[HBR] HR-10 to HBR project

[Walt Hutchens]

Pictures of the project can be found here:

https://picasaweb.google.com/105255380483660395824/Walt2011Project

The first four (from upper left) are the original HR-10.   Then there are
two with the front end area stripped; the bandswitch assembly (coils ...)
have also been removed, though you can't see that in the pictures.
Finally, six pictures showing the HBR-ized version.   Points of interest:

1. The shield plate between the antenna and mixer coils.   It has to be this
large to prevent oscillation.

2. Relocation of the CAL capacitor from left of the ANT TRIM cap to below
it.  (Saw off the bracket, new hole in panel and new mounting holes in
chassis.)   This frees up the space just to the right of the tuning cap for
the coils.

3. Slide switches gone -- replaced with miniature toggle switches.

4. Large toggle switch does ON/OFF function, replacing the switch on the
volume control.

5. S-meter aligned with panel markings.

6. Three more tubes -- all 6BH6's -- for the three oscillator functions.

7. Extra holes for the 100 kcs calibrator function at the right rear of the
chassis are a mistake: This function is now in front of and just left of the
tuning cap.

8. Blocks of wood taped to top of power transformer (in some pics) allow the
set to be flipped over without resting on the shield plate.

9. In the new rear view you can see that the 3rd IFT (tall) and BFO coil
(short) have been exchanged.  The added BFO tube is visible between the
output transformer and the filter cap.

The BFO was formerly the triode half of the 2nd IF tube -- an impossible
location if you don't want the BFO getting in the IF and taking over the
AGC.   

10.  The coil in the 'accessory' socket at the right rear of the set is the
(Hartley) oscillator coil.   It stays cool enough there that there's almost
no drift.   The 6BH6 ECO LO is partly visible to the right of the oscillator
coil.   The tube between the 1st IFT and the rear of the tuning cap is the
12AT7 mixer.  

11.  The new power line connector and BNC antenna connection are visible on
the rear apron.  

12. Barely visible on the meter terminals are the two strings of 1N4001
diodes I use to protect meter movements on projects like this when the meter
is on the HV side so any short will cause loss of the smoke from some
part(s).   

I was pleasantly surprised at how little chassis hacking was required.
Holes had to be added for the new tubes and the new panel is a pretty big
job but most of the work wasn't too difficult.

There is one loose end in front panel layout.   The BFO TUNE variable cap
got moved to the former location of the RF/IF gain control.   (To shorten
the leads and thus reduce coupling into the IF.)  That worked okay as did
installing the power toggle switch in the former BFO control spot.   However
the RF/IF gain control really doesn't fit in the former bandswitch knob
location: It has to be wedged in forward of the drum for the tuning cap
string drive.  It works, but ... There needs to be a better answer to this.
Maybe right above the current phone jack hole?   NOT great human factors
there ...

Or move the ON/OFF switch above the phone jack hole and use the former BFO
TUNE control location for the RF/IF gain control?

Probably I will solve the RF/IF gain control location problem on this set by
using an extension bushing and a pot with a longer shaft so the pot itself
will be behind (clear of) the drum.  However unless you have a very well
filled junkbox, those extensions are probably unobtainium so another
approach should be found for any future build.

Bottom line: I think a satisfactory W6TC HBR design can be built on an HR-10
chassis provided that you use the crystal lattice filter and only the 1681
kcs IF instead of double conversion to 85 or 100 kcs.  The dial and tuning
cap are adequate, the plug in coils worked out fine, the selectivity is
pretty much perfect, there's plenty of room on the chassis ...

My set is close to an HBR-11 except for using the Heath 1681 kcs IF.   I
also used different second detector and AGC circuits but the W6TC circuits
would fit fine and are easier to build.  My version of the HBR-11
uses dual section tubes for the two IFs (eliminating the 12AT7) and omits
the voltage regulator tube; it thus gets the count down to 10 sockets,
counting the rectifier.   You could do it with 9 sockets by using a dual
section tube for the mixer and LO but there's plenty of room and I didn't
want to do that.  

At this point I can't fault any of the choices of tubes: The 6LM8 semiremote
pentode/triode IF stages are good, the 6EH7 RF stage can't be beat for that
job and the (lower powered) 6LF8 audio tube is plenty.

(FOOTNOTE: The 6LM8 IF tube seems to be rare:  I got four from Antique Radio
but they have no more and eBay lists a total of three: Two at $4 and one at
$18.   An alternative would be using the original 6EA8's and tweaking the
AGC circuit to apply a quarter to half voltage to these stages.  You can see
how this is done in the HBR-13C circuit.)

The best way to approach this project would be to get the project HR-10
working (to test all the parts) then strip it to a bare chassis.  The
oddities of original builder wiring, the fact that the original wiring
rarely has more than 1/4" of slack and that Heath recommended wrapping
connections, together with the extensive changes needed, make a rolling
conversion (as I did on this project) a time consuming and
not-too-satisfactory option.

> The hum audible at full volume was coming from the detector stage.

In the end I changed the HV filtering from resistors to a choke input filter
followed by another choke.   The improved regulation -- about 16V difference
from full gain to IF stages cut off, instead of 60V or so -- made a bit more
trouble seem worth it.   These are a pair of command receiver chokes;
they're rated 3H at 60 mA and the current doesn't go much over 70 mA even
with no signal so they should be fine.

With that change and just a bit more filtering for the BFO, the hum is GONE
and if all goes well I'll probably start the new AGC circuit in the next day
or so. It's easier to do that before working on the shortage of gain because
uncontrolled high gain causes so much distortion.

Also, the most obvious place to raise the gain is the 2nd IF which has the
original 470 ohm cathode resistor.   Changing that to a more suitable value
will likely require changes to the S-meter parts values since the meter is
in the plate circuit of that stage.   Unfortunately the S-meter uses a 2 mA
movement (800-820 ohms resistance) so it needs pretty high driving power.

The AGC change is complicated: It needs a negative HV supply, the RF/IF gain
circuit gets replaced, AND there's the AGC amp stage itself with a new rear
apron control.  

Walt
KJ4KV