BC-348 Receiver

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

  1. W6MQI

    W6MQI Member

    What I meant to say is don't work on your 348 unless you have to be glad it works these things seem to be real fragile.

    Yes I did yesterday also received a bag of 2N3439's. Before I start building the power supply I need to get this thing working again like I said earlier one step forward two steps backward its been frustrating to say the least.
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  2. W5HRO

    W5HRO Administrator

    It will have been worth it when finished. That's a great receiver for CW because of the super slow tuning (gear reduction), but you will need to add an external "Q" multiplier if you really want to use it for that.

    Oh, don't even try to use that NPN transistor without a heat sink and don't forget the case is hot with the input HV.
     
  3. W6MQI

    W6MQI Member

    Okay here's my fix for the APC variable capacitors that keep breaking. The first picture shows what happens when the pressed on nut no longer holds its grip due to cracking of the nut, when this happens the rotor slides back because of spring tension thus shorting out the cap.
    20180126_202526.jpg
    Picture 2 shows the nut still in place on this cap, but most of the time it just falls off.
    20180126_202547.jpg
    In picture 3 the cap is placed between the jaws of a vise, the vise is tighten till the stator, and rotor are centered between each other. Once centered drill a 1/16" hole through the nut and capacitor shaft.
    20180126_202658.jpg
    Picture 4 shows the 1/16" brass pin pressed through the nut, and capacitor shaft holding the rotor from sliding back against the stator. I'm having to do this to just about every capacitor in the radio.
    20180126_203236.jpg
    20180126_203306.jpg
     
  4. W5HRO

    W5HRO Administrator

    So, you drilled a hole through the nut and shaft and inserted a brass peg? That's what it looks like in your pics, but the description isn't fully clear.
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  5. W6MQI

    W6MQI Member

    Yes in the last picture you can just see the 1/16" brass pin on the side of the nut this pin goes through the nut, and the capacitor shaft so that the two are locked together. Others have mentioned they used JB weld to fix the nut in place with good results this didn't work for shit in my case so I went for the pin method.

    20180126_203236.jpg
     
  6. W5HRO

    W5HRO Administrator

    Can you no longer adjust the cap after putting the brass pin in? I'm trying to understand what it does and if it makes the cap fixed after you do it. If so are you adjusting the cap first, then taking it out to drill the hole and insert the pin after? The cap in your pic has the plates fully meshed so that’s why I’m asking.
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  7. W6MQI

    W6MQI Member

    The cap is fully adjustable the nut turns with the rotor shaft.
    In the picture the flat spring pushes back against the bearing plate which is attached to the rotor shaft this pushing action from the spring wants to push the rotor shaft back against the stator plates. Upon centering of the stator, and rotor plates the nut is pressed onto the rotor shaft ONLY this nut bears against the spacer which acts like a thrust bearing keeping the correct stator to rotor spacing. If the nut comes loose the rotor shaft will slide back from the spring tension pushing against the bearing plate. Also remember the pressed on nut is not threaded it just slides onto the rotor shaft for a press fit, so again if the nut is cracked it will loose its grip on the rotor shaft allowing it to slide back.
    Again no the cap is fully adjustable so once the pin is in place the cap is reinstalled in the radio for later adjustments.

    Unfortunately all the caps have been reinstalled into the radio so I can't make a video showing how the caps works. Do I need to have a YouTube account to post videos here or is there another way? There's no better way sometimes then a video to show the process.

    Cap 1.jpg
     
  8. W5HRO

    W5HRO Administrator

    That pretty much answers my question and then some. I've never tried to adjust those in mine yet and at first, I was assuming it was a lock nut you would screw down to lock it after alignment.
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  9. W6MQI

    W6MQI Member

    I ended up repairing 11 caps and now the receiver is back up and running. If you ever have to work on you 348 be prepared to repair some of those caps an 1/8 to 1/4 of a turn and you'll know right away something is wrong.
    So now the 348 is back running it works on all bands equally well in AM mode in CW its not so good. While listening to CW I find the tone of the CW station some what distorted and kind of shaky sounding (lack of a better word) SSB can be tuned in, but again distorted. I have tried different B+ voltages from 200-230 makes no difference. With regards to the I.F. alignment not really sure I did this right I turned on the crystal filter and sweep back and fourth around 915khz never really heard a strong peak just a tone that changed pitch. So maybe some of my issue with audio distortion the IF could be off frequency I guess any thoughts?
     
  10. W6MQI

    W6MQI Member

    Well found out it runs a whole lot better in MVC then AVC it actually behaves like a normal receiver tunes SSB and CW just fine. Listening to 9Y4D loud a clear right now think I'll revisit to IF adjustment again.
     
  11. W6MQI

    W6MQI Member

    Did some reading and yes the receiver should be run in MVC for CW-SSB operation, and for best results AVC position for AM operations. I ran the 348 for about 1 1/2hrs tuned all 6 bands listen to CW-SSB and AM I gotta say this receiver works great the audio quality really sounds good. Brian you mentioned possibility of adapting a Q multiplier well I decided hook up my SCAF two thumbs up to this combo :icon_thumbup::icon_thumbup: Wow I think I could actually use this receiver for CW op's. Now to get going on the power supplies so I can button this thing up and start using it I'm stoked:biggrin:
     
  12. W5HRO

    W5HRO Administrator

    Isn't the SCAF just an audio filter? What you really want is like a Heathkit QF-1 or similar and modify the two coils so it will work with the 915kHz IF.

