[HBR] HR-10 to HBR project

[Walt Hutchens]

In an effort to build up an HBR that:

1. Is based on an easily available set of basic parts, and,

2. Could be duplicated

I bought a somewhat tired Heathkit HB-10.   The first round of work was

> ... to relocate the calibrate and antenna trimmer capacitors so there'll be
> room for the antenna and mixer coils.   After that, it's into the front end
> with the wire cutters and nut driver ... "Strip to a bare chassis and lay out
> new holes as follows ..."   However it shouldn't take more than a couple of
> days to get the major chassis work done.
  
The front end has been rearranged, 80M coils wound, the new oscillator tube
added, the mixer circuit/tube changed, and the necessary new panel made and
installed: All of that went pretty well, though I may need to add one turn
to the mixer coil to get perfect tracking.   I did do these two coils
without trimmers and that seems to work okay.

I then started at the back end.  Things were more complicated there: To even
start the testing I had to replace the audio section because the 6EB8 audio
tube (two sections -- triode-pentode) socket was so loose that you couldn't
get all pins to make contact for more than a few minutes.   Rather than
clear out the whole area to allow building the new circuit I redid the
original circuit using new parts and a new socket.

Having shown that signals would go all the way through the set, I tackled
the other changes in that area: moving the BFO and giving it a tube
of its own, changing the detector circuit, and rewiring that part of the
panel using the new toggle switches.

Unfortunately I recycled the detector socket and got some of the same
trouble as with the audio tube so I now have to rebuild my own work starting
with a new socket.  If a tube socket in one of these sets doesn't grip
firmly it ought to be tossed right at the start.

The BFO -- now a 6BH6 --  was relocated over by the detector. With the BFO
tuning cap moved one hole to the left (on the panel) all the connections are
shorter and the tangle (both electrical and mechanical) of IF and BFO has
been eliminated.  This has to be better than using the triode half of the
2nd IF stage as a BFO and I'm hoping it will get me most of the way to being
able to use AGC and BFO at the same time.

The original HR-10 design, however, has NO decoupling of various stages:
Everything is on the same filament buss and most of the tube plate/screen
voltages come straight off one B+ line just as in an All American Five BC
design.  So it's not as simple as just replacing stages: The interaction
between stages has to be fixed at the same time.

The one-step-at-a-time approach may make sense for developing this project
but my guess is that if it gets to be possible to duplicate it, that should
be done from a bare chassis.  (Maybe just leave the power supply.)   Between
Heathkit compromises (like cheap sockets), worn out or failed parts, and
original builder mistakes and workmanship issues, there's a lot of waste
motion in a rolling rebuild.

It will be important to get the foundation receiver working first since
that's the easy way to smoke out parts and other problems.  So far I've
found one dead crystal, one rosin joint, one unsoldered joint, two bad
sockets, and a worn-out volume control.   Building these into a new set
(even as part of one step at a time with testing) would not promote builder
mental health. 

(The second set I had to buy to get a good pair of crystals came with a
near-frozen bandswitch and I've seen others complain of the same. Since that
part of the set is unneeded, that's not a problem.)

The current status is that all stages sort of work, either in the original
or as rebuilt but partly because I don't have enough AGC voltage and partly
for other reasons, it sounds terrible.   After I replace the detector socket
I'll start at the audio section and work backward just as with any new
receiver.  

Tube lineup:

6EH7 -- RF 
6BH6 -- 100 kcs calibrator
6BH6 -- LO (ECO) 
12AT7 -- Mixer 
6LM8 -- 1st IF & AGC bias rectifier
6LM8 -- 2nd IF & AGC detector
6BH6 -- BFO (ECO) 
12AX7 -- detector 
6LF8 -- 1st & 2nd audio
6X4 -- B+ rectifier

A gain of three tubes (to a total of ten) plus comprehensive changes to
both tubes and circuits.   The 6LM8's are one of just a couple of
pentode-triodes with a semi-remote cutoff characteristic so I'm looking
forward to seeing how they work out.

I think I've made all the parts of this work before but of course a new
chassis and 'details' mean that it'll take time to get them working
here.  

Pictures shortly.

Walt 
KJ4KV