[Premium-Rx] WJ-8711A Main Tuning Encoder

[Charles Steinmetz]

Much has been said (mostly by a previous generation of "authorities") 
about the shaft encoders WJ used in the 8711/8711A/HF1000 receivers -- 
particularly as related to transmitted (leaked) and conducted encoder 
switching noise.  Most of those opinions are incorrect.

Specifically, WJ's own "solution" was to replace the metal-shaft encoder 
with a plastic-shaft encoder, which dropped the externally-radiated 
interference to a dull roar.  *However,* in addition to being only a 
half-measure with respect to the externally-radiated interference, this 
did nothing to reduce the radiated, re-radiated, and conducted 
interference *internal* to the receiver.

The only proper fix is to use a metal-shaft encoder that is very well 
grounded at the enclosure through-hole, and to "soften" the switching 
waveforms by filtering the encoder feed lines.

Grounding the encoder shaft can be accomplished with Tempest-style metal 
mesh bushings (such as the ones used on the HP-8640B or 
Tempest-qualified equipment like some Harris RF-590s), or by creative 
use of small, gold-plated safety pins installed planar to the front 
panel so that their bodies are firmly grounded and the free legs are 
spring-loaded to bear firmly on the shaft.  I generally use three of 
them, radially disposed at 120-degree intervals and treated with 
Cramolin Red then Blue.  Finally, the front panel must be securely 
bonded to the chassis at multiple points with silver-plated copper braid.

I've been trying to find my notes regarding the encoder filtering I 
worked out, so far without success.  I do recall that I used two-pole LC 
filters on the encoder outputs (series L toward the encoder, shunt C to 
ground at the other end of the L, all *very* close to the encoder), and 
four-pole LCLC filters to the power line (ditto).

Best regards,

Charles



> Ok, I opened up both 8711a's and did a little research.......
>
> Oddly enough, both steel shaft and plastic shaft encoders have the same
> mfg's part number. (HRPG-ASCA#19F) mfg by Broadcom (Agilent?). Mouser has
> them listed as non-stocked with a 14 week lead time. Digikey lists them as
> a minimum qty 50 order.
>
> On my 8711a with the steel shaft encoder, there is a small tantalum cap
> soldered across two of the 5 pins on the back of the encoder. (noise
> reduction?) I removed the knob (two small hex head screws) and there is a
> compression washer on the shaft. I put the knob back on pushing against the
> compression washer while I tightened the set screws and no more wobble...
> great... . On my other 8711a with the plastic shaft encoder, I'm thinking I
> might be able to drill and tap two holes in the knob and add two set screws
> like on the other knob, add a compression washer and wah-la!.. another fix.
> Of course it would be best to find new replacements but that's not looking
> too promising right now. We'll see....
>
> There's an E-bay seller that from Israel that has four used one's (steel
> shaft) listed for $145 (ouch!) but used??...
>
> I haven't examined the data sheet to see what the part number suffix "19F"
> means but maybe a substitute is possible....
>
>
>
> On Tue, May 7, 2019 at 8:55 AM Gore, Greg <ggore at hmsracing.com> wrote:
>
>> Glenn,
>>
>> Which encoder do you have, plastic shaft or metal shaft? The plastic shaft
>> encoders were subject to wear if they weren't lubricated periodically which
>> resulted in a really sloppy fit. I have one here that was so bad the tuning
>> knob rubbed hard on the front panel and wore through the paint. There was a
>> service bulletin which explained the reason for plastic shaft encoders was
>> to correct a noise issue. As far as I know the plastic shaft encoders were
>> special parts and are no longer obtainable except from another receiver.
>> The original metal shaft encoders are still available. Tuning knobs are
>> different for each type, plastic shaft used a push on type mounting with a
>> flat side while the metal shaft knobs used round with set screws.
>>
>> Greg;
>> WA1KBQ