Germanium diodes are a staple of electronics. It appears as the detector in both radio and television, the tone chain of musical instrument amp, and plenty of other places. This part offers a low voltage drop, low capacitance, and relatively high speed. They used to be cheap and everywhere, and still have plenty of uses - especially for experimenters.
Unfortunately, germanium diodes aren’t common these days, and aren’t made in any great quantities. There’s also the online hysteria surrounding the part, where unscrupulous sellers are more than happy to label a schottky diode as germanium. While these are fine for detectors in radio, they aren’t germanium and if you’re expecting the properties of germanium, you won’t get it.
However…
The USSR made germanium parts right up until the end. There’s still millions of germanium diodes out there from this era, and, while even those are starting to get harder to get, you can still get them relatively easy from online sellers and auction sites. These diodes are perfectly good germanium parts, and came in many different styles and ratings. For most experimental purposes, there’s no difference between parts, and this isn’t going to discuss that. There is, however, a singular difference between Western and Soviet parts.
Here’s an old GE 1N60 diode. This image came from…the Internet. Somewhere, I’m sorry I don’t remember where.
Note that the package states the cathode, i.e. negative-most terminal, is denoted by the band, or the bar on the graphic symbol (the diode symbol) printed on the glass. In this case, the cathode is pointed towards the left. This is the way Western diodes are marked, and is the way pretty much every diode made today is marked. There are some exceptions, but there’s almost always some identifier to indicate what’s what.
Here’s a bunch of diodes.
I’m going to test these parts. We have, from top to bottom:
A traditional 1N34A glass diode.
An unknown germanium - this may be a 1N60.
A Soviet type D9A.
All of them have well denoted bands. I’m going to toss in a 1N4007 Si rectifier diode (not shown) as well, just for comparison.
Here’s the parts as viewed through a magnifying glass. Of particular note is the Soviet part - you can clearly see the flying lead and the point of contact on the germanium crystal.
1N34A
Unknown germanium
Soviet D9A
Testing the diodes
What’s the purpose here? Well…the Soviet part has something interesting about it. If you’re familiar with how the diode works, you’ve already noted the issue.
Let’s test the parts, I’m going to be using my old reliable, a meter I purchased many many years ago. It has a diode function that tells you the voltage drop across the diode’s junction.
For diodes, these are the “perfect” theoretical voltage drops across a junction:
Silicon: 0.7VDC
Germanium: 0.3VDC
In reality, it’s closer to 0.5VDC and 0.23VDC, respectively. Let’s test the parts on the bench. First one is the silicon diode, for comparison.
Note the negative lead of the meter is on the banded side of the diode, so we know that’s the cathode. We see the expected 0.5xxVDC drop of the junction.
Here’s the 1N34A:
That’s in line with the expected drop.
Here’s the unknown germanium:
Again, the expected drop.
Here’s the Soviet D9A:
It’s correct, that’s zero. There’s no drop across this device, indicating the unit isn’t conducting. It’s connected correctly, isn’t it?
No - Soviet diodes put their bands on the anode. That is, they are marked completely backwards from what we accept as diode marking. Here’s the device properly biased:
There’s the expected drop. Completely backwards from what we expected.
Why is this important?
Soviet germanium diodes are the most common Ge diode available at this time, so they show up in a lot of places. Radio kits, fuzz boxes for guitars, small signal rectification circuits - anywhere a diode with it’s properties are needed. For a radio kit, it’s not really terribly important which side of the information you recover from the carrier, but if you’re trying to rectify a signal it’s very important. You need to make sure you install them correctly, and if you follow “accepted” conventions you’re not going to accomplish that goal.
Measure your part - that’s the best way to verify what you have.
The first ‘fest of the season has come and gone. Notably, the dearth of CB radios from the past few years has started to fade. Unfortunately, all of the older stuff has started to fade away. There was still some interesting things to be seen, and I picked up a few interesting things for later projects and checkouts.
A Commodore 64, now 40+ years old.
These machines were not cheap, that's ~800$ today.
A unique clock kit. I took it home.
I don't know, some…thing with cool meteres.
cf2026-diskettes-wereboar.jpg
cf2026-equipment-wereboar.jpg
Another table of random things. These are getting rare.
OSHA? No sah!
A homemade flight sim rig.
Nothing much to hear these days.
Just junk on the floor for your parts needs.
People love those deaf-as-a-post Knight radios.
Some old meters. I took the Keithly and Heathkit.
A Hong Kong special. Probably deaf when new.
I never understood those weird Mac packages.
There's the orange “T” from Coshocton.
A lamp for your porch. Welding goggles not included.
A box of probes. I took the “EICO” style for my tracer.
Rat Shack Radios for days.
These used to be $10 all day. Not no mo!
A nice RCA radio.
A 27MHz RF curing machine. Could be someone's new linear.
Look at the size of that tube.
The business end of the cure. Cures…life, probably.
Almost got this and the next one as a real challenge project.
In bad shape…
Gimme one of those S-Pecans.
These are always a lovely piece of history.
We topped the day off with a stop at Arthur Treachers’ Fish and Chips, and then headed home.
Next up is Dayton, I’m planning on going all three days this year. As a reminder, you can still mail order a ticket - internationally until Wednesday, and domestically until May 1st. See you there!
The cutoff date to purchase tickets and have them mailed to you is rapidly approaching. For international orders, you have until Wednesday, April 15th to get your order in. For domestic orders, you have until May 1st. After that - tickets are held will-call at the door. If you’re going, the time to order is now!
As the carbon film resistors on the range switch decided to go way out of tolerance as they were being worked with, I removed them all and decided to dump the switch assembly in the ultrasonic bath.
Watching the dirt lift off was quite fascinating…it just drifted away.
It’s not the easiest to see because the dark wafers don’t show much difference, but they feel different now. The ceramic wafer at the bottom is also much whiter, and some of the green growing on a previous owner’s mod is gone.
Once the new resistors are installed, I’ll give the rings a shot of cleaner-lube, and put some dielectric grease on the detents.
As the King of Hearts said: “Begin at the beginning, and go on till you come to the end: then stop.”
We’ve seen this thing is an absolute mess inside. It was built…ok, but then someone came along later and j-hooked EVERYTHING. I cannot figure out why, even Mister Wizard would probably be at a loss to explain this one. So where to actually start?
First of all, I did some cleaning on the device. The front panel was wiped down with some gentle degreaser, and all of the knobs/terminals were cleaned.
The only problems I’ve run into with the ultrasonic bath and these parts is the white paint on the chickenheads tends to come out - but that will come out with just water and a cloth as it’s so old and brittle. I’ve thought about this, maybe some white ink in a syringe or on a toothpick would be a way to re-mark them. The other issue is the pointer tends to change shape a little when you wash it in warm water. This old clear plastic bent outward - I actually don’t have an issue with this because they like to bend inward and scrape the face up. It is what it is.
Moving forward:
The top of the chassis, of course is a good place to start. This is the easiest (and most open portion) place.
On this top part of the chassis, we have the following delights:
These are good quality parts, being orange drops…but the weird installation method. I assume that the person just hooked in when the old paper poppers were removed. I will remove these, and may use them in this device assuming I can figure out the date of manufacture. I’m guessing 70s, so…maybe not?
Sure would have been nice if I’d put that image here in the first place…
We also have this beauty. Again, a good quality part, put in with meh installation methods. I’ll remove it and put it in the “bench use bin.” Besides…the new part is of better tolerance than this one, which is essential for the circuit this one is in. Thankfully the hole that it runs into was insulated!
Last thing, other than wires, is the eye tube socket. This one, unlike the Olson TE-189 unit, at least has the wires soldered to the socket properly. I’ll clean those up and run new wires and resistors. Nothing critical here, but I’ll use 1% metal films just because they’re cheap.
On mica capacitors
That red rectangle behind the chaos is a mica capacitor - or so we assume. (more on that later) It’s made by stacking layers of mica with metal foil - often with some silver in the mix due to silver’s better than copper and gold conductivity. Most will tell you these never go bad.
Yes and no. Silver can migrate in this type of device. You see this in radios where you get arcing in the IF cans on the little silver-mica capacitors - the silver oxidizes and creeps out, changing the value or even shorting. It’s a problem and will only get worse as time progresses.
So these can go bad. They can get silver migration, called Silver Mica Disease. They can get cracked cases and allow moisture ingress. The coatings can be suspect, collecting water or dirt that allows leakage. Just because you haven’t run across a bad one yet, it doesn’t mean you won’t have a bad one later. There are plenty of reports of people seeing these start to go funny in the 90s, and we’re now 30 years past that point. These devices are now 80 years old.
Why does this happen? It’s a combination of many things. The resin gets cracks in it you can’t see. These cracks, or the resin itself, allows and traps moisture. When you apply a high potential across the part, the moisture allows silver ions to start dissolving and moving towards the opposite potential. This creates dendritic growth that both changes the capacitance of the part, and eventually (and that’s a long eventually) it will short and die. If the case was improperly sealed, then the danger is double because then you have a known ingress point for water.
