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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 In most radios even just a year or two later, the 37s would likely have been replaced by 76s in a high-end radio like this one.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.
Depression-Era Radio: 1934 Grunow 460 Tombstone
I’ve had this Grunow 460 sitting in my queue pending for a little while and it’s finally on it’s way home looking great! There was a fun extra bit of detective work to identify and solve the issues that came up along the way.
The Grunow 460 was made by the General Household Utilities Company at the height of the Great Depression, in 1934. I follow eBay for Grunow radios specifically and saw a few of these for sale last year and have been curious what they’re like to work on. The cabinet materials do reflect being made at a time when many consumers were incredibly cost-conscious, and this was the most economical radio offering they made that year. The General Household Utilities Company did deliver an attractive design though with the contrasting color diamond in the center of the grill cloth and a light bevel around the dial face.
The design is a very simple 4-tube superhet receiver using the tubes 6A7 6F7 42 80. It’s functional, and would’ve worked well enough for daily use in cities with near-by radio stations. The 6F7 tube contains both the pentode section IF amplifier stage and a triode section Detector/1st Audio stage. There’s a lot of space under the chassis but a lot of components are stacked into one side.
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The chassis tags were in pretty good shape.
Unfortunately, one tube broke in shipping, the 80 rectifier. It turned out, with further testing, that all 3 other tubes needed to be replaced as well, so this radio got a full set of replacement tubes and should be good to go for many years.
Intake checks revealed a few problems and evidence of dubious repairs using highly variable components:
This connection to a floating ground lug wasn’t even soldered.
an IF transformer was loose
The exposed antenna coil was broken in several places and partially unwound. Fortunately, there are universal broadcast band antenna coils available, so I ordered one of those from my supplier and continued working.
I replaced the antenna coil with the universal model, attaching it to a terminal strip by a solder lug and reusing the existing screw and chassis hole:
You can clearly see where the power transformer was replaced previously in the radio’s life – the new one uses a vertical core; the stock Grunow transformers use a horizontal core and the bell housing end fits through the chassis. There’s no difference in how the styles work, just a different shape. The replacement is held on with only 2 of the screws since the base won’t line up anymore, but it’s a small transformer and the two mounts are plenty strong.
Also note there’s only one visible IF transformer can. Most radios have two (or more) of those cans which provide shielding and protection for the IF transformers. This Grunow has a single shielded IF transformer with the second IF transformer unshielded under the chassis, another way they saved costs in manufacturing and passed the savings onto the consumer.
This radio needs about a 6′ wire antenna attached to the white wire antenna terminal on the back. The radio was missing tube shields when it came to me, which were absolutely necessary for proper operation, but I was able to supply them from stock. They’re important to keep the tubes from picking up interference directly, or introducing interference to the other tubes – some generate a fair amount of noise if you leave them unshielded, and you’ll end up with a radio that squeals but can’t receive much else.
Not pictured, I also replaced the cord with a new polarized cord, the old one was cracking.
That did it! It was time for an alignment. I peaked up the IF trimmers which were off by a fair amount – the volume increased significantly after the adjustment – and fine-tuned the other adjustments. The dial tracks quite nicely within 10kc after the adjustments. I didn’t take photos of this process, unfortunately – doing an oscilloscope stage alignment on this radio would take a lot of time but not give any benefits over doing the classic signal generator/output meter checks.
At this point, I had the radio up and playing and was getting ready to send it along when it cut out. Back to troubleshooting. It turns out a section of the candohm resistor had opened. It’s likely it just failed at a terminal lug as nothing was shorting anywhere else in the radio to have caused a damaging current draw, it just died. Fortunately I had resistors on hand to replace it and mounted a terminal strip to use as new tie points.
The antenna coil is pretty cheaply made, and the solder lug was only weakly glued onto the cardboard coil form. The glue separated when being adjusted, so I was left with the last resort of wrapping it with electrical tape. It’s not readily visible and doesn’t harm the operation and is the most cost-effective fix for the problem.
This one will polish up nicely and look great on its owner’s shelf! I really like working on these Grunow radios – they have interesting cabinet designs, circuitry that usually has a couple of interesting tricks to it, and very good published schematics. I’ll be fixing up my World Cruiser when my workload dies down a bit.
Up next are two more Bose 901 Series 1 equalizers, a Philco 66, and a Silvertone 1708! Expect to see new articles more often than the last couple of months.
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.
Referred Radio Repair: 1950 RCA 9X561
A friend I’ve made through the antique radio community is also a repairman himself, but ran into some scheduling difficulty with a piece of work on his bench and referred the chassis over to me for service while he dealt with his scheduling conflicts: an RCA 9X561, from 1950.
