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Mods for the BC-610




 
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Steve - K4HX
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« on: December 16, 2012, 12:56:48 PM »

Mods for the BC-610

      by Joe, N3EYR



 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.
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« Reply #1 on: August 18, 2013, 11:20:56 PM »

Joe
After a year and one half, did you run into other projects or did you finish it and move on?
I am about to acquire a 610-E and it may benefit from some of these mods but I have yet
to take ownership of it (next month) and once I do, I will get a better idea of what it will need.
Mine will include the 614 speech amplifier. I will not be using an SX-28, so I have to consider
using my 51J-4 instead. I am tempted to pair it with an HRO of some sort.
Out here on the West Coast, Brian NI6Q has a tremendous sounding 610 he uses most evenings
and it has very good audio. Not sure of the mods, but I shall inquire and compare notes.
Thanks for your post.73 de Billy N6YW
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« Reply #2 on: August 19, 2013, 12:00:30 AM »

Those mods are from the 1980s/90s. I haven't heard Joe on the air in many years.
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« Reply #3 on: August 19, 2013, 12:09:26 AM »

Steve
Thanks for the fill. Perhaps I will pick up where he left off and report on my results in
case anyone is interested. So far, mine looks like a very well cared for and often used
transmitter. I am stoked to say the least.
73 de Billy N6YW
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« Reply #4 on: August 19, 2013, 11:08:38 AM »

If he was going to use the same tube layout, I wonder what scheme he was going to use? Seems to be a straight ahead design for the time but for the sake of discussion and putting the BC-614 aside, what would anyone else use in it's place? A friend of mine uses a McIntosh MC-60 mono amplifier to feed audio to his exciter. On air recordings of his transmissions are breathtaking. Aside from this expensive approach, I am curious what others would do to feed audio sufficient to drive the 610 to 100% with good fidelity.
Thanks,
Billy N6YW
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« Reply #5 on: August 19, 2013, 01:33:19 PM »

I don't like using tubes that are hard to find in good shape.

Back when I had a BC-610F the decision was made to change the modulator to hi-mu triode connected 813s.

Moved a few things around in order to stuff in a Thordarson T11M78 mod transformer connected to allow high positive peaks.

Worked OK zero bias on high tap. Later BC-610 can be configured for 4 power levels. It would take some positive bias on the 813 grids at lower power to do it correctly.

Audio drive was a cheap stereo output chip rated at 60 or 80 watts out. Used a pre-war Thordarson push-pull to speaker multi-tap xfmr to grids with something like 5khom swamping across each grid. Also needed plate resistors for parasitics on the modulator power tubes.

Modified the tuning units for use with an external exciter. Just added another coil around the cold end of one of the grid tanks. An unused oscillator variable was configured to the coil to allow easy measurement of drive by normalizing to 50 ohms.

Originally I swapped the 250TH final for a 3-500Z which worked OK and was a relatively easy swap. Later I retrofitted an 833. A little more neutralizing C put the power out max, the grid current max and the plate current dip all exactly in the same place. This held on 160 through 20.

A few years back I ran into an old timer who had put a U.S. Army BC-610 on 10 meters while stationed somewhere everyone wanted to work. Several of his pals around the globe were doing the same! Statute of limitations etc........

If you notice any strange plate current increases with reduced drive after modification of tuning units look for spurs. I used to get a 40-50 mill increase in final plate current when reducing the grid drive below 80 mills on the 833. When transmitting on 3885 an additional AM signal would appear down around 3700!
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« Reply #6 on: August 19, 2013, 10:26:01 PM »

If he was going to use the same tube layout, I wonder what scheme he was going to use? Seems to be a straight ahead design for the time but for the sake of discussion and putting the BC-614 aside, what would anyone else use in it's place? A friend of mine uses a McIntosh MC-60 mono amplifier to feed audio to his exciter. On air recordings of his transmissions are breathtaking. Aside from this expensive approach, I am curious what others would do to feed audio sufficient to drive the 610 to 100% with good fidelity.
Thanks,
Billy N6YW


Many years ago -- around 1974 to 1978 -- I had a BC-610E which I modified for higher fidelity. I drove the 100TH grids with a 20-Watt Linear Standard audio output transformer hooked up backwards. I loaded the grids with around 10 kilohms each (I think that was the value) and then drove the 16 ohm tap on the transformer with a modified Knight tube hi-fi amplifier which used a pair of EL-34s. I loaded the 16-ohm line with a 32 ohm resistor to linearize the load seen by the hi-fi amp. I then used a cap followed by a resistive voltage divider right off the modulated B+ and fed that through a potentiometer to the (newly unbypassed) cathode of one of the earlier stages in the Knight amplifier and adjusted for about 6 to 8 dB of negative feedback. Worked very nicely.

This was the rig I used for my "QST, QST" announcements when I was battling Docket 20777, which would have made AM illegal.

