KF7LZE Radio & Technology

I’ve finally had the time to finish one of my long-running projects repairing GE’s most powerful radio offering from 1937, the GE F-135. I picked it up from Craigslist back in November but other responsibilities kept me from getting much done on it until the 1st of this year. Finally, after a few months waiting, it’s finished and playing!

The radio came to me complete and in decent shape for the age. It’s missing the glass for the dial, but is otherwise completely intact and the finish isn’t in bad shape despite a few scratches here and there.

This radio is the current king of my collection. The best radio GE sold in 1937, it has a lot of innovative features – early APEX hi-fi reception, dual 6L6 output tubes putting out 20W of audio to a large and rich-sounding 12″ speaker; a total of 13 tubes including a tuned RF stage, dual IF stages, separate oscillator and “station seeking” automatic frequency correction.

Under the chassis it’s in decent shape too. It looks like it has been worked on before a few times – there are some ’40s, a ’50s and a couple of ’90s capacitors installed. There’s a sticker from a Seattle-area Jukebox Repair store on the back which probably explains the more-recent repairs; I looked up the address and they don’t seem to be around anymore.

First thing first after doing the complete set of intake checks on the radio, I gave the cabinet a thorough scrubbing and hit it with Howard Restore-a-Finish and it came out beautifully. The intake checks were uneventful so I didn’t take any photos, but the power transformer, all the IF transformers, oscillator and antenna and RF coils and the speaker transformer and coils were just fine. After applying the Howard’s, it really cleaned up the fading and covered over a couple of small scratches and really brought out the natural shine on the wood.

Then I tested all the tubes and found that most were in good shape (especially the 6L6G tubes installed) there were a few 6J5 and 6K7 tubes that needed replaced. I had these in stock and so it was easy to solve that problem.

I worked on this one under my kitchen’s vent hood as it’s currently too cold to vent soldering fumes outside. The chassis stand is the right width though so that’s perfect.

Every paper and filter capacitor needed replaced, as well as nearly every single one of the resistors which had drifted as much as +100% in value in some cases. Fortunately the coils are all intact or this could’ve been a much messier repair.

I also found a couple of places where the previous repairman who worked on the set may have been dyslexic, as there were a couple places were numbered resistors were reversed – i.e. R23 being in R32′s place and so forth. I imagine that couldn’t have made it work any better, anyway. I tested and replaced going along from the bottom up as needed. These are in-progress shots, so you might see a couple places where leads aren’t trimmed or components aren’t yet soldered. All of those issues were taken care of but might not have made it into the photo series.

Small caps dealt with, it was time to replace the multi-section caps. There is a 4-section can, insulated from the chassis, mounted up top with a set of 2 filter caps and 2 cathode-bypass caps. These all ran to a hole in the chassis where they went above. I snipped the long cross-chassis leads and moved the components close to their intended locations. In this photo, I’ve replaced the 10uF cathode bypass capacitor with its replacement. I like using bipolar caps for the large-value cathode bypasses but that’s just my preference and what I keep in stock (I keep those values around for repairing crossovers in old speakers) but you could use a standard polarized capacitor there.

Here’s the totally-complete underside shot.

There’s still a matter to deal with above the chassis, though. This is an AFC radio which uses a special and complicated transformer heading into the diode which recovers the audio. And it has a small resistor which is reading double it’s value and needs replaced as well, or it won’t align right.

Then I sealed the can back up:

I hooked the speaker and pushbutton assembly up on the bench and gave it a test run – it fired up immediately and started pulling in a few stations even on the Shortwave bands. The dial was off alignment a bit though, so it was time for that.

For the alignment, I pulled up the signal generator and started with an IF alignment before going back to the RF stages. This radio has a special IF arrangement with a procedure, so I aligned the 3rd IF primary, second IF secondary and primary, first IF secondary and primary, then went back and aligned the 3rd IF secondary that feeds into the diode. Aligning that discriminator was a maddening 10 minutes spent trying to nudge the adjustment ever so slightly. My goal was to get 0V between two segments, but it approached that point at an incredibly steep slope. I managed to get it there, though.

The original 0V spec was made with a primitive meter; I’ll take 0.01 on a more sensitive modern instrument. That’s perfect IF alignment. It was definitely worth it though. Now onto the RF, which involved tweaking something like 16 trimmers in a precise order with an RF signal generator at various frequencies.

