Archive
1934 Silvertone 1708A Repair and Restoration
I recently had the pleasure of working on a 1934 Silvertone 1708A which was brought to me for repair locally. This was great – having a radio repaired can be a big decision, so I’m happy to show off my workspace and chat for a few minutes and go over the radio briefly in person. This particular radio itself is very interesting, too. Sears, owner of the Silvertone brand, liked to re-use model numbers. I discovered 2 completely different radios, one with two slight variations, both sharing the same model number so it also involved a bit of detective work.
The Silvertone 1708A is an 8-tube radio with a dedicated oscillator and two IF stages for additional selectivity, and a tube line-up that showed it was still in a bit of a transition period: 6A7 78 78 37 37 37 42 83V. In most radios even just a year or two later, the 37s would likely have been replaced by 76s in a high-end radio like this one. The 83V is a bit unusual, too. It’s functionally not much different from an 80, and in fact upon a close inspection, it even had an 80 in place when it came to me.
The more knobs the better, and with five, this is near the top of the line. Power, volume, tone, tuning and AM/Shortwave. I went through some intake checks and found 4 tubes were bad, and that transformer looks especially nasty and tested an open winding as well. Underneath was otherwise in decent condition.
It showed evidence of being worked on a few times, and one of the filter caps was put in across a failed capacitor (as was common, but still very bad, practice back then) but no major issues. The speaker was fine too:
Testing showed the other components to be good, so off to replacing parts. I tested the resistors; within tolerance were left alone but others were replaced:
A 2W flex resistor broke along the way. These are incredibly fragile and break if you look at them wrong; they can be replaced with a standard resistor.
With most of the parts erplaced and ready to go, I replaced the bulb and managed a first power-up using a bench clipped replacement transformer.
The lights are on but nobody’s home – and despite good voltages coming off the unloaded transformer, and a normal current draw, there’s only about 20V B+ available. Closer inspection and testing of the bias circuit revealed the resistor in the B+ was cracked and reading very high, around 500K, when it should have been 350 Ohms. I replaced it with a very close substitute with some extra capacity.
She powered right up after that, and while I was poking around, I discovered the original transformer appeared open because of a break just a little ways back; I was able to re-solder the connection to the rectifier and all was well. In my opinion this was one of the nicest radios I’ve worked on – there was plenty of room to work and attention was paid to make sure everything was wired neat from the factory. (Contrast with the Simplex Model P Dual Band from the same year.)
I also added a line input; a simple resistive stereo to mono converter into the high side of the volume control. This way, you can use the radio’s volume control for the input source volume too.
It was time for an RF and IF alignment using my vintage signal generator and digital storage oscilloscope.
The generator puts off a messy waveform, but it comes out as a nice sine on the radio side. Tube AM circuits are pretty forgiving.
While I was working on the electronics, the radio’s owner spent some time reconditioning the cabinet and it came out incredible.
This radio is going to play beautifully for many years to come and will look great in anyone’s living room – especially with the upgrade of adding a stereo line input, it’s also future-proof.
Want to Buy: 1931 Westinghouse Power Transformer
I’m seeking a replacement power transformer for a 1931 Westinghouse WR-8 Columnaire grandfather clock-radio.
The radio uses the tube line-up 24 24 27 24 24 27 45 45 80; any similar 9-tube radio with a similar tube line-up is likely also sufficient. The Westinghouse radio uses the same chassis as the Radiola 80, shared by many models.
The main power transformer from any of these contemporary models will work:
RCA: Radiola 80, 81, 82, 86
Westinghouse: WR-5, WR-6, WR-7, wR-8
Graybar: 700, 770, 900
General Electric: H-31, H-51, H-71
Majestic: 90-B (*90 with no suffix is not compatible)
Period service replacements are:
Stancor P-713 (direct replacement)
Stancor P-6006 (universal replacement)
Electrical specifications:
Line to 700V (350-0-350) @ 120 mA
5VAC center-tapped 3A
2.5VAC center-tapped 12.5A
2.5VAC center-tapped 3.5A
Please reach out via the e-mail address on my About Me page if you have one of these components for sale!
