It also can check a capacitor under its actual operating voltage, a feature that makes it very handy for restoring vintage tube radios and TVs. After cleaning the controls and replacing a few capacitors, it works like a champ. Below is a photo of the device in action. In the previous photo, I'm measuring a capacitor's value.
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Post a Comment. Total Pageviews. Search This Blog. I mentioned earlier that I had purchased a Capacitor Tester from an online auction, in this post.
Since that post, I've had the opportunity to remove the case and inspect the wiring. I found a really good website explaining a lot about the Eico B here.
If you don't feel like following the link, I'll share this bit - most of these were sold as kits to hobbyists. Here's what it looked like when it arrived. Notice the bottom left knob. You can apply up to volts DC to a capacitor connected to the two right-hand terminals.
That will put a hurt on you if you aren't careful! The Eico and many other vintage instruments uses a bridge circuit to measure unknown values for capacitance and resistance. Bridge circuits are very sensitive to small changes in the electrical values on either side of the circuit.
Below is an example of a bridge circuit. A voltage is applied from a DC power source at the left the output of the rectifier tube in the picture above. Resistors R1 and R3 are a matched pair - of known value. R2 is an adjustable resistor. At this point the bridge circuit is "Balanced", and current flows equally through both branches, from A to C.
The value of adjustable resistor R2 at which the bridge is balanced, will be the same value as "Rx", our unknown. The cool thing about a bridge circuit is how very sensitive it is. Even a minute imbalance between the right and left side of the circuit will cause the center meter to deflect quite a bit. Even though the circuit is simple, there is a lot of amplification going on.
With the Eico , voltage developed in the bridge circuit between points D and B is applied to the grid of the tuning eye tube , rather than a meter in the diagram above. When the bridge is unbalanced, the eye looks closed, below. When the bridge is balanced, the eye opens up at the bottom, as seen below.
At that point, you read the scale where the pointer landed, and now you know the value of the unknown resistor or capacitor. Below, I had cleaned off the knobs, and was on my fourth or fifth wipe-down of the face. There are still some white-ish cigarette tar smears on it. The knobs and posts came clean pretty well. The pointer plastic has yellowed - I tried to clean it and that is not cigarette tar. I pulled the tuning eye forward a little, and wire-wheeled the screws attaching the face plate to the cabinet.
I kinda gave up trying to get all the haze off the face plate. I didn't have the free time available to make it perfectly haze-free. I disliked the mottled look of the face plate, and getting it clean was taking a lot of time.
I finally removed all the hardware from the front panel and got to work with some aluminum polish and a small pneumatic buffer. Labels: Bridge Circuit , capacitor , vacuum tube , Vintage Electronics. No comments:. Newer Post Older Post Home. Subscribe to: Post Comments Atom.
EICO 950B – Repair/Restoration
Recently I have begun what might be the start of a nostalgic journey in vacuum tubes. I credit this addiction to Paul of Mr. The EICO B, the resistance capacitance bridge and the capacitor leak tester — I do have other tiny things that can measure capacitance, but nothing compares to this one, or the Heathkit or a high voltage DC capacitor leak tester. It is a vacuum tube scope and I need a leak tester to check the caps in it. But I digress. I was looking for a decent heathkit or the EICO to help me restore it when I stumbled upon this little treasure.
Users browsing this forum: Google [Bot] and 15 guests. EICO vs B. Posted: Oct Wed 23, am. I have 2 Eico units that I have had for years. Both silver faced and look the sane as b.
Post a Comment. Total Pageviews. Search This Blog. I mentioned earlier that I had purchased a Capacitor Tester from an online auction, in this post. Since that post, I've had the opportunity to remove the case and inspect the wiring. I found a really good website explaining a lot about the Eico B here.