- 2024
- Sep
- 12
The McMurdo Silver “Sparx” Model 905 Signal Tracer - Update
I decided to sit down with this device last night - the intention wasn’t to fix it, but to clean up the pilot lamp wire and get it prepped for a little more troubleshooting this weekend.
Whomever added the pilot lamp did a great job mechanically, but a terrible job electrically. I didn’t take a picture of the “before,” but this is the “after” wire that was removed:
The pilot is tapped off the filament voltage for the eye tube, and all filaments are in parallel. The tube socket for the eye has two eyelets for each pin, so you can do exactly this - but the person just kind of tacked the wires into the holes with a big blob of somewhat cold solder. It worked, but there was pieces flying everywhere and enough solder to do all the pins. I removed that, wicked it clean, re-seated the filament wires for the tube, and continued with my cleanup.
I’m using an LED #47 replacement to keep the current draw low - the device wasn’t designed with a pilot in mind so I want to keep the transformer happy. A 2.7k resistor of the proper vintage and some small heat shrink does the job - the resistor is needed because the LED bulb is blindingly bright, and I wanted to cut it back to a more reasonable “I’m on but not lasering your eyes” brightness.
I probably should have J-hooked a longer wire on to the resistor and moved it closer to chassis to keep AC from flying around, but it’d not enough for me to worry about. There are other filament wires winding all over the place, the little bit of emission from the resistor isn’t going to do squat.
While I was sizing the resistor (I have a junk bin of older parts) I bumped the input 6C4, and the unit crackled to life. I finished up my resistor installation and hooked the signal generator up - 1kHz came blaring out of the speaker. Turns out the sockets are a bit flaky, which is not a surprise for a device that’s looking 80 years old square in the face!
The speaker is a bit rattley, so it (and the filters) will need to be replaced. The screen also needs to be knocked out, so I’ll do the speaker and screen at the same time - assuming I can find a properly sized speaker, that is.
Beyond that - and after a liberal dose of Deoxit to quiet the moving parts - the fire bottles are full of fire and the device is happily spitting out audio from a crystal radio.
A new cord, some maintenance, and this guy is going out in the main room to be used as a small amp. Strap on your cap gun and get your mask, the Lone Ranger is on soon!
- 2024
- Sep
- 3
The McMurdo Silver “Sparx” Model 905 Signal Tracer
I was recently browsing a local antique mall for an unrelated purchase when I came across a booth with a lot of electronics. Near the back was some test equipment - this unusual signal tracer being a part of that display. It was cheap enough, so I picked it up, noting the name and asking where I had heard that before.
Turns out that this was made by McMurdo Silver, a company known for producing visually stunning and electrically over-engineered radios. I’m not going to go into those here, but there’s plenty of information online as they have quite the fandom.
This is just a signal tracer. It would have had a crystal diode probe at one time for tracking down audio in a RF signal, or simply following it through the amplifiers after demodulation. It does all the usual signal tracker things, like amplifies, gives you a visual indication via the eye tube, allows you to tap the plate and B+ of the final amp, etc. Nothing unusual here, other than the manufacturer.
Tube lineup is pretty simple, consisting of a 5Y3 rectifier, two 6C4 amps, a 6AK6 power final, and the 6E5 eye. The 5Y3 is somewhat hard to get, and the 6E5 is unobtanium. I’ll probably try and pick up some spares at Findlay, if they’re available.
Beyond that, you can see a little of McMurdo’s over-engineering in this device as well. The lever function switches on the front are pretty stout for consumer gear. But the back is another story, the cord simply goes into a large hole where you could easily reach in and touch the transformer taps!
The front panel is sparse, but there’s an anomaly here. The literature for the unit shows a lamp above the speaker. This one has the lamp off to the side - but is it original? I have not opened it up to see how it’s connected. The schematic for the device doesn’t even seem to show a lamp!