This RCA is a simple and straightforward radio which came to me with 75% of the work completed; my task was to finish off a few odds and ends to bring it up to 100% condition. This turned out to be a bit more interesting of a task than I’d suspected it would be.
I apologize in advance for the poor quality of all photos that will appear on my blog now and into the foreseeable future. Both of my digital point and shoot cameras have since given up the ghost after 5 and 7 years of service respectively – I’ve been taking these photos for quite a while using my cell phone’s built in camera. It delivers acceptable quality in most situations, but unfortunately, its lens has been scuffed beyond the point of buffing out and all future images are going to be somewhat poor definition. (If you find my writing here useful and care to donate securely via PayPal, it would be much appreciated!)
The radio, speaker and chassis arrived alone without the case.
Two capacitor leads were broken, as was a lead from the second IF transformer.
I replaced it with a new segment of wire to replace the now too short broken one.
One of the capacitors which had come disconnected was C3, a 0.05uF bypass capacitor located physically near the oscillator section shown in the center of the photo below.
I reconnected and set out the process of final tests.
The IF chain was fine, but the radio seemed not to want to oscillate. I went through a fair bit of troubleshooting – inject the IF signal into the cathode and grid of the 12SA7 mixer to replace the local oscillator, swapping known-good tubes, using another radio to find an oscillator beat note – nothing at all. The odd part was that it wouldn’t work even with an injected oscillator signal – something which should definitely get the radio going again, and generally identifying the problem with the oscillator.
This one is very simple. A set of coils, a resistor and a capacitor. The coil was intact on both ends and the resistor and capacitor were within tolerances, so I took another peek under the circuit. It turns out that I’d made a mistake reconnecting the broken lead. C3, the bypass capacitor, was connected at the junction of C2 and R2 (indicated above in red) which had the result of completely bypassing the local oscillator to ground – including when I would inject a signal from the generator on pins 5 or 6. The correct location for the connection was the lower left circle indicated in green.
I moved the capacitor from the converter tube over to a tie point on the secondary of the first IF transformer, and the radio pulled in stations instantly.
That missed connection was an easy way to kill a few hours troubleshooting, but it all turned out in the end. The radio is back to my friend to return to his client later this week.
First Look: 1934 Grunow 750 “World Cruiser”
I’m getting ready to sell some of my radio collection to make room for new ones, but until that happens I’ve been under something of a self-imposed moratorium on new purchases. This one came up on eBay a little while ago for a steal so I cheated a bit and had it sent my way.
This is a huge tabletop radio – a “grande” coffee from my local coffee shop is provided for scale. It’s a 7-tube model using the tubes 6D6 6A7 6F7 75 76 42 80 – the AM Broadcast Band and the lowest Shortwave band are double-tuned with a single-stage RF amplifier for extra selectivity; on the two higher shortwave bands the second stage of tuning is disabled to increase sensitivity. With an 8″ speaker, it should sound pretty great. There are a couple of minor scratches on the front that will buff out nicely, and it’s otherwise original – this should be a nice, fairly quick radio to bring back to life.
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!
Fixing a 1934 Simplex Model P Dual Band Vintage Radio [Antique Radio Repair Log]
I picked up this 1934 Simplex Model P Dual Band radio for my personal collection about 7 months ago, but haven’t worked on it much until this weekend. It sat on my articulated chassis stand in the corner waiting for work while I took care of other projects, but its number finally came up.
It’s not in the best shape by any means, with non-original knobs and some chips in the veneer – but the price was right, and the deco-style tombstone cabinet has a lot of potential. I did get an excellent deal purchasing this model due to the cabinet condition. The tube line-up is the ubiquitous 6A7 6D6 75 42 80, receiving the standard AM Broadcast Band and Shortwave 6-16MHz. While that tune line-up is found on low-end radios all the way up through fairly premium tabletop sets, this one is a higher-end table radio with a glass dial (not celluloid), four knobs and a continuous treble roll-off tone control.
You’ll see in this next photo why I wasn’t in a huge hurry to dive in – the wiring (which looks factory to me – there are few replaced components visible) is a complete rat’s nest of free-floating solder joints and spaghetti wiring.
The wiring looks more intimidating than it actually is, so I dove right in, first replacing the capacitor from AC to Chassis with an X1Y2-rated safety capacitor for noise suppression, then continuing on through the rest of the radio. This model uses a negative-filtered power supply with an RLC network between chassis ground and the center tap of the transformer; less well-engineered radios frequently used only a resistor or a field coil in that location.
The tie points for the RLC network weren’t very convenient, and were getting pretty beat up, so I installed a terminal strip in place of the dead filter can, and moved the electrolytic capacitors and the filter network to their new home.
