One day I noted that the power was not dropping to zero after unkeying. My investigation led me to the BC-456 modulator and the High Voltage relay K-52. I found that when unkeying, there was an arc between the relay contacts causing partial HV to go to the transmitter.

Here's what the relay contacts looked like:

When an inductive circuit (the dynamotor) is opened, the sudden collapse of the dynamotor's magnetic field generates a very high voltage spike that arcs through the partially open contacts melting the contact material (probably silver). This arc deposits melted contact material onto one relay contact (forming a “hill”); the melted material is from the other relay contact (forming a “hole”). The deposited material gradually builds up into a larger "hill" which then allows the relay contact to stick by the "hill" sticking into the "hole".

The solution is to put a filter (called a “snubber”) across the relay contacts. It consists of a capacitor in series with a resistor. It was recommended to use a 0.003 uF 1600V capacitor in series with a 1K resistor. I didn't have a 0.003 uF capacitor, so I used three 0.001 uF capacitor in parallel.

Here's a photo of the snubber installed in my BC-456 modulator:

Here's where the snubber goes in the BC-456:

I have not made this modification to my MD-7 modulator, but I would think it goes across the contacts of K-52 as well.