    See the attachment below...
     

    Attached Files:

  13. W6MQI

    W6MQI Member

    Yes switch capacitor audio filter (SCAF) this is not one of those Idiom press models preset for 500hz mine has a 4 gang thumb wheel for all sorts of different combinations of AF frequency's. I have looked at the QF-1 option as well as a Q5er using a BC-458 both look like good options.
    Whats your AF gain pot feel like mine is stiff and has a sorta grinding feel to it almost feels like the shafts aren't concentric with each other or something is binding, also all the audio seems to be in the last 3/8-1/4 turn of the pot (strange). The main tuning control isn't the easiest thing to move either. Would have been nice if the AF, and RF gain controls were separate instead of locked together that was either a cost saving idea or somehow planned into the design.
     
  14. W5HRO

    W5HRO Administrator

    Yeah, the pot on mine was the same. I was planning on using some WD-40 and work-it down into the shaft to clean and to try and get it going first. The pots are sealed and the windings could have corroded or become rusty inside, but it may just be grime buildup and from not being turned in decades.
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  15. W6MQI

    W6MQI Member

    Well that's good I guess:icon_eh: I still might try disconnecting the back 330k pot, and hook up a separate pot just to see how the receiver behaves.
    Bad news on the HV power supply I wasted my brand new 270DX transformer. Had everything clipped together with clip leads and was running the 348 at 220v no problem for about 15 minutes. When I moved the receiver to get a better view of the dial a clip lead from the ground side came in contact with one of the secondary transformer taps grounding one side just before the full wave rectifier . The receiver was working fine until I heard bubbling sounds coming from the power supply it was the transformer boiling. I never once even smelled hot transformer DAMN IT.
    You would have seen one of these from San Jose if the 348 was toast.
    upload_2018-1-29_21-1-4.jpeg
    Ah....Shit fire looks like I need to get another transformer Damn it.
     
  16. W5HRO

    W5HRO Administrator

    I've never tried it, but you might be able to drill a tiny hole into the sides of those two pots just enough where you can stick a needle in and fill the pots with WD-40 and then let it drip out. It may just be that the shaft has sort of frozen over the years and just needs lubrication, but it could also be the inside of the pots.
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  17. W6MQI

    W6MQI Member

    Sounds like a plan, but if a piece of metal were to get loose inside one of those pots Hmm.... not sure I'm willing to go that far. If the part wasn't unobtainium I might be willing, but with my luck no way. Since I need to order a new transformer anyway:mad: I'll order a couple of pots and try the separation of the two controls for a test.
     
  18. W5HRO

    W5HRO Administrator

    I mean a really tiny drill bit. If you reversed the drill when pulling it back out it should be ok. If those windings are rusted inside that the wipers spin around on then that's about the only thing you can do.
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  19. W6MQI

    W6MQI Member

    I found a brand new in the box Miller 4409 coil for the Q-multiplier mod all I need now is a Miller 4411 which I think could be made from one of the coil forms from my junker URM25D signal generator.

    20180130_200851.jpg
     
  20. W5HRO

    W5HRO Administrator

    I found a couple of those coil forms with the different colored slugs back when I was working on the receiver using the QF-1, but I never got that far with it before putting it aside. I just never got back to it again, but I still have the coil forms.
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    Update: Here are those coil forms and the slugs. I had them wrapped up and sitting on top of the QF-1. I need to find the two that will work best at 915kHz when I eventually get back to working on it again.
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    IMG_6403.JPG
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  21. W5HRO

    W5HRO Administrator

    Did you get your replacement transformer yet? I'm guessing with your construction business you took advantage of the heatwave we had for the two weeks. It’s turned cold again now. The sky here turned dark yesterday afternoon and the cold wind started blowing hard.
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  22. W6MQI

    W6MQI Member

    No never did order one right now the 348 is running on an external supply one of these days I'll fix it kind of sick of working on it right now.
    Did an alignment on the SX-28 this weekend sounding real nice now used it to work the Sunday morning military collector net. I forgot how damn heavy that thing is moving that beast around on the bench almost caused me to blow a gasket.
     
  23. W5HRO

    W5HRO Administrator

    I kept watching for the below because I assumed I would see it in the direction of Livermore after you reported that you had melted down the transformer :razz:

    6f9.jpg
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  24. W6MQI

    W6MQI Member

    Yep a real Einstein move on my part still makes me sick thinking about it. Sorta took the steam out of me for a while with regards to working on the 348.
    I've been looking at the SP-600 wondering if or when I should tackle this beast did some reading recently and found out the JX-17 version which I have has a crappy AGC design, and probably would need modifying. So with that in mind do I keep the receiver or sell it? Would love to have a R390A the few I have looked at locally were way over priced POS. One R390A didn't even have a PTO and the guy wanted $250.00 for it sorry charlie, another one the guy picked it up by the power cord then drug it across the garage floor for me to look at:icon_thumbup: People are idiots
     
  25. W5HRO

    W5HRO Administrator

    Forget the R390A and get a R388 or the 75A-4, but you will pay for the A-4.

    P.S. Most all of those old tube receivers had crappy AVC designs, it's nothing new.
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