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 Post subject: B&K 747 calibration question
PostPosted: Nov Fri 10, 2006 9:38 pm 
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Joined: Nov Fri 10, 2006 9:18 pm
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Location: Sacramento, CA
I am trying to calibrate my B&K 747 with limited success and was hoping someone would have some info that may help. First problem is the number 1 55 pilot bulb is out and will not light, even after changing with known good bulb. The other 55 pilot bulb lights ok. The 2 fuses are good and the unit powers up and the front panel on/off light works. I don't get any reading on my voltmeter from pin 1 of tube 1 when I try to do the line level calibration adjustment. When I do the bias adjustment test I get 6.00 to 8.00 volts adjust range and cannot set the trim down to 2.5 volts, as called for. I appreciate any help on this.


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PostPosted: Nov Fri 10, 2006 11:01 pm 
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Joined: Jul Sun 02, 2006 9:15 pm
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Location: Norwalk, CT
Signal and bias will not calibrate until you get your #55 ballast lamps working (preferably with cleaned reliable connections to the bayonet bases).

Sounds like one of your 16 ohm ballast resistors is open. Pull out the lamps and read the resistance of those. These are available from Mouser or Newark.

If open, then why did it blow? Wrong settings? Tube shorted to grid? Check the 470 ohm resistor on the end of the switchbank also.

The bulbs must #55 units. Stick with the brand that you calibrate it with. I use only the GE brass base. Very interesting circuit, that ballast.

Good luck.
Denis S.

EDIT: Forgot - the solder lands, for the lamps, on the PC board, sometimes break off and need to be shored-up with some bits of buss wire etc. Also all solders on that board need to be checked. Parts often pull out of cold solders there. I think B&K soldered them at night with an open flame in a dust storm in the desert.


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PostPosted: Nov Sat 11, 2006 8:06 am 
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Joined: Nov Fri 10, 2006 9:18 pm
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Location: Sacramento, CA
You were correct about the solder lands . I found the 16 ohm resistor felt loose and sure enough, it was not making good contact on the solder side. Both lamps light now and it should be ready for calibration. I will let you know how it goes. Thanks very much for the tip.


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 Post subject: Re:
PostPosted: Oct Thu 22, 2020 9:42 pm 
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tentrax wrote:
You were correct about the solder lands . I found the 16 ohm resistor felt loose and sure enough, it was not making good contact on the solder side. Both lamps light now and it should be ready for calibration. I will let you know how it goes. Thanks very much for the tip.

Quick question on the short calibration. Do we adjust till the bulb stays on or just till it flickers? Please advise and thank you.


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Thu 22, 2020 10:09 pm 
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You're responding to a 2006 post. :-D. Hopefully those folks are still around, but if not, I think there are others on the forum that know that tester and may respond.

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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Thu 22, 2020 10:40 pm 
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Barry H Bennett wrote:
You're responding to a 2006 post. :-D. Hopefully those folks are still around, but if not, I think there are others on the forum that know that tester and may respond.

Lol.. I keep doing that here .. geez .. yes, hope someone answers , thank you.


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Thu 22, 2020 11:38 pm 
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Location: Dallas, TX
I think just flickers is OK. It isn't an extremely accurate indication. In other words, does it matter whether a tube has as much leakage as a 1 Megohm resistor or a 1.1 Megohm or a 900K resistor?
Not a definite quantity, in the neighborhood is what you want to know about a tube. Somewhat depends on the circuit the tube is going to be used in also.

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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Thu 22, 2020 11:52 pm 
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Notimetolooz wrote:
I think just flickers is OK. It isn't an extremely accurate indication. In other words, does it matter whether a tube has as much leakage as a 1 Megohm resistor or a 1.1 Megohm or a 900K resistor?
Not a definite quantity, in the neighborhood is what you want to know about a tube. Somewhat depends on the circuit the tube is going to be used in also.

Thanks, I had a GEC 6as7g that failed miserable on the BK shorts and emissions test last week, the needle flipped all the way to the right almost and that was a first for me since my recent acquisition of the BK. Sent it out to get retested on a Hickok and it passed. So thinking I need to recalibrate the short and emissions on the BK 747. The tube will be used on my headphone amps.


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 1:47 am 
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Found out that the short light is constant when activated, just finished a calibration on it, kinda, I'm a fish out of water with this sort of stuff but can follow a youtube vid like the best of them :D anyways, for those familiar with the B&K 747 Signal Level Calibration I am maxed out on the signal level calibration potentiometer R21 and am only getting 1.3v, it's supposed to be at 1.5v , ?? what to do now? any ideas? All I can think of is to start replacing resistors..maybe all of them for a shotgun fix.


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 2:02 am 
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To follow up on my question above, I'm finding that it could be any of the following. And I understand half of it..

R21, R22, R23 Off-Value or FL1, FL2 Defective

If R21 is off value I might be totally screwed, as these things probably aren't even made anymore. now this FL1 and FL2 ..that I might be able to replace, what are they? :)


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 2:13 am 
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FL1 and FL2 are lamps, standard number 55. Should be readily available. I think 6V and a slightly larger bulb than most.


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 2:18 am 
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bob91343 wrote:
FL1 and FL2 are lamps, standard number 55. Should be readily available. I think 6V and a slightly larger bulb than most.

Thanks!!


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 2:27 am 
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bob91343 wrote:
FL1 and FL2 are lamps, standard number 55. Should be readily available. I think 6V and a slightly larger bulb than most.

And the R22 and R23 is??


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 3:11 am 
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Both are 16 Ohm 5% 5W resistors. They are wire wound and the parts list says P.F. whatever that means.

http://www.tubebooks.org/Books/Atwood/B ... ematic.pdf


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 4:40 am 
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From your posts it is very evident that you are in over your head with this tester. Without knowing how to identify components, know what they do in a circuit, or read a schematic, it will be very easy to turn your tester into a parts caracas. I would strongly suggest putting this tester on the shelf until you have gained more electronic experience or getting a knowledgeable person to help you. I would really like to see you gain more experience but this tester is not the way, you need to start on something much simpler and work your way up. I would rather see you learn and enjoy this hobby than get frustrated and give up.

DM


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 5:34 am 
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devilsmist wrote:
From your posts it is very evident that you are in over your head with this tester. Without knowing how to identify components, know what they do in a circuit, or read a schematic, it will be very easy to turn your tester into a parts caracas. I would strongly suggest putting this tester on the shelf until you have gained more electronic experience or getting a knowledgeable person to help you. I would really like to see you gain more experience but this tester is not the way, you need to start on something much simpler and work your way up. I would rather see you learn and enjoy this hobby than get frustrated and give up.

DM

You are so right and I should've included that information, I do have a friend helping me out, I'm trying to figure some of this stuff for myself without ruining the initiation he gave me for that help., if that makes sense. We've been going back and fourth for a week, I just know he's gonna ghost me soon.. :D There's not that much info out there for the BK 747, a few youtube vids and this forum, that's it.


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 5:39 am 
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bob91343 wrote:
Both are 16 Ohm 5% 5W resistors. They are wire wound and the parts list says P.F. whatever that means.

http://www.tubebooks.org/Books/Atwood/B ... ematic.pdf


Thanks, I'll let my guy know and we'll decide if I have to send it back to him or if he thinks I can manage the work myself. Thanks guys, Your all awesome. :)


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 4:19 pm 
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Location: Dallas, TX
Make sure you are measuring the signal right.
Since the two lamps are used in a symmetrical bridge regulator circuit the two bulbs should be replaced as a pair (identical pair). Also problems with the lamp sockets is a know weakness, the metal parts can develop
poor contact between the pieces. A stranded wire can be soldered to the end of the center plunger contact and the other end soldered to the metal piece (bracket) that holds it. Also the cylindrical lamp base holder can be soldered to the bracket that holds it. By the way these lamps only glow dimly if at all when in use.
The circuit regulates because the filament resistance varies with temperature which varies with the size of the signal. The lamps would have a very long life because they don't burn bright, unless the tester is dropped.
I find that the 747 finds many tubes have grid emission.
By the way, on any tube tester if a short or excessive leakage is detected there is no point testing for emission or transconductance, because those measurements cannot be done correctly. That is why the short/leakage test is always done first.
Wire wound resistors are very reliable as long as they haven't been overloaded.

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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 6:20 pm 
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Notimetolooz wrote:
Make sure you are measuring the signal right.
Since the two lamps are used in a symmetrical bridge regulator circuit the two bulbs should be replaced as a pair (identical pair). Also problems with the lamp sockets is a know weakness, the metal parts can develop
poor contact between the pieces. A stranded wire can be soldered to the end of the center plunger contact and the other end soldered to the metal piece (bracket) that holds it. Also the cylindrical lamp base holder can be soldered to the bracket that holds it. By the way these lamps only glow dimly if at all when in use.
The circuit regulates because the filament resistance varies with temperature which varies with the size of the signal. The lamps would have a very long life because they don't burn bright, unless the tester is dropped.
I find that the 747 finds many tubes have grid emission.
By the way, on any tube tester if a short or excessive leakage is detected there is no point testing for emission or transconductance, because those measurements cannot be done correctly. That is why the short/leakage test is always done first.
Wire wound resistors are very reliable as long as they haven't been overloaded.

Thanks for the info, my friend installed new bulbs and he informed me last night that he was able to get the pot up to 2 v so he believes the pot got stuck when I was making the adjustment. I'm gonna try again this time on a table and not the floor.


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 Post subject: Re: B&K 747 calibration question
PostPosted: Oct Fri 23, 2020 10:06 pm 
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UPDATE

With the guidance and push form my friend Bill Cowen, I was able to achieve that magic number of 1.5 volts. I noticed that them two bulbs dim or brighten when adjusting R21, what I thought was maxed out was really at minimum! With the help of my two daughters, one holding the test button the other taking photos, I was able to get this dialed in. Thanks for your help guys!!


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