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 Post subject: Weston 981 "Paper Tube" Rectifier Replacement
PostPosted: Nov Sun 08, 2015 3:54 pm 
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Joined: Aug Tue 18, 2015 10:24 pm
Posts: 27
Hello All,

Weston 981 tube testers have a rectifier/diode component in a yellow paper tube labeled "Federal 1170".

The parts list identifies this component as an 8 MA (max DC current output) selenium rectifier, Weston part number ND-26778. The name "Federal" is the Federal Telephone and Radio Company, or "FTR" for short.

The one Federal 1170 rectifier/diode in my Weston 981-2 tested bad using two different multimeters, so I replaced it with a 1N4007.

Now, I'm starting to worry a little about the "V drop" increase of switching from a selenium to a silicon diode. Most of my research suggests that selenium replacement with silicon should be accompanied by added resistance to offset higher output voltage.

At this time I don't have the equipment or the knowledge to measure the current MA in the circuit.

So, I'm wondering if anyone else who has replaced the selenium diode (the Federal 1170) with a silicon diode (a 1N4007 or other) in a Weston 981 has added a resistor to offset the higher output voltage?

In my 981 type 2 tester, the original Federal 1170 8 MA (max DC current output) selenium diode is connected to a 1/2 watt, 1.8 K resistor on the cathode side of the diode. Can any deductions or approximations be made with this information regarding adding resistance to a 1N4007 in that circuit?

Thanks!


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 Post subject: Re: Weston 981 "Paper Tube" Rectifier Replacement
PostPosted: Nov Sun 08, 2015 7:09 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 9388
Location: Santa Rosa, CA
A "bad" reading on a DMM is irrelevant for selenium rectifiers. The forward voltage drop is higher than most DMMs can test.

Rich


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 Post subject: Re: Weston 981 "Paper Tube" Rectifier Replacement
PostPosted: Nov Sun 08, 2015 8:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30698
Location: Pocasset, Cape Cod, MA
What does it do in the circuit?


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 Post subject: Re: Weston 981 "Paper Tube" Rectifier Replacement
PostPosted: Nov Sun 08, 2015 10:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11765
Location: Mpls, Minnesota
This is in the leakage circuit, voltage has to be correct for the meter to read properly. I think it was about 125 volts and one of the resistors has to be adjusted to get the proper reading on the meter using a calibration resistor.

Dave


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 Post subject: Re: Weston 981 "Paper Tube" Rectifier Replacement
PostPosted: Nov Mon 09, 2015 2:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1765
Location: 07450, New Jersey
Here is the explanation of the leakage circuit from the Heathkit TT-1A manual. As has been stated before, the TT-1A is essentially the same as the 981. Seems like the series resistor could be adjusted so that a 250K resistor between two socket pins reads 250K on the meter when testing those pins for leakage.


Attachments:
TT1 Leakage.jpg
TT1 Leakage.jpg [ 130.49 KiB | Viewed 1065 times ]

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Tim

"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne
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 Post subject: Re: Weston 981 "Paper Tube" Rectifier Replacement
PostPosted: Nov Mon 09, 2015 4:02 pm 
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Joined: Jan Wed 26, 2011 5:42 am
Posts: 536
Location: Clayton, NC
I currently have 15 Weston 798, 978, and 981 testers, including a 981-2.

I avoid using anything other than NOS selenium and germanium solid state replacement components in these instruments, as it will be a major headache trying to get it to calibrate and otherwise behave properly. The one you refer to is difficult to get, but not impossible.

It is a good idea to stock up on these components as they become available. The 1N158 rectifier is the hardest to come by, often those that do come around are overpriced.

The Heathkit TT-1 is significantly different from the Weston 981-2, it was actually patterned after the 981-3 (and 978). The 981-2 utilizes the same method that the 981-1 and 798 used, in which AC is on ALL tube elements, and the grid bias is actually pulsed on the negative swing of the 60Hz grid bias voltage. The 5KHz signal used to develop plate current is filtered from the rest of the 60 Hz voltages and sent to the meter for a Gm value.

The TT-1, 978, and 981-1 (and subsequent 981 models) use filtered DC on all tube elements except the filaments.

FYI one place that seems to frequently drift out of spec is the voltage divider circuitry that drop the 5KHz signnal depending on what Gm range you have selected. These are 1% resistors and are readily available on many electronic supply sites.


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 Post subject: Re: Weston 981 "Paper Tube" Rectifier Replacement
PostPosted: Nov Fri 13, 2015 3:05 am 
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Joined: Jan Wed 26, 2011 5:42 am
Posts: 536
Location: Clayton, NC
Correct the following to: 981-3 and subsequent models use filtered DC on all tube elements.


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