Finally, it was all set! Time to reassemble.

At this point, the radio plays beautifully and pulls in stations from all over, and I’ve added a line input to let me hook up an audio source. The hassle of the AFC calibration was definitely worth it, it’s nearly like magic to watch it work. With the switch off, the radio tunes sharply and a station comes in over just a few degrees of rotation. With the switch activated, it’s like an entirely different radio – the same station will come in across about a quarter-turn of the knob, 2 divisions in either direction from the center frequency and it will block quieter stations from interfering.

The radio sounds great with a pretty good frequency response and more volume than I know what to do with, too. The relay for the motor is burnt out, though. I missed that on the initial checks so when I went to test the pushbutton function…I got a whole lot of nothing. I’ll make another post here when I do get the motor resolved but for now I’m going to hang this one up and start playing it. This was a very fun and enjoyable project and I have a beautiful radio with a commanding presence to enjoy for many years to come.

I had an interesting call the other day with a gentleman about a radio he’s working on, among other topics:

That’s a Philco 46-420. They’re nice little bakelite radios with 6 tubes designed to receive the AM broadcast band. He’d come across some unlabeled wiring while repairing and had dealt with it but we were talking about what it’s purpose was.

I generally work on pre-WW2 radios so haven’t run into this particular arrangement personally, but I’ve read a few different articles by other collectors on this topic and recognized it immediately. The coil, wound 8 turns around the capacitor and connected at one end to the chassis, is a type of wave trap designed to cancel out the inductance of the old capacitor. This helps to prevent interference – both received, picked up through the cap as if it were an antenna, and radiated interference from the signal passing through the cap. Philco used these capacitor wave traps in most of their radios from 1946 and on. There’s an article at the Philco Repair Bench describing one style; this is a slightly variation with the same effect.

Modern caps are constructed out of metalized polymer films that have very little inductance, but these older capacitors were just concentrically coiled metal foil sheets with a lot of natural inductance.

Bypass cap from my client's Grunow 589

Filter cap from my GE LF-116

1950s "Bumblebee" Cap Exploded - MyLesPaul Forums

0.1uF 400V TubeTime.us

Modern caps don’t have that physical property, so it’s safe to replace the wrapped capacitor with any modern replacement and either shove the new cap through the coil, or remove the coil entirely.

Thanks to Bob from Old Tyme Radio for these photos of his project, and for distracting me from being snowed in for a bit!

I’m always taking mail from readers with interesting anecdotes, photos and questions so feel free to send them over either as comments or through the e-mail address I’ve posted in my Repair Services page.

I was talking with a fellow antique radio hobbyist on the phone the other day about a repair he was working on. A TrueTone battery radio, he’d taken care of all the important steps – checking out the tubes, replacing capacitors, replacing the broken wires, that sort of thing. The radio worked before the service but wouldn’t play after and so we talked through the steps to see if there was anything he’d missed and get a second set of eyes on it.

This set had a lot of rubber covered wire which has degraded over time and turned into bare wire, so some of that needed to be replaced. This started to become a problem around the second half of the 1930s up through WW2 and sometimes after…you can replace the wire or unhook one end and cover it with heat shrink tubing. Everything sounded good until I heard about replacing some wires “on top of the variable capacitor.”

RF issues in these old things are insidious and tough to locate and it takes a lot less than you’d think to throw it off. Wire diameter, wire length, physical positioning and shielding are all involved to some degree. Wiring changes in the front end are the first thing to take a look at. He’d mentioned a broken wire on top that he’d replaced.

That rang a bell. My next question was, “Were they twisted around each other?” They had been. Problem identified.

This twisted-wire “fake” capacitor is called a gimmick and was a way to save a few cents on the manufacturing cost. It doesn’t take much to make a capacitor. All you need is a two conductive charged plates separated by something non-conductive; two wires twisted around each other don’t provide much but but can make a few pF. Just enough to couple a small bit of a high-frequency signal like in the RF or IF frequencies.  It’s not always obvious that a bit of twisted broken wire is actually a circuit feature, though – especially if it’s in bad shape from age, so replacing it with a new piece of wire is a pretty obvious thing to do.

Unfortunately in this case, though, the circuit as connected is shorting the oscillator and antenna sections of the variable capacitor together and the radio won’t receive anything. It was a really quick fix, though. Just twist a little tighter and snip!

Doesn’t look like much but it gets the job done – the radio fired right up and received stations after the quick change.

