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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Apr Mon 30, 2018 6:31 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
Here is a pic of the rebuilt power supply. As I was double checking the power cord connections I looked at the schematic and saw that the radio already has the choke and caps for filtering on board. So I guess the person that originally paired up this power supply with the radio either didn't know that or wanted super doubled filtering. I'm kind of embarrassed that I didn't realise this til now. But I'm going to leave it as-is.


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power supply2.jpg
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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Apr Mon 30, 2018 9:53 pm 
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Joined: Mar Sun 14, 2010 2:51 am
Posts: 392
Location: Newport, Oregon, 97365 U.S.A.
You did a great job on bringing this receiver back and it looks good. But don't waste your precious time with a crystal filter for it; that's nonsense. What are you going to be listening to in this band? Do you think the fast dial travel of the
tuning will work with high selectivity? It was built for wideband FM. This is one radio that will have to just be a working curio, with no application other than display or for some military vehicle completion.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Apr Mon 30, 2018 9:55 pm 
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Joined: Mar Sun 14, 2010 2:51 am
Posts: 392
Location: Newport, Oregon, 97365 U.S.A.
If there are caps and filter choke onboard the receiver, these are for filtering of the dynamotor ripple, and would likely
be insufficient for filtering from a 60 Hz ripple anyway.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Apr Mon 30, 2018 10:06 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
I made my own simple AM detector board. Aside from looking cool for display I can tune 4 channels of CB receive. So its not completely useless. :wink: Around here most people talk on 3 channels far enough apart so its not as though the wide IF is going to give me overlapping conversations on adjacent channels. Its better than nothing. (slightly)

If I came up with a way to narrow the I.F. I would build a SW converter to run in front of it. The radio was free and I have spent maybe $15- $18 in parts and paint. So if it stays a shelf queen I'm OK with that.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Jul Tue 24, 2018 5:08 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
A small update. I decided to have another look under the hood of the radio. I noticed a while back that the radio has a BFO tube which is odd for an FM receiver. It was used only for zero beating a signal while tuning. That other half of the BFO tube was rewired for something else. I reconnected B+ to the plate and tapped off the BFO signal with a cap to my detector board. When I connected it to the diode input as my first test point, I had a very noticable increase in audio fidelity and output. Looking at the schematic it made no sense why it happened. For the heck of it I measured DC voltage at the output end of the BFO plate coupling cap and I measured maybe 15-20 mv. My guess is that was just enough to bias the detector diode into a better range than it was. I was able to use an existing switch to turn the BFO on and off. Now I'm waiting to hear some SSB chatter so I can adjust the level of injection. Its certainly not going to blow my socks off for performance but at least I have a radio that is now usable for 10 and 11 meter reception instead of only a shelf queen.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Jul Tue 24, 2018 7:30 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
I found a few weak SSB signals to test the mod. It worked surprisingly well. Stable with decent audio. The big drawback is the wide range tuning dial is way too touchy to tune in stations so close together. I found it easier to vary the BFO frequency by twisting the adjustment shaft sticking out of the can. Not an option when the case is put back on. Its not as though this radio will be a daily driver so I might just leave it as-is. Ha, thats what I said after the last time I put it all back together. :wink:


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Aug Wed 08, 2018 5:07 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
I decided to build a new detector board to replace the bare-bones one I made before. This one also has an ANL circuit added. I followed the schematic of the ANL circuit from an old tube type CB radio that I knew worked good and re-drew it to make it easier to follow.

Building it went quickly as I was able to lay out the parts on the perf-board just like the schematic. I then grabbed the radio and it's schematic and was about to wire it in. Suddenly I was reminded that one leg of the I.F. transformer I need to connect to has one output leg grounded to chassis. Its almost impossible to reach with a soldering iron. Plus, if I remove the ground wire to that leg I am also removing the internal ground connection to the adjustment assembly for that can. It may or may not introduce instability.

So before going down that road I want to figure if there is a way to adapt the new det/anl to the existing IF can of the radio. I have seen solid state radios with a grounded IFT leg but they are using a small voltage tapped from elsewhere in the radio for the ANL stage.

If I read the schematic properly, the ANL diode under no-signal conditions will conduct on the noise pulses. But in the presense of a signal, a small voltage (from the detector) gets applied to the diode to keep it from conducting which allows the audio to pass by without any clipping. Or something like that.


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det anl2.jpg
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Last edited by forumuser on Aug Thu 09, 2018 3:06 am, edited 1 time in total.
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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Aug Thu 09, 2018 2:57 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
I slid on my best pair of geeky magnifying glasses, took a few deep breaths to steady my hands and dove down deep into the chassis to break and make the connections I needed for the new detector/ noise limiter board.

SUCCESS. It knocked down the impulse noise I get from electric company Smart Meters by about 80 percent. What little is left is mixed with regular backround noise now. It sounded like spark plug noise from the old days of running 2 way radios in my car. The board has no detectable affect on audio clarity. It works as good in this radio as in the radio where the schematic came from. The only change I need to make is increasing the last cap from 470 pf to around .003uf to knock down the high frequency response more to my liking.

Its a proven winner of a circuit so feel free to copy it to your archives for future use. I forgot to bring my camera to the basement for a pic of the board. I'll post a pic tomorrow. Its nothing fancy but I'm riding high on success and want to show it off. 8) I really thought I was going to hack the radio because of how hard it was to reach where I needed to go.

I edited the schematic pic to remove the original pinout of my radio's IFT because it is no longer relevent.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Aug Thu 09, 2018 12:10 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20215
Location: Warner Robins, GA
How would one adapt that circuit to a BC-455A receiver that uses a 510K resistor between the third IF transformer secondary bottom terminal and ground and the detector diode between the third IF transformer secondary top terminal and ground.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Aug Fri 10, 2018 9:00 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
I guess you would have to do what I did. Disconnect everything from your present detector and replace it with this.

Going to the board will be:
2 wires from the I.F. transformer. 1 wire from chassis ground. 1 wire from your AVC line. 1 wire from your audio input stage.

I remember seeing ANL schematics for radios with the grounded diode leg like your radio has. Its a slightly different version of the same thing here. Google is your friend.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Aug Sat 11, 2018 5:35 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20215
Location: Warner Robins, GA
Oh ok.

Will have to look it up


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Sep Sun 23, 2018 11:48 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 2066
Location: Milwaukee,WI
I've been watching a bunch of Youtube videos about old military radios. I was surprised to see a BC-923 setup starting just before the 40 second mark of this video.

https://www.youtube.com/watch?v=ODxBa0b556Y


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