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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 In most radios even just a year or two later, the 37s would likely have been replaced by 76s in a high-end radio like this one.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!
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Bose 901 Series 1 Equalizer #29827 Repaired!
I recently got to fix up another Bose 901 Series 1 equalizer which I received for repair. These are some of my favorite electronics to work on – they’re easy to work on and each one has its own history. Every one of these I’ve seen has been slightly different and this one was no exception.
This one in particular has 4 separate repairs. One is especially interesting.
The last one is somewhat clever. A 10K resistor, probably 5W, across those terminals is the modification to run this equalizer on 240V in Europe or similar. It’s been jumped with a solid piece of copper bus wire taking it out of the circuit but still leaving it in the equalizer if conversion ever needs to happen again.
Top-off testing was next. The neon indicator lamp in the power switch was flickering badly – it had likely been losing neon through the metal-glass interface very slowly over the past 40 years. It’s a neon lamp attached directly across the AC mains with a voltage dropper/current limiting resistor in series. The total power consumption is a few mA at line voltage.
Here it is removed from the circuit. The lamp/resistor combination is actually a single component – they’re welded together. I replaced it with an NE-2A/150K resistor combination, I believe the resistor is 1/8W the draw is so small. The envelope size of the new bulb is about half that of the old one, but it fits in well from the bottom to let wire tension keep it in place better.
After burn-in testing, the equalizer checked out perfectly! It has incredibly clean switches. The others I’ve serviced are much improved after cycling but can hang up the first time they’re used and these didn’t even need cleaning.
This one is going to be a great performer for a long time, and these are a lot of fun to work on.
(If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
Bose 901 Series 1 Equalizer #35793 Plays Again! [Repair]
I recently had a chance to repair another Bose 901 Series 1 equalizer. This makes quite a few of these that I’ve written up on here. One of my favorite things about seeing copies of the same model is getting to pick out the individual variations that happened in the production run and any repairs that have happened over the years, and this one is no different.
This one is new to my bench, it’s a 240V model! I haven’t had this one across my bench before – but the circuitry is identical except for the addition of a single extra resistor. Fortunately, I’m equipped for that!
This one looks like it’s in great shape except for a tiny corner that’s cracked but not yet separated.
It’s an incredibly simple switch. A 3W resistor dissipating about half that amount in series with the AC and the transformer, dropping the line voltage to 110V and feeding the standard circuit. It doesn’t look like this model has been repaired.
Capacitor replacements went according to plan, although one set of capacitors ended up being defective from the factory so I ordered a different set. Shown below are the good replacement filters.
All replaced! Precision 2% tolerance resistors, German-manufactured film capacitors, and modern replacement electrolytic capacitors. This equalizer powered right up and sounded great on every setting immediately with no further troubleshooting required.
Quite a few parts were replaced during this process.
(If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
First Look: 1934 Grunow 750 World Cruiser
I picked up this Grunow 750 “World Cruiser” radio from eBay a little while ago for an incredible deal and now it’s time for it’s turn on the bench. These radios are fairly uncommon and frequently sell for several hundred dollars, so I was excited to be able to pick one up for under $100 with shipping.
It’s in remarkably good shape, despite the eBay seller packaging it in form-fitting cardboard with no padding whatsoever and the chassis unsecured in the cabinet. The fact it arrived as anything other than a pile of broken wood, bent metal and shattered glass astounds me – it was by far the worst packing job I’ve ever seen an Internet seller provide.
The radio looks like it sat somewhere very dirty, and possibly was briefly inhabited by a rodent. There are a few chewed-on spots, and some fiberglass insulation was dragged into the cabinet. It doesn’t look like whatever lived there was in it very long, however, as there’s no rust, the damage is very minor and there wasn’t a lot of “fill” material brought in.
I set to cleaning and examining. One IF transformer is missing it’s grid cap, that’ll be a bit annoying to replace.
You can see around the edges where it looks like a rodent did some chewing.
It looks like it also chewed through the output transformer leads.
