An EICO 950A R-C Bridge Part 2: Parts!
Tuesday, April 7, 2026 at 06:04:23
Welcome to visitors from blogosphere.app and lobste.rs. This blog is a personal account of the old tech? junk? I work with on a personal basis. This is just for my own education, not to make money or anything else, and with the hope that some of the things I present here will help others in their quests to keep this old stuff going. There’s a couple of pages you may be interested in, the first is the main project hub, and the 2026 Hamfest list / show pictures page. All of these are on the sidebar, including previous years’ pictures.
Thank you for visiting. Now, on to the show:
In the last post, we determined that this device is going to need some serious corrections, aka complete rebuild, in order to be safely functional again. The previous owner, while they used decent quality parts in the unit, attached them in such a manner that I have to wonder what was going on. It’s just…what? You can find the link to that post near the bottom of this page.
Since then, I’ve acquired a few things:
A manual, essential because there’s no schematics online for this particular variant, and a pre-made kit of parts containing all of the capacitors for the device. The latter is more of a convenience thing, but it seems to be a kit of good quality parts at a reasonable, one-off price.
Speaking of schematics:
This is a 2500px-wide image, and should be decent enough to print on an 8.5×11 sheet for use.
As part of the process, I’m going to take all of the knobs off and dump them in the ultrasonic bath. But first, where is the main dial’s limit?
And they all come off easily except the binding posts.
The binding posts need to come out of the chassis to be cleaned.
I’ll unsolder those, clean them, and remount before anything else happens. The case itself, as rusty as it is, will also get a good scrub. I don’t repaint these, I like the used look.
Let’s do a quick test on one of the parts in the unit. This is the main 2.0μF “precision” measurement capacitor, and this one was replaced at some point by the previous owner. Datacode is 70s, but that doesn’t mean anything more than that’s when the part itself was made.
It’s under 2%, that’s pretty good. The new one:
That’s about 1.1%, so this one is marginally better.
The next thing to do is go through my parts and see what I have, and then order some resistors. I already have the big 500Ω 5W part, this is leftover from the Olson TE-189 Rework project. That device and this one share very similar schematics - but then again, it’s not like there were many ways to do this type of thing with line frequency and a tube…
A quick note on parts here: In tube devices, you’re often dealing with voltages much higher than you’d encounter elsewhere - this unit, for example, has 540+ VAC on the transformer output. Not only should you take caution, but you need to be aware that certain parts must have certain ratings. Capacitors, sure - but resistors as well. Take this 30kΩ resistor for example. This is R7 on the schematic, and takes the full brunt of B+. This can be in excess of 700V under the right conditions, and this will destroy a normal resistor.
It’s hard to see in this picture, but the resistor is longer than others - this is to provide more isolation between B+ and the load / ground / etc. In this case, make sure your resistor is rated properly - I’m going to use two 15k 500V devices to halve the potential on each, which should reduce risk. I’ll do a better comparison once the parts are out of the unit, including a measurement post.
I have all of the parts ordered, but the Cuyahoga Falls Hamfest is this weekend and I am going to see if I can get a few of the components there. If not, I’ll order parts and we’ll start the rebuild in earnest. Stay tuned!
Next part of this series: Coming soon
Previous part of this series: https://wereboar.com … -did-they-do-to-you/
Wrapup and final thoughts: Coming soon.