The AM Window: Tech: Mods: BC-610

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 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.

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 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 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 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 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.