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.
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.
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.
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.
I started working on a Jamo MPA-101 amplifier back in August for a friend and after some early work it sat for a while. He’s re-doing his audio system at home so I spent a few hours to finish troubleshooting while waiting on parts for every other project on my bench right now.
The MPA-101 is a nice compact desk amplifier for a stereo speaker system or a subwoofer. It’s 50W/channel into 4 Ohms or 100 into 8 Ohms bridged mono and has a very quiet cooling fan which is almost totally silent and doesn’t even come on all the time. Great understated styling, too. They’re still in production and you can even buy one on Amazon for about $200. This one was $20 at a thrift store, if I remember the story.
Jamo is a part of the Klipsch group, and these amps are pretty well regarded. They’re daisy-chainable with cascading inputs, so several of these would make a nice independent amplifier system when paired with a digital speaker controller or similar.
I e-mailed Klipsch and they sent me the schematic to help with the repair process. You can download a copy here.
The amp wasn’t coming out of protect or when it was, it was incredibly distorted and with basically no volume control, only loud crashing. It looked like the power supply had suffered a failure at one point, with the resistor being discolored. Some of the capacitors looked pretty suspect so I shotgun’d it and replaced all the capacitors on this board with new ones. The power supply board was solder jumpered to the main amplifier boards at an edge connector, which was a pretty annoying connection method.
Jim KJ7QT wrote me a note talking about his experience with a similar problem on this model:
I pulled the boards out of the amplifier, and carefully examined them with a 10x magnifing glass – a 220MF electrolytic capacitor (labeled C39 on the schematic) showed signs of leakage at its base, and less than 3 Ohms resistance across its plates in circuit – which should have been around 1K Ohm based on the value of resistor R88.Capacitor C39 is part of a sensing circuit that takes 32VAC from the main transformer, rectifies it to a 12VDC reference voltage, which is compared by the amplifier’s protection circuitry. I’m assuming that this circuit is intended to sense an overload on the transformer caused by a short-circuit and shut down the amp – so when the capacitor failed, the voltage dropped, and the amp was shut down.We also replaced resistors R78 (2.2K, 2W) and R85 (2K, 2W) with 5W parts, upgraded R90 (39 Ohm, 1W) to a 2W part, and re-flowed the solder joints on all of the main power transistors, as the back side joints were quite dry, and one had been visibly arcing under load.
I did also get my new Rigol oscilloscope, which really let me see what the amplifier was doing at each internal stage.
I used Audacity to generate a 600 Hz test tone about 25 minutes long and saved it as an MP3, then played it back from the laptop. The garbage waveform it produced and the laptop rendered makes me want to move my HP 200CD precision audio oscillator up the repair queue, it needs its power supply reconditioned and to be calibrated. One probe was attached across a dummy load at the output terminals. The other probe I held on to and used it to probe the amplifier stages from the back forward. The idea was to compare the waveforms and see where the distortion was being generated in the circuit.
It’s almost 600 Hz.
I started probing the input ICs on the preamp stage.
It was handy having the entire schematic visible at the same time, more or less, working right under it.
Output trace with the volume turned about half-way up. Terrible distortion.
Even worse when the amp was being adjusted
The signals phase better when both are connected together. I assume it’s something to do with the triggering; I’m still learning how to use the new oscilloscope since even this functionality was just not possible on my old EICO 460. This new scope has around 60 years worth of improvements built in.
Here I am probing one of the driver transistors on the amplifier board. The distortion has cleared up a bit it seems.
And at an earlier stage. At this point all I’d really done was clean some connections, cycle the volume knob completely a few times, and reseat connectors but it looked like the amplifier stage gain was working properly. I decided to switch to some music.
I’m not entirely sure how to make the scope snapshots a consistent size. The software isn’t the most intuitive. The communication protocol has been pretty well hacked, though, someone might write a replacement UI for the scope. I switched to probing the volume control, since the distortion only came back when it was moved.
It looked like the volume control might have a broken track internally. It worked fine when not being touched, and must have been worked into making a better connection by moving through its travel but was still very badly distorted and didn’t seem to be getting any better at the low end. I was feeling confident enough to attach an actual speaker to it at this point.
I could hear the distortion, but it sounds much better than it did before.
I ordered a http://www.mouser.com/Search/ProductDetail.aspx?R=RV122F-20-15F-A50Kvirtualkey14860000virtualkey313-1240F-50K from Mouser for about $4, and set to replacing.
I removed the control from the mini-board it was mounted on. Here you can see I split the control to see the carbon tracks under it.
There was one minor hold-up where the new control has a different footprint than the old one. Not the end of the world: each control has 3 wires, so I used a section of Ethernet cable and removed the extra pair. It’s about 4 inches long. These new controls unfortunately had the reverse pinout of the previous ones so I had to remove and re-solder the outside connections for each one to make the control work in the correct direction.
I mounted the potentiometer board to the LED board using a common screw and hole. That’s convenient!
I reassembled everything and set to test waveforms with speakers hooked up and my oscilloscope. Yellow is left channel, Blue is right channel. 600Hz synthetic wave software-generated MP3 tone:
Alternative Endurance streaming station:
Looks perfect to me. It didn’t sound like there was any excessive hum or buzz in the dead time. The original volume control had an additional grounding lug which the replacement doesn’t have. I’m betting this isn’t a significant issue, but if it is, I can reconnect it fairly easily.
Looks and sounds great. These large 6800uF 50V snap-in capacitors fit within about 1mm of the footprint, it can be kind of hard to find good-fitting parts out of all the possibilities out there.
This was a really fun project where I got to use a bit more in-depth troubleshooting techniques, and the end result sounds as good as you’d expect from something by the Klipsch group. I’m excited to hear it out powering a set of Bose 901 Series 1 vintage speakers.
I’m working on another Bose 901 Series 1 equalizer. It looks to be the same manufacturing run as the first one I wrote up last July, uses the same components, and those components look like they’ve had the same failure mode.
The rather dubious EM-branded capacitors have the same discoloration on the top middle I saw on the other Bose, as well as on the Farnsworth K-262P which had been repaired once before. I’m curious what the physical failure is, as the discoloration would suggest to me overheating, but that seems very unlikely in this circuit and application. Newly manufactured replacements aren’t likely to have this problem as the film technology has improved significantly in the last 40 years.
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.
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.
It’s interesting working in a radio that’s been repaired before in its life. The quality of the work varies greatly. I’m working on a Grunow 460 (which will be written up later) and I found a late 1930s repair attempt which hadn’t been soldered – just wrapped around the end of another component’s connection to a lug.
I doubt this helped the radio’s operation much.
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.
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.