He sent over some construction photos to show how you can make your own gimmick replacement:

Thanks Steve for the photos! I’m glad the TrueTone is back in operation.

I’m always happy to throw out some advice about antique radios and radio repair, so if anyone reading has any questions feel free to drop me a line through my About Me page, or make a comment reply and I’ll get back to you. Feel free to share photos and stories as well, I’ll post the most interesting ones on here so everyone can benefit.

Part of a continuing series:

Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement 
Part 5 – Finished!

I’m continuing to work on this 1942 GE radio which I’ve been enjoying in my office for a year, and now it’s time to make it play again. I’ve pulled the chassis out of its cabinet for inspection, made a test jig, and rebuilt the multi-section can capacitor above the chassis due to lack of room underneath for mounting replacements. In this issue, I’m going through the capacitors and cleaning up a few other issues that cropped up along the way.

As we’ve already seen, there’s a fair amount going on down here. For all the empty space under this large chassis, General Electric’s engineers decided for one reason or another to use only about 1/3 of the available space and pack everything into that area as tightly as possible. There may have been some interference concerns, but I suspect a cost-saving measure for one reason or another that’s long lost to history. It does make it a lot of fun to work on, as capacitors are tucked in between the band switch, three layers deep under wiring and resistors, or otherwise made as annoying as possible to access. I’ve been spending a good bit of time with a needle-nose in each hand and that requires a special amount of coordination that makes for slow going.

This radio uses an interesting arrangement. On shortwave or AM bands, the first tube is a tuned RF amplifier helping with distant reception. On the FM band, the first tube is switched into the first stage of a cascade converter system where there’s a two-stage stepdown to the 4.3MHz intermediate frequency – this was done because at the time, tubes didn’t have the bandwidth to perform the conversion in one step without losing quality. It makes for a crowded and more complicated circuit, though, as quite a few coils are switched in and out depending on what’s being requested at the time.

The radio bears evidence of having been serviced many times throughout its life with several different brands of capacitors from varying ages, date codes ranging from 1940 up into the ’60s. There’s the usual poor soldering in a few spots, clipped component leads left on terminals, and general quick re-work but by and large it’s in decent shape and doesn’t appear to have been “hacked on” very much.

I began replacing capacitors one by one, mixing radial or axial styles depending on the location, and came to a resistor that had burned out – just to the right of the red clip I’m using as a marker.

My copy of the schematic wasn’t very readable, but another hobbyist was able to supply me a better scan and I ended up purchasing the complete set of high-resolution media as a result of seeing this sample.

With this wiring snip, the full schematic diagram, and some confirmation from another hobbyist I was able to identify the burned out resistor as R-11, 2.2K Ohms 2W, which supplies B+ voltage through IF transformer T1 to the plate of the 6SG7 converter. Capacitor C-32, a 0.02uF coupling capacitor, was shorted which passed B+ directly to ground and caused it to burn up quickly. This obviously happened at least once in the past, as C-32 was a replacement as was that resistor.

Capacitor replacement followed pretty unexcitingly, assisted by my Hakko for cleaning up terminals. I rebuilt the above-chassis capacitor block in Part 3, linked from above. It’s slow going due to the large amount of brittle, crumbling rubber wire and tight quarters. Many restorers advise either replacing or covering with heat shrink this wiring as if it crumbles, it could short out. Mine is mostly intact, and by taking extra care not to bend – just to push – the wiring around I’ve avoided having any crumbling accidents so will not be doing that time-consuming step unless it turns out later that it’s absolutely necessary.

Very unfortunately, though, a tie point snapped off the socket below the molding while trying to replace one of its connections. These loctal sockets seem more fragile than the octal sockets used in 9 of 11 tubes in the radio. I ordered a set of brand new ceramic loctal sockets from Angela Electronics who boast “Since 1977 we’ve supplied thousands of hard to find items to musicians and tube audio enthusiasts worldwide.” Hard to find no question, as they’re the only site I’ve found that sells loctal sockets – as well as brand new ceramic 5- and 6-pin sockets! On this fact alone, they’ve got all my business for new sockets going forward.

The socket arrived; I carefully remove the wiring from the terminals and drill out the rivets for the socket to replace.

