[Milsurplus] Bendix Skipper 135: OOPS! and Opinions

[MICHAEL ST ANGELO]

Dave,

I used to repair and tune Marine MF (2 to 4 Mhz) radios on boats when I was in college 45 years ago. I had to adjust the output series coil to resonate the 20 foot whip antenna. My memory is vague but the coil was used to resonate with the fiberglass whip on board the boat; the load was not necessarily 50 ohms.

If you have an antenna analyzer of one of those VNA's this is what I would try.

BEFORE YOU DO THIS MEASUREMENT VERIFY THE AMPLIFIER IS POWERED DOWN AND PLACE A SHORT ACROSS THE POWER FILTER CAPACITORS TO DISCHARGE THE CAPACITORS!

- Place the RF Tube in it's socket. This will simulate the tube capacitance.

The ARRL handbook gives the following approximation formula for calculating the load
resistance of a vacuum tube power amplifier:
Vp
Rl = ----
KIp
where K is 1.5 for class AB, 1.57 for class B, and 2 for class C.

Calculate and install a non-inductive resistor from tube late to ground to simulate the Plate load.

- Install the Analyzer or VNA on the RF Output connector

- Set the PI Network Capacitors and Coil for each band and adjust the capacitors (and coil tap if necessary) to get the output impedance close to 50 +j0 ohms.

The plate formula is an approximation but these measurements will get you into the ballpark.

- Remove the resistor and short before operating the amplifier.

Since marine radios were transitioning to the VHF band I would recommend getting a new VHF radio instead of spending money fixing the MF radio but some owners didn't want to spend any cash on new equipment.

Mike N2MS


>     On 10/26/2021 1:08 AM David Stinson <arc5 at ix.netcom.com> wrote:
>      
>      
> 
>      Remember the Bendix Skipper 135 Marine radio with the
>     High Voltage problem?  Well, I think I have that sorted-out
>     and it wasn't the switching transistors; it caught a case of
>     the "oops."    You know, voltage doublers work so much
>     better when you connect both electrolytics with the
>     proper polarity.... DOH!
>     I ran out of time (as usual) before I could correct it,
>     but I'll get back to it as soon as work lets go of my hair.
> 
>     Anyways- here's the latest request for opinions.
>     I'm attaching the transmitter circuit diagram.
> 
>     Reduced the five crystal channels to one on the diagram.
>     These rigs typically have movable taps on tank coils,
>     and loading coils (think of an ARC-5's roller),
>     one tap for each channel.
>     I don't want to fuzzle-around with swapping coil taps
>     and tuning caps with the PA "live" drawing excess current
>     while I "muddy the waters" looking for the right taps.
>     So here's the plan- I will disable the High Voltage, light
>     the tube filaments and use a signal generator to inject
>     3885 KC on the PA signal grid, connect the rig to a
>     50-ohm dummy load, monitor for the signal at the
>     load with a scope and move taps/tune caps for max
>     signal across the load.  If it does the "Milrig" deal
>     where it won't tune 50-Ohms, a series cap will fix it.
>     That way, when I restore High Voltage, the rig
>     should be close to correctly tuned.
>     What do you think?
> 
>     TNX OM ES 73 DE Dave AB5S
> 
>     P.S.  Grid dip meters are a non-starter.
>     I never have good results with those things.
>     In fact, I've given-away the ones I had.
> 
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