Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Help with my 6146B homebrew CW transmitter - Some progress today?  (Read 2879 times)
KC9KEP
Member

Posts: 208


WWW

Ignore
« on: December 18, 2012, 03:20:46 PM »

Hello all and a profuse "thank yous" to all who responded to my query about debugging my
circa 1965 CW 6146B transmitter!

Two things seemed to make a big improvement today:

1.)  I moved the neutralization capacitor in order to shorten the wire leads that connect to it.  Actually, I still had issues
with PA oscillations even after re-locating the capacitor.  So, I was just going to remove it from the circuit and see how
the transmitter performed without it.  I discovered that the capacitor's rotor is close enough to the 6146 to capacitive couple
even with no wire lead connected between the capacitor's rotor and 6146B plate!  (To neutralize, I used the technique of removing
the plate and screen voltages to the 6146, ran the oscillator, and adjusted the neutralizing capacitor for null at the antenna output.)

2.)  I moved the 150 pF to ground as per KC2ZFA's suggestion.  (The newer evolution of this design is wired in this manner
in the ARRL handbooks.)

Bottom line, I can run my screen current at 2.5 mA with no no-oscillator-start-up problems so far.  My plate current is still about 150 mA but
my cross point needle meter says that I'm delivering at least 80 Watts into my Heathkit Cantenna.

(I may experiment by changing the 150 pF to 1500 pF and add a 10 Ohm resistor to the grid circuit as the ARRL handbook illustrates in newer
versions  ..)

73

--KC9KEP
Logged
KC2ZFA
Member

Posts: 15




Ignore
« Reply #1 on: December 18, 2012, 05:10:24 PM »

Good job OM !

(To neutralize, I used the technique of removing
the plate and screen voltages to the 6146, ran the oscillator, and adjusted the neutralizing capacitor for null at the antenna output.)

if your PA is not neutralized then you should be getting lots of grid current variation as you turn the plate tune capacitor (with a dummy load on and no plate/screen volts). So another way to neutralize is to tune the neutralization capacitor so that the grid current stays constant as you turn the plate cap through its range.

Logged
KC9KEP
Member

Posts: 208


WWW

Ignore
« Reply #2 on: December 18, 2012, 05:37:33 PM »

I don't know what I'd do without this eHam resource and all the helpful Elmers here.

Building equipment from the ARRL handbooks is kind of like building an automobile from scratch, but
having never driven one before. 

The obvious and common hands-on knowledge just can't be beat!

'Uhh .. is it normal for the car to not start when you leave the headlights on over night?"

Seriously though, thanks again to all :-)

73

--KC9KEP



Logged
K8AXW
Member

Posts: 3900




Ignore
« Reply #3 on: December 18, 2012, 09:37:17 PM »

KEP:  I built a 2 X 3-500Z linear from one of the ARRL Handbooks.  THAT was a trip!!  Still use it.  If you check QRZ.com it's that black beast to the right of me in the photo.
Logged
GW3OQK
Member

Posts: 153




Ignore
« Reply #4 on: December 19, 2012, 04:59:29 AM »

Hope you had luck making the 150pf bigger. I reckoned it much too small. It needs to be 1000pf or greater to give rf ground return to the bottom end of L1,2 & C1 I believe. Then as a potential divider with C2, it serves to neutralise 0.22 pf anode-grid capacity, if neutralising is required at all.

I think RFC4 & 22R has been drawn in the wrong place, and is intended as a VHF parasitic stopper to be connected to anode and other connections.

Wonder who'll shoot me down.
Andrew
Logged
KC9KEP
Member

Posts: 208


WWW

Ignore
« Reply #5 on: December 19, 2012, 05:31:46 AM »

GW3OQK,

Yes, I've noticed that the parasitic suppressor has been moved in the schematic diagrams for
all the newer versions of this transmitter in the ARRL handbooks.

Another nit that I've noticed is that several extra "fly-specks" are present in the 1965 schematic
that usually indicate multiple connections at a given wiring junction.  These extra fly-specks go
nowhere .. i.e., they are situated where a wire takes a 90 degree turn.  It's as if someone was
editing the schematic and didn't finish the job ..

(I think they're called "fly-specks" .. the little "dot" that indicates that two wires connect where
they cross each other)

73

--KC9KEP
Logged
KC2ZFA
Member

Posts: 15




Ignore
« Reply #6 on: December 19, 2012, 08:28:39 AM »

if you have a 500 or 1000 pF feed-thru capacitor use it instead of grounding that 150 pF (or its
replacement) under the chassis and then running a wire through the chassis. The neutralizing
cap can be soldered directly to the top side of the feed-thru capacitor and, if properly placed,
the bottom side of the feed-thru can be soldered directly to C1 or S2 (whichever is closer).
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!