I did give it a quick try. It lights up. There’s no hum of any sort, so I’m not sure if it works, but fortunately I have another signal tracer to check my signal tracer with!
I was sent some literature on the device, so if this is yours and you don’t want it here, or need attribution, poke me on mastodon.
An article from Radio News, published in 1946. Note the lamp placement on the front panel.
“Sparx at work” from Radio Service Dealer, 1947. This article talks about using the instrument and it’s probe construction.
There’s not much more to say about this device other than stay tuned - I’ll fire it up some weekend later this year and see what it does.
Well…either that or I’ll fire it up immediately after I wrote this post. No hum, but I get noise when I switch the output selector. I assume the speaker and output transformer are working, at least. There’s a dozen things this could be, so it goes on the bench to work with right after the other signal tracer gets rebuilt.
I did take a minute to investigate the lamp placement. It appears this was added after the fact, but not that long after as the wire type matches what’s in the unit itself. This will need some cleanup as well, as it wasn’t installed in the best manner.
- 2024
- Aug
- 14
Has it really been 30 years?
It’s been a while, hasn’t it, old friend? I didn’t think you worked anymore, but it looks like you’re still sitting there patiently waiting for input. Your power connector is loose, so we need to see about that, but I’ll see if I can find your manual and make some magic.
- 2024
- Aug
- 4
The dial lamp can be more important than you think.
You’ll notice there’s a #47 lamp (Part# LM1) in the circuit for a lot of transformerless sets like the S-38C. It’s fed by a special tap on the rectifier tube:
It’s just a light bulb, right?
Yes and no. If you look at where it’s at in the circuit, it’s actually helping feed input current to the plate of the rectifier. This has the effect of both providing said current for the tube, and limiting the current available to the tube - you can draw about 60mA in this configuration.
So, do you need this bulb?
Again, yes and no. There are configurations where the tube is fed directly from the AC line, and this is a perfectly acceptable method. However, in our case of having the bulb, you have a bit of voltage drop to the plate which results in a lower output voltage, so you’d need to take this into account if removing the bulb - i.e. you need a resistor for some voltage drop. There’s also the fact that it is providing the AC current input - if you remove it, all of your current is now being sourced by the connection from the heater, and you’re stressing the tube. The radio won’t quit - but it just told you “Hey, I need service.”
In this case, the bulb is a component of the circuit that does more than provides light - it’s ballast for the rectifier and is providing a path for input current. It’s as necessary as the rectifier itself. You can’t replace this with anything other than a incandescent bulb without modification, so don’t grab an LED replacement for this one.
Fortunately, places like Antique Electronic Supply sell packs of 10 for under a fiver, so you can pick up a bunch and drop them in your parts bin. You probably want to do that anyway to get one that isn’t more tungsten boil-off than light bulb.
- 2024
- Jul
- 31
Replacing the filter capacitors in a Hallicrafters S-38C
One of the things I picked up at Dayton this year was a really nice example of an S-38C multi-band radio. Completely untouched inside, it of course has bad filters. That’s par for the course for one of these. (It probably also has some silver mica disease, but that’s another day’s problem!)
Replacing them is easy enough, but it all depends on how you want to do it. Some prefer to keep it original, opening up the can on the multi-section device and putting new components inside. Others just wrap capacitors wherever there’s a place to put them. I’m thinking of going a different direction with this one.
Most of Hallicrafters low-end transformerless sets used similar multi-section capacitors and dropping resistors.
(Schematic from the Riders Manual.)
Some, like this one, used a 220Ω and 1kΩ resistor, others (like the S-95) used a 470Ω instead of the 1K.
So…why not create some sort of universal PCB that can drop in and replace all of the filter capacitors, sans the cathode bypass capacitor used in the audio stage of some of the units?
I had thought of using axial devices, but those can get expensive. What’s not expensive, and are compact and readily available? Surface mount parts. A small board could be created with three capacitors and space for the two dropping resistors, pre-assembled (or partially, depending on what you’re trying to fix,) and either double-side taped in or bolted down. The board wouldn’t need to have any through holes, wires could simply lay on pads and be soldered down. 5W resistors and 160V capacitors exceed original specifications, and would be easy to install on the board.
There’s certainly plenty of room in the device once the original can has been removed.
Wow, those feet are in bad shape…need to find some replacements!
I know that idea will offend some purists, but that’s not my concern here. I want to get the device back up and running with good quality parts that fit in the space required, without drilling a lot of holes or simply shooting parts all over the place.
I just need to brush up on my PCB skills, and lay out the board for sizing. Stay tuned!
- 2024
- Jul
- 17
A cheap Chinese radio kit.
I picked this radio kit up from a popular import site. I’ve seen a couple of them built on random videos, and the price was right (under $10 shipped…) They work, but yeah, it’s cheap.
The kit consists of a case, board, parts, and “instructions.” No alignment notes or anything, so I hope this thing is 455Khz…
The instructions are literally just a schematic and parts placement diagram of sorts. It assumes you know how to align the radio, I guess!
If I get around to putting it together, it will be a bit of a challenge. Who knows when I’ll get to this.
- 2024
- Jul
- 7
The RCA 151 Oscilloscope
This is probably the second oldest piece of equipment that I have in my collection, but this one is known working.
The RCA 151 Oscilloscope (or, oscillograph as it was known at the time) was one of RCA’s early test equipment products, and was supposedly one of the first scopes that was considered “affordable” enough for a small radio shop or well-off hobbyist to purchase. It’s not calibrated in any way, shape, or form, and doesn’t have the ability to lock on a trace - it’s just a purely visual indicator to see waveforms as they travel through a device. It has a top end of about 15Khz, and utilized a 1” RCA 913 tube for it’s display.
Of note here is the overall good condition of the device, both inside and out. I don’t think it was used much. Also of note is the input jacks - I believe these were biding posts at one time, but someone replaced them with banana jacks. Note the burn mark on the right where a hot lead hit the chassis instead of the jack!
The device (and others like it) were made in Canada. This one is unusual in that it was meant for the Niagara Falls power standard of 25Hz - a system that was available until 2006, when equipment failure wrote out it’s final chapter.
It works fine on 50 or 60hz, the 25hz functionality being provided by some extra filter capacitance. As far as I can tell, nothing else changed.
If you’d like to read about that power system, the IEEE published a whitepaper on it in 2008. You can retrieve it from their website by clicking this link or, in the event that they no longer offer it, by using this link that will download it from my document storage site. This is a PDF document.
The cord and grommet appear to be original, from pictures I’ve seen. The technician plug, however, is probably fairly modern - no earlier than the 60s.
So, let’s get some stuff out and try it! My trusty CSC generator has outlived pretty much anything else I’ve had, so we’ll set it up.
The display tube itself isn’t very bright. It’s just enough to see in a lit room, but works well enough. I don’t think this is age, I just don’t believe these were very bright to begin with.
It doesn’t lock because there’s no sync of any sort, so you have to mess with things to make it stable - and then it generally starts rolling again. As said, this was simply to show you the waveform.
What’s inside? It’s a relatively simple circuit consisting of a Type 80 rectifier, two 6C6 for amplifiers, and an 885 Thyratron for the sweep - essentially this is like SCR sweep in a late Sony CRT Television.
Everything is in really superb shape here - no rust and very little oxidation on anything. This thing must have been a prized piece that sat on a bench.
Note the capacitors - that big box capacitor in the middle has been spliced in and the joints coated with old-school cloth electrical tape. I would assume that RCA would have used their own brand parts at this time, and not Aerovox capacitors, but I’m not sure - but this most certainly was repaired at some point. The old dry Aerovox ‘lytics are still good, to this day, so whatever technician made that choice - I salute you, sir.