Ceramic sockets are nice and durable. But, naturally, there was another issue: the mounting tabs on the 7K7 socket weren’t spaced evenly with the chassis holes AND unless I wanted to mount with rivets instead of 6-32 screws, I wouldn’t be able to get the tube to seat properly – the mounting was interfering with the base. At this point, my significant dislike for loctal sockets was solidified and I said forget it, grabbing an Octal socket from my parts bin. I mounted the octal socket above the chassis held in by the retaining clip to ensure there’s enough space to seat the tube properly.

A 7K7 tube would go into the loctal socket; for the octal socket, the same tube is labeled 6AQ7GT. I don’t have one of those in stock so ordered from eBay for $2. They’re both double-diode/triode tubes, serving as discriminator and phase inverter for the audio output. And, annoyingly enough, they’re laid out somewhat differently. Whether this was for any particular technical reason or just a rivalry between companies, I don’t know but the socket required re-wiring beyond just hooking the leads back up.

7K7(Pin) 6AQ7GT(Pin)	Description	Visual
1	 8		Heater		Grey Resistor/Ground Tie
2	 6		Triode Cathode	Orange Resistor
3	 5		Triode Plate	Orange Cloth-Cov. Wire
4	 4		Triode Grid	Orange-Drop Cap and Res.
5	 1		Diode 2		Green Cloth Wire into Can
6	 3		Diode 1		Green Rubber Wire into Can
7	 2		Diode Cathode	Cloth Wire into Can
8	 7		Heater		Black Rubber Wire to Tubes

With that mapping completed, it was time to re-wire. This required slightly extending some of the wires coming from the discriminator transformer.

This radio is by far the most frustrating to recap of any I’ve worked on yet. It might actually be the most complex one I’ve worked on so far anyway, but the construction – layers upon layers of tight components with sensitive lead dress requirements buried deep inside the radio. Much of the time I was replacing some of the deeper components with a pair of needle-nose pliers in each hand, and I even had to remove the output transformer from its mounts to replace three capacitors located basically under it.

Around the output transformer and push-pull output tubes, there were a handful of 1000V-rated 0.05uF capacitors. I don’t have 1KV metal film capacitors in stock, so I used 1KV-rated ceramic multilayer capacitors with a Z5U temperature coefficient…should be mostly sufficient for the application. Z5U-rated capacitors operate between 10C and 85C with a maximum variation of -56% to +22% capacitance, and honestly the original manufacturing tolerances of paper, foil and wax were probably broader than that in the first place.

A resistor bypassed with a capacitor. The resistor itself is a 1.2K 5% resistor, but it’s actually drifted by about 10%; I’m hoping this won’t be a significant issue but if there’s an issue around the oscillator circuit, this’ll be the first place I revisit. If I’d noticed the tolerance marking before reconnecting, I’d have replaced the resistor outright as well.

These capacitors have identical ratings – 0.005uF and 1000V tolerance. What a difference better manufacturing technology makes. The smaller physical size actually caused some mounting issues of its own, though, as the replacement component had shorter leads that required extending with a small piece of jumper wire in a few locations.

Naturally, it was bound to happen that something else would break during this process. I heated this joint with my Hakko to clear some solder, and the lug cleanly separated the instant I did so – it was held on by solder alone, it seems, the underlying metal having broken from thermal stress or a past workman’s abuse sometime during the last 69 years. Fortunately, very very fortunately, this is just a tie point – it’s not connected to the actual switching pads. I soldered a jumper to the rivet for physical stability and replaced the capacitor as normal. If this had been an active switching lug, this could have potentially permanently crippled or even rendered the radio unservicable.

Finally, I reattach the rebuilt can capacitor. I ran the common to the can’s mounting lug, then a jumper from there to a near-by ground tie point and soldered the lug to the chassis, the wire to the lug and the jumper to the lug as well. This ensures a solid ground even in the event the chassis soldering didn’t  take very well. I ended up using my Hakko as a soldering iron in this case, as the thermal mass of the tip allowed it to heat the chassis to soldering temperature without cooling; the iron I use for adding solder to most connections is a thin point that cools too quickly when heatsinked.

I did a tube-less power-up to make sure there were no immediate shorts, such as a stray piece of solder, and found that the Beam of Light lamp was burnt out. Fortunately, I have #44 dial lamps on hand:

With the radio’s electrics fully serviced, now I can continue on to the first power-up just as soon as the tube I need arrives. Stay tuned!

Part of a continuing series:

Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement 
Part 5 – Finished!