This is a big radio with a big chassis to match, accepting 7 tubes 6D6 6A7 6F7 75 76 42 80. It can receive 2 shortwave bands and the AM Broadcast Band, features a tuned RF amplifier, and double-tuning on the broadcast band for extra selectivity. For the double-tuning, it uses a 4th segment on the tuning capacitor. It’s very rare to see a 4-segment tuning gang on a superhet and it’s a definite indicator of quality.
The underside is built a bit like a tank, with multiple sets of shielded coils. Fortunately, the sides of the chassis are bolted on allowing easier access to the components. It would be impossible to work on otherwise.
With the sides off and the coil covers removed, it’s a lot easier.
I’ll be working on the radio this week, testing all the coils and transformers and then replacing the out of tolerance resistors, new capacitors, and repairing the IF transformer grid cap and output transformer leads. The radio will also need a new cord as the old model was badly frayed and chewed and so it was discarded.
Stay tuned.
Bose 901 Series 1 Equalizer Repair (Deux)
A client recently engaged me to repair his Bose 901 Series 1 equalizer. This second one was in a bit worse shape electrically than the previous one I’ve written about, with one channel entirely dead and scratchy switches. I received the equalizer and got right to opening it up to check out the internal components. It shows evidence of having been serviced a couple of times in the past, with several of the electrolytic and output capacitors replaced as well as some of the .015uF signal capacitors (the white rectangles near the center and center-right of the image.)
Right away, one problem is immediately visible – one of the output capacitors (C12, near the top right) has split down the side and shows visible leakage from the positive lead. The originally fitted capacitors are the silver cans; the black cans were fitted some time later. They must have been lower quality replacements, as it’s one of the previously replaced units that’s failed most obviously.
My client requested I replace all resistors outright, so I didn’t spend extra time testing the carbon composition resistors before getting straight down to replacement. In the past example I discussed, many of the resistors had drifted far beyond their printed tolerance markings, and as all carbon resistors are going to behave similarly, it would have been a purely academic exercise to see drift figures on these resistors which were going to be replaced anyway.
The circuit board is very easy to remove. All connections, except one set, are along the far side of the board. I de-soldered and cleaned the terminals of the output connections to make it easy to remove.
The underside of the board. Not visible in the photo, but visible when closely looking, I could see where flux had flowed on the board during previous rework. This is normal during rework, and if you know what you’re looking for it can give away previous service if you’re not sure if a board has been worked on before or not.
I started off with the electrolytic capacitors. The two primary filter capacitors were replaced with Sprague Atoms capacitors. Sprague has been around for many, many years and these were manufactured in the USA. I frequently find Sprague capacitors in the vintage equipment I service, it’s great to see they’re still being produced right here at home. The rail filter capacitors I replaced with high quality USA-made CDE capacitors.
Then on to the resistors and signal capacitors. All resistors were switched with metal film replacements which are temperature and time-stable with precision tolerances of 2% or better, with the overwhelming majority being replaced with 1% or better. My supplier sent some with a different body style, so several are physically larger but specified identically. The signal capacitors were replaced with polymer film models at 400V or 630V tolerances, the up-rating ensuring they’re being run well below their tolerance and so should be stable for longer than the life of the speakers they’re attached to. Finally, the output capacitors were replaced with Japanese production Nichicon capacitors which are frequently found in high-end audio equipment.
Fortunately, all 10 transistors (2N5088) were good and didn’t require replacement, but I do stock new replacements just in case.
Ceramic disc capacitors such as the 10pF model in the center of this shot aren’t subject to failure the same way the other models were, and so the originals were retained.
I used control cleaner followed by repeatedly cycling the switch positions to clean their contacts. The Tape Monitor switch remained a bit touchy, but the others cleaned up nicely. My client preferred to preserve the originality of the appearance, and the switch works fine with just a couple seconds of extra touching to make it connect, and so it was left original.
With these repairs, the equalizer should last nearly forever barring some unfortunate outside circumstances. Only the highest quality parts were used to ensure perfect sound reproduction and long life.