Interestingly, I also used a TMC master oscillator to drive the 610 -- it was very stable!

All the best,

Kevin, WB4AIO.
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« Reply #7 on: August 20, 2013, 01:31:05 PM »

Kevin
Thank you. That sounds interesting. So, when you say "linearize" you mean placing the 32 ohm resistor across the 16 ohm load? And, the two 10k resistors going to the grids of the 100TH's are used as swampers? The variable feedback design allows you to tailor the audio peaks to your voice characteristics?
I appreciate your response,
73 de Billy N6YW
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« Reply #8 on: August 20, 2013, 11:44:15 PM »

Kevin
Thank you. That sounds interesting. So, when you say "linearize" you mean placing the 32 ohm resistor across the 16 ohm load? And, the two 10k resistors going to the grids of the 100TH's are used as swampers? The variable feedback design allows you to tailor the audio peaks to your voice characteristics?
I appreciate your response,
73 de Billy N6YW


Yes, the purpose of the resistors (both the 10k ones in parallel with the grids and the 32-ohm one paralleled across the 16-ohm line from the audio driver amp) was to even out the impedance seen by the audio driver over the course of each audio cycle. In a class AB2 modulator, that impedance varies quite a bit as the modulator grids to into and out of their grid-current-drawing region.

And the negative feedback functions to lower the audio distortion and flatten the frequency response somewhat. If done right, it just makes your amplifier more correctly reproduce the input waveform than it would without feedback. And accuracy is good. There are pitfalls, though, and it's easy to generate ultrasonic birdies if you're not careful. Here's a good article on the subject by K1DEU:

http://hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/negative_feedback_design.htm

73,

Kevin, WB4AIO.
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« Reply #9 on: August 22, 2013, 07:26:36 PM »

Kevin
Again, thank you.
I think what I will do first, is to get the 610 on the air and make my decisions based on need in order of
priority, IE: Power supply, frequency stability, etc. The talk of using an external VFO excites me (pun intended) because the obvious ease of correcting frequency, but I beg the question: Which VFO  scheme makes the most sense?
I ask this because there are a variety of them to choose from. The Johnson 122 has been used by others but I
don't have a real rhumb line of comparison. The TMC unit you referred to sounds great but where can one be found?
After having my ART-13 on the air for the better part of a year, I have gained some useful knowledge with Am transmitters. My recent article in the CCA Signal magazine tells of the building of the power supply and it was
a lot of fun to be the one who energized and brought her on the air after decades of neglect and non-use.
Still, this is all a learning process that has taken time and I have to be careful of where I choose my battles when
it comes to equipment over 65 years old. Getting it on the air, on frequency and keeping it on frequency is job one. Getting it to sound better or great follows that. I have enough audio engineering experience to make great audio but how its implemented into the 610 is what really counts. I don't want to make the same mistakes others have.
Baby steps while reading a lot on the way to walking and chewing gum.
Thanks again, great conversation.
73 de Billy N6YW
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« Reply #10 on: August 22, 2013, 07:35:48 PM »

Billy --

You're right -- the TMC master oscillators are very rare these day, and they were never plentiful to begin with.

If it were me doing up a BC-610 today, I would use one of the DDS kits that are available, with a buffer amp if necessary. Standards have improved in that department since 1944 -- and 1974!

73,

Kevin, WB4AIO.
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« Reply #11 on: August 23, 2013, 09:28:02 AM »

Yes, I agree with the DDS approach. Who makes a good kit?
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« Reply #12 on: August 23, 2013, 09:49:06 AM »

Yes, I agree with the DDS approach. Who makes a good kit?

I haven't tried any of them -- I am currently using an old Flex SDR-1000, which would also make a good driver for the 610 and give you SDR reception too. But there are probably others on here who know more.

Here's one I have seen:

http://www.pongrance.com/super-dds.html

73,

Kevin, WB4AIO.
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« Reply #13 on: August 25, 2013, 11:51:05 AM »

Thanks for the link. That is a nifty package and rather inexpensive. He has a 2 transistor buffer circuit
posted on his website that looks very easy too. I might have a swing at this one as it's a quick and easy work around.
Just so I get it correct, the concept here, is to bypass the plug in tuning units altogether, right?

 Although I own several Collins receivers, I think for this station, I will use a SDR rig of some sort. The Perseus SDR is amazing and flexible enough to where I can get rid of the idiot in the Midwest  who insists on keying up his Digital birdie box on 3870 every night. Perhaps one day in the not too distant future, when we sell our house in the canals and move onto some acreage, I may be able to put up a curtain for 75 meters. With a kilowatt, I will affectionately call it "The Midwest Mute System"  Grin

73 de Billy N6YW
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« Reply #14 on: August 25, 2013, 02:07:50 PM »

Thanks for the link. That is a nifty package and rather inexpensive. He has a 2 transistor buffer circuit
posted on his website that looks very easy too. I might have a swing at this one as it's a quick and easy work around.
Just so I get it correct, the concept here, is to bypass the plug in tuning units altogether, right?

