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 Post subject: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Fri 07, 2019 5:36 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
This is a project I shelved awhile back after I reached the saturation point of my patience. I'm regrouping now and hoping to get through it this time. The issue my unit has seems to be hereditary: the iffy & fragile "BIAS" circuit which is blocking me from enjoying this tube tester.

I've searched Google for a couple of days copying & pasting AR Forum comments on this. So I have two pages of "hints" but I'm afraid there are so many leads that I'm not sure how to go about it. The only clue I have is that when the unit sits a long time, when it first powers up, the bias meter works in a glitchy way and then stops working completely. This 500 uA meter is hens teeth I suppose so I am thinking of trying to measure the bias voltage at the external bias connect posts at the top and live with it that way or just replace the meter with anything that will fit, the appearance be "da- - ed".

My first question is will this work? Is the actual bias voltage available on the top without bringing a wire out from inside?

Thanks, Doug


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Sat 08, 2019 12:57 am 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4385
Location: Sunbury, Ohio 43074
Does the bias meter ever work properly? I would look for a cause other than the meter itself. I’ll look into this further when I get to a computer larger than 3”

Yes, there are certainly other ways to measure this voltage whether or not it is brought out to the panel already

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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Sat 08, 2019 1:24 am 
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Joined: Jan Thu 24, 2019 1:22 pm
Posts: 121
Location: Centennial, Colorado 80122
Doug, Its common for these meters to fail and if its the round type they are sealed. Here is an easy way to adjust the bias with Your digital multimeter. After you have set your selector switches for the tube to be tested put the probes in any other socket that has the pin numbers as shown on the selector switches for cathode and grid. if you have old socket savers for the octal and 9-pin it might be easier. Check with me as I might be able to help you if your meters are round.
Bill


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Tue 11, 2019 4:59 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
Sorry for delay. I was looking for responses in my email.

To Barry: Yes but only for a brief period of seconds, enough to set the bias for a tube but then it falls to zero and then is not responsive. When it does indicate, it jumps or glitches. Most of the time it is dead when I turn it on.

To Bill: I have the square type meter. To do the test you describe, do I put a tube in that I am setting the switches for ?

Questions: At the shunt external world connection posts, there is a shortening bar. How/when should that be in place shorting the terminals? I repaired the DAVEN 1 to 1K (or whatever) 3 pot accessory and it now reads correct on my VOM.


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Tue 11, 2019 5:58 pm 
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Joined: Jan Thu 24, 2019 1:22 pm
Posts: 121
Location: Centennial, Colorado 80122
Doug, The square meter can be taken apart and sometimes corrosion where will cause intermittent operation. This you will see once you open it up, but care must be taken. If its been in a humid climate it happens all to often.
The specs on the meter are as follows:

200 ohms
500 uA
100 mV

Three ranges
0-5 Volts Bias
0-50 Volts Bias
0-100 mA VR Current

There s not a shunt directly connected across the meter. The spool shunt resistors are mounted on the board along with the 1% scaling resistors.

Replacing the square Meter is a real challenge, since the meter is the mounting for the resistor board and these meters were not used beyond the late 50's. The were replaced with the round sealed units that have a separate mounting frame for the resistor board. one for the 539B could be used, but you would have to exchange the meter scales.

The meter is the same as the square ones used in the 539B's electrically, but the scales are different 0-10 and 0-50 Bias Volts and 100 mA.

If you just want to be able to use the tester put one lead of your digital multimeter in either of the cathode jacks located on the top left of the unit. The other can go in any tube socket pin designated by the selector switch for the grid. Like for a 6L6 where the selector grid is 6 count counterclockwise on any socket having at least 5 pins.

A more permanent to use your digital multimeter is would be to mount a couple of pin jacks n the front panel like they did for the AC Meter. These jacks will need to be connected to the bias pot one on the common (bottom of rotation) and the other to the center terminal of the bias pot.

Of course it would be possible to replace it with digital, but keep in mind the meter will need to be configured to match the Analog meter. You would also have to fabricate a power supply and some kind of a mount for the meter board.

As you can see there is nothing real simple, but hopefully you can get some idea of the challenge.

No need to hesitate and asking more questions as I or someone else will surely respond.

Bill


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Tue 11, 2019 6:42 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4385
Location: Sunbury, Ohio 43074
Some of the symptoms you note might be due to intermittent switch contacts

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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Tue 11, 2019 11:58 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
This afternoon, I took the tester panel loose from its cabinet. Testing things with my VOM, I was so clumsy, I could. not get repeatable results. So, reading & heeding the description of measuring from the wiper terminal of the bias pot to the zero side, I attached test clip leads to those terminals and brought them out to my Fluke 27/FM military meter. The two scales move from 0 to slightly above their needed high points of 5 and 50. Very smooth, glitch free, I was happy with that.

Before I drill the panel for test points I'd like to ask where is the best place to put them.

Still. hoping to get these two questions answered: (1) with the Daven accessory disconnected, how should the shorting bar be stored? (2) When the Roll Chart says "6 Lo" what is meant? there is a 600 and a 6000 position on the "Low SHUNT" (right side) switch. Is it one of these or ????

Meanwhile, while I work up the nerve to pull the square meter for disassembly, I'm going to explore the possibility of a dedicated DMM.


