[TenTec] Comments on "Repairs"

[Larry DiGioia]

Robert & Linda McGraw K4TAX wrote:

>Larry:
>
>One concern that surfaced from the description of your station and grounding 
>technique.  You indicated that you have a common ground connected to an 
>outside driven ground.  Is this ground rod connected or bonded to the AC 
>mains ground?  It should be in order to prevent a difference in voltage 
>between the station ground and the AC mains ground.  Typically your radio is 
>connected between these two and thus any current differences are flowing 
>through the radio equipment.  Not good.
>  
>
There is no simple answer to this, I will describe: I have a total of  5 
ground rods spaced around the outside of the house that directly attach 
to the house wiring... I have a #6 copper "backbone" running the length 
of the house along the basement ceiling (shack is in the basement.) This 
is what the (electrical system) ground rods are connected to.

However, I also have the HyTower out on the front lawn. There are 4 more 
rods spaced around the HyTower, all connected to the radial system (77 
approx. 80' long radials) with copper strap (plus cadwelded #6.) There 
is a spark gap at the base of the HyTower. Then, a #6 wire runs from 
there, parallel with the feedline (both buried) all the way back to the 
house, where there is another rod, and the box with my Polyphaser 
protectors in it, connected to the rod with strap. Finally, this ground 
system continues inside where it meets the electrical backbone near the 
center.

I did all custom electrical wiring for the shack, and all outlets are 
grounded to the backbone. So I guess you can see why it is difficult to 
say where one system leaves off and the other begins...

>With balanced lines into the shack, if they are indeed balanced, then there 
>should be NO circulating currents.  I would certainly question this aspect 
>of the station.  Why would you have circulating currents or RF in the shack 
>if the line is balanced?  It is normal for a small amount of imbalance to be 
>in existence due to different ground conductivity under the legs of the 
>antenna.  I find it acceptable to have current differences in each side of a 
>balanced line to be less than 10% worst case.  More like 2% to 3% in my case 
>in practice.
>  
>
Well, I guess it is a question of "quantity." I didn't mean to make it 
sound as if the shack is "live" with RF, it is just that, with some 
audio equipment getting left on, I can hear the hum while tuning up on 
the balanced lines sometimes. Even with the Hytower, it will turn on a 
touch-switch light my wife keeps on the night table if I get on 40m. But 
it's not like I'm getting RF burns.

Where I personally see a possible problem is what I mentioned earlier - 
the fact that I may have as much as 10 feet of coax total between the 
rig and the balanced tuner (depending on coax switch settings, which 
rig, etc.)

>With the tuner and its internal SWR metering system you get good results. 
>OK, I'll buy that.  However, what is the SWR the radio is seeing?  This must 
>be measured at or in the radio.  Now to that end, with the ground system you 
>describe I could easily suspect that one of the diodes D2 or D3 on the Low 
>Pass Filter board could be defective thus caused by a nearby lightning 
>strike and a step voltage between two different grounds.  With either diode 
>at fault the radio ALC and power control system may be failing to protect or 
>control the radio.  I've seen this before.  Others relate the same story.
>  
>
>Now I know this doesn't explain why the Yaesu works fine and the Tentec 
>doesn't.  Could it be that the Tentec stays connected in the system all the 
>time or most of the time and the Yaesu sits detached from the system most of 
>the time?  A recent nearby lightning tap may have taken out D2 and or D3.
>  
>
Um, yes, but the radio had only been back from the factory after the 
last repair (diodes, among other things, not sure which ones, replaced) 
for about 4 hours of operation before quitting again, there was no 
lightning or exposure to lightning.

I am seeing a pattern here, though - for instance I had the same thing 
happen after Field Day: I took it out (power supply and all,) to Field 
Day and used it with an 80m vertical of my own design, tuned with the 
Nye, ran the whole weekend without a hitch. Got it home and a few days 
later went to operate 20m into my HyTower, poof. This time, after 
getting it back, I operated 80m SS (HyTower) for 4 hours with no 
problem, then 2 weeks later started to tune it up on 20m into a dipole 
Thanksgiving day, poof.

>Regarding commercially made RG-213 jumpers.  OK good I'll buy that but I've 
>seen new, commercially made jumpers in two way and broadcast equipment found 
>to be defective out of the box.  Spin the shell off of the PL-259's and 
>check to make sure the braid is soldered and the center pin is soldered.
>
It is.

>  A 
>good check with an ohm meter is wise.  Better yet, a dummy load on one end 
>of the cable with the MFJ bridge on the other end and do a wiggle and jiggle 
>check. 
>
A wiggle and jiggle test turned up an intermittent open on the 
previously-mentioned arc-plug suppressor, that's why it's gone.

>Frankly, I don't care for the RM-35M power supply unless 
>you have a 20A fast acting magnetic breaker between it and the radio.  It 
>has the ability to instantly dump 35 amps of current into the PA which is 
>only rated for about 26 amps max.  OK so it works with the Yaesu but they 
>use a totally different over current protection scheme.  Tentec does not 
>monitor or control emitter currents on the PA or driver stage.  They use the 
>SWR and ALC circuit to control drive power.  This area could be the source 
>of your evils.
>  
>
Yes, this is sounding more and more like the area I should be looking 
at! I am going to either get the TenTec supply (if I can find one,) or 
the breaker.

>More good dialogue please.  We'll solve the problem to component level if 
>you wish.
>
Thank you for the generous offer! So, are you implying that you think 
"this time" may be only another bad diode or two, for example? I am fine 
with a soldering iron and board work, but I don't want to take any 
chance of hurting "my baby," if there is any doubt, I'll just pack it 
off to Tennessee again, I think the next DX contest (that I care about) 
isn't for about another month!


-- 
Larry  N8KU

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