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.
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.
A little while ago, I picked up a set of seven LCD monitors in various states of not working to work on repairing and maybe eventually reselling. The first one was quite easy – it just needed a wire reconnected internally and works perfectly. I grabbed the second one, a ViewSonic VP191b. It’s nothing hugely special, 19″ with two VGA and a DVI offering resolutions up to 1280×1024, a 16:10 aspect resolution.
Nothing I’d use as a main monitor, but a decent consumer device. And it turns out this one’s a little more complicated to repair than the last few I took care of.
Taking the case off, you can see the high voltage power supply which takes 12V DC and converts it to a thousand or so volts AC to power the backlights; in the center is the logic board and on the right the switching power supply.
The power supply is encased in a plastic insulator to keep it from shorting to the case.
Unfortunately, this one wasn’t in as good of shape as the others. In addition to having a few bad capacitors, it turns out that the resonant transformer is also bad (the yellow square to the right in the photo above.) If this supply lost regulation when a part failed as it was running, it could cause a nasty cascade taking out transistors, transformer windings, anything really and that looks like what happened.
The capacitors used in this model are:
- 470uF 25V
- 1000uF 16V x2
- 470uF 16V
- 120uF 400V
I don’t have a spare resonant transformer, and wasn’t able to locate another one online…maybe the end of the road? Nope! I checked the voltage ratings on some of the logic board components and they were all rated 16V…the rule of thumb for capacitors is you overrate their voltage by sqrt(2), or 1.414 times. These were rated 16V, so I estimated from this the logic board wants a 12V input. That’s handy, and pretty easy to supply.
I need a 12V bench supply for a few other projects I have coming up, so I ordered one from eBay. This one’s inexpensive and considerably bigger than I need, but it’ll be good in the future. The ViewSonic draws ~35W, and the eBay power supply can supply up to 120W. It came without connectors, so I hooked up a line cord socket that I’d scavenged out of a dead Ethernet switch. As always, when you’re working with electricity, take proper safety precautions – don’t touch power things while they’re energized and double-check your connections.
I’ve removed the power supply from the back so it doesn’t get in the way, then depopulated it to save the remaining good components for something else:
I ended up recovering 4 small signal transistors, a bridge rectifier, several misc. resistors and small capacitors, two choke coils, an unidentified standard transformer and an NTC thermistor.
Here’s a shot of the back with the power supply removed, ready for other connections:
For the first trial, I’ll just run jumper wires.
And let’s see…
Looks like it works! My estimate about the voltage proved correct. I mount up a terminal strip just like I do with a radio and wire the new power connection to that so it can be accessed from outside the case later. I’m using a computer power molex as the new connector as that’s what I have on hand, preserving the coloring.
Testing out one more time before reassembly:
It joins the ranks of my other spares I’m not sure what to do with yet:
Mission accomplished. I’ll just get it a power supply of its own, a power brick this time, and it’ll be finished! You can only tell it’s been worked on by the dangling wires hanging out the bottom.
After seeing the repair I made on the Samsung LCD monitor, a friend gave me a few-years-old Westinghouse LCD/TV that had quit working – it wouldn’t power on anymore. It’s a Westinghouse SK-19H210S, 19″ LCD accepting VGA or HDMI up to 1440×900 resolution (somewhat smaller than true 1080P) and can also tune ATSC and NTSC television signals to receive HDTV over the air.
It’s apparently a very known fact this one has a weak power supply – all over the web. I opened it up and grabbed the power board:
Tucked away all in the back is one capacitor that’s visibly failed, which means it’s likely several are bad or will be soon.
New parts arrived from Mouser.com:
Using my trusty Hakko, I replaced six capacitors. 4 caps in total showed signs of leaking from the bottom as well (discolored board below), 2 seemed okay but I replaced anyway because why not. I’m getting better at using the Hakko and doing this kind of PCB rework in general, the entire process from start to finish only took about 15 minutes this time.
1000uF 25V x 2
Interestingly (or maybe not), these bad caps were the same brand as the bad caps from the Samsung: CapXon. Obviously those have reliability problems, or are just the cheapest they could buy.
Reassembled and powered on. The first power-up would come online but drop off immediately and it was making a hissing noise; it turns out I hadn’t firmly connected the backlight leads. After fixing that, I snapped everything back into place. Consumer electronics these days aren’t made to be opened up, so the case doesn’t quite fit back together the way I’d like it to around the control panel on the side, but it’s not visible unless you look for it fortunately.
Another one fixed! This one was about $12 of parts. Looks like this one goes for around $80 these days, so I’m half-way to getting my money’s worth out of that rework station already.
My next TV repair will be somewhat more ambitious. I got this Samsung HL-P4663W, a 46″ DLP (720p) HDTV for free from Craigslist. It needs a new bulb, and some other rework, and it’ll be worth a few hundred dollars after I get it sorted. I don’t intend to keep this one (as I already have a 46″ Samsung LCD that does full HD resolution) but just to repair and sell most likely.
I found a Samsung 225BW LCD sitting on top of my apartment’s dumpster, and figured I’d drag it upstairs. It’s a few year old model but it’s better than the current older Dell LCD that I’ve been using (1680×1050 versus 1440×900). A quick check showed that it would power on, sort-of, but the power light would flicker constantly and there was no backlight.
I popped it open, suspecting a problem in the power supply – and turns out that was right. Several capacitors on the board were showing signs of failure. Capacitors are the main component I replace in the vintage radios but cost-cutting OEMs are often known to use caps that fail after only a few years when new to save a few cents on each part that goes out the door on new things as well. In this case their 330uF and 820uF @ 25V caps had failed and the logic board was no longer getting good power.
Modern electrolytic caps fail by bulging and leaking out the top and/or the bottom, it’s easy to see at a glance. The top two are bulging and leaking; the bottom ones are bulging only which is a bit difficult to make out in the photo.
This project is one of the reasons I bought a Hakko 472D desoldering tool. It’s made for reworking through-hole and point-to-point boards, and works by melting the solder and then applying a strong vacuum through the center of the nozzle sucking it out of the way and cleaning the connection. It wasn’t cheap, but I thought it’d be important to have one of these as I do more types of electronics hobby work. I tested it out on an antique radio and it works perfectly for the annoying old joints.
This board is pretty easy to work on, the components are widely spaced and marked.
Even though it’s not bad, I’m replacing the large main filter as well – just in case. It’s the same brand as the failed ones.
Here I’ve depopulated the bad components from the board and have placed the main filter back in position, with the old one above it for comparison.
The new caps are larger than the old ones – for the same ratings, a larger size capacitor is going to be a bit more durable. For example these 330uF 25V models:
Slid the components through the top, spread the leads to hold them in position while soldering and reattaching:
Bad planning on my part meant I forgot to take a photo of the board post-repair, but it only took about 30 minutes to do the entire thing – most of which was spent figuring out how to adjust the Hakko. And for the power-up:
Success! Back to life. This LCD goes for around $150 online even today and I’ve been meaning to add a second monitor to my desk anyway, so I’m about 1/3 of the way to recovering the cost of that desoldering station after the first project. One down, two to go….This project required 3x330uF 25V capacitors, 2x820uF 25V capacitors and 1x150uF 450V capacitor which came out to $9.83.