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Archive for the ‘DIY’ Category

Introducing Easy-Kits!

July 3, 2013 Leave a comment

I’m proud to announce a new venture from KF7LZE in partnership with KF7LZR!

Easy-Kits strives to provide easy to find parts to complete the projects you find on my site, not to mention offering other interesting projects and parts to help you make your build successful. We’re launching with a selection of the most popular items from featured KF7LZE projects:

Bose 901 Equalizer Rebuild Kits to fix your Bose Active Equalizer

RTLSDR with HF Converter Built In designed by KN0CK for very easy reception of weak HF signals

Tools and Supplies to make your DIY project a success.

We’ll continue to add new items – look for each project completed here to have repair items available from the Easy-Kits store, and many more products coming – accessories for the RTLSDR, speaker repair kits, audiophile grade discrete opamp kits, and a selection of repair parts for modern and antique electronics. Stay tuned!

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Partsim: Circuit Simulation Made Easy

June 17, 2013 1 comment

I came across Partsim, a free and easy to use circuit simulator that runs in the web browser, and would definitely encourage you to check it out if you’re looking for a tool to design and simulate circuits.

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It supports a wide variety of components, and even supports Digi-Key integration to make it easy to buy your project once you’ve seen its results. That’s a great feature! Anything to take some of the pain out of generating a Bill of Materials is welcome in my book.

Check it out! Partsim

Overloaded 1931 Transformer [Fried Electronics]

June 7, 2013 2 comments

I’m working on a 1930/31 Westinghouse WR-8 Columnaire clock-radio which had a bad transformer. The filter capacitors failed and shorted the high-voltage secondary, burning it open and causing a lot of heat and melting.

 

The replacement is a General Electric service transformer from a Radiola 82, which shared the same chassis. Production revisions led to an improved design over the original, with a separate 2.5V winding for the #45 output tubes to reduce hum and the rest of the RF tubes on their own independent 2.5V winding, so the new transformer will offer a noticeable performance advantage over the original, too!

A Most Convoluted 1934 Philco 66B Antique Radio Repair

May 31, 2013 9 comments

Recently, I took in a beautiful Philco 66B for repair. Manufactured in 1934, this chassis ended up in several different models – a couple of tombstones, a cathedral, and at least two console radios. They’re all 5-tube radios with the AM Broadcast Band and 1 Shortwave band.

Philco’s designs spanned the entire range of quality, with entry level sets being subject to various interesting design quirks of junior engineers and more advanced sets designed with tight tolerances. They did tend to use potted components longer than most other manufacturers that I’ve worked on, though, and that coupled with quite a few other issues made this one of the most challenging repairs I’ve completed with a lot of unexpected detective work.

The tube line-up of 6A7 78 75 42 80 is very common. The 78 tube is effectively identical to the 6D6 tube, although they were developed separately. After testing, this radio needed a new 6A7, 78 and 75 tube which I replaced from my stock. A few spiders once lived inside but were clearly long since gone and were vacuumed out easily.

Something happened to the speaker at least twice in the past. There’s glue, and two different types of tape applied to the cone.

The underside looked untouched, or was serviced only at an authorized Philco retailer which replaced with branded components. I couldn’t say for sure.

This model did have a terminal strip, stacking components in two layers. I had to disconnect a lot of wires to remove it to get at the connections below.

I replaced out of tolerance resistors and capacitors as normal, including the molded bakelite capacitors which I replaced with terminal strips and discrete capacitors. It would have been much easier to work on if Philco had switched to cardboard capacitors for all parts instead of only some.

Time for reassembly.

The first power-up was a success! In the sense that nothing caught on fire, but it wasn’t making any noise – even when probing various circuit points listening for activity from the speaker. I spent quite a few hours troubleshooting and it turned out to be quite a few very subtle problems which only turned up after a lot of diagnostics. Each resolved problem revealed something new.

All the coils checked out, and initial checks revealed voltage all the places I expected it.

