Old Radio Detective Work

Posted on March 25, 2011 by jwk

I’m still working on my 1941 Stromberg-Carlson 520-PG and am running into a few new problems I’ve never come across before. Back in the era of tube radios, everything was serviceable on a component level and most people had enough aptitude and desire to learn that it wasn’t uncommon for people to attempt repairs themselves. You also had “radio technicians” who may or may not have been reputably trained and were frequently turned loose on people’s equipment for the lowest bid.

This was definitely the case with my radio, as I’m discovering evidence of someone else having been inside and attempted repairs that were of dubious quality and may never have worked properly in the first place.

It’s pretty easy to tell if someone has made repairs before. Component brands is the easiest method, at least for major-brand radio sets from that time period. Most of the capacitors in this Stromberg-Carlson set were branded with that name, but Mallory capacitors made a couple of appearances too. These were clearly replacements, S-C .01uF capacitor on top and a Mallory .01uF capacitor on the bottom. Making it worse, the solder job was so bad I was able to slide the entire joint up and down along its wire – I doubt there was ever an electrical connection between the wires, even though they were physically fixed together.

There’s also an extra part not listed on the schematic, bypassing the high voltage plate resistor for the audio amplifier tube to ground. I speculate this was done to eliminate some interference from getting into the audio, but it’s another modification that is of unknown quality. The capacitor, the block with colored dots, has started to fail after ~70 years and occasionally introduces some static into the audio as it’s playing. (The capacitor is across R-6 on the schematic snip slightly lower on the page.)

Finally, there are wiring changes made that don’t match the schematic. Is the schematic wrong, or is the wiring in the radio wrong? Schematics of the day were hand-drawn by draftsmen who frequently worked long hours revising and publishing schematics and service data, and mistakes are not unheard of.

I’ve been in other radios that have shown evidence of previous repairs, like this Packard Bell 35-Late which has 5 different brands of capacitors from 3 distinct eras of materials, but were all wired correctly. Pretty much every one of the cylinders except for the tiniest ones is a capacitor in this photo:

or this Zenith 7-S-363 which used both Zenith- and Mallory-branded capacitors from the factory, and also contained Aerovox and Solar later replacements but also worked perfectly after repair:

This Stromberg-Carlson is the first radio I’ve serviced where there were clear mistakes made along the way. It’s an entirely new set of challenges on top of the already-difficult antique radio repair process, but it does add a level of fun and discovery that a straight-up easy “recap” repair doesn’t offer.

Edit: After some peer review, it turns out it was a draftman’s error between the schematic diagram and the wiring diagram! Annoying. I should publish some errata, maybe I’ll do that soon.

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Review of Victor 3165 Frequency Counter

Posted on March 14, 2011 by jwk

A frequency counter is an important part of any electronics service bench where it’s necessary to deal with unknown signals. I used them to verify the operation of my RF signal generator while performing alignments on tube radios, because I’m using period equipment that might not always track on frequency as well as it should.

“Good” frequency counters have also tended to be pretty expensive, but have come down in price a lot lately. I used to use a vintage Systron-Donner frequency counter, from the ’50s or ’60s which still used Nixie tubes. It worked well for a while, and was a Cadillac in its day – four input channels, variable attenuation, ranging and differential measurement between channels. It could compare signals as well as count frequency, pulse and period. It has calibration stickers indicating it was being maintained into the ’70s and was used in telecom service for AT&T. Unfortunately though mine has lost the ability to count, and tracking down the problem wouldn’t really be worth it. It cost $100 when I bought it used at a surplus shop.

I needed a replacement, and there’s really no reason to purchase laboratory-grade equipment that could service telecom grade communications links. Old equipment simply won’t take advantage of the features the high-end counters require, but there is some bad stuff out there at the low end to avoid. I ended up settling on this Victor frequency counter:

It’s a small, Made-in-China unit (with all the labels and controls in Chinese, which makes it much more exciting to use) with range from 0.1Hz to 2.4GHz in three steps. It can only measure one channel at a time, has fixed selectable 20dB attenuation, and is good for 30V DC or 30V P-P AC Coupled input up to 50MHz, and only 3V input from 50MHz to 2.4GHz. Signal level stuff only. Input for low-end is 1M Ohm, input impedance on the high-end is 50 Ohm.

