[Heathkit] Troubleshooting a Mohican, transistor subs, more.

[Philip Atchley]

Hi,

I'm in the process of restoring what was an inoperational Mohican 
receiver.  It had been passed from local Ham to local Ham with nobody 
doing anything with it.

This is a "nostalgia" set for me as I bought and built one while 
stationed in Spain in the late '60s.  This one arrived here rather dirty 
and very dusty inside. However, it is complete and cleaned up fairly 
nicely (with some white paint specks and scratches on the case.  But 
that's the least of my worries.

When I loaded the battery case I had a number of issues to deal with 
before getting power to the set.  Things like a broken wire in the 
battery cable.  Then the volume control turned out to be open, requiring 
you to turn the volume up all the way to get any audio at all.  Luckily 
I had a suitable control with the correct shaft, BUT a pull-on power 
switch (like the BFO) instead of the more usual twist Clockwise to turn 
it on.  No big deal in my book either.

Operating the set, it turned out to be very nearly deaf,connecting an 
antenna to the antenna jacks did nothing for reception (it has the 
antenna hi-lo impedance slide switch, NOT shown in the manual). 
Apparently somebody in the past thought that the antenna switch was the 
problem as it had been totally rewired with wire that IS NOT Heathkit, a 
stranded steel wire like the old military field phones used.  Solder in 
that area was sloppy and the connections loose as the wire didn't take 
solder.  So, I rewired that area satisfactorily, with stranded copper 
wire.  STILL no signal through the antenna input.

Then I tried touching the various sections of the tuning capacitor, 
something I should have done in the first place.  Touching the mixer 
section brought in local broadcast bands at decent level, touching the 
RF amplifier section resulted in no response.  Eureka!  Dead RF stage!

Checking voltages on this stage disclosed the same voltage on both 
emitter and base of the transistor, with low current through the stage 
(it's a common base amplifier circuit).  Unplugging the transistor and 
"diode checking" it with the DMM disclosed satisfactory diode action,. 
Hmm.  This set uses the old germanium transistors and I 'HATE' trying to 
test those critters, in or out of circuit.  Good ones will often test 
"leaky"  and bad ones often test "good".  Basically the only real test 
is "does it work".

It is a 2N1396 4 legged critter with a grounded metal case, thankfully 
plug in.

I dug through my junkbox, but the only germanium PNP RF transistors I 
found were some 2N3284's.  A smaller metal can, 4 long leads, a cutoff 
frequency of 500MC instead of 250MC (so a better high frequency 
performer).  It turned out that the pinout is identical 8^)

So, I plugged one into the RF Amplifier socket.  IT OSCILLATED TO BEAT 
THE BAND ACROSS ALL THE BANDS!  Hmm, it's "too good"! So, I checked to 
see what the mixer device was in the set.  It's a 2N1225 that crosses to 
the same NTE number 126 that the 2N1396 did.

So, I plugged the "higher performance" transistor into the mixer 
position and the mixer transistor into the RF position.

Double Eureka!  The set came to life on all bands and seems to have 
pretty good sensitivity.  The RF amplifier appears to be working as it 
should.

In fact, I have an additional 2N3284 and may try using it in the RF 
position and see if I can work the oscillation problem out of the set by 
replacing bypass capacitors etc.  It 'might' perk up operation on the 
higher frequencies.

Moral of the story, if you got germanium transistors that "diode check" 
good, don't assume that they'll work in the circuit.  Kind of like some 
tubes in that regard.  (I find that silicon transistors are usually 
either good or blatantly bad).

Since this set is wired "POSITIVE ground, I can't really wire it into my 
12 Volt station battery for normal operation.  That would make the case 
"hot" to adjacent equipment.  Batteries are expensive in the long term, 
so I figured on building a regulated -12 Volt supply for the set.  If I 
can find one I may build it into an old battery case from a "parts" set 
if I can find one (I don't want to modify the battery case in this set). 
If not, I may just build it on a small aluminum plate that can be 
mounted where the original battery case mounts.

Anybody have any thoughts on this idea, or other suitable suggestions 
concerning this set?

73 de Phil,  KO6BB
991 Different NDB's heard to date.

http://www.geocities.com/ko6bb/
Merced, Central California, 37.3N  120.48W  CM97sh