_Potter.jpg)
Thoughts on HD Radio
I’m still around – there are actually 3 projects on my bench currently that are ongoing, and I’ll hopefully have a few posts in the next couple of weeks were I fix a Philco Model 66, a Silvertone 1708A, and a Westinghouse WR-8 Columnaire. Until then, I’m quite busy, so things are moving a bit more slowly than they otherwise might.
I bought an inexpensive aftermarket car stereo about six months ago, and it’s sat unopened in my closet until this past weekend when I was able to find 30 minutes and actually put it together. The process in my 2003 Honda Odyssey was incredibly simple – six screws was all it took. And now I have a great, brand new Blueetooth and Pandora-enabled car head unit!
The feature I already enjoy the most, though, is the HD Radio receiver. I’ve never owned anything with HD Radio before, nor can any of my SDRs decode HD Radio as it is heavily protected by iBiquity patents. Let me tell you – it’s like night and day. FM Radio around the Seattle area is always spotty for some reason which I’ve always attributed to the geography. Even a big station like KNDD (locally, 107.7 The End) pushing 18.2 KW of transmitter power has static intrusion, fade, and general sub-par audio quality beyond the normal loudness war audio degradation.
Every time I tune a station on this new head unit, it’s like an immediate A/B comparison between Analog and Digital radio. The tuner first locks onto the analog channel, and there’s some hiss and crackle and the sort of noise you expect from radio. Then the “ST” indicator disappears from the dial and “D” appears when the digital sideband locks in, and the audio quality jumps, the sound field expands, and the artifacts melt away entirely. I swear, it’s like I’m listening to a CD. Except for the commercials.
It’s just amazing. It’s by far the best feature on my head unit, because it makes the experience of being lazy and passive in my music selections better like night and day. Now the music I listen to when I’m bored of my CDs, sounds as good as the music I’ve put some planning into bringing with me. If you don’t have an HD Radio already, you should definitely get one.
An interesting mystery disc discovered.
I stumbled across an interesting post over at Arcane Radio Trivia where the author discovered an old Home Recording Acetate, in bad shape, but still intact enough to recover some information.
From around or before 1940, these “phonograms” were used to record programs from the radio or from an attached microphone at home. This one had a flimsy paper core which apparently didn’t hold up well to the test of time, but Arcane Radio Trivia managed to grab 1:57 of audio from it.
First Look: 1934 GE M-125 Radio
I was lucky enough to find a very rare, high-end radio for sale on Craigslist and jumped on it as quickly as I could. This particular one is the 1934/35 General Electric model M-125, their highest end offering for that year. And it’s both visually and electrically very impressive.
It’s in a stately (and very heavy) burled walnut cabinet with closeable front doors to hide the controls, and the small feet common to the mid-’30s console radio styling. It’s just as impressive with the front opened up:
Inside there are a total of 7 control knobs: Sensitivity, Volume, Treble, Tuning, Bass, On/Off, and Band Switch. On these old radios, more knobs means a higher-end radio with a more complex circuit and there aren’t many other radios with this variety. Some of the complicated McMurdo / Silver and Scott radios have similarly complex control schemes, but it’s quite rare to find surviving examples of such high-end pieces.
The radio uses the square “clock dial” face, with a large double-sided pointer to indicate tuning position and a smaller sweep second hand in the center for fine tuning. Tune the main knob to approximately the right frequency, then fine-tune with the second hand and mark the position in your log for next time to perfectly tune in the station. A couple of trim pieces are gone on this radio, but they should be able to be replaced or refreshed with new ones without too much trouble – everything else is complete.
Electrically, the radio is the same as the RCA 281 – they share the same chassis, but with a different cabinet and dial face design. This radio is a 12-tube receiver with a tuned RF stage, separate oscillator, two IF stages, and push-pull drivers (76) coupled two a pair of push-pull 42s for ~10W of audio output. It can receive the AM Broadcast Band, the Long-Wave band (<375 KHz, not much broadcast there anymore) and AM Shortwave stations up into the high reaches of the VHF band.
All the tubes are present, although it’s missing grid cap shields on 3 of the tubes in the back – although this is more cosmetic than functional – and all the labels are also in excellent shape.