It’s stable, it’s small, it’s easy to use once you memorize the controls – there are only two buttons that really matter – and the best part is it comes in under $100 frequently including shipping. Even if it eventually dies after a couple of years, at that price it’s cheaper to replace than it would be to buy an H-P or other lab unit. It works great with my old equipment, too, accurately measuring the output of my EICO 324 signal generator all the way through its range of 150KHz-435MHz.

Shown here verifying a 450KHz IF signal used for calibrating a radio. Easy, cheap and effective – I recommend this one for use as a general hobbyist device. I guess at this price “it does what it claims to do” is a pretty good endorsement, especially since there’s a lot of junk in the bottom end of the equipment market. Product specifications and distributor finder at Victor’s web site.

8-digit LED display, clear reading
Range and gate time adjustable continuously (100ms – 10s)
Selective range
High stability
Frequency measurement: 0.01Hz – 2.4 GHz
Period measurement: 0.5µs – 10s
Input sensitivity: 30mV(rms)
Power: AC 110/220V ± 10%

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Burned Up Aiphone

Posted on March 10, 2011 by jwk

This is what happens when you connect the high level output of an audio amplifier, to an input designed to be hooked up to an MP3 player or other line level audio source. This Aiphone PA amplifier was incorrectly installed by a technician and it made this 1/2W resistor dissipate about 40W. It held up…briefly…but quickly turned into a charred mess. It was somehow managing to pass a terribly distorted signal but didn’t damage any other components. After replacing the resistor on this pictured unit, and another identical one damaged the same way, it was back in service.

It’s not always overly aggressive cost engineering that makes things fry. Sometimes it is actually operator error.

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Seeking Information: 1941 Stromberg-Carlson 520 Radio

Posted on February 16, 2011 by jwk

If anyone reading this has information about a Stromberg-Carlson 520-PG from 1941, I would appreciate if you contacted me. I’ve been trying to find a photo of one that someone else owns, or even a picture of one in a printed advertisement, to help determine how it needs to be refinished – but as far as my research has indicated, I might have the only documented example of this model.

The Radiomuseum doesn’t have any more information than I do (in fact, those are my photos on their site). There’s nothing in the Radio Attic Archives, a huge collection of photos of restored radios from all different models. The Antique Radio Forum gallery of Stromberg Carlson radios has a bare chassis photo of another model in the 520 family. And even “Made in Rochester“, a site dedicated to a lot of S-C audio equipment, has only the scanned engineering data. A fellow radio collector has identified it as having a highly desirable Ingraham cabinet, which is a plus.

There’s this ad, showing the 520-PL which has the chassis mounted under a hinge in the top, and the record player is accessed from above:

Another radio enthusiast on the board found a 520-J which uses the same chassis but in a tabletop format, that’s the closest I’ve found so far:

Posted in Commentary, Radios and Tubes, Vintage | Tagged , , , | 7 Comments

Antique Capacitor Color Code Chart

Posted on February 16, 2011 by jwk

I’m working on restoring another radio, and it has antique capacitors with color code markings that are different from the commonly used 3-digit codes found today. It took a bit of searching to find a good chart with these, so I’m making another copy of this scan available so more people will be able to see it.

Some photos of the antique capacitors, they can be quite colorful.

The reference below:

This one looks like a 6-segment capacitor but it is in fact only a 3-segment display capacitor, and a tolerance value. They went for the cheaper marking option. Which means it’s Violet-Green-Black, 75×10 = 75 pF capacitance. The second is a bit harder to read as due to wear from heat the paint has chipped, but a faint bit of black remains giving Green-Black-Red, 50×100 = 5000 pF capacitance. From looking at the schematic, it’s possible to determine the value of the other capacitor (which is 700 pF) because we’ve eliminated the one it isn’t even though all the paint is chipped off.