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Wed 12, 2019 3:04 am 
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Joined: Aug Wed 31, 2011 11:23 pm
Posts: 486
Location: Tucson, AZ
I initially used a pair of pin jacks for the bias meter (see photo), but eventually put in a pair of meters to measure the bias voltage and plate current. You need to use isolated power supplies for each meter. I am using calibration standard set to a Gm or 1500 on the 3000 range. A number of other mods to this tester.


Attachments:
Hickok 539B.JPG
Hickok 539B.JPG [ 335.37 KiB | Viewed 412 times ]
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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Wed 12, 2019 3:48 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
Very nice. Thank you for sharing.


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Thu 13, 2019 6:41 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
Still hoping to get these two questions answered:

(1) with the Daven accessory disconnected, how should the shorting bar be stored? What is it used for?

(2) When the Roll Chart says "6 Lo" what is meant? There is a 600 and a 6000 position on the "Low SHUNT" (right side) switch. Is it one of these or ????


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Thu 13, 2019 10:44 pm 
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Joined: Jan Thu 24, 2019 1:22 pm
Posts: 121
Location: Centennial, Colorado 80122
Low 6 simply means low signal 6000 range. The jumper to its position across the two cathode jacks. Its only removed when you want to dial in a cathode bias resistor for some frame grid tubes. By the way I would suggest you pick up a manual if you plan on using this tester.


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Fri 14, 2019 4:23 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
Thanks Bill.

I had not checked the BAMA/EDEBRIS site because I assumed they would not have it but sure enough, my exact model was avalable as a nice scan of the original Hickok manual.

I'm still researching the meter issue


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Sat 15, 2019 10:05 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
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How good is the KS-15750-L1 guys? And how hard is it for a newbie to tt to calibrate??


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Sun 16, 2019 12:43 am 
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Joined: Jan Thu 24, 2019 1:22 pm
Posts: 121
Location: Centennial, Colorado 80122
Here is the url for the calibration instructions Hickok provided in 1964. http://bama.edebris.com/manuals/hickok/1575/ The RD1575 is exactly the same as the KS-15750.


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Thu 27, 2019 4:39 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
Bill, et.al., looks like I am dead in the water.

I installed two female banana jacks on the upper left side corner of the unit to measure the Bia voltage direct. Seems to do the job although both ranges over shoot the stated max. The 5 V position can read up to 7 volts and the 50 volt position can read upwards of 60 volts. Still, I can set the rheostat for the desired Volts and that should suffice.

So, I tried to test a tube or two and nothing. The large meter does not respond. The pointer jumps just very slightly when switches are pushed in but it doesn't read at all.

I did download the RD1575 doc which has what is called a "calibration procedure" but it does not. It lists a couple of pages of check points and what should be correct measures at those points but it is void of any information on how to align the unit. I've spent a couple of days wasting all why time looking for clues and more information and I have had enough.

I think I am clearly over my head in this. A couple of years ago I sent this unit to the California bandit who regularly advertises repair service and more or less got scr____d just getting an estimate. So not going to ask for anymore recommendations. But, can anyone venture what this unit is worth? AC reads & adjusts and meter works fine, bias works now, all other switches. fuses & charts are fine, just no output meter. Maybe a decent eBay sale is the way to go and then put that money towards a working unit. Or does anyone have a working tester for trade?

Thanks,

Doug


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Thu 27, 2019 7:48 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4385
Location: Sunbury, Ohio 43074
What part of the country are you located in? Perhaps someone nearby might be able to assist you. Not sure what they are selling for these days, but it'll be a LOT less if it's not working. They should not be that difficult to fix, but of course, that depends on your level of experience and troubleshooting skills.

Don't give up yet.. it's a dandy tester when it's working

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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Thu 27, 2019 9:48 pm 
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Joined: Jun Fri 07, 2019 4:52 pm
Posts: 22
Hello Barry,

I'm in Baton Rouge, Louisiana. Easy access to New Orleans and southern LA in general.

I also have a Hickok Model 600 that has a flaky (intermittent) main output meter.


Doug


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Thu 27, 2019 10:03 pm 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1148
Location: Brooklyn, NY 11217
I would go through ALL the spool resistors. Sounds like you have issues there...


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Thu 27, 2019 11:30 pm 
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Posts: 22
You're referring to the KS-15750-L1 ?? The spool resistors are those sewing machine size donuts which are stacked on a screw?


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 Post subject: Re: Hickok KS-15750-L1 TubeTester.Bias Circuit & Calibration
PostPosted: Jun Fri 28, 2019 12:40 am 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4385
Location: Sunbury, Ohio 43074
Yes, those are spool resistors. They are just a wood bobbin with resistance (or even sometimes regular) wire wound around them to the desired resistance. Sometimes one of those will open up due to either a bad solder connection on the end, or a break where it is soldered to the lug on the bobbin. You can unwind one turn, clean the wire, and resolder if necessary.

Spool resistors do fail this way, but not really all that often. But it's easy enough to check. If one is open, you can be sure it's a culprit. If not, and the resistance is way off, chances are pretty good that there is some other circuit element in parallel messing up the reading. Wirewound resistors don't change value much, if at all, so you should read pretty close to the factory resistance.

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