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Archive for the ‘DIY’ Category

Evidence of Past Repairs

September 16, 2013 1 comment

It’s always interesting to see what’s happened with equipment that’s been worked on previously. It’s often a mixed bag with some great repair jobs, some that have a lot of room for improvement, and some that really just don’t measure up. I like to think I’m in that first category, but I let my work stand for itself backed up with a set of photos.

Sometimes I’ll get lucky and find a good quality repair or even an upgrade, as was the case in this Bose 901 Series I equalizer which had upgraded first filter capacitors.

There are quite a few that probably worked well at the time, but the repair has exceeded its working life, or something else has gone bad.

I believe these to be 1960s or 1970s film drop capacitors. They’ve been bad in every piece of equipment I’ve found them in, and are often even slightly physically discolored in the center. Not to mention, this one had the speaker wired incorrectly, so it’s unlikely it actually worked after whatever service was done to it that included this capacitor replacement. (Farnsworth K-262P)

That type of bad capacitor turns up in the Bose equalizers, too. In this case, the one with the upgraded filters, had original defective film capacitors. (#31131)

As did this one.  (#35793)

This unknown 1940s Gilfillian radio had been serviced a few times. The original paper capacitors are intact, then later sealed paper capacitors, and finally the same ’70s era film capacitors were installed.

Sometimes it’s a little less pretty. Like when a previous technician destroys a solder pad, and manages to leave a pretty poor solder joint after scraping a new pad on the trace. I suspect those two things may have been connected. This work was performed locally in Seattle, although I don’t specifically know which shop.

It was pretty common back in the day to add additional capacitors to a circuit, without removing the old ones. This sort-of worked, but was very poor practice. This poor http://blog.kf7lze.net/2012/03/10/1936-grunow-566-repair-finished-part-2/ had this treatment: the on-chassis replacement failed and was replaced with the 8/16…and then three more 10s across different places in the circuit, including one connected in parallel with the field coil for some reason.

That same radio, though, did have the electrodynamic speaker replaced with a (very beefy) permanent magnet speaker and substitute resistor in what is actually pretty good, and likely a modification from the 1940s, so fairly period.

There’s also this, where “they should know better”. A shop nowhere near-by serviced this one fairly recently, and it failed shortly thereafter. There are several eras of components installed, but most notably, the newest set was installed after it was a well-understood best practice to replace all those sorts of components preemptively, as if they aren’t bad now, they will be soon.

I like seeing the history of previous repairs and doing some detective work to find out why that might have happened, but sometimes it can be frustrating to have to go in and fix mistakes which might have been the reason these devices fell out of service in the first place.

Do you have any stories about surprises left in your work by someone who’d been there before? Please share!

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KN0CK HF Converter Rev. 4 is Released!

July 25, 2013 1 comment

The complete Rev 3 KN0CK HF Converter has sold out and been discontinued. That’s okay, though, because it’s been replaced with the KN0CK RTLSDR for HF Revision 4! This model incorporates a lot of the feedback from the community about the old stack – especially the expanded tuning range, since the new model can now upconvert from the 6m band (54 MHz) versus 30 MHz with the previous model. It’s even smaller and still manages to be easier to manufacture, too, and it retains the core Mini-Circuits pre-amplifier and 120MHz local oscillator frequency.

Receiver Closed Up

Receiver Opened Up1

It’s a pretty great performer, too.


Buy Here

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!



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.

partsim snip


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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