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 Post subject: EICO 950 expanded ranges
PostPosted: Nov Sun 03, 2019 1:18 am 
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Joined: Mar Sun 20, 2016 11:04 pm
Posts: 655
Location: Mesa, AZ 85206
Hi All

I pulled out my Eico 950B to tune it up so I can pass it along. I had been using it principally for leakage testing.

All caps and resistors test fine, and C4 is accurate and does not leak.

Well, both the expanded ranges on the cap and resistor are wonky...the eye is small at the low end, and larger as the pot is turned...there is never a point where the eye is maximally open close to the value being tested. (these are switch positions 4 and 8 on S1)

All other ranges and comparator functions are good.

Theoretically, the only difference is in the addition of the 250K resistor R2. It tests fine...and as far as I can discern the wafer is working with good contacts for this resistor.

This has been addressed before (https://www.antiqueradios.com/forums/vi ... 8&t=307819) but no answer was found there.

Interestingly, the C4 cap is connected to #3 on P2 (the on/off and power factor), and #3 and #2 on the unit are joined....this looks opposite to the schematic, but this unit appears to have been factory made in 1971 (has inspection signature). I have attached the general block diagram as well...it is #3 and #5

I am quite annoyed with myself that I could not solve this in the last few hours....

Thoughts?

Thanks!

Steve
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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Sun 03, 2019 5:30 am 
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Joined: Mar Thu 15, 2018 2:23 pm
Posts: 422
Try a 90k resistor in place of the 250k at R2. For more accuracy you can substitute a variable resistor at R2.


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Sun 03, 2019 12:12 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9985
Location: Long Island NY
It’s not sufficient for the bridge caps to be low in leakage. Their dissipation (DF, power factor, tan delta, etc.) has to be very low too. If this started happening after the bridge caps were replaced, I’d suspect you got some lousy caps—C4 in particular—and need to try some others.

Another thing to check is the power factor control which needs to be in good condition and the proper value.

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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Sun 03, 2019 2:33 pm 
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Joined: Mar Sun 20, 2016 11:04 pm
Posts: 655
Location: Mesa, AZ 85206
Thanks!

I had already considered the cap and power control...the power control tests fine, and I substituted another C4 just to be sure. Same issue exists.

There is something I am missing...the bridge is acting as if it is approaching the null state for most of the range.

I will throw a 250K resistor in parallel with the R2 later today and see if that changes anything.

Steve


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Sun 03, 2019 7:16 pm 
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Joined: Mar Thu 15, 2018 2:23 pm
Posts: 422
Here's the Eico 950B reference source for R2 resistor...

https://tubesound.com/rc-testers/

Attachment:
Eico 950B.jpeg
Eico 950B.jpeg [ 378.4 KiB | Viewed 1394 times ]


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Sun 03, 2019 8:06 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1658
What this added resistor does is spread the main dial calibrations near high capacitance/short area. Just how much spread you get depends on the added resistor value.

Of course the dial has already been calibrated for a particular value of added resistor so it only needs to be adjusted to match the dial.

If there is a poor null, it's due to imperfect components or some parasitic leakage somewhere. One approach is to short the added resistor and see that the bridge works as it does on the otherwise highest range. If not, there is a switch problem. If so, you can gradually increase the added resistor and see what happens.

If it still makes the null sloppy, there is a problem in the detector circuit. It may be loading down the bridge. One way to check that is to disconnect the detector and substitute an external detector, say a scope or audio voltmeter. A poor null with this test means a bad switch that's loading the bridge.

One other source of trouble might be the applied ac from the transformer. Make sure it's a normal value; I seem to recall something near 50 Volts, which of course stresses the unknown capacitor (or resistor) quite a bit and can cause failure. All these service oriented bridges do this. That's another reason I don't use my 950B.


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Sun 03, 2019 11:42 pm 
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Location: Mesa, AZ 85206
Thanks, Bob (again).. you got me to the home plate.

I shorted the 250K resistor in the exp range and tested a lower value (in the 3rd range) and the meter worked as designed. (wish I had thought of that...)

Replaced the 250K resistor, works now....

So, the old 250K resistor tests fine on the bench, but apparently doesn't work in the circuit....

I have not seen a resistor test fine and not work in circuit, but now I know....

You know, when I was in high school, I called my dad ( now passed away..a mech engineer) when my car's engine wouldn't turn over. I insisted it must be the solenoid or some such....he lifted the hood, gave the battery terminal a shake, and it turned over. He looked at me and said "start at the beginning...it's almost always the obvious".

Steve


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Wed 20, 2019 6:27 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1658
If the 250k resistor added in series with the main pot expands the range to higher values, then wouldn't it expand to lower values if inserted at the other end of the main pot? That should make resolution of small value resistors and capacitors much better.

Accourding to the schematic, the bridge excitation voltage is 56 Volts I think. Add to that the modern power line voltage of around 120 V and you get nearly 60 Volts. This can appear on the unknown device, and cause a lot of stress. A resistor of perhaps 7k Ohms will dissipate a half Watt with this much across it, and more if lower resistance. Of course any capacitor rated for less than 60 Volts will be overstressed.

So be careful that you don't ruin a good part by measuring it with this bridge. The other brands of bridges of the same sort, such as Heath and Solar and Sprague also are like this.

If you want safe measurement, use a laboratory type bridge; they often measure with stresses in the millivolts.


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Thu 21, 2019 12:21 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9985
Location: Long Island NY
Some of the later capacitance bridges like the Heathkit IT-11 and IT-28, and the Sprague TO-6 and 6A, put an amplifier tube ahead of the eye tube. This gives greater sensitivity so the bridge voltages could be much lower. These later testers came out in the "transistor" era and were designed to test lower voltage capacitors found in transistor circuits without damaging them.

It also has to be remembered that the bridge voltage is applied through the range resistors and pot, so the current is greatly limited. Test voltage is not the same as open circuit voltage in most cases. However, the capacitors most likely to be damaged by this kind of abuse are electrolytics and tantalums, and those are the ones you are most likely to want to test!

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

Thomas A. Edison


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Thu 21, 2019 2:21 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8197
Location: Sunbury, Ohio 43074
I wonder if your original 250k resistor was just soldered in poorly perhaps? Or the resistor body was dirty enough to be leaky. Or perhaps someone in the distant past put the wrong type in?

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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Thu 21, 2019 6:23 pm 
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Joined: Mar Sun 20, 2016 11:04 pm
Posts: 655
Location: Mesa, AZ 85206
I just don't know...Heck, I even hooked the resistor up to a voltage source and verified it's functioning via Ohm's Law.

My best guess is that the puppy was not well soldered in.

Stev


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 Post subject: Re: EICO 950 expanded ranges
PostPosted: Nov Thu 21, 2019 10:59 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
How does the bridge perform in camparator mode?

Suggest remove the tubes, connect your two channel oscilloscope,
in differential mode * across R10, and try various pairs of known
similar parts.

See if the main dial will actually balance at the zero.


Then repeat the test, with the tubes in, and see how your
scope null compares with the eye null.

The ground on R10 picks up 60 HZ phase from the filament winding
whereas the bridge null circuit uses the two brown wires brown
54 volt wires from the transformer. Would swapping the brown
wires change the eye null, but not the tubes-out scope null ?

* need to read scope manual for this mode.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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