Debunking the Collins KW-1 Modulation Transformer Myth

W5HRO

Amateur Station
#1
Debunking the Collins KW-1 Modulation Transformer Myth
By W5HRO

There has been a lot of misinformation on the web and within the amateur radio community in regard to the modulation transformer used in the Collins KW-1. Both the KW-1 and the Johnson Desk KW used the same transformer with only slight variations and they were both based on the old Chicago CMS-3 transformer. The transformers were designed for a pair of 810’s in push-pull (12K plate-to-plate) to the 4-250’s or 4-400’s in the RF amp for around a 6.25K RF power amplifier load. The Johnson Desk KW transformer did have additional low-voltage taps that were used for low-power mode, but towards the end of the KW-1’s production run Collins added additional connections for 18K plate-to-plate to increase the primary impedance. It was done because many hams were blowing those transformers due to the talk-back and etc. However, it was a poor workaround to solve a problem that was actually being caused by something else. That something else was the splatter choke (L-503) and its two .006uF caps to ground which creates a pi-network. That spatter circuit is one of the biggest mistakes Collins ever made.

If you understand how a pi-network circuit works, like in a class C RF amplifier for example there is a dip on the input side of that network. When you dip the plate, you are dipping it to a resonate null. Unfortunately Collins didn’t realize those pi-network audio spatter filters would produce the exact same null. When you turn the audio level up it will hit that null and if you calculate it out using the choke and the 2 capacitor values you will see where it is and it happens on the input side of the filter as can be seen in the spice simulator screenshot below. That is what creates the talk-back and slowly damages the transformer over time. It tries to resonate the modulator which creates a bad match each time it happens. The null happens around 1.8kHz to 2kHz depending on which choke taps you use and it destroyed many of those modulation transformers over the decades. It was simply a design screw-up that Collins never revealed. They either didn’t know or they did know and their solution was to just send out a service bulletin to try and sell people another transformer with the additional 18K connections on the primary.

KW-1_Null.png

The problem is a pair of 810’s are 11.6K to 12K max and not 18K. The Johnson Desk KW transformer never had any 18K connections because it simply didn’t need them. It had the 12K connections and the 9K taps for low power mode and they never used the splatter circuit which is why the 12K always worked correctly. Collins just should have told people that all they had to do was eliminate the spatter circuit to eliminate the talk-back and protect the transformer, but instead they increased the primary impedance which was not a good solution at all. Much of the talk-back went away, but the dip at the filter input was still hard on the transformer regardless. For full 810 modulator power 12K on the primary is correct. 18K is simply too high.

The solution to the problem is to just bypass (short across) the splatter choke (L-503) and disconnect the two .006uF bypass caps (C-503 & C-504) going to ground on each side of it and the problem is thus eliminated. Then the 12K primary connections will work correctly and the transformer will no longer be destroyed.

KW-1_Splatter.png

A big part of the problem within the amateur radio community has always been that the majority of Collins collectors think Collins was like God and incapable of ever making such a design mistake. Unfortunately, it’s that very attitude which has led to all of the misinformation regarding issues such as this and it’s not the only one. Nobody was perfect and Collins did make their share of mistakes.
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