Unfortunately, as you can see in the top right of this photo (somewhat in the shadow), the power transformer on this radio is toast. It’s spit out its tar from the bottom. Someone during this radio’s life plugged it in before repairing it electrically and it suffered catastrophic electrical damage. I’m lucky that I have a large stock of transformers lying around, but I will have to dig them out of the depths of storage and a transformer replacement is never a small job.
With the transformer gone, and the speaker field coil used in the power supply circuit, I hope the speaker isn’t also damaged – but I do have a suitable replacement or two lying around if necessary.
One interesting choice is in the huge cabinet, why they went with a small 10″ speaker design. The Tone Equalizer cabinets on the sides are resonant chambers which will help with fidelity a bit, an early nod towards evolving hi-fi designs, but for such a high-end piece the audio seems a bit under powered.
I’m very much looking forward to starting work on this radio next month. It will likely be a several part series, as there are a few big jobs to deal with – transformer replacement, recap, and alignment. Stay tuned!
Shortwave Radio Reception with the RTL-SDR Dongle
I bought and did a quick setup on my RTLSDR dongle using SDR# a few weeks ago, where I used it to listen to FM radio stations around my area and a few public safety frequencies. That’s all well and good, but I’m much more interested in shortwave listening – when the weather is good, I can pick up a fair number of stations on my Hallicrafters receiver and there’s even more out there that I can’t tune in with that old equipment.
The RTLSDR tunes from around 64MHz up through around 1800MHz, but shortwave frequencies are much lower – only up to around 30MHz. Using an RF mixer, it’s possible to shift the signal into the RTL’s tuning range. Portuguese designer CT1FFU developed a mixing upconverter which adds 106.25MHz to the incoming signals, shifting them up into the correct receiving range and filtering out signals about 50MHz to prevent interference. His version comes as a kit which requires surface-mount soldering, but German retailer Wimo offers mostly-assembled versions of the kit which only need the antenna terminals and power connector soldered.
Finding those adapters was a bit challenging – I have a helical antenna which terminates in that alligator clip, feeding into a coax break-out, with an SMA-Coax converter. On the other end is an SMA gender-changer and an SMA to MCX adapter. Ultimately I ordered them from eBay and they work as intended. The USB port provides the +5V power supply for the converter’s operation but otherwise isn’t connected.
Reception is acceptable. With the aid of the SDR software, I can see where signals are more readily, but issues with my antenna setup and local interference are keeping it from performing as well as the Hallicrafters. I can identify human voices on more stations, but it seems there are fewer I can actually listen to with this equipment. I’ll probably try building a tuned loop antenna similar to this one, and see what I can do with better noise rejection and directionality. I might also add a low noise amplifier after whichever better antenna I end up using.
If anyone has a favorite, easy-to-build loop antenna for 10-160M I’d love to hear about it.
World’s Most Complicated FM Radio [RTL-SDR]
After many hours of trial and error, I’ve turned my computer into one of the most complicated FM radios imaginable – and it’s nearly free. With just a cheap ($20) TV tuner based on the Realtek RTL2383 chipset and some free software for Windows/Linux/Mac, you can have your own Software Defined Radio receiver that can decode nearly every type of transmission imaginable from 50MHz-1800MHz, with the right antennas.
I’m using it to listen to my local FM radio station, KNDD 107.7 The End.
In this graph, you can clearly see the HD Radio sidebands (HD1 and HD2) as well as the main carrier. It’s present in both the waterfall, and the waveform. The sidebands are digitally encoded on either side of the analog carrier on the center frequency. This station is centered on 98.9 MHz (KLCK-FM/ HD1/ HD2 “The Click”) and plays a a mix of Top 40 and Alternative/Rock.
I’ll be posting more about RTLSDR in the future, including a guide on getting set up with RTL-SDR. In the meantime, back to experimenting!
What’s been your most difficult challenge at work, and how did you overcome it? [Crazy Boat Connections]
I was swapping war stories with another technology professional the other day and we got to talking about our “best problem” scenarios. I happen to have an interesting story from my days as IT Manager of the Las Vegas Casino Lines, and how I solved a vexing issue with our shipboard Internet connection that had been around for a year and was costing the company money every time the ship sailed.