I’m continuing to work on the GE LF-116 radio, an AM/pre-FM radio manufactured in 1942. We left off where I’d created a jig to hold the radio upside down because it couldn’t be mounted the way I normally do, and now I’ve started to go through and refurbish the components. I quickly ran into a bit of an issue which spawned an entire new post, which would’ve normally just two or three photos in part of a larger article: It’s cramped down there!

In the center just below the “orange drop” capacitor hanging upside down is the base of the multi-section can capacitor. Seen from the top circled in red:

This capacitor houses the first second and third filters, and the output tube’s shared cathode bypass capacitor. There’s just nowhere good under the chassis to mount a terminal strip and new capacitors, so I’m forced to actually go ahead and restuff this can with modern replacements – it’s a time consuming process I’ve mentioned not finding to be a good use of my time in the past, but necessity dictates I do it this time. This is the first can I’ve restuffed, and it didn’t come out quite as well as I’d have liked but it’s passable to anyone but a purist.

I begin by removing the leads from the terminals on the bottom of the can and marking which they came from. The body of the capacitor is the common negative for all four segments and is tied to chassis ground which makes it slightly less messy than if it were an insulated can.

The lugs are marked with a shaped cutout in the phenolic base, and the mapping is indicated on the side of the can.

In this case we have:

• 30uF 450V (C-73A) First Filter
• 15uF 450V (C-73B) Second Filter
• 10uF 450V (C-73C) Third Filter
• 20uF 25V (C-73D) Cathode Bypass

I’m always keeping my eyes out for interesting vintage hi-fi speakers from the ’60s-’70s, and found a nice set of Sansui SP2500s on Craigslist. Produced between 1960 and 1974, these beautiful walnut speakers are solid and very heavy – definitely high quality. I’ll share some photos below, and then dig into reconditioning them!

Here are the vintage ads for the Sansui speakers. Click on the image for the full-size version!  The web site is down apparently, replaced with a placeholder park page redirecting to a spam site. Sorry, no vintage ad anymore! I should’ve hosted it here instead.

Unlike newer speakers (including a pair of Sansui SP7500X that are still waiting to be refurbished), these are very easy to remove. Six screws to remove the crossover panel from the back, disconnect the driver leads from the spade terminals, and pull out of the back. Like all older electronics, speakers also have components that can fail inside. The crossover network, distributing and filtering the amplified audio to the different drivers, contains capacitors which can fail with age just like the capacitors in an antique radio or in your modern electronics. It’s easy to forget speakers have parts that can degrade inside, when they’re almost always treated like a single box unit.

If you’re buying speakers from Craigslist and they have adjustable crossovers, make sure to check them on all settings to help assess their condition.

With the crossover removed, it’s easy to see the components that need replacing. The three blue cylinders are the crossover capacitors, in 2.2uF, 4.7uF and 10uF varieties. These are bipolar electrolytic capacitors, a special type that is slightly more expensive than standard electrolytic models, but they are required because speaker current is AC and must flow both directions across the capacitor. A polarized capacitor would be likely to explode if used in the same application.

Parts Express sold me everything I needed for these speakers, and a few others:

The old capacitors are glued to the board and I didn’t want to damage it, so I snipped the leads as close to the end as possible and bent them into terminals, then attached the new capacitor to the wire and soldered together.

Repeat for the other capacitors, and after trimming the leads, much nicer:

Two speakers means two crossovers:

Reinstalled and ready to rock:

My only complaint now is the diffusers send the high-frequency audio straight into my carpet, when placed on the floor. I suspect these were meant to sit on stands somewhat. I’ll experiment with different positions for the speakers in my living room, but if it ends up not helping, the hole is symmetrical so I can just rotate the top assembly and now the speaker will send its sound up, towards my ears.

I have several more pairs of speakers – Sansui SP7500X, Bose 601 Series 1 and Cerwin-Vega D-5s. I’ll post photos of refurbishing their crossovers when I get to it.

My parts list:

• 2.2uF 100V Non-Polarized Capacitor
• 4.7uF 100V Non-Polarized Capacitor
• 10uF 100V Non-Polarized Capacitor

I used 100V capacitors both because that’s the lowest voltage in this type my supplier offered, but also because they’ll last longer if they’re intentionally de-rated and with any luck, this will be the last time I’ll need to work on them.

Total cost: $1.38 + $6 shipping to restore these speakers to original working condition after 40 years of service.

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.

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.

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.