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 Post subject: Tel-Ohmike Circuit
PostPosted: Nov Wed 06, 2019 4:23 am 
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Posts: 341
Location: Mesa, AZ 85206
Hi All

I apologize if this question is too basic, but I have been looking at the circuit of the TO-5 to understand it's flow.

I am having trouble understanding the role that the 2x12uF 450VDC caps play in the circuit (C3 and C4)..normally, I would be looking at filtering functions, but I am having trouble seeing it.

I have spent time searching the 'net, but have yet to come up with a clear explanation.

I would appreciate some edumacation!

Steve


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Wed 06, 2019 5:58 am 
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Location: Seattle WA US
Well, the pentode V3 is acting as a grid-controlled rectifier tube. Ignore the supressor grid (pin8) and notice that the lead from R3 continues on to pin 7 which is one side of the filament (or cathode) of the rectifier tube. So, this lead from the cathode is the positive output of the rectifier, which is current limited by R3 and continues on to the positive end of filter capacitor C3. The voltage here is up to 600 volts, so we have electrolytic filter caps C3 and C4 in series to provide an equivalent capacitance of 6mfd with a breakdown voltage up to 900 volts. Resistors R1 and R2 are equalizing resistors, selected to be much lower resistance than the leakage resistance of C3 and C4, causing the rectifier output voltage to divide (nearly) equally across the two capacitors. You can ignore the screen grid of V3 as it is connected to the plate, making the tube more like a triode ( even with the supressor grid that we are ignoring also). The control grid of the tube is used to reduce the conduction of the rectifier to provide a low voltage (60 volt) output mode. When the 600volt mode is selected, you can ignore the control grid, too.

Why ? because 1619 tubes were available for pennies in great quantities from WWII surplus and this cut a few cents off of the manufacturing cost of the tester.

So, you were right, C3 and C4 are indeed the power supply filter caps. They were well hidden.

-Chuck


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Wed 06, 2019 6:27 am 
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Awesome! This was extremely helpful; I don't think I could have gotten here on my own.

Steve


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Wed 06, 2019 3:53 pm 
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Joined: Mar Thu 06, 2014 6:01 pm
Posts: 163
drworry,

AS it happens, I just finished repairing my TO-5 and documenting how it works. I sent a copy to Barry Bennet who has agreed to make it available on his site. I don't think it is available yet; however. If you will send me your email address, I will send you a copy of it. In the meantime, here is a sample. Included is a redrawn schematic of the power supply and some color coded schematics with the current paths colored to make it easier to follow.

Regards,

Mike


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I. Fig. 8.JPG [ 118.21 KiB | Viewed 397 times ]
X. Fig 18.JPG
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O. Fig. 14.JPG
O. Fig. 14.JPG [ 131.24 KiB | Viewed 397 times ]
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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Wed 06, 2019 5:42 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8946
Location: Long Island NY
Grid controlled rectifier circuits were around a lot longer than the Sprague Tel-Ohmike. Using mercury vapor thyratrons and ignitrons, the concept was used in industrial control since the late 1920s. Since they had to have some kind of rectifier anyway, it was actually a pretty clever way to control the test voltage in a capacitor tester with a conventional power pentode, but Sprague was not the only company which used the circuit. They had it in the TO-1, TO-16, and TO-2, all of which predate WW-2. Aerovox and Solar had it in some of their prewar testers. Post war, it appeared in the Pyramid CRA-1 and CRA-2, and the military ZM-3/U (with an 807 tube). No doubt some other makes and models of capacitor testers had it as well.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Wed 06, 2019 5:53 pm 
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Joined: Mar Sun 20, 2016 11:04 pm
Posts: 341
Location: Mesa, AZ 85206
Thanks, Mike...I have sent you my email.

I thought about sending Barry a PM on this last night, but I decided it would have been too intrusive.

Learning vintage gear in my 60s as I approach retirement from my day job (which has nothing to do with electronics...) has been a slow process. So, sometimes I feel foolish asking these types of questions here...

Steve


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Wed 06, 2019 8:00 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
Mike, thanks for the reminder. I got sidetracked. Again. I am going to make a section on capacitor testers eventually starting with your TO5, my TO4 and 6, and a few others. I’ll try and get yours posted up tonight.

Steve, the only foolish question is the one NOT asked.

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Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Fri 15, 2019 9:39 pm 
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Joined: Mar Sun 20, 2016 11:04 pm
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I just wanted to add to this thread (in case someone in a search ends here) The excellent work that Mike has done in detailing this circuit and what Barry has kindly agreed to put on his website!

In Barry's own words:

https://www.bbtvtestequipment.com Navigate on the top bar to PHOTO GALLERIES and choose the CAPACITOR TESTERS button.

Thanks!


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Sat 16, 2019 5:49 pm 
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Joined: Jul Tue 21, 2009 1:38 pm
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Location: SW WA state
drworry wrote:
Learning vintage gear in my 60s as I approach retirement from my day job (which has nothing to do with electronics...) has been a slow process. So, sometimes I feel foolish asking these types of questions here...

Steve


Steve,

Don't ever feel bad about asking questions! We're all here to help each other.
After nearly 40 years in electronics, I still find myself asking questions. It's all good.
Every day is a learning experience for me as well.
Best of Luck!

-Tom


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 Post subject: Re: Tel-Ohmike Circuit
PostPosted: Nov Sat 16, 2019 10:10 pm 
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Joined: Sep Wed 10, 2014 7:13 pm
Posts: 567
Location: Urbana, Illinois
Test gear schematics are often very difficult to read and understand the signal flow and functionality of different stages. It’s worse when there are lots of rotary switches. If you can find a full manufacturers service and operation manual it might have a “theory of operation chapter” to explain things. At times I’ve created my own “detail” schematic to help understand how certain sections operate.

-EB


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