At some point in the past, the bottom lug of the volume control had snapped its solder joint off the chassis. This would cause the volume to increase (as the volume control wouldn’t be functioning as a voltage divider anymore), so I resoldered it back using my heavy-duty soldering iron. Then continuing with the capacitor replacement.
With all the important capacitors replaced, it was time to reconnect the speaker. Unfortunately, but perhaps not surprisingly, the primary of the output transformer was open. This could’ve happened through age or excessive current draw, but fortunately I happened to have an extra similar-sized output transformer for the #42 output tube on hand that was an easy substitution.
With the speaker repaired, I set the radio up and turned it on – no smoke! But no sound, either. I did the “stage test”, tapping a screwdriver to the top caps of the tubes from the 75 working towards the front-end. A click in the speaker means that stage is passing signal, and the click stopped at the 6A7 converter. (The 6A7 tube, affectionately called the converter, is the mixer+oscillator tube responsible for converting the incoming RF to the radio’s lower Intermediate Frequency or IF.)
It was quite dead.
With the converter replaced, it did start to receive some stations – but weakly and with distortion. The only capacitors I hadn’t replaced were the molded paper caps near the detector, so those were the next to be replaced with a pair of ceramic discs. While there, I also replaced the associated resistors, a few of which had drifted more than I’d like but not technically outside their tolerance.
The final step was to address the slipping dial. The cord was intact but had lost some tension due to the spring stretching and the mounting gaskets sinking. This was an easy fix, though: I re-tensioned the spring using a trick I learned: just hook it half-way through. This cured the slipping dial problem perfectly.
A reassembly power-up before going back in the cabinet:
And back together!
I may work on the cabinet at some point in the future, but the real reason I went ahead and fixed this one is I needed the chassis stand to be free for another incoming project, and wasn’t going to allow myself just to push this project down the road for another day. This project took about 10 hours of hands-on time, and the radio should be good to play for many years in the future. After a full alignment, it plays beautifully and fills the room with clear, selective sound.
Feel free to leave comments or questions!
Repairing the Farnsworth K-262P Vintage Radio [Antique Radio Repair Log]
A local client engaged me to repair his antique radio, a Farnsworth K-262P from between 1948 and 1951. His wife bought the radio to listen to records, and it came to me with reports that it had been working for a time but had inexplicably gone bad with distortion and fading, and the dial was very difficult to turn.
The Farnsworth (the same as the inventors of television) is a 6-tube AC/DC (series string) radio. Unlike all but 1 set in my portfolio, this radio does not use a power transformer but instead connects all the tube heaters together in a series string across the mains voltage AC line directly, then rectifies the AC voltage directly into around 170V DC. This is a less expensive arrangement than using a transformer, and the K-262P occupied the territory about one shelf up from the most economical offerings, so it’s to be expected. Even though it’s a more economical design, AC/DC radios can produce excellent sound and liven up a room the same way a transformer powered radio can, so it’s no matter for the effective performance. It uses the tube line-up 12SK7 12SA7 12SK7 12SQ7 35L6 35Z5 (notice how the voltage values – the first digits – add up to around 120?) with the first tube an untuned RF amplifier, a loop antenna, and of course there’s a phonograph on the bottom in the pull-out drawer.
The radio is of course the most important part, as without that nothing else would work, so that’s the first step in the process. I pulled it out from the cabinet and found it’s in mostly good physical condition, not too much dirt or grime or rust from the years.
The underside definitely had some evidence of work in the past – replacement electrolytic capacitors (the large yellow cylinders) and either ’70s or ’80s issue Cornell-Dubilier film capacitors shown in red (with one 1960s era replacement in the very far top left.) Cornell-Dubilier capacitors are Made in the USA and are generally a good quality, and these are modern capacitors, so my first instinct was to replace the electrolytic caps and the one old cap and see what happened.
It was on to testing tubes next, and I did discover that three were bad – the oscillator, 12SA7, the output 35L6, and the rectifier so I replaced these from my stock and reception improved to the point I could pick stations out of the noise but it was still very noisy. Then I noticed a shocking wiring error made by a previous repairman: the speaker was connected directly to the output tube, instead of through the transformer like it is supposed to be. This results in about a 1500:1 impedance mismatch and a ton of distortion and reflected power. This is a very bad mistake, and is likely the reason the output tube was failed when the radio came to me. I re-wired it correctly and powered it up, and now the distortion changed. It sounded like the kind you get with a leaky capacitor applying random voltage to different tubes.
Closer inspection of the new capacitors revealed they were in fact also bad! In this photo you can see the reddish capacitor with visible discoloration on the top side. With that, it was off to replacing the bad CDE caps with replacements from my stock. I also replaced the vintage Sprague while there, even though it wasn’t obviously bad, if the similar age ones were going it is only a matter of time.