(If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
Bose 901 Series 1 Active Equalizer Repair [Vintage Hi-Fi]
(See other Bose 901 Series 1 Active Equalizers I’ve fixed, and get info about your own repair: Bose 901 Active Equalizer Repair Page)
While some other projects have been pending information and parts, I’ve had a chance to work on the Bose 901 Series 1 equalizer that’s been on my bench for a couple of weeks.
The equalizer is a necessary component of the Bose 901 speaker system, Bose’s highest-end hi-fi equipment. The speakers are specially designed in a way that requires the signal to be equalized and pre-amplified, and if you don’t use the Active Equalizer they’ll really sound pretty bad. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, but the lack of using an equalizer might be where derogatory slogan “Bose: No Highs, No Lows” came from.
This particular model came to me from a client complaining about distortion and eventually signal loss in one channel, and general sub-par sound. He had the equalizer for about a year after purchasing the set from a collector, and it never sounded quite like it should and rapidly degraded from there. The capacitors in the unit had probably been going bad for a while but only crossed the threshold to completely dead after some time in use.
The Active Equalizer offers 30-some combinations of curves to select and enough pre-amp gain to maximize the speaker’s output.
The equalizer is fairly simple construction, using a single-sided PCB, ten low-noise 2N5088 transistors (five per channel) and an assortment of capacitors and resistors and a few inductors to do the work of changing the frequency curve of the audio signal.
With the top cover off, you can see inside clearly. This particular equalizer came to me with reports of distortion and low gain. Obvious immediately are the large orange and red capacitors. The red models especially have visible discoloration at the very top – exactly like I saw in the 1951 Farnsworth K-262P last month. This is both good and bad: it means the problem was easy to diagnose, but unfortunately, it also means that even 1960s-70s era metalized film capacitors can be nearing the end of their service life and that doesn’t bode well for a lot of other equipment from similar and slightly later years. I also noticed that many of the resistors are the original carbon composition type, which is known to absorb moisture and change values. As the resistors in the signal path, any drift can change the equalizer’s effectiveness. I spot-checked a sample of the resistors, and found that 30% of them had drifted past their stated tolerance and were also going to need replacement.
The filter capacitors. 2x500uF and 7x100uF @ 25V.
The EQ is easy to service. The board is secured with four screws and lifts up.
The back of the board is the business end. This is a simple, single-sided etched PCB common to the era. Very easy to work on with my Hakko desoldering tool. Since both capacitors and resistors have begun to fail, it’s only inevitable the rest of them won’t be far behind so I elected to replace most everything on the board, filters included.
On to the metal film signal capacitors, I’ve replaced them with identical capacitors with a higher voltage rating, between 400V and 630V. These should be more durable and last longer before requiring service again. Miniaturization means these beefier parts occupy the same footprint as their lower-rated vintage predecessors.
Next up, resistor replacement. I made a checklist to follow as I replaced each channel pair of resistors. I’m using high-precision metal film resistors instead of carbon composition resistors, all rated at 1W (versus 1/2W for the originals) and a maximum of 5% tolerance with many coming in at 1% to ensure long-term precision and stability. The original resistors were rated at Metal film resistors are less subject to drift in the first place, and selecting precise values ensures a long life and stability.
Some of the wiring had become brittle with age and needed to be re-soldered to the board after being flexed a couple of times. No big deal.
This project required about 5 hours of touch-time for the re-work and a couple more hours of research as it was my first 901 equalizer on my bench.
Quite a few replacement parts:
With brand new capacitors and high-precision resistors, this Bose 901 Series 1 Active Equalizer should be good to go for many years.
It’s likely that all the Series 1 and Series 2 equalizers could benefit from a rebuild at this point, and possibly even the Series 3 equalizers. (If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
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.
More Strange Coiled Wires [Mailbag]
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.
How Television Benefits Your Children [Vintage Ad]
Motorola, leader in television, shows how TV can mean better behavior at home and better marks in school!
Own a Motorola and you know you own the best.
Does that clown look terrifying to anyone else?
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