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Project Idea: Digital Vacuum Tube Tester

Posted on February 15, 2011 by jwk

I’m in the early stages of brainstorming another project idea that will combine vacuum tubes with modern microelectronics in a useful and innovative way: a computerized vacuum tube tester.

Tube testers themselves aren’t very complicated devices – the simple ones, are just a transformer wired to many different sockets and a meter to measure the tube emissions. Better quality testers employ multiple circuits for each tube element: dynamic testers split the tube plate (high voltage) and grids (control voltages) into separate circuits to provide a more detailed reading, and transconductance testers take it a step further to measure certain tubes, like audio amplifier tubes, under actual operating conditions to allow for matching, a feature highly desired by audiophiles. But ultimately no matter how many separate circuits are employed in the testing, they’re all just a series of switches that are engaged or disengaged depending on the requirements for the tube under test.

This Precision tube tester, a dynamic tester, is my current bench model. It’s somewhat difficult to operate, though – look up the lever positions in a chart on the computer, set the levers, turn it on and wait a few minutes, engage a series of switches sequentially then hold the button to read the meter.

My idea is to, at first, construct a simple emission tester – except instead of switches, use a microcontroller to trigger triacs and relays based on either a lookup table or supplied values. There’s already been one attempt at a computer-driven tester, but that model is a bit less automated than I’d really like. Not to mention, it looks like it’s no longer in production.

Once I’ve worked out the logic, I’ll expand the design – add multiple sockets, enable USB communication, and hopefully work up to producing a fully-automated transconductance tester capable of measuring every tube used from 1920-1970, on full auto. I’ve placed a parts order for a basic microcontroller (the Arduino), a few small tube sockets, and am looking into substitute meters – my current thoughts on that are a digital multimeter with serial output – the multimeter handles the measurement of the electricity and sends it to the Arduino, which will handle communication from the computer regarding settings and back to the computer with the resulting data. There are hacks out there to enable serial on multimeters that don’t come with it, but I might just buy one with the protocol built in for ease of use.

I expect to start designing the circuit schematics for this tester in the next month or so, once I’ve cleared out my backlog of radios, and get into construction of a product over the summer.

Update: I did pursue this project for a good while, and bought some parts and sockets, but ultimately decided that if I wanted to go this route I’d use a uTracer, but since I wasn’t going to be selling curve tracer matched tubes or anything like that, it wasn’t a project that was worth fisnishing.

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Antique Radio in “Firefly” Episode 2

Posted on February 7, 2011 by jwk

I was watching Firefly the other day, and noticed something interesting. Firefly is set in the year 2517 centered around a small smuggling ship named Serenity manned by a crew of mercenaries, former freedom fighters, a pair of escaped convicts, a high class prostitute, and a comparatively normal mechanic and pilot. On one of their adventures in the second episode of the series titled “The Train Job”, space gangster Niska contracts the crew to rob medical supplies from a train. In the background of his office, you’ll notice something interesting:

In the lower left corner of the frame, you’ll see a 1940 Zenith model 10-S-464. It looks like it’s in good shape for being 577 years old, too. I wonder if there are any tubes left.

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Found: His Master’s Voice (HMV) 444 Radio

Posted on February 3, 2011 by jwk

I’ve added a new radio to my collection, and I think I have a new favorite. This 1934 His Master’s Voice model 444 is a small 5-tube console with Medium-Wave (MW, also known as AM Broadcast) and Long-Wave bands in a beautiful art-deco cabinet.

This radio is originally British – how it ended up in the Pacific Northwest is anyone’s guess. It hasn’t been plugged in since moving here, though, as it has the original 220V plug installed with the pin ends snapped off. I’ll be replacing the power cord and installing a step-up transformer inside the cabinet to provide the right line voltage; tubes should be available on eBay and capacitors are the same in all countries. With only five tubes, it shouldn’t be a challenging repair.

His Master’s Voice, HMV, was the Commonwealth licensee of RCA’s trademarks, including the Nipper, but were never sold in the United States because of licensing issues – both companies couldn’t use the same trademark simultaneously.