The Las Vegas Casino Lines were, as the name implies, a gaming establishment that operated out past the International Boundary out of Port Canaveral, FL. They went out of business a few years ago (spurring my move to Seattle), but while they were in business had a fairly sophisticated network on- and off-shore to allow tracking systems, cash- and game-management systems, security, navigation and communications to all continue without interruption even when the ship was out at sea.
This WWAN connection was provided by a Verizon Wireless air card and an external antenna. The only trouble was, every time the ship was out, it would randomly lose connection. Without a connection, patrons couldn’t withdraw cash from the onboard ATM, charge gaming credits to their credit cards, or do anything really. If you had money, you could keep playing, but if not you got to enjoy a 4-hour smoke-filled boat ride.
The ship’s antenna for the WWAN connection is circled in red above. The perspective makes the antenna appear a lot bigger, but it really was only a six-foot fiberglass antenna mounted on the railing. The antenna is on the right side of the ship (facing forward) near the bow on the middle deck. I rode along with the ship for a few days while monitoring the connection signal and started to work out a pattern of when the signal dropped off: only when the ship faced certain directions, and more on the return leg of the journey than on the outbound leg.
I had a hunch: the signal is clearly dropping when we face certain directions. What are we connecting to? I visited the FCC’s web site and pulled the listing of fixed cellular transmitters along the Space Coast from our route, and eliminated towers that didn’t belong to Verizon; then plotted the tower locations on the map along with our ship’s turn area and boundary markers. Then, I rode along again and took a GPS track of our ship’s position, which I annotated with comments about signal connectivity.
As it turns out, we lost connectivity when we were on the margins between cell towers, and when we were facing the wrong direction. With the ship traveling north, the antenna was attempting to transmit through the (grounded metal) superstructure of the ship itself to reach the tower. Combined with the fact that microwaves don’t travel as well through salty humid air over the ocean as on land, and it’s a perfect recipe for signal loss. The shipfitters had installed the antenna in a poor location for our application. It was an easy fix, after that: one of the crew helped me run new coax to relocate the antenna to the top of the ship’s tower.
The signal was completely lost at this point – I’d overlooked a small issue. Adding 250 feet of additional coax cabling had introduced a lot more cable loss into the system, so now we needed an amplifier to get the signal out of the ship. Finally, though, the amplifier was installed and the system worked perfectly. Patrons could now charge gambling expenses to their credit cards while out in the middle of the ocean, and everyone was happy.
Antique Car Radio Repair: the 1951 Firestone 4-B-31 “Roamer” Vibrator Radio
A local friend is building a rat rod out of 1920s-1950s parts, a custom collection that ultimately will turn into a very fast car powered by a huge V8. He found a vintage car radio to go with it, the perfect addition and gave it to me to fix up. He requested to leave the metal cabinet alone so he could paint it to match after the car’s color scheme is finalized, so don’t worry too much about the finish.
This radio, the 4-B-31 “Roamer” was built by Firestone Tire & Rubber, the same company that today makes tires interestingly enough – they used to have a bigger product line when consumer buying habits favored combination stores. It’s a six-tube radio with a broad RF amplifier stage. Most likely the radio bolted up under a pickup truck’s dash and connected in the back to the firewall.
The tubes are 6SK7GT 6SA7GT 6SK7GT 6SQ7 6V6GT 6X5GT. The radio operates off a 6V car battery. With the low voltages it’s only about 1.2W of output power so will never be that loud, but when highways were new it was a lot quieter on the road and probably sounded better.
The battery directly powers the 6.3V filaments of the tubes, and the high voltage is provided with the help of a vibrator power supply. The 6V is fed into the electromechanical device which rapidly vibrates between two contact points turning the DC into a square-wave AC which is fed through a transformer to step the voltage up, then into a conventional rectifier power supply.
One of the pins was broken on this original vibrator, so it was the first to go. I replaced it with a solid-state replacement that uses a few transistors in a multivibrator circuit to accomplish the same effect, and should never need to be replaced again. I also replaced the 6X5 with a pair of 1N4007 diodes in an octal tube base, although this isn’t shown in any photos.
