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 Post subject: BFO quit on BC-348N
PostPosted: Feb Sat 04, 2017 8:41 pm 
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I was doing an alignment on a BC-348N I recently picked up, went to loosen the "BEAT FREQ" knob because zero beat was off, couldn't get knob to turn separate from shaft so pulled it off. Knob came off hard after removing both set screws. Now the BFO won't oscillate so I figured the BFO coil was damaged. Checked plate voltage on VT-233 (6SR7) and find it goes to ~35v when the "CW OSC" switch is in the "ON" position... manual says it should 55v. I think it's close enough to oscillate.

Since the BFO coil knob was the last thing I touched before it quit I suspect that is the issue. But I'm open to any other suggestions especially from those with experience on the BC-348 series. The BC-348N is the same as the "J" and "Q" models.

Anyway I'm looking at taking apart the BFO coil to see if I can repair it.... Any pointers here are a appreciated.

John


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 Post subject: Re: BFO quit on BC-348N
PostPosted: Feb Sat 04, 2017 9:32 pm 
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Location: Monterey California USA
As I recall, on the J/N/Q sets the BFO assembly is mounted to the front panel, unlike the older and more costly design that use a flex cable from panel to chassis. On one of mine, the BFO failed to work because the postage stamp mica capacitor on it was bad.

In your case, I presume the BFO worked before you removed the knob. Are you sure the BFO isn't still working, just working at a frequency outside the IF center range? The front panel assembly, if unmolested, should have a knob stop affair under the knob so that the knob cannot rotate around more than 270 Degrees (or less) so that the BFO tuning shaft can't crank the slug past that range in either direction. It has more than one turn. It's pretty hard to diagnose things from a distance on-line but what I would do is turn on the BFO and turn the BFO shaft, without knob, through its range to see if could recover the BFO signal, then set it so that it is more or less centered and then re-install the knob. By the way, if I remember correctly, the BFO knob is different from the others in that it has little nubs inside to catch the stops.

If that fails to work, I would then start looking at the BFO assembly, first the mica capacitor and then continuity of the coil, moving on to the circuit at the tube after that.

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WB6NVH
California Highway Patrol Radio
Bell System Mobile Telephone History
http://www.wb6nvh.com


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 Post subject: Re: BFO quit on BC-348N
PostPosted: Feb Sun 05, 2017 7:51 am 
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Geoff Fors wrote:
As I recall, on the J/N/Q sets the BFO assembly is mounted to the front panel, unlike the older and more costly design that use a flex cable from panel to chassis....
In your case, I presume the BFO worked before you removed the knob. Are you sure the BFO isn't still working, just working at a frequency outside the IF center range?.

Correct. The radio was on and BFO working while pulling knob off. After the knob came off no BFO. If BFO is oscillating yet tuned beyond IF freq it should still effect RX audio to the point one should notice... More hiss or distortion. This isn't happening, no noticeable change in audio between BFO on or off. But... I'm not going to say it's not working, it just might have gotten tuned way off that it doesn't have much effect. I'll pull the scope out and see if I can find a sine wave.

Quote:
The front panel assembly, if unmolested, should have a knob stop affair under the knob so that the knob cannot rotate around more than 270 Degrees (or less) so that the BFO tuning shaft can't crank the slug past that range in either direction. It has more than one turn.

Correct again.

Quote:
It's pretty hard to diagnose things from a distance on-line but what I would do is turn on the BFO and turn the BFO shaft, without knob, through its range to see if could recover the BFO signal, then set it so that it is more or less centered and then re-install the knob.

Yes, I thought the BFO shaft without the knob should turn more than one rotation but it is only rotating about the same as a potentiometer. This has me thinking there's something binding the works inside the BFO can.

Quote:
By the way, if I remember correctly, the BFO knob is different from the others in that it has little nubs inside to catch the stops.

The BFO knob has a pin on the inside hollow portion between the center and skirt at the same location as the pointing arrow. The pin is underneath and can't be seen unless the knob is remove and turned over. There's also a thin piece of sheet metal captured under the nut holding the BFO shaft and can to the front panel. This piece of sheet metal is flat against the front panel, it extends downward till it clears the nut about 3/16" then does a 90* bend to come straight out under the knob between the center and skirt. When the knob is turned too far the pin makes contact with the thin metal stopping the rotation of the knob. The sheet metal also has a contact point that rubs against the inside of the knob skirt to give some tension to the knob rotation so it won't be easily moved if jarred or bumped..... Not that this has anything to do with the problem at hand... Just a little trivial pursuit. :D

Thanks for the advice Geoff. Will be giving a closer look when I have time in the next couple days.


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 Post subject: Re: BFO quit on BC-348N
PostPosted: Feb Tue 07, 2017 5:01 am 
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The BFO is working... Nothing was wrong with it except the shaft was turned out about 3 turns! I think it was turned all the way out. Just like you said Geoff, BFO freq was way off from IF freq. Seemed like the knob came straight off. Was trying to be careful to keep close to zero beat but I must have rubbed against the shaft when the knob finally popped off.

Funny, the shaft would turn in easily about 270* then get stiff. I gently turned it through the stiff part then it got a little easier, after about 3 turns I heard the beat note.

Glad that's all it was. I wasn't looking forward to taking it apart.

Thanks Geoff.


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 Post subject: Re: BFO quit on BC-348N
PostPosted: Feb Tue 07, 2017 6:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Monterey California USA
While you were doing that I checked my BC-348Q, which is apparently missing the end stop parts, because the BFO control went round and round quite a bit, turn after turn. I tried to be careful and put it back to where it was, as the set is in storage. If not, I have scuppered myself as usual and I too will be looking for the BFO center tuning spot when I get it back in use.

That's good you checked back in with the solution, most of us like to read the outcome of these mysteries...

Geoff

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 Post subject: Re: BFO quit on BC-348N
PostPosted: Sep Tue 21, 2021 10:05 am 
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Location: somewhere outside the coalition of the willing
Years Later....

The thing is that some people break the BFO transfomer slug. It can be re-joined (outside the transformer)
using super-glue. Work gently with it...not rip tear and bust!

The most difficult check is measuring the lesser part of the coil..."0.21" ohms. Sure one can measure the
whole coil or include a high quality resistor of an exact "000' in series with the short-end using a really high
grade VTVM...and may see .21 at the end of the new reading. I strippd a lot of junky work from my nice "Q".
Getting it working was quite a job....I replaced all off-value resistors and caps. The BFO still appears to be
not working, though at present I have it desoldered for new silver mica's...tossing up whether to inspect it
internally. Many sets have factory hand-picked micas to meet specs. Mine were off-cap other than the 50pFd
however caps can mesure ok on value and still leak dc. I decided to change the three of them. The 250pFd
was not available at Jcar (surprise surprise??)..a prty useless place also selling the worst multimeter probes!
I went to Evatco, muh better selection as they dal wih valve-amp nthusiasts.

My BFO transformer measuements using the terminals with caps all off SEEMS to have correct coil resistances.
Opening it up (it obiously has-been open before...maybe during its services rebuild. They also made brilliant
access to the oscillator area. The Services would not have done the horrible power supply, the transisor or the
other mods. A transistor was connect acros the BFO...probably as a diode detector or as a signal injection amp.
I didn't bother to suss that out...which my have saved me a lot of time had I drawn up the mods. I was keen to
get it back to +/- original circuitry. That was about 18 months ago....

I now have an extra turn on the 'faux-cap' pick-up. I have split the af and rf gain pots an used a switch-pot for
the AF..but am having some weird result with the new 20 k ohm RF pot.

That the modifier may have misaligned the set is on the cards The BFO switch itself wasnot functioning reliably.
I could neither find another no fix mine. I found a paxolin swich to tidily an without complications do the job
(in the parts-box) but may have to install a shield aound it.

There's also a pair of wires in mid-air fromt the coil adjacent to the BFO transformer ...mine had been cut back.

Supposed to be 'about 4 pFd it is discussed in the 348Q manual. I'll have to (likely) extend them uing silver-coated
wire.

I'd like to have it all finshed but gout and RSI have brought an end to my traditional Ham days...I will have only
two single ended typ 348's left whn my last two Q's are gone. One of the R's has dynamotor, mounting pad and
p103 (I think it is....) cable plug_socket for the rear. Difficult to find and quite a bother if pins dismantled I would
no longer hesitate to buy an 8 pin socket closely fitting the mounting bracket and carefully file it (the bracket ...
not the terminal block/plug/socket) and changeover the connections. I have also installed3 pin mains connctor
that way....to get rid of th nuisance of cord hanging around...especially 2 core, singly insulated lamp cord
Americans area addicted to using Do you B- connection properly ...at the output transformr choke and earth
the chassis through doubly insulated power-cord.

Back to the "Jonesa-plu (p-104) nuisance. Don't be dismayed!...You can use a six pin and get rid of the IF control
if not using with a transmitter. You could even go down to 4 pins for Ham/SWL us. Essentially you need filaments
and B+/B- (four connections) make it all tidy. Solder the disused wires to a tag strip mounted about an inch an
an half behind the 3 pin mains connector...using existing set screw...easy!

With onboard mains supply you need no external wiring...unless wanting to disconnect IF section...which can be
done using a switch on font panel. Use the P-104 bracket for the 3 pin mains..(same as used on computers) but
solder all P104 wires to that 8 tag tagstrip. Take the time to make it really 'professional''.

Use high quality tag stips not common Chinse junk.

Do your BFO testing on MVC 'position'. Record you changes/mods' and photograph them (clearly) ...store the
photos..

Cheers


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