With the capacitors all replaced, the radio powered up just fine and sounded perfect. This did leave a couple of interesting issues, though: the previous repairman had put a resistor in series with the volume control, limiting its range on the top end through all inputs. This resulted in the line input being too low with the radio proper volume. After I removed the series resistor, it became apparent why it had been there – with very strong local stations, so much signal was coming through that the radio played nearly full volume even when turned all the way down.
To compensate and knock the signal down a bit further, so my client can listen to the radio with his children asleep, I added a 100K resistor to the output of the second IF transformer ahead of where the RF hits the detector. This knocks the radio volume level down significantly but leaves the input un-attenuated. Finally, it was time to move on to working with the phono. The mechanism had been replaced in the ’80s to let it play 33/45/78, but uses a magnetic cartridge. The older phonograph pickups used a ceramic cartridge which put out a much, much higher level output; the aftermarket phono was far too quiet to be audible. With the volume turned up as high as it would go, you could just barely make out the absolute loudest parts of the songs playing.
I used my Eve 6 album for testing; I don’t own a record player myself nor do I collect vinyl so I only have a few discs on hand for testing. It quickly became apparent I needed a phono preamp. Fortunately, I had one lying around; it amplifies from 30mV to 2V and was enough to push plenty of volume up to the amplifier.
The current owner added the extension cord and master-off switch in the back; the preamp can rest nicely on the floor below the speaker. And it cured the volume problem! I played through the record, then switched back to the radio for a 4-hour burn in while I enjoyed my evening; at the conclusion of the test, she’s ready to go!
And back into final form.
This radio was an interesting repair experience, on account of the fact someone had been in there before and had done a poor job of it, making several mistakes along the way. It just goes to show – not everyone who says they can fix something should really be trusted with the job! My work speaks for itself with full photographic documentation of the process from start to finish so you can know you’re getting quality professional service. It’s also a great example of the benefits of flat-rate pricing as a repairman charging by the hour would quickly have added up to a very large bill!
This radio is returning home to live with its owner next week, and should perform admirably for many years to come. Feel free to leave any comments or questions you have here, or inquire about engaging me to fix your antique radio.
1938 Philco 38-C-12 “Coronation” Radio Repair
I’ve had this radio on my bench for a little while, and she’s finally back up and running – and it plays great! It’s a 1938 Philco 38-C-12, special edition to commemorate the crowning of King George IV and Queen Elizabeth.
Since we don’t have a King and Queen of the United States, it’s fairly obvious this radio isn’t from around here. This particular model was made in Canada and brought to the U.S. sometime later, and was actually rediscovered in Florida. I can only assume a retiree seeking warmer weather brought it with them sometime in the last 75 years, and that’s how it traveled the long distance itself.
Period Canadian radios came with a Ontario Hydroelectric Power Commission approval stamp, which makes it easy to tell their origin.
The power transformer also gives it away – the 25Hz transformers used in some parts of Canada at the time are considerably larger than the 60Hz transformers found on most American radios. (You can use a transformer rated for a lower frequency, as the greater amount of metal makes it less likely to saturate, but you generally can’t use a transformer rated for a higher frequency on a lower one.)
This radio has seen a little bit of service in the past – both can capacitors have been removed; one was cut open and used to hold its replacement in a partial restuff and the other was replaced below the chassis. All in all, though, the radio came to me in very good near-original condition. This radio uses five tubes 6A7 78 75 41 84 – which is a little different from the typical tube line-up, but allows a smaller power transformer to be used as the rectifier can take advantage of the same 6.3V windings for the rest of the tubes, rather than needing a dedicated 5V winding. Its schematic is the same as the American Philco 38-12.
Capacitor replacement was uneventful on this radio, but I also had to replace the tuning capacitor mounting gaskets. These isolate the frame of the tuning gang from the chassis to keep it from shorting out. Over time they shrink and harden. Renovated Radios has many types of reproduction chassis gaskets. The new ones have restored the right tuner height and isolation. This part was pretty labor intensive, but needed to be done for best performance.
You can see in the next shot, the braided ground strap from the tuning gang ready to be tacked back to the RF ground. Desoldering braid works perfectly for this application.
On top of all these problems, the dial pointer shaft was seized up. I removed the shaft, cleaned with 99% isopropyl rubbing alcohol, re-applied lubrication, re-mounted the shaft and restrung the dial cord. This one is very easy – spring mounts on the gang itself, and 1 1/2 turns around the tuning shaft for tension.
As you can see in this shot…the reproduction gaskets don’t fit 100% the same way the originals did. They work, though, and this part of the radio isn’t visible while it’s in the case. So it’s time to put it all back together and get the shields on:
Back in the case:
Fully serviced, this radio will continue playing for many years!
Feel free to leave comments and questions.
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.
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