HG-10 VFO out issue's

[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:

         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.

[KX5JT]

I too have an early HG-10 with my early DX-60.  I have noticed less grid drive on 80 meters.  I just compensate with the drive pot but I cannot hear the spotting heterodyne on 80 meters making it a PITA to use on the band. 

Could swapping those coils solve this problem too?

[KK4RF]

Gentlemen:
     Interesting topic. I bought an old HG-10 vfo at a hamfest in September. It seems to work OK but I do have quite a hum with it when I use it with an old HW-16, which I don't have when I run the HW-16 with a crystal. My manual shows the plate choke to be a 28 uH unit. Think I will tear into the vfo and make the swap. By the way, I was impressed at just how well built the vfo was. It's a very solidly made vfo, frankly better than some of the competitors' vfos of the day. Enough said. Thanks again.   ---Marty, KK4RF---

[W3GMS]

Marty & John,
I would swap the 2 chokes and I am sure you will have much more grid drive and better performance from a drive standpoint.  Mine was so bad that I could not get enough drive with the drive control turned all the way up on 80M. 

Pete,
I will send you an e mail....

Gary, W1GHW is going to send me some scope readings of his which appear to have the very low value inductor in the plate circuit.  I am sure it will look like mine that is wired per the manual but offers low drive on 80M to the DX-60. 

Very interesting find!

Regards,
Joe, W3GMS 

     

[KK4RF]

Joe,
     I found a 1 mH choke in my junkbox and stuck it in. Did not touch the cathode choke. Left it alone. VfO works FB and my spotting signal now much louder on 80 meters. Formerly it was fairly quiet. I need to hook it up to my other HW-16 and see if the 40 meter hum disappeared. Anyway, I alerted the members of the YAHOO Group HW-16/HG-10 users group about this topic thread and I'm sure there will be quite a bit of interest in this topic. Thanks for your observations, Joe. ---Marty, KK4RF---

[W0GSQ]

Tube substitution guide shows 6CU8 will sub for the stock 6CH8. It shows higher gain.
Pros, cons, yes, no, maybe?

[W3GMS]

Marty,
The 1 mh works great.  As Stu, AB2EZ noted above, the 28 uh choke just does not have enough inductance especially at 3.5 mhz.  Its clear that some users are getting away with 28 uh, but the engineering analysis shows that its not the correct value at the lower frequencies.  Both amplitude and waveform quality suffer.  Another interdependency  could be the differences in DX-60's.  The oscillator topology is different when used as a xtal oscillator compared to when the VFO is used.  Maybe the gain on my 6CL6 is somewhat low and that would explain maybe why I can't the grid drive when others Dx-60 users can.   At this point, I am going to make the HG-10 perform properly by putting the 1.1 mh choke in the plate circuit and using the smaller inductor in the cathode circuit. 
Thanks all for the comments on this subject.
Regards,
Joe

[WA2ROC]

I had my late model (1975 vintage) HG-10B out of the case this morning and it has the 350uh RFC in the plate circuit and on the schematic and parts listing.  I relocated the physical position of the choke (the original builder put it in a straight line point to point) and I now get full drive at the top end of 75 meters.

I also developed, troubleshot and fixed a problem too.   After the choke was moved, I got no output at all.  After puttzing around, I found that the RFC was open!  I was able to re-solder the small wire to the lead and everything worked fine once again.

[W3GMS]

Dick,
One thing for sure, you will get more out of the VFO with the 350 uh choke than the 28 uh.  In fact with the 350 uh choke I am sure the DX-60 would be able to develop enough grid drive.  I did not have the 350 uh choke in mine so the wimpy 28 uh was moved to the cathode and the 1.1 mh was placed in the plate circuit and the beast is operating fantastic. 
Thanks for your input on your late model HG-10B. 
Regards,
Joe, W3GMS   

[Dave KA2J]

Here are two waveforms observed on the output my HG-10 VFO.  One with the existing choke on the plate of V1a (not sure if it is a 28uh, as it looks like it may be a replacement choke) and a 350uh I purchased from Mouser.  There is more grid drive on 3.7 MHz, as the gain pot went from a position of 3.6 to 1.3.

Notice how their is less energy in the harmonic content and more available for drive at 3.7 MHz.

Dave KA2J

[w1vtp]

Dave

What's the scale on that scope reading?  What kind of load was it looking into?

Al

[AB2EZ]

I descided to hook my HG-10 (working, but rarely used) to my SB-10 sideband adapter. As expected, the signal that the HG-10 developed across the input of the SB-10 was too low, particularly on 75m

Note that the SB-10 has an input circuit that looks roughly like a 1000pF capacitor (around -j40 ohms on 75 meters) in series with two parallel +/- 45 degree phase shifting networks. Each of the phase shifting networks has an impedance of (roughly) 50 ohms - j50 ohms on the selected band of operation.

Note that the HG-10 has a cathiode follower output circuit (could drive a low impedance)... but there is a 100pF capacitor (about -j400 ohms on 75 meters) between the cathode and the HG-10 output jack.

This 100pF capacitor in the HG-10 output path, in combination with the input impedance of the SB-10, acts like an approximately 10:1 voltage divider at 3.885 MHz.

Since the HG-10 was intended to produce a lot of harmonic content in its output waveform, one can understand why the the plate load on the emitter coupled oscillator is an inductor (higher impedance at higher frequencies)... and one can understand why there is a 100pF capacitor between the output of the cathode follower and the output jack (lower impedance at higher frequencies).

I am going to increase the value of the 100pF capacitor in the HG-10 output path to 1000pF... in order to substantially reduce the voltage divider effect on 3.885 MHz. I'll also check the inductor value to see what it is. I may increase the inductor value, and place a 10kohm resistor in parallel with it, to produce a load on the plate of the emitter coupled oscillator that is around 10kohms at all frequencies... since I don't use the HG-10 at frequencies above 7.3MHz, and I don't need a lot of harmonic content in the waveform.

Stu

[k4kyv]

I seem to recall from somewhere that the 80m oscillator is actually oscillating on 160m, just as in the  case of the older VF-1, so that the 80m output is the 2nd harmonic.  Not sure if this is true, but might be worth checking out, and if so, it shouldn't be too difficult to modify the unit for output on 160m.

If you are using it to drive one or more class-C stages, harmonics in the output is of no consequence, since the class-C amplifier is inherently rich in harmonics in any case, depending entirely on the output tuned circuit(s) to filter them out.

Don't you love how "new and improved" often means useful features discontinued, such as Heathkit deleting the 160m band when they "upgraded" from the VF-1 to the HG-10, and from the DX-100 to the Apache?