His Master’s Voice lives on today as the HMV Group, a UK media retailer and artist/venue management firm directly descended from the record label and gramophone producer of the 1920s-1950s. They’ve even updated Nipper to keep up with changing times. I kind of like it.

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Refurbishing a Capacitance Tester

Posted on January 20, 2011 by jwk

Parts Express sent me an advertisement for a $45 capacitance meter today. This actually a pretty good deal, and it’s a useful thing to have around if you’re doing electronics work. I’d probably have bought one myself, if I didn’t already have both a $14 model.

The trouble with the small meters is they only test at a low voltage. The working voltage ratings of many capacitors are in the hundreds of volts, and if they’re in a circuit where those voltages are present a low-voltage tester may pass a cap that is in fact faulty at higher voltage. That’s where vintage gear comes in, it’s a little harder to work but has a lot more features. I managed to purchase a pair of these EICO 950 capacitance/resistance bridges for $5 each from a salvage shop. They’re ’50s era bench devices to measure, test and compare unknown capacitors and can not only measure unknown values, but perform leakage tests at up to 500V.

They’re a little bit more complicated to work than the digital meter, but a bit more versatile.

The EICO 950 can measure from 10 pF – 5000 uF. Not as low on the low end, or as high on the high end, as the digital model, but I’m not working with any capacitors smaller than about 50pF or larger than about 50uF, so the shortened range isn’t a big deal. These are bridge instruments, where you’re comparing your capacitor to either an internal reference or to an external standard. Turn the dial until the indicator shows maximum shadow, and read the corresponding number off the dial. If you can’t get a fully expanded shadow, you know the capacitor is defective.

Cover off, the back view. A very simple design only uses 2 tubes – one to indicate, and the other supplies the power. The old capacitors will need to be replaced as they dry out and leak over time – exactly what the instrument is designed to test for, but it won’t work without being serviced itself.

Since I have two identical models, I only refurbished one. I’ve used the unrestored model and the restored model together in this post as representative “before” and “after” models. This is the “after”: capacitors replaced by newer, heavy duty models. Not all of them needed replacement so I left a couple of the original ones intact.

Vacuum tubes have a nice glow while running, it’s very attractive to look at.

Bright eye tubes in original equipment are tough to find. The eye is similar to the phosphor inside of a CRT television; it wears out and grows dim with use. At the bottom, the wide dark sector is the shadow; currently shown at its widest point where you would take the measurement.

The reassembled tester is checking out a modern Sprague 8uF +/- 20% capacitor. It’s reading a little lower than 20% but that’s because I haven’t recalibrated the tester after changing the parts. In addition to moving the dial, you can select the leakage test (paper-mica or electrolytic test) on the dial. This disables the capacitance/resistance dial and ties the action of the eye tube to the working voltage selection. If the eye stays approximately fully open through the full working range of the capacitor, it does not leak. If the shadow closes and the eye goes fully green anywhere in the range, it means it is leaking at that voltage and is not suitable for use.

Modern, small capacitance testers can’t do that.

In total, the pair of vintage testers from the salvage shop and the replacement parts set me back about $15 total, plus about 20 minutes to replace the parts. I really only use it for leakage tests, as the small meter is I hate to admit faster for telling me if the cap is dead-short or dead-open, which is more common on the old sets anyway.

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Cost Engineering in 2011

Posted on January 18, 2011 by jwk

Cost engineering to pick the exact right time of failure has gotten considerably more precise since then. The warranty on the array controller in my enterprise backup server expired on Jan. 2, 2011 – just over two weeks ago. Today, a surface mount capacitor decided it had enough, I’m impressed with how close to the end of the warranty it lived.

My last computer was dead-on perfect, though. A voltage regulator at the base of the board decided to end its life in a spectacular fashion while playing an online game, releasing the smell of ozone into my apartment and leaving me staring at a black screen for a minute wondering if my computer really did just explode or I was imagining it.

The time of death? One day outside the warranty period.

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