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 Post subject: B&K 747B Tube Tester Calibration
PostPosted: Dec Thu 07, 2017 3:54 pm 
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I just bought this tester and replaced the electrolytic caps. I am having trouble with the first part of calibration, though. I'm only getting 1.25v and can't adjust R21 to get the required 1.5v. I have not attempted to move on from that step yet. Looking at the schematic, I verified that R22 and R23 are good. I don't know what other parts related to this calibration I should check, other than the R21 pot itself. I appreciate any suggestions there.

(schematic attached)

Also, I am attempting to calibrate with a digital multimeter. I don't have a volt ohmmeter or a vacuum tube volt meter as the manual suggests. I understand that is all the technology that existed at the time, and I didn't think it would matter, but is that a problem?

Attachment:
B&K Model 747 Dyna-Jet Tube Tester Schematic.pdf [157.55 KiB]
Downloaded 18 times


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Fri 08, 2017 2:40 am 
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Have you checked the bulbs FL-1 and FL-2? Both the bulbs should be replaced at the same time. They regulate the signal level. Also check the bulb sockets, they are a possible problem area. Bad contact between the socket housing and the solder lug, if I remember. I have one of these that I fixed up. Bad contact in the test switch could also cause this problem. Check R27 and R28.
The digital meter should be OK.

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Fri 08, 2017 11:33 pm 
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AntKnee wrote:
.........
Also, I am attempting to calibrate with a digital multimeter. I don't have a volt ohmmeter or a vacuum tube volt meter as the manual suggests. I understand that is all the technology that existed at the time, and I didn't think it would matter, but is that a problem? ....


You should always use test equipment that has the same characteristics as the ones specified unless you are sure it will make no difference. There have been a zillion posts on this forum about bum readings caused by using meters that didn't load circuits the same as the ones specified.

RRM


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 12:15 am 
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Bulbs are good. R27 and R28 are good.
I did measure across the R21 pot itself, which should be a 10 ohm and only got 5.1 ohms maximum. So, that seems to be the culprit here. Could I just put a resistor in series with the pot to make up the difference I need?


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 1:32 am 
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Did you remove the wires from the pot before testing?

Dave


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 2:11 am 
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easyrider8 wrote:
Did you remove the wires from the pot before testing?

Dave


No. Dang it. I knew better. It tests fine.

The only other thing I can think of is that I'm not getting enough power out of the transformer to begin with. Schematic shows a 9.5 vac on the transformer there. If I measure from the transformer side of R21 to ground, should I be getting 9.5 volts?


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 4:48 am 
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AntKnee wrote:
Bulbs are good. R27 and R28 are good.
I did measure across the R21 pot itself, which should be a 10 ohm and only got 5.1 ohms maximum. So, that seems to be the culprit here. Could I just put a resistor in series with the pot to make up the difference I need?


Does not matter if the bulbs are "good".
It's the variable resistance of the socket bayonet connection that can mess up this step of the calibration.
Just went through this recently. Soldering a wire directly to the bulb's outer base and to the socket's solder lug is what finally got it to calibrate properly

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 5:47 am 
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processhead wrote:

Does not matter if the bulbs are "good".
It's the variable resistance of the socket bayonet connection that can mess up this step of the calibration.
Just went through this recently. Soldering a wire directly to the bulb's outer base and to the socket's solder lug is what finally got it to calibrate properly

The socket problem was what I was driving at. Since the bulbs don't burn bright they should last a long time. However the bulbs are not expensive and are important.
The manual says VOM or VTVM, not critical. This is a relatively recent model so DVM (bench) may have been available.
I would measure the transformer voltage between R21 and ground. Also measure the primary transformer voltage. If the "Life Test" switch is set wrong it will decrease the primary voltage. Are you sure your meter is accurate?

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 2:28 pm 
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processhead wrote:
AntKnee wrote:
Bulbs are good. R27 and R28 are good.
I did measure across the R21 pot itself, which should be a 10 ohm and only got 5.1 ohms maximum. So, that seems to be the culprit here. Could I just put a resistor in series with the pot to make up the difference I need?


Does not matter if the bulbs are "good".
It's the variable resistance of the socket bayonet connection that can mess up this step of the calibration.
Just went through this recently. Soldering a wire directly to the bulb's outer base and to the socket's solder lug is what finally got it to calibrate properly


+1, solder the bulbs in and you know for sure, plus have the correct bulb number, bulb substitutions here do not work well.

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 5:49 pm 
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Sorry, gents, but I'm not clearly understanding where you're saying to solder the wire to the bulb. Do you mean directly to the bulb, or to the socket somehow? Should I remove the sockets entirely?

I'm measuring 10 VAC from the transformer. That's not the problem.

Attachment:
20171209_104148_a.jpg
20171209_104148_a.jpg [ 198.24 KiB | Viewed 251 times ]


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 8:05 pm 
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Location: Dallas, TX
AntKnee wrote:
Sorry, gents, but I'm not clearly understanding where you're saying to solder the wire to the bulb. Do you mean directly to the bulb, or to the socket somehow? Should I remove the sockets entirely?

I'm measuring 10 VAC from the transformer. That's not the problem.

Attachment:
20171209_104148_a.jpg

Sorry I didn't remember that the sockets were PCB mounted. Don't really need a wire, just a blob of solder on the socket base, (were the slots are) to the tab against it that goes to the board. Definitely remove the bulbs before you solder. That joint is just held together by the pressure of the metal bent when it was assembled. The joint can loosen with time. This isn't really causing your problem at the moment, the voltage would be too high if it was open, but it is a common problem.

EDIT
I took my tester apart and found I did also soldered a short section of stranded wire between the center socket 'pin' and the tab that it passes through. That is probably the more important thing to do.