The bottom shows the parts of the era. Dogbone resistors and lots of capacitors here with minimal marking. It’s all on a terminal board, so completely rebuilding this would be easy, if it was necessary. I don’t ever plan on using this thing on a daily basis- so it’s not.
There’s more of those Aerovox capacitors. Maybe that was original? Did RCA just take an off-the-shelf unit and splice in an extra capacitor for the 25Hz model?
The entire thing has that “smell” of old electronics. You know what I mean.
Last - the paper label inside is still intact.
Time to put it back together and back on it’s display shelf.
There’s plenty of information out there on this device, including ads and manuals. A good source for that information is located at The Oscilloscope Museum, which displays it’s own example of the device. An advertisement for the device (also from the aformentioned site, check it out!)
_
My device cost $44.95 - that’s $1015 today!
Maybe not so cheap for a well-off hobbyist, but certainly invaluable to the radio shop…
- 2024
- Jul
- 5
The Westinghouse 54884B Watt-hour Meter Standard.
This is probably one of the oldest pieces of equipment in my collection that could sensibly be considered “electronics.” This is a watt-hour meter made by the Westinghouse Company of Pittsburgh, PA, and was designed to be used as a calibration standard for other measurement systems.
While the case itself has seen some better days, it’s still relatively intact for a device from 1919. A few cracks, and the hinges were missing when I got it. It has a lovely brass nameplate on the top:
I love the patent dates here. That’s 1895.
The actual range is selected by a big knob in the middle. There’s no clunk of detents, but the knob has two pointers. They’re lableled “1” and “2” - the knob turns all the way around so I assume that it’s a brush inside the device riding on something that connects a shunt for the current portion of the meter. Not sure at this point, however. I have to assume that the numbers are for the incoming voltage range - but again, not really sure.
The really cool thing about this device is the instructions and original calibration card are still in the lid:
And even more cool? There’s a calibration sheet with notes from 1930 and 1933 on it:
But the million dollar question here is does the thing still work? No idea right now. I need to open it up and make sure there’s nothing inside that’s broken first, and do a little study on the instruction card. For now, it’s just a neat display piece, but could potentially be put into service in a pinch - if the innards are still good!
Stay tuned for pictures from inside the device.
- 2024
- Apr
- 29
An Eico 145 Signal Tracer - Part 4
Not much this time, just some prep work for the big stuff.
First was to unsolder and remove the RF jack on the front and find something to put in the hole. This piece of plastic from…who knows…almost fit and will be secured with some RTV later in the project.
You’d never know, but the case was washed.
The handle and some hardware was given an ultrasonic bath to clean it.
The location and year of purchase was noted inside.
And the old, almost hard as a rock cord was removed.
Next is to start removing components. I haven’t decided if I want to do it all at once, or try to do it a part at a time. I can certainly get cleaner results removing everything and doing it all at once. I have the assembly manual, so - probably all at once. Stay tuned!
Next part of this series: https://wereboar.com … ignal-tracer-part-5/
Previous part of this series: https://wereboar.com … ignal-tracer-part-3/
- 2024
- Apr
- 23
A Radio Shack 28-177 Crystal Radio Kit - Conclusion
After returning home to my high-signal area, I hooked the crystal radio up to it’s antenna, and gave it a ground through the building to earth. As expected, several stations were available for listening.
As for the kit itself, it performs pretty much like you’d expect. Selectivity is poor, but the stations I did receive came in loud and clear. The only problem I had was the earpiece appears to have a broken connection somewhere - not surprising, seeing as it was coiled up for 30+ years and is probably a little crispy. It’s not like those were high value items to start with! (They’re still available if you want to pick up some from a well-known auction site on the bay.) I simply grabbed the trusty signal tracer for my listening pleasure.
Overall? Works as expected, and I had fun putting it together. Now to listen to…eh…nothing much of interest. Oh well.
