HG-10 VFO out issue's

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W3GMS:
I pulled off my HG-10 VFO off the shelf yesterday to mate it up with my DX-60 yesterday.  The DX-60 had plenty of grid drive when used with a xtal but very low grid drive with the HG-10 attached.  I did all the normal stuff like replacing the oscillator tube but nothing helped the output.  I did all the normal DC voltage checks and everything looked fine.  The output was very distorted and almost had  a double hump to the output sine wave. 

I then went to my spare really bad looking HG-10 and decided to hook it up to the DX-60.  It worked great with plenty of grid drive on the DX-60.  Then I took off the bottom of the one that worked and I noticed two RF chokes were swapped location wise in the one that worked compared to the one that did not.  I then went to the manual and the one that is wired correctly with the chokes is the one that does not produce enough output.  The one that has the chokes swapped works great!  I then took the swapped choke version and put it back the way the manual stated and sure enough, the ouput is low and the distortion is just like the original one that did not work.   

I was discussing this issue with Dave, KA2J and he sent me a later schematic of his HG-10B and low and behold one of the RF chokes they had increase in inductance.   

The chokes we are talking about are the plate RF choke which in mine is 28 uh and my cathode choke which is 1.1 mh.  When I swap them location wise it works great!  I can't believe Heath had that big of a documentation error since a lot o HG-10's were produced.  What's interesting is that the later versions of the HG-10 apparently have the inductance in the plate circuit increased to 350 uh. 

Does anyone have any experience dealing with this possible issue with the HG-10's? 

Regards,
Joe, W3GMS 

WA2ROC:
I just had a similar problem with a "brand new" HG-10B.  It was almost completed from the kit when I got it, but needed the front panel, dial stringing and alignment done.

I tried the 80 meter calibration and it worked OK up to about 3.8 MHZ.  There the signal dropped off rapidly.  I had to relocate several buss wires from the 80 meter coil to the places they were supposed to be routed and I have output all the way up to 3.95 MHZ.  

40, 20 and up work just fine.

I am not in the shop right now but will look at the RC chokes when I get home.  I do have a photo of the wiring and maybe you can determine how the chokes are installed.

UPDATE!

Just pulled the HG-10B schematic from EBAMAN and the plate RFC shows 28uh but is crossed out and penciled in with 350 uh.  The cathode RFC is 1.1mh. 

This pencil chance was done by whoever was building this VFO.

Maybe there was an addendum to the manual that was included in later kits.

There is also an HG-10 (non-B model) schematic there that has 28uh plate and 1.1MH CATHODE RFC's shown.

AB2EZ:
Joe
I heard you and the other folks talking about this on the air... so I got out my HG10 schematic.

I did some quick calculations, and it was apparent that increasing the plate choke from 28uH to a higher value was appropriate. I.e. there would be only 650 Ohms of plate load impedance at 3.7 MHz; and with an estimated plate current swing of +/-10mA, this implies only +/- 6.5V of plate voltage swing from the oscillator half of the 6CH8. The impedance of the plate load is twice as high on 40 meters; which implies twice the voltage swing. [Note that the total capacitance between the plate of the oscillator and everything else in the pentode half of the 6CH8, plus the total capacitance between the grid of the triode half of the 6CH8 and everything else in that half of the 6CH8, is only around 6pF; which implies a capacitive reactance of more than 7000 Ohms at 3.7MHz... so the total plate load impedance is controlled by the 28uH  plate choke]

Making the plate choke much larger (and thus making the plate load much larger) would not be a problem. The oscillator would just produce a sine wave + several harmonics... which is fine for this application.

In addition, it appears the a cathode choke is much larger than it needs to be. Given the 500pF of capacitance in parallel with it (only 86 Ohms impedance at 3.7 MHz), the 28uH choke would be big enough to do the job.

Thus.. swapping the two chokes makes sense. Changing the 28uH choke to a 350uH choke also makes sense. It would then  cancel most of the plate capacitance at 3.7MHz... producing an even higher load impedance.

Best regards
Stu

W3GMS:
Dick,
It sounds like your HG-10B has the same problem as I observed with my very early 1962 HG-10.  We both have the low value of plate RF choke.  If you look at the output of the VFO you will see reduced amplitude along with a very strange waveform.  Swapping the 2 inductors should fix the problem.  You will notice that the output waveform is not only higher in amplitude but the waveform is much cleaner.  The before and after pictures tell the story.  With the VFO chokes swapped, you should have plenty of grid drive and the grid drive control will be at 10 oclock or so on the DX-60.  With the 28 uh choke in the plate circuit with the control fully clockwise you will have only about 1.8 MA of grid drive.  As a reference, with a xtal installed the grid drive control on 80 is right around 9 oclock.   All my observations were done on 80 Meters. 

Stu,
Thanks for comments and quick analysis.  It sure correlates with what I observed.  What amazes me is that I don't remember anyone mentioning this issue before.  The tell tale sign is lack of grid drive when interfaced to the DX-60.  I will probably just leave my chokes swapped for now although its tempting to see its performance with Heath's updated value of 350 uh shown in some of the schematic published by Heathkit. 

It certainly sounds like an errata sheet from Heath could have been a possibility. 

I would post the schematics but not sure if that would be legal from a copyright perspective. 

Basically I have 4 versions of the schematic that people have sent me. 

HG-10 showing 28 uh
HG-10 showing 350 uh
HG-10B showing 28 uh
HG-10B showing 350 uh

You would think that by the time Heath got to the HG-10B that the documentation would have been consistent!

Thanks all...
Joe, W3GMS

       

Pete, WA2CWA:
As far as I can tell, Heath never changed the document for the HG-10 to make the RF choke change from 28 to 350 uh.

In 1973 they issued:
Quote

         February 7, 1973
HG-10B                  Bulletin No:
VFO                  HG-10B-1

            Hum On 40 Meters

Replace RFC-1, 28uH [PN 45-18] with a 350 uH choke coil,  [PN 45-82].
This change will be put into production soon.

This change was incorporated in the issue 4 of the HG-10B document, 595-886-04. If you have a HG-10 schematic that has the 350 uh RF choke, it was probably cobbled from a HG-10B schematic with a bit of whiteout in the right places.

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