From my notes I had to replace R21 on mine, I don't recall what problem it caused. R21 is 3 Watt as it says in the parts list. It would be kind of hard to find a replacement, luckily I had another 747 for a donor.
If you are getting 10V at the transformer just follow the voltages along and see were it is dropping too much.
It could be bad contact in the test button or further along.

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Last edited by Notimetolooz on Dec Sat 09, 2017 8:34 pm, edited 1 time in total.

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sat 09, 2017 8:25 pm 
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When you are reading the voltage are you using a 1 uF capacitor? I don't think would want to use it with a DVM. Make sure all the programming switches are set all the way down. You might have a short somewhere like a tube socket. R21 might have a short to its case which is probably grounded. Did you replace C8?

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sun 10, 2017 4:05 am 
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"bulbs are good" isn't enough. Both should be from the same lot, any single replacement could be be a problem. And contact may not be so great after all thee years.


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sun 10, 2017 4:47 pm 
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glue_ru wrote:
processhead wrote:
AntKnee wrote:
Bulbs are good. R27 and R28 are good.
I did measure across the R21 pot itself, which should be a 10 ohm and only got 5.1 ohms maximum. So, that seems to be the culprit here. Could I just put a resistor in series with the pot to make up the difference I need?


Does not matter if the bulbs are "good".
It's the variable resistance of the socket bayonet connection that can mess up this step of the calibration.
Just went through this recently. Soldering a wire directly to the bulb's outer base and to the socket's solder lug is what finally got it to calibrate properly


+1, solder the bulbs in and you know for sure, plus have the correct bulb number, bulb substitutions here do not work well.


Also, modern production #55 bulbs will not work here. I recall having a similar problem to the OP when fixing my 747. Found some NOS #55, and finished the calibration with ease.


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Sun 10, 2017 8:11 pm 
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My bulbs seem to work fine. They appear to be from the same lot, same brand. They measure the same resistance. I ordered some NOS bulbs anyway. I will update when I install them and reattempt calibration.


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Mon 11, 2017 7:45 pm 
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AntKnee wrote:
My bulbs seem to work fine. They appear to be from the same lot, same brand. They measure the same resistance. I ordered some NOS bulbs anyway. I will update when I install them and reattempt calibration.

Actually I doubt that the bulbs or the sockets are the cause of you problem unless the filament resistance is very low. You aren't seeing enough voltage at the tube socket, a poor bulb contact would make the voltage too high.
Set R21 to minimum resistance and measure the AC voltage at a couple of points, EE (junction of FL-2 and R22), and both ends of R27 (one end of which is connected to pin 1 of tube socket 1. Measure the voltages with and without pushing Test Button 1.
I had some internet problems yesterday.

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Tue 12, 2017 12:07 am 
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Notimetolooz wrote:
Actually I doubt that the bulbs or the sockets are the cause of you problem unless the filament resistance is very low. You aren't seeing enough voltage at the tube socket, a poor bulb contact would make the voltage too high.
Set R21 to minimum resistance and measure the AC voltage at a couple of points, EE (junction of FL-2 and R22), and both ends of R27 (one end of which is connected to pin 1 of tube socket 1. Measure the voltages with and without pushing Test Button 1.
I had some internet problems yesterday.


VAC at EE: 0.75v

R27 (socket 1 side): 0v
with test button pressed: 0.75v
opposite side of R27: 0v
with test button pressed: 0.75v


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Tue 12, 2017 12:16 am 
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AntKnee wrote:
Notimetolooz wrote:
Actually I doubt that the bulbs or the sockets are the cause of you problem unless the filament resistance is very low. You aren't seeing enough voltage at the tube socket, a poor bulb contact would make the voltage too high.
Set R21 to minimum resistance and measure the AC voltage at a couple of points, EE (junction of FL-2 and R22), and both ends of R27 (one end of which is connected to pin 1 of tube socket 1. Measure the voltages with and without pushing Test Button 1.
I had some internet problems yesterday.


VAC at EE: 0.75v

R27 (socket 1 side): 0v
with test button pressed: 0.75v
opposite side of R27: 0v
with test button pressed: 0.75v


My mistake!
opposite side of R27: 0v
with test button pressed: 1.25v


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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Tue 12, 2017 1:39 am 
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I opened my tester up and did similar tests. I got some results a little different than I expected.
Point EE is basically the same signal as pin 1 of socket 1.
At point EE, with or without the button pressed I got about 1.5VAC, actually a bit higher without pressing the button. I took the bulbs out and pretty much as I expected I got about 10V, (button not pressed), basically the transformer voltage. With the bulbs back in I put a jumper across R21 to simulate turning it to a minimum, (I didn't want to change the calibration). Now without the voltage drop across R21 you would think, (well I would of thought), the voltage at EE should be higher. It went down to about 0.45 V! So the regulation effect of the circuit over-corrected. I suppose that with the higher voltage across the bulbs they drew more current so the voltage drop across the resistors (R22,23) increased to the point that the voltage at EE dropped a lot.
So this means that the bulbs might be your problem. You might want to double check that R21 does go to a max of at least 10 ohm and that there are no dead spots or jumps in resistance as you vary it. This is a case where a analog meter is better because you can see that a variable resistor works smoothly.

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 Post subject: Re: B&K 747B Tube Tester Calibration
PostPosted: Dec Tue 12, 2017 2:41 am 
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Thanks very much for going to that trouble.

I have checked R21 and it measure 10.1 ohms max.

I am going to replace the bulbs just to rule them out, if nothing else. I also think they might be the problem after all. I'm measuring 2.1 ohms resistance across both bulbs. If you still have your tester open, would you mind checking your bulbs?

Thanks for the help!


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