BC-348-Q restoration

[KK4RF]

Fred,
     I'll have to cogitate on this one for a while. I'm driving up to D.C. tonight for a 2 day conference up there and will be reviewing  the schematic and my other articles on this tonight and tomorrow night when I'm not in the conference.  I won't have access to a computer, however,  for the next 48 hours. What I'm concerned about is either trying to isolate the chassis of the BC-348 from other rigs that all connect to the same antenna, or if there is a way to do this that doesn't   ground out this vital bias. I think you know what I mean. Thanks again, Fred.   ---Marty, KK4RF---

[KA2DZT]

Marty,

You not going to ground out the -18V.  This method of creating the -18V bias is not unusual, in fact it is a fairly common way to obtain a negative voltage with respect to chassis ground.  The only thing that is a little different is that the BC-348 uses the aux winding as part of the total resistance that drops the -18V.  You could not use the aux winding and just substitute a resistor in place of the aux winding.

Remember the metal chassis is NOT floating above (or below) ground, it is ground, the same ground as all your other equipment, coax shields, etc.  You can connect the BC-348 up to anything in your station, cables, grounds, etc.  This will have no effect on the -18V bias.  The only way to short out the bias voltage is to ground the connection point (under the chassis) where the Negative lead of the PS (CT on the power xfmr) is connected to the aux winding.

Having said all of this, my PS is built into the receiver in place of the dynomotor. 

If you are using an external power supply which is mounted on some external chassis and connected to the BC-348 with a cable,  THEN there WILL BE some issues that we need to look at.

Fred

[N0WEK]

I haven't played with mine since the 80s but I'd like to get back to it someday.

The AC supply that somebody put in the Dynamotor spot seemed to add quite a bit of heat to the receiver. I always thought that it'd be nice to make the supply external.

[KA3EKH]

The original Dynamotor had its B- isolated from ground by returning it thru T-155 "B" winding and then R-108, a 50 ohm resistor to ground. All this requires is isolating the B- from the external power supply and returning that to ground by R-108. I have removed the original output transformers on some radios and have others that the transformer was removed before I got them and think the original transformer is not that good to begin with anyway, unless you're planning to use high impedance headphones or speakers so I have used old audio output transformer from old tube type television sets and they work great with the 6K6 The B set of windings that the B- use to go thru were not necessary in any of the sets I have done and do not appear to make any difference in the radios performance that I can determine.  Have had the issue of heat buildup in older set someone else converted but they used a 5Y3 and that developed a lot of heat. The last receiver I built I used a 6X4 being that it's getting harder to find tube transformers with five volt heater windings, the 6X4 is a smaller tube and for some reason have a almost endless supply of them. Solid state diodes may be better yet although I did not want to have the B+ instantly applied to the radio before the filament warmed up so did want to use a tube. Also a lower voltage secondary transformer is better than a 250 volt or above being it makes little difference in the radio s performance between running 190, 200 or 250 volts on the plates but the radio runs cooler with lower plate voltage. Couple more pictures to show that an AC supply fits easily into the radio and where R-108 is now located just under the chassis by where the power is brought in. The power supply in this radio is built from an old and useless URC-35 receiver that I was given at a Hamfest that provided the power transformer and the choke. The mounting plate took a couple days to build. The new output transformer is from an old RCA color television that had a 6AQ5 output tube so its close enough to the 6K6 and having a three ohm audio output is way better for driving normal speakers.

[KA2DZT]

If you look close at the schematic you'll see that there are two 50ohm resistors, the second is in a different circuit area.  Must be a few volts of neg bias used in another circuit.  I still use the original output xfmr that has a 8Kohm and a 300ohm output.  I use a LS-3 speaker that has a 8Kohm xfmr built into it.  Seems to be a perfect match and the receiver has plenty of good clean audio.

If you use the -18V bias for the 6K6 your DC supply voltage will be 18 volts less to ground.  I think mine runs about 240 B+ to ground plus the 18 volts, so the supply runs about 260 volts total.  I added a second 6.3V filament xfmr (removed the large metal cap to make room for the xfmr) connected in series with the 6.3V winding in the power xfmr.  I connected the filaments to run two tubes in series for 12 volts.  I did all this 48 years ago.

