HP 650A Output Divider Cable

65A-16D schematic

Someone on the Antique Radio Forum was asking about the HP 650A test oscillator. They are beautiful vacuum tube R-C test oscillators. The 650A was originally supplied with a Model 65A-16D Output Divider Cable, but they are invariably missing when you acquire a 650A.

The manual provides a complete description of the 65A-16D cable, so I constructed one for my 650A. 

IMG 0580

I used two small Hammond die-cast aluminum boxes. Rather than hard-wire the cable, I added BNC connectors at each end, so I can use whatever length of BNC patch cord is convenient for the current bench setup.

IMG 0581

The top side features a dual five-way binding post for the 300Ω output.

IMG 0589

I used fiber insulated shoulder washers to isolate the banana plugs from the case.

IMG 0591

Here's the inside of the source side, with the 594 ohm series resistor.

IMG 0590

And here's the 6Ω termination end.

IMG 0594

I also made up a 12:1 divider to provide about a 50 ohm output impedance.

IMG 0597

The 50 ohm output is the BNC connector.

IMG 0593

The divider consists of a 549Ω resistor and a 49.9Ω. This gives about a 48Ω output impedance, factoring in the 600 ohm output impedance of the oscillator. I should have used a 52.3Ω instead.

© Steve Byan 2011-2016