As it happened, I accidentally flicked off the power strip with the workbench light instead of the strip with the radio on it, and glanced down in the dark at the tubes to see a bright blue glow in the #42 output tube. That was the first failure. It wasn’t readily visible in the black getter tube under bright lighting, and the tube tested good on the first pass. It must have finally given up during the time it was powered on for troubleshooting. I replaced it with one from stock, and was able to get a few clicks and some minor static, but nothing significant. On a hunch I tested the resistance from various points in circuit to ground, and quite a few had drifted – but the resistors had been replaced! In other cases, the end of a capacitor to ground was several hundred ohms. The 1934 solder joints seemed to have failed. After I tightened down my new grounds and re-soldered others, the resistance was fixed, but it still wasn’t making noise.

I removed a test jumper but noticed I wasn’t getting the right voltages, and it turned out now the #75 detector didn’t have plate voltage. Due to an error on the schematic from the draftsman in 1934, the capacitor’s connection to B+ was omitted.

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.)

With the jumper back in place, the radio powered up and immediately tuned static across the range and it was on to final tweaks. This radio is very susceptible to interference even with the shield in place, but it picked up stations immediately with a 3′ antenna although some were weaker than others.  I hooked up my signal generator and oscilloscope.

The Philco 66 uses a 460 kHz IF, so a nominal frequency of 458.7 kHz is close enough. The signal generator is from the 1950s, and even though it’s been reconditioned, it’s just not very stable – the frequency randomly fluctuated on either side of the center. I’d like to get a synthesized signal generator at some point. This was the same equipment that would’ve been in use at the time (or better), so it’s perfectly suitable for alignment.

Somehow this Philco managed to keep its metal plugs to prevent accidental adjustment to the IF trimmers. I went through the alignment and peaked the dial at the appropriate locations. Then, everything went back together:

This model of Philco went through quite a few design revisions over its lifetime, which complicated the repair efforts – each variation had slightly different arrangements to defeat interference this model was very vulnerable to. Even perfectly repaired, this radio showed sensitivity even to switching on and off a work lamp near-by and feedback from ambient electronic noise. That’s just the reality of modern electronics life – there wasn’t the same kind of EM spectrum pollution back then there is now, and antique radios often just don’t have the ability to reject interference the way modern electronics do.

Even with the possibility of interference, this Philco came back to life beautifully and tuned across the entire range of AM broadcast stations, perfect for listening to Oldies or the Mariners’ game.

If you’re in the Seattle metro area, I can help bring your antique radio back to life – contact me!

Australian TV Tuner Offers 4 RTLSDR Tuners in One Internal Card

May 13, 2013 Leave a comment

Over at RTL-SDR.com is a report of a fascinating development with the DigitalNow Quad DVB-T Receiver, apparently well suited to working as a very powerful multi-SDR solution.

I just wanted to let you guys know that the Digital Now Quad DVB-T Receiver (http://digitalnow.com.au/product_pages/Quad.html) works. It’s a PCI-e card with 4 tuners on it, linked up internally via USB. This has pleased me no-end – I might finally be able to get DAB+ working on my media centre!

I had to add the following line to librtlsdr.c

{ 0×0413, 0×6680, “QuadDVBT” }

I wouldn’t be surprised if this patch makes its way onto the slightly more user-friendly Windows pre-compiled drivers before too long. If you live in Australia, or don’t mind paying for shipping, you could have one for AU $179, with the Australian Dollar roughly at parity to the U.S. version. For $45 per tuner it’s a great looking integrated solution and is much more elegant than a USB hub and a stack of dedicated dongles. Looks like this one has a PAL connector.

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1934 Silvertone 1708A Repair and Restoration

April 23, 2013 Leave a comment

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.

Kegerator with Arduino-Based Beer Level Indicators

April 14, 2013 1 comment

Andrew  of Andrew’s Telephony/IT Blog came up with a fascinating project where he built a 3-tap kegerator out of a 7 cu. ft. GE chest freezer, an Arduino, and a variety of flow rate sensors to display exactly how much beer is left in any keg at a given time. It’s a very in-depth project, and it looks like it’s turned out both awesome and functional.

Go check it out!

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