You don't need the winding in the xfmr to get the -18V bias,  you can just use a resistor of about 240ohms plus the two 50ohm resistors.  I measured about 270 ohms total for the winding and the two 50ohm resistors.

Fred

[KA3EKH]

Good point, I completely forgot about the DC resistance of the original coil.  Do not know the current bias voltage of the 6K6, may have to look and see!  The first BC-348 I ever owned I bought from a Army Navy store back in 76,  maybe the first real HF receiver I ever owned. Whoever did the modifications on that did just about anything you can think of. It had a 6SN7 with one triode as a voltage amplifier stuck just behind the band change gear and that drove a 6F6 in the audio output. That 6F6 ran so hot that the paint would cook off the metal skin of the tube. Got my fingers burnt several times by touching it. The same person also installed separate controls for AF and RF gain, a S meter circuit, antenna trimmer and a internal AC power supply built from some old radio transformer that had bleeder resistors that were huge and generated tons of heat, so the case had huge sections cut from it with window screen attached and painted black to keep things from falling in. also remember something about all the 6SK7 tubes being swapped out with 6AC7 tubes. That had to be one of the most modified and chopped up receivers ever built. Have owned several since that receiver going from all original and all complete working examples to the one I have today that’s what I consider the best of the "Ham" versions of the receiver. May have to find a hacked ARC-5 transmitter to put with it for 80 meter CW, have no idea what ever happened to the one I had forty years ago, think I gave it away sometime after high school.

[KA2DZT]

The current running through the bias resistors is the total current drawn by the receiver.  All the current runs from the chassis ground through the 270 ohms back to the negative return in the PS.  18V/270ohms= about 67ma.  I also include about 8ma of bleeder current.

The receiver draws about 60ma, (I think I'm getting about -19V of bias so the current may be a little more than 60ma) with about 240VDC B+ to ground.

6F6's can run hot.  6K6 draws .4amp filament, 6F6 draws .7amp filament.  Fixed bias for the 6K6 is -18V with about 250 volts on the plate.  Bias would be less with lower plate voltages.

You don't need the extra audio amp. The receiver, when properly set up, should have plenty of audio with the correct load on the output xfmr (8K ohm or 300ohm).

Fred

[KK4RF]

Fred,
     Just got back home from my D.C. (Washington) trip. I studied my schematic of the '348-Q  when I was up there. I'm going to work on it this weekend, modify  the power supply and check voltages. I think I was mixed up the way I was thinking about it. I want to check the voltage of chassis to a ground. I believe it will measure zero. The minus 18 volts should be from the grid of the 6K6 to ground. Anyway it's late here and I won't have any time to work on it until this weekend. Thanks for the assistance. I'll let you know how it goes.   73s,  Marty, KK4RF

[KK4RF]

Fred,
     I modified my old power supply (lifted the power transformer's center tap from ground to make the ungrounded B minus, and lifted the negative side of the electrolytic from ground and connected it to the B minus)  and hooked it all to the receiver. Everything seems to work fine, no 60 cycle hum in the receiver. I measure about 0.7 volts between the chassis of the receiver to the chassis of the power supply. Not sure why that is, but I won't lose any sleep over it either, HI...
     Fred, how would you recommend muting the receiver during transmit periods? I suspect there will be lots of different ways to do this. I would welcome ideas on this from others as well.
     Thanks again to all who had input on this topic. I still have a fair amount of work to do on this which probably won't get done until my week off after Thanksgiving.
          ---Marty, KK4RF---
 

[KA2DZT]

On my BC-348J there is a pair of wires that came out to the original Cinch plug on the back.  Those two wires were the mute feature.  They went to a relay elsewhere in the plane for muting.

The leads open the B+ to a portion of the receiver.  I just finished bringing those leads out to a terminal strip on my 348.

You need to look close at the schematic to see which wires are involved.  In fact, it is three wires, two are connected together for one terminal and the third wire for the second terminal.

If your set no longer has the Cinch plug, then those three wires would have been tied together as one, since they were connected to the Cinch plug.

OK on the lifting the CT and the filter cap neg lead from chassis ground.  You need to then connect those two leads to the AUX winding on the output xfmr.  They will reach chassis ground through the winding and the two 50ohm resistors.  It is that connection point that is at -18V with respect to chassis ground.  That point is also connected to the grid resistor circuit of the 6K6.