There’s another issue. Is this really a mica? Some manufacturers packaged paper parts like this too, thinking the same thing that later manufacturers did - that the coating will protect the paper, already known to be a problem item. It did, but you still get water ingress over time, and that’s all she wrote on that piece of soggy paper.
How about the part in this one?
It reads, value-wise, ok. Does it leak? I won’t know until I get it out, and it may not after I hit it with heat. Who knows?
The kit I bought for this device doesn’t have this part in it for the above mentioned reason of “these don’t go bad.” However, as I was told not that long ago: “You found this car in a barn, all hoses and belts are original. Do you want to drive it, or look at it?”
I want to drive it. Therefore…I’m getting an order of resistors and some other parts together for the unit, and will put a capacitor in the cart as well. I’m removing pretty much every other passive in this unit because it’s out of tolerance, is of poor installation quality, or is just bad. I’m not going to skimp on a single $4 part.
If you’ve worked on tube devices, you may have seen a resistor that looked like a higher wattage part, but was strangely long. For example, this 30kΩ 20% part in this EICO 950A RC bridge. It’s somewhat longer than other carbon parts:
Why is that?
Simply put, it’s for voltage ratings. This particular circuit has that 30kΩ part bearing the brunt of B+, and the entirety of 500VDC can be across that resistor A smaller carbon resistor would arc across and flame out.
Here’s a couple of examples. The first is a comparison with a 1W resistor.
And a comparison with a 2W reisistor.
For the 1W comparison, the higher voltage part is 16mm long, whereas the 1w part is 13mm. The 2W comparison offers a 19mm higher voltage part, and a 17mm 2W device.
This is purely to provide physical spacing between the two leads. More space, less chance to arc. If you see one of these unusually long parts, there’s something special about that circuit, and it demands your attention.
This is something that I just learned not that long ago, so…now you know!
A “crystal” radio, or an AM radio that works without any kind of power other than that generated by the radio signal itself, is one of the staples of basic electronics. It’s something that’s always fascinated me, and while I’ve never wound one by hand I’ve built a few kits over the years and generally enjoyed them. It’s so named because originally, these would have used a chunk of some natural semiconductor like galena to make the detector - but in a modern sense we use a small crystal of germanium in a 1N34/1N60 type device.
This particular device is sold on a popular auction site, and is offered by the creator. What’s in the box?
Packing material! Just what I wanted. No, not really.
A couple of sheets of instructions, a couple bags of parts, and a PCB.
We have:
Two variable capacitors
A germanium diode of type D9K
An earphone jack
Two screw terminals
An inductor
and, a Resistor.
In the other bag, we have:
Two cut clipleads
A Piezoelectric earpice
Those of you who have been around for a while will recognize that earpiece as the same one included in every Radio Shack kit ever made. There are lost tribes using these as jewelry somewhere.
Also included is this PCB.
The board itself looks nicely made, and uses the copper as part of the inductance for the circuit. The only thing I see right off is the diode is going to be a tight fit with the leads bent down right at the body of the part. That’s not really good for a glass part, and hitting that with heat can cause damage - especially if you’re not careful to get in and out asap.
I spoke with the creator about the diode, he indicated that the lead spacing was widened in the latest batch.
But, I think it should be ok. We’ll see, I’m going to build this kit within the next few weeks. There will be one more post about the building and testing, stay tuned!
The Cuyahoga Falls Amateur Radio Club hamfest is happening this weekend. Located in it’s namesake of Cuyahoga Falls, Ohio, this show occupies most of the floor space of an old grocery that was converted to event space. I’ve always been able to get some unique stuff from this show at good prices, even during what I would consider bad years.
You can also visit what was up until last year, one of the few remaining Arthur Treacher’s restaurants. It’s literally down the road a mile or so from the event.
I’ll have pictures from the event posted ASAP.
See you there!
Cuyahoga Falls ARC 70th Annual Hamfest
Emidio & Sons Party Center
48 E. Bath Road
Cuyahoga Falls, Ohio 44221
Saturday April 11 2026
8A-1P
Admission $10
Instead of removing the range switch from the good unit and trying to rebuild it, I’ve decided to use the switch from the parts unit. It has some modifications to try and correct poor connections, but a good cleaning with Deoxit on the wafers seems to have taken care of this.
In order to try and save a little time, I measured the components already on the switch. There are quite a few of them that fall within tolerance:
Quite a few of them, in fact.
However…
These are most likely old carbon deposition resistors, aka carbon film. The minute I hit them with a soldering iron to remove their out of tolerance neighbors, they started to change. A lot. I kind of expected that but was hoping it wouldn’t do that. So…
They were all removed and will be replaced with new metal film parts. In the meantime, the switch has a date with the ultrasonic bath, and will be re-greased once done.
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!