 Although I own several Collins receivers, I think for this station, I will use a SDR rig of some sort. The Perseus SDR is amazing and flexible enough to where I can get rid of the idiot in the Midwest  who insists on keying up his Digital birdie box on 3870 every night. Perhaps one day in the not too distant future, when we sell our house in the canals and move onto some acreage, I may be able to put up a curtain for 75 meters. With a kilowatt, I will affectionately call it "The Midwest Mute System"  Grin

73 de Billy N6YW

I just fed the master oscillator into the grid of the crystal oscillator stage through a cap, if I recall correctly. So the tuning units were set to the crystal position, with no crystal plugged in, and their buffer/driver tuning controls were still active.

Good luck with the global muting control!

73,

Kevin, WB4AIO.
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« Reply #15 on: August 25, 2013, 09:40:28 PM »

I used to drive my 610 direct to the crystal socket shunted with a 50 ohm non inductive resistor. On 160-80 I had to turn the IC-746 all the way down but for 20 meters, it required about 5 watts to get the proper grid drive. I never tried it on 17 meters but a 610 should go up there. With the resistor in there, you never have to worry about too much drive or matching a solid state rig.

On your modulation meter, it should gently kick up to 150 to about 180 mills on average. If you go higher than that you will become a splatter master. Resting current should be 40-50 mills.

On the final grid, I run as much as I can get but you should see close to 100 mills of grid drive on 160-80 and about 60 to 80 mills on 40.  20 meters will be about 55-60 mills. If you get more, use it. The only way to drop your grid drive is to de-tune the different stages. Not a good idea. I think if I ever took another 610 apart, I would put a big pot in place of the screen resistors or use a separate adjustable B+ supply for the 807 tubes. By doing what I have suggested, you'll be able to tune all stages for max efficiency and have real control over your final grid current.  One last thought, if you get the grid drive as posted above and your are getting downward modulation, you most likely have a soft 250TH tube.
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« Reply #16 on: July 04, 2015, 08:57:52 AM »

Sorry to bring back a dead thread, but perhaps the authors have the notification function switched on.
 Kevin, the Hamtronics link is expired, I'd like to read the K1DEU article.
I believe I am experiencing ultrasonic birdies on my t-368.
 Occasionally, a signal is noted on my carrier appx. 28 khz. either side of center.
 Said signal is about 60 dB down , but sometimes is cause for comment when the band is quiet.
 Thanks, Ed AB3HT
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« Reply #17 on: July 04, 2015, 01:38:09 PM »

Sorry to bring back a dead thread, but perhaps the authors have the notification function switched on.
 Kevin, the Hamtronics link is expired, I'd like to read the K1DEU article.
I believe I am experiencing ultrasonic birdies on my t-368.
 Occasionally, a signal is noted on my carrier appx. 28 khz. either side of center.
 Said signal is about 60 dB down , but sometimes is cause for comment when the band is quiet.
 Thanks, Ed AB3HT

When you run into a problem with a dead link, try using the Internet Archive web site. Try this:
https://web.archive.org/web/20100528071210/http://www.hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/negative_feedback_design.htm
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« Reply #18 on: July 04, 2015, 04:28:55 PM »

Sorry to bring back a dead thread, but perhaps the authors have the notification function switched on.
 Kevin, the Hamtronics link is expired, I'd like to read the K1DEU article.
I believe I am experiencing ultrasonic birdies on my t-368.
 Occasionally, a signal is noted on my carrier appx. 28 khz. either side of center.
 Said signal is about 60 dB down , but sometimes is cause for comment when the band is quiet.
 Thanks, Ed AB3HT

I see that Pete has given you an excellent solution -- archive.org is a great service in finding information that used to be live on the 'Net.

I found with my T368 and BC-610 setups I could get somewhat over 10 dB of feedback before instability set in. But 10 dB was plenty to help the fidelity quite noticeably!

73,

Kevin, WB4AIO.
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« Reply #19 on: July 08, 2015, 10:42:38 AM »

With my T368 I use an HP 3325a frequency synthesizer as a signal source. The 3325a has plenty of output. It runs at half frequency into the FSK input.
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« Reply #20 on: May 23, 2016, 01:50:19 PM »

Been using a Collins 310-B to drive the the grids of the 807's with excellent success...pull the other
RF tubes, plug in the appropriate TU, and with a couple of "C" adjustments you are set...
Don't forget to use a 100 pF 1000 vdc coupling cap in the "rf feed" to the grids
otherwise you'll smoke the 310-B!
73
Steve 8TOW
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73  W8TOW
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« Reply #21 on: May 23, 2016, 03:40:29 PM »

was by Joel N3EYR, he changed his call WA3JS!
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