The BC-610 Page

	The BC-610 Page
	By N3EYR
Here lies my thoughts and experience with the Hallicrafters BC-610H
The author assumes no responsibility for its content or accuracy. It is assumed that anyone using this information has a through understanding of radio engineering and uses proper safety precautions when working with high voltage equipment.
READ AND USE AT YOUR OWN RISK!
Not long after acquiring my 610 it became apparent that some work would be necessary. First fix some problems and bring it to "specs". Then make more serious mods to be presentable on 75 meter AM.
Problems:
1) Hum. Slight but noticeable.
2) Modulation "Talk Back". Strong enough to cause feedback.
3) Arcing. Intermittent and disturbing.
4) Drift. Tolerable but annoying.
5) Fidelity. Fair but light on the bottom, Lacking definition.


HUM:

 First thoughts were tired old filter caps. Not this time, Turned out
to be a shorted choke in the low voltage power supply. I think I would
have overlooked this possibility if it had not been for the coating
of tar on the inside of the cabinet. Tracing its origin revealed the 
culprit. An exact replacement was found in an old T-368 carcass.
 Hum was reduced considerably. Just a small amount proportional to mike 
gain. I felt content with that as the audio stages would be getting a
complete work over.


Talk Back:

 This problem bugged the heck out of me. Sounded like there was a 
rattling tin can for a speaker in the rig.  Suspecting the mod 
transformer, I tightened the screws. No change. Next I rigged up
tape recorder and played my voice through the thing so I could get
a better look at where all the racket was coming from.  On the power
supply deck was a box shaped cover, Seemed all the noise was in there.
Upon removing the cover it was apparent that a previous owner had
spent considerable time insulating it to dampen the sound. Under this
cover are several relays. Looking at the schematic there is a "plate"
relay used as over current protection. Its coil between B- and ground.
Hence final and modulator currents flow through it and if excessive will 
energize the relay and open the PTT line. Side effect is the varying 
modulator current produces the "Talk Back". I paralleled the coil with a 
100mf 450v electrolytic. Talk Back is totally gone now.
 
 Note: If the relay coil were to burn out, The cap would have full B+
across it. Not wishing to use a huge oil filled cap with sufficient voltage
rating it is assumed that the electrolytic will explode in the event of 
relay failure. I believe this is expectable as it is enclosed on all sides.
Use your own judgment!


Arcing:

This has yet to be completely resolved but has been reduced to rare 
occasions by cleaning insulators and replacing high voltage wiring. May
just be "normal" mod transformer spark gap events(or the gaps need widened).
 In any case is expectable and so far nondestructive.


Drift:

 The 610 tuning units are notorious for there hum and drift. Mine are no
exception. Used with a x-tal they are expectable, Other wise I would 
recommend an external VFO.
 Originally I used a Missner Signal Shifter feeding RG-58 to the x-tal 
socket. It worked but was not optimum. Drive was low the and the cable 
length needed to be kept short.
 For some time I've possessed a TMC O-330 master oscillator from the GPT-1k.
It is very stable, Digital freq. display{mechanical}, And a native low Z 
output. Seemed like a perfect use for it.
 I added a BNC connector to the side of the 610 {they fit in the square vent
holes nicely} and a short jumper from there to the x-tal socket. There is
over 20 feet of RG-58 between the 610 and the oscillator with no loss of 
drive. I've played a bit with driving the buffer/multiplier stage but it
needs a higher drive voltage. A 4 to 1 transformer should do it.
 The rig at this point was reliable and fairly clean.



FIDELITY:

 Starting at the bottom, The power supplies got a work over. I solid stated
all except the final/modulator B+. That was left in its tube state mainly
because I didn't have rectifiers sufficient to do the job.
 Bias and low B+ use two 8mf oil filled each. Removing these leaves 2 one
inch holes from each oil filled. I had some 40mf can electrolytic
that fit the holes nicely. Each supply now has some 80mf up from 16.
 Noticing that the transformer primaries have two taps 110v and 125v, I
connected to the higher tap to compensate for the voltage increase from solid
stating. This worked well as the rig is designed to run on poor AC mains
and voltages seemed to be a bit high anyway. Next the final/modulator caps
were replaced by a pair or 16mf 4500v oil filled. This really stiffened
things up and increased modulation percentage slightly.
 I'm not particularly fond of the bias supply being used as the driver plate
{actually cathode} supply as well. It is simple and clever but lots of 
energy is converted to heat via the low ohm bleeder. A future mod will be
to ad a regulated bias supply and invert the B- to the driver. For now it 
will do.
 
 I replaced the input and driver transformers. The original units are not to
bad but far from optimum. For the input I used a 500ohm to 30kohm split 
sec. found at a fest. The driver is a UTC S8. The split input will work
nicely for push-pull feedback  schemes.  
 Some care was taken to find the correct loading resistor values for the 
transformers to control ringing. I use a square wave generator at 1khz on
the input, A scope and decade resistor box on the output. Decrease 
resistance until the leading corner begins to round off then increase 
resistance a few steps. I doubt if this is audible but if you plan on using
feedback around transformers it will be a life saver.
 The stock mod transformer is still in place but as soon as I find a good
modulation choke it will have to go too.
 Audio is much cleaner now but the speech amp is holding it back.


The Speech  Amplifier

When I started to tweak the BC-614 speech amp many voiced the opinion
that is was a waist of  time "Just use some hi-fi amp and be done". While
that was tempting I had my reasons for wanting to see what it could do.
First is its simple compared to six feet of processors. Secondly it will
make a good backup when a new processor decides it doesn't like RF.
And third it looks neat, Well OK it looks like a black crackle bread box
but I like it.

 The first few attempts at simple changes were unsatisfactory. Upping the
coupling caps made more hum. High end was being rolled off by poor
internal wiring design. Multiple ground paths precluded any simple fix
to the hum. It became apparent that a more aggressive approach was needed.

 Break out the chain saw, Well wire cutters anyway. The military wiring
technique was making changes difficult so it had to go.

NOTE: Collectors should not read further less they become ill.

 I chopped out the entire wiring harness, Ten feet of  "crunchy" coax and 
ripped the pots and mike connectors from the panel. No turning back now.

The basic plan was.

 1 .    Star grounding.
 2.     Line input.
 3      Hi impedance mike input.
 4.     Negative feed back loop.

 The original circuit contained a simple limiter and sidetone amp., Neither 
are retained in the new circuit. Tube line up is unchanged and I'm still
using the stock output transformer, Though  I recommend replacing it if
possible.  

 A single piece of #14 solid runs from SO 102 {the output connector} to the 
chassis just below the mike connector. All circuit grounds tie to it and it
has a single connection to the chassis.
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