You really need to get a schematic of the receiver to more clearly see all this.

I downloaded a lot of info on the BC-348J, which is also good for 348Q.

Fred

[KK4RF]

Fred,
     I do have a good manual on the '348-Q which has a good-sized schematic. I did indeed put the B minus to the auxiliary winding on the rig. Will check the voltage to ground from that point (which I forgot to check last night.) Most of my work at present is clip-leaded together. The Cinch-plug is long gone on this rig. Making a permanent power cord to the power supply should not be too hard to do. Thanks again for all the help.---Marty, KK4RF---

[Noddyswl]

The B- is confusing. Yes. What seems to be missing in all the comments is that the Choke was not put into the
Audio transformer to use up left-over wire. The bias system was commonplace and a development from using
grid battery in the 1930's when the set was designed and worked well.  The BC 348 power supply was filtered
using a choke in the B-line and provided bias to the audio tube at the same time. I admit that the power supply
situation is poorly explained....The explainers and presenters of BC348 mains supply design do not see the
confusion or don't understand what's going on. I didn't even after reading numerous articles. Then it clicked..


Yes you can substitute a resistor but why?...because you don't understand that all your B- should go through the
choke? and don't understand that the designers actually knew what they were doing unlike the ruinous mod's
from USA in the 40's-70's. Henry from Boulevarde likes to filter to the B- line and also the chassis...I don't know why
that is but he knows the sets well...Just look at you power supply as B+ and B=, the B- being to the audio choke
rather than the chassis. You can earth the chassis but dont simultaneously earth the B- line. If you don't run the B- to
the choke but instead to chassis you will have distortion and overheating of the audio tube.

If you bypass the choke and use a bias resistor you lose some good filterin. Use the choke and keep B+ less than
220volts filtered dc. If usng diodes organise some delay in switching-on B+ until heaters have warmed...say 30 seconds.
The simplet way is to switch the B+ after say 30 seconds from turning on the set. I used a switch pot for the volume control
so as to feed the p/s. I  neatly placed an RF gain control to right of the tuning dial but you could swap the lighting
rheostat for a 20k w/w switch pot to separate the AF and RF controls (thus the change also to 300k or thereabouts switch
pot for audio ) .

I'm notmuch in favour for using the AVC-OFF-MVC contacts for switching B+(or B-). I'd rather not put that switch at risk as it
was ok more or less carrying 4A (plus start-up) at 28 volts in its design. You could use the BC348 switch to operate a 6 or 24
volt relay of course but again they are not rated (usually) for dc though sure as eggs are eggs someone will have dozens of
suitable relays or small timers or some solid state delay which will take the dc B+  voltage. You could of course switch the
B- line..noting rong with that.

Rather than switch 220v dc on a 240v ac rated switch I'd look at using a relay with HV rated contacts OR....just use a tube in the
power supply. Then heating up will pretty well take care of itself.

[Noddyswl]

Re KA's inquiry...you can switch the B- line or disconnect the B+ to the IF's for example. The set was built with
connections to just that purpose (terminals 6 and 2 on the Jones socket at rear of set) and you have to join
hem for operating the Rx... or open for Tx. If the Jones plug has been removed, follow the circuitry and reinstate
the wiring for the stand-by.

[WU2D]

Oh and the dynamotor on these BC-348 sets is very quiet. Like the one in the BC-652, it is hardly an issue noise wise. Now the ARC-2 is a different story! That thing will drive you out of the room!

I just did a cursory clean up on the dyno and it has been sweet for 15 years. Mine came complete, but with no knobs. ha.

[MikeKE0ZUinkcmo]

For those interested in properly installing an AC power supply, Hallicrafters made a retrofit kit for the military to do just that.   HERE YOU WILL FIND the original documentation for that modification, for virtually ALL versions of the BC-348, including the original schematic diagram.

I installed my AC power supply in '75 and although it is a SS version, it has served quite well.   It is important to that you make sure you return the B- to the original B- Choke, AND NOT TO GROUND.   As mentioned above it supplies the Grid Bias for the audio output amplifier tube.