The chassis was decent to work on. It had open sides which made it easier to get things in with tight tolerances. The resistors tested decently, but all caps did need to be replaced as always. Several had blown their ends off already.
This radio was of course designed to be used in a car, and that means used with a car radio antenna which is a specific length and has certain transmission line characteristics – not quite as simple as just stringing out a long-wire. It’s a standard antenna, though, so I ordered a replacement that cost something like $10 with free shipping from Crutchfield.
It arrived in interesting packaging. The box was clearly broken in half, but both halves made it to my door without actually being connected somehow.
The antenna was in the bigger section. Go UPS?
A terminal strip in the radio was broken. This was a problem because the broken terminal happens to be the positive power lead-in and it couldn’t be salvaged. Only one terminal broke, though, so I improvised, screwing a screw lead to the mounting bracket and securing as shown, then running the wire out of the case.
Reassembled and testing with a bench power supply that was okay to check functionality. The switching power supply introduces too much hash to receive any stations, but it was good enough to do an alignment with a signal generator by injection. I then switched to a lantern battery for final tweaks which had a disappointing life of about 10 minutes. Clearly these were meant to be run off lead-acid batteries or linear power supplies only. It draws around 4A.
I also rewound the dial indicator. The dial tuning drum was still wound properly but the dial indicator string had broken so the pointer no longer moved. I used string that was a bit too thick but it worked out okay and is perfectly functional. No photos of that available though, it was pretty quick. The service manual had a full dial string diagram and pointer adjustment procedure. Unfortunately I ran into a problem as I was reassembling everything: the volume suddenly dropped off massively even with the control maxed out and it wasn’t coming back for anything. A check of the voltages showed that I had tens of volts on the screens of most all the tubes, where there was supposed to be a few hundred. I was at a loss about why this happened and finally resorted to the poke test.
The poke test is what it sounds like: poking or tapping on pretty much every part in the radio. I gave decent raps on all of the solder joints, tube pins, tie points and finally came to one that would make the volume cut back and forth: R7, the B+ dropping resistor for the screen voltages, a 15K 1W carbon resistor. Apparently it was internally cracked or otherwise defective. I replaced it with two 30K resistors in parallel to form a 15K 2W resistor, and a few others that shared the same tie point or were otherwise looking rattier than I really like even if they were in spec.
With that repair completed, the radio fired up perfectly with loud volume. This was a fun project, but power supply issues mean I don’t think I’ll take on too many of these in the future.
Don’t Trust Everything You Read (On Old Radio Schematics)
Electrical drafting is primarily done on a computer today, with software such as EAGLE or KiCAD. This wasn’t the case back when tube radios ruled the airwaves, though – schematics were drawn up by engineering draftsmen by hand. And as with any process with a human element, they didn’t always get it right.
I’m working on a 1934 Philco 66. It came to me in excellent original condition with little evidence of having been service, and throughout the process, I’d been relying on the schematics to guide me in the right direction. Unfortunately, along with a laundry list of other issues, my reliance on the schematic to be “the truth” led me around in circles longer than I needed to be to resolve a power supply problem.
Below is a schematic snippet of the power supply and audio sections of the 1934 Philco 66, with the RF chain to the left of the #75 Detector/1st Amplifier tube hidden for simplicity’s sake.
In green, I’ve highlighted the path B+ (high voltage) is supposed to flow from the rectifier cathode to the plate of the first audio amplifier. It’s a very straightforward path…if the draftsman had indicated that tube was supposed to be connected to the power supply. In red, I’ve indicated a missing connection symbol. Without it, there was no power being supplied to the first tube in the audio amplifier stage and the audio signal was being killed at that point before it could make it to the final output amplifier. Using an alligator clip, I restored that connection to test, and the radio sprang to life making noise on the next power-up.
The second filter capacitor should have been connected to both B+ and to the plate path for the #75 tube, rather than just the plate path. (Incidentally, the two capacitors are both at the same potential, so under the correct connection scheme could have been replaced with a single capacitor of a larger value.)
It’s not done yet, but I’m inclined to believe the final wiring issue has been corrected, and it’s on to performance.
Share this:
Like this: