BC-348 Receiver

Discussion in 'Technical' started by W5HRO, May 21, 2016.

  1. W5HRO

    W5HRO Administrator

    Couldn't resist starting this one since we also have an AN-ART-13 transmitter topic.

    Below is my BC-348-K receiver

    Receiver Front

    Receiver Top

    Receiver Bottom

    Inside sticker

    Homebrew AC Supply

    Mounted AC Supply
    mounted supply.jpg

    The supply has a +24Vdc 2A regulator that regulates the filaments of all of the tubes.
  2. W5HRO

    W5HRO Administrator

    The one thing I have not done to it yet is modify my Heathkit QF-1 "Q" multiplier and hook it up per the attached modification article.


    Attached Files:

  3. W6MQI

    W6MQI Member

    Okay where did you find that in a time capsule it looks brand new! I'm jealous. From the looks of your power supply the BC-348 doesn't pull a lot of current fairly simple design. Putting together a power supply for the ART-13 might be a different story locating a power transformer for the HV.

    Nice looking receiver Brain.
  4. W5HRO

    W5HRO Administrator

    I found it in eBay a few years ago for around $400. With shipping it was around $475 I think.

    They actually made an AC power supply for the Navy, but they were rare. I think Hallicrafters made it for them. Below was the schematic. It used a 5Y3GT and mounted where the dynamotor went.


    Below is my equivalent and improved homebrew supply schematic with the 24V regulation.

  5. W6MQI

    W6MQI Member

    Picked up a BC-348P today at a customers house I saw it sitting on a shelf in the garage so I asked if they were interested in selling and sure enough the lady took the bait. It powers on other then that unsure if it works or not the power supply transformer is leaking tar smelled that as soon as the case was opened. So I now have a matching receiver for the ART-13 and another project.

  6. W5HRO

    W5HRO Administrator

    So I assuming someone built the AC supply and it was mounted where the dynamotor use to be, or is the supply external?
  7. W6MQI

    W6MQI Member

    Yes there is an internal 110v power supply that someone built using a 6080 there's also a 6AU6 on the power supply deck with a potentiometer not sure what that's all about?
    The fuse has been disconnected why? There is also a small transformer located where the power plug was mounted another mystery. So it looks like I may have some work ahead for myself right now though I'm really not in the mood to work on this thing going to put it on the shelf for a future project.

    20170420_194005.jpg 20170420_194042.jpg 20170420_194109.jpg 20170420_194118.jpg 20170420_194203.jpg 20170420_194156.jpg
  8. W5HRO

    W5HRO Administrator

    Above in this topic is my HB supply and then the original supply circuit that was made for the navy. Those are impossible to find so everyone just makes their own to match it.

    One note, that B- ground connection must be floating from the chassis.

    That small transformer on the bottom may be a separate filament transformer for all or one of the tubes. It looks awful small though. Without knowing if there is a filament winding in that main transformer there is no telling. It may be like a small 40V transformer and someone tried to build an adjustable regulator using that 6AU6. Just guessing though. It could also be the filament transformer for those 2 tubes on the HB supply or the circuit is used to regulate the HV supply voltage in some way. I would definitely redo that whole design though because it looks kinda shitty. You could use that same 6080 socket and plug-in a 5Y3GT and find another transformer if necessary.
  9. WD5JKO

    WD5JKO Member

    That power supply addition is pretty neat. The 6080 can be used as a series regulator tube. My Gonset G76 power supply was built by an unknown OM who went to extremes in many ways. One neat thing was he added a 6080, a 12AV7, a pot, and a stud mounted zener diode. The LV B+ hangs tight at +275 using that arrangement. On the BC-348, look for a regulator reference. Could be a zener, a neon bulb. I see a 991 tube in there. Those are neon...what was that used for in a BC-348?

  10. W5HRO

    W5HRO Administrator

    That's what I was thinking too, the 6AU6 might also be shunt regulating the HV circuit. The BC-348's do have a bulb in there for regulation.

    Regulating the HV in a BC-348 is not really necessary though so it makes you wonder why someone did that if so. It only needs regulation at the oscillator tube which would be normal. Adding regulation to the filaments is way more helpful.

    Below is what they did using the 991 bulb. All it needs is a zener installed in its place which would be better than a bulb, it will still move around a bit with the bulb.
  11. W6MQI

    W6MQI Member

    I was looking around the web trying to find a schematic that maybe close to what I'm seeing in the BC-348 didn't really find anything.


    This schematic with a 6AU6 maybe close just guessing right now until I can trace out and draw a schematic of what's in there.
    I can see good filtering, but why regulate the B+ seems like a lot of extra work, and parts to me.

    Anyway the weekend is real busy probably won't have time to dig in to see what's going on in there maybe next week I'll try and do a schematic then post it here for evaluation.
  12. W5HRO

    W5HRO Administrator

    If they did use the 6AU6 as a shut regulator then the circuit would look similar to the below. Yours would just have the pot connected to control the output level. The circuit below is for a fixed voltage output and yours would just be adjustable.

    shunt regulator with pentode driver.png

    If that's what they did then they may have removed the bulb too. All you need to do is put a zener there and eliminate the HV regulation at the supply itself. Since the 6080 is a twin triode they could also be using it to obtain 2 different output voltages. You could have one fixed voltage and then one adjustable voltage for the oscillator tube so anything is possible. Then at the same time they could be using that 991 bulb as part of the supply like Jim may have been thinking and that could be possible too. You won’t know until you pull it out and look everything over. I think they probably did something really simple though from the looks of it at first glance. I’ve even dragged home junk through the years where someone had used a twin triode tube as a twin diode rectifier so that could be possible too. That 6080 could just be the rectifier tube and I’m serious :lol:
  13. W5HRO

    W5HRO Administrator

    P.S. I just looked at your pics again and the 991 bulb is there. It even says RCA-991 in red letters beside it.

  14. W6MQI

    W6MQI Member

    I saw that thinking what's this all about now you have my curiosity going now. I have a Mt.Bike race in Monterey this weekend the Sea Otter Classic when I get back I'll do some exploratory surgery on the P.S. with a report in a few days.
  15. W5HRO

    W5HRO Administrator

    That little transformer underneath also looks like it could be for the audio output. It could be a small 8-ohm transformer they threw in there to drive a modern speaker. I still need to replace the one in mine. Hammond makes one that can be mounted were the stock one is. You can just cut a small plate and put it where the stock plate is. Then the question is where is the 24V filament transformer? If that transformer is for audio then they might be using the HV regulated down to the 24 or 28V. That’s possible too. One fixed side of the 6080 going to the plates and the other side going to the filaments which is adjustable. Just one more possibility. It would really load down that HV transformer though, but that could have been what they did.

    Anyway, the only thing I did to mine was build the supply, replace the main caps, and then used it for a little while. I left the 991 bulb in. I was going to go through the whole thing and do the rest and also connect the Heathkit Q multiplier, but I have to modify the multiplier’s two coils first for the 915kc IF. I finally just put it all on the shelf because I had and still have other projects I wanted to finish up first like that NC-303 Plus modifications. I didn’t plan on having to redo the filament supply, but I did that instead of working on the BC-348.
  16. W5HRO

    W5HRO Administrator

    By the way, it looks like they have mounted that small transformer where the male power/interface connecter use to be.

    Here is a pic of mine where that connector is suppose to be.

    Then here is the female connecter that plugs into it that was pulled out of a B17.

    Then here is a spare spinner knob I picked up still sealed in the original military package.

    This stuff will pop up on eBay from time to time, but it's all getting really scarce and it's coming up less often. It might be really hard to find the male power jack they ripped out of yours to mount that small transformer.

    The person to ask would be KC4TOS. He has a lot of old BC-348 junkers saved for parts and restoration. He might have an old scrap one with the male connector still in it.
  17. W6MQI

    W6MQI Member

    Did about two hours of wire tracing today on the 348 not a whole lot to show for it.
    Looks like the B+ is switched by the dial light switch, and the filaments off the AVC-MVC switch, now I know why I see some 348's with a separate B+ switch on the front panel the dial light switch doesn't look to robust.
    That small looking transformer is a audio transformer schematic shows how its hooked up. I suppose the 10 ohm resistor is there for some type of load on the output? There is no external speaker jack seems the previous owner used headphones of some unknown impedance the transformer is unmarked. I haven't done any measurements of the transformer yet.
    I also traced a bunch of unknown wires floating around the cabinet I now have them sorted out, next is to trace out the power supply.
    Awhile back a BC-348 with the dyno-motor installed went for some where around +$550.00 on Ebay it was real nice looked completely unmolested.

  18. W6MQI

    W6MQI Member

    Guess its kinda obvious the head phones or what ever he was driving must be around 4000 ohms.
  19. W6MQI

    W6MQI Member

    Here's the power supply its real close to the one I posted in a earlier post. I didn't have the correct drawing for the 6080 so I combined two triodes, T1 is also all one transformer. There's an unknown value zener I believe on the 6AU6 pin 7 its hard to see it also has a wire wound resistor across it?

    Power supply.jpg
  20. WD5JKO

    WD5JKO Member

    The schematic attached might be closer to what you have. The 6AS7 is essentially a consumer grade 6080, and the 6SH7 is an Octal variation of the 6AU6. The schematic attached is from the 1970 Radio Handbook by W6SAI.

  21. W5HRO

    W5HRO Administrator

    Yeah, and I don't think Dave's circuit is 100% correct either. Not sure why they would tie the 6080’s filament lead to the cathode. Unless I am just missing something that has to be a mistake. The only thing that would make sense is those two solid state diodes are not really there and they are rectifying and the controlling output level both with the 6080 tube. If those diodes are there then that cannot be right. Not only that, but unless someone rewired all of the filaments up in parallel their voltage should be 24V.

    I hope someone didn't do that, but that could be what they did. The BC-348 filaments are all in series if it’s stock which is better because it requires much less current. That could be why there is no separate filament transformer because that was going to be my next question again. Per Dave's drawing the 6AU6 and the BC-348's tubes are being powered by a 6.3Vac winding. The 6AU6 would die with 24V so that would indicate someone did convert the tubes to parallel instead of series and may be why the transformer is dying. I doubt that one small power transformer has enough current to power up all of those tubes to 6.3Vac and sustain it.

    In addition, the center-tap of that first main HV winding cannot be grounded. That has to be a floating above chassis ground B- connection. That's a common mistake BC-348 owners often make when they go to build a supply for it.
  22. W6MQI

    W6MQI Member

    I found a mistake on the power supply schematic attached is the updated schematic. Tried to trace the filament circuit which wasn't very easy, and I believe the filaments are in series 99% sure.

    Rechecked three times they are tied to the cathode.

    For such a relatively simple receiver I find it real hard to trace out especially since someone has been in there cutting, adding, and removing parts hope I can pull this off. Attached are a couple of pictures of the power supply you can see how the transformer was leaking all over the radio case, and the bottom of the transformer is coated as well.

    power supply.jpg
    5086_20170423_190830.jpg 5087_20170423_190836.jpg
  23. W6MQI

    W6MQI Member

    Missed this quote I'll double check this and verify.
  24. W5HRO

    W5HRO Administrator

    Yikes, that transformer is toast :eek:

    Here's the problem, there is no reason for the 6080 cathodes to be shorted to its filament. The supply does have the two solid state rectification diodes and I can see them in your pic (at least one) unless that's the zener, but I don't think so. The way it's connected it has AC shorted to it’s DC output at the cathodes. That makes no sense at all.

    If that is the way it’s connected, then maybe in the beginning someone had a single 5V rectifier tube in that octal socket, then they later, or someone else decided to regulate it down adding the 6AU6 and they plugged in the 6080 and never bothered to disconnect the filament pin from the output. I bet that second transformer winding is a 5Vac 3A winding, but the voltage is high enough to light up the 6080 at only 2.5A. That could all be the work of 2 or 3 different owners too.

    The 6AU6 is the driver for the 6080 series regulator, all it does is control the 6080's output level so you have no other filament transformer for the rest of the tubes. That means they must have connected them all in parallel to make it 6.3Vac instead of 24Vac.

    Yeah, you have your work cut out for you on this one, but it’s still restorable. At least you have one now.
  25. W6MQI

    W6MQI Member

    Yep I'd say it was time to drop kick the power supply right into the trash can and start over. What the hell was someone thinking why convert over to 6.3Vac whats the point? Sounds like a whole lot more work to me to convert over hopefully the receiver is okay after all the butchery well at least the outside looks nice anyway. This will probably be the last you hear about the BC-348 I'm going QRT for a while the building where the shack is located has been condemned by me. In the next few months we will be building a new wireless room 12' x 28' fully insulated, AC, and a floor that won't fall through when walked on. So ham radio is out for a few months I'll post some pictures as work progresses.

    73, Dave