for the BC-610
by Joe - N3EYR
Used to link to Joe's page but it is no longer
on the Web
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.
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.
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.
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!
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.
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
xtal socket. It worked but was not optimum. Drive was low 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, has a mechacal digital frequency
display, 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.
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
Bias and low B+ use two 8 uF oil filled each. Removing these
leaves 2 one-inch holes from each oil filled. I had some 40 uF can
electrolytic that fit the holes nicely. Each supply now has some 80 uF
up from 16 uF.
Noticing that the transformer primaries have two taps 110v and
125v, I connected to the higher 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 16 uF/4500v
oil filled. This really stiffened things up and increased modulation
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 add 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 500
Ohm-to-30 kOhm split secondary type found at a fest. The driver is a
UTC S8. The split input will worknicely for push-pull feedback
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.
26 August 1997