole again! I am attaching a picture of it is with its original owner, sometime in the late 1930s or early 1940s.

[David C. Poland]

Looking at the entire July 1937 diagram, suggests the T10 transformer used was an "off the shelf"  push-pull for both primary and secondary winding, but the primary side is wired together as a bigger single primary. I just wonder why a more conventional interstage transformer with p-p secondary be a reasonable choice for now to get it running. 2nd AF is a single 6J5 as I read the diagram. Or does a proper orientation of the pair of primary windings become necessary for proper action of the secondaries? (but I have no experience with with the later McMurdo sets)

Your  M-S 6 set up for the expander and of the 2nd AF drivers is different than the Scott Philharmonic, although  Scott also uses an  interstage audio transformer that is p-p both primary and secondary.

[Jonathan Tipton]

Since these original transformers are rare, I would recommend going to the Lundahl Transformer website. They have many varied interstage transformers to meet your circuit needs. I have used them in my Philharmonic which had a blown interstage too and the Lundahl PN: L1660S P-P worked great and it fits in the space provided! Give it a try.

[Kent King]

I agree with your technician, you can't extrapolate from the DC Resistance values. Those secondary windings...even the direction they are wound may be relevant. I looked at my transformer catalogs of the period - I don't see anything that looks "close enough" to even try. But I agree with Dave - at this point in the MS lifecycle, they probably were not doing custom parts - many MS VI sets were built by Howard. It is probably an off the shelf transformer, if we can identify it.

Kent

[Alex R. Whitaker]

I would think that, with the resistance values of each winding, you could extrapolate the turns ratio.  You would have to take apart the transformer I sent you, and measure the wire gauge of each winding.  Then, look at the ohms/foot specifications for each gauge of wire.  You could then get the winding measurement in feet, based on the resistance measurement.