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Archive for the ‘DIY’ Category

Homemade North Korean Defector’s Radio [Photos]

July 25, 2014 Leave a comment

Radio might be taken for granted here in the U.S., but in other parts of the world it’s still a vital link in receiving underground information in repressive regimes. This story out of North Korea from NK News shows just how important it is in some other parts of the world.

Doesn’t look like much, and apparently it was pretty tough to operate without a tuning dial and with such a small tuning knob on the front:

The radio therefore leaves users to depend totally on delicate hand skills to turn the manual module in micro-inches every time. NK News staff could, however, easily experience the frequency changing with slightest turn of the module either to the left or right.

That knob would be controlling the variable capacitor on the board, shown bottom left:

This is interesting. It’s clearly an adapted PCB from another device, which looks to have been modified with a few additional capacitors and other components on the front end and possibly the amplifier area. It’s tough to tell how much is in use for this modification, though. This might have been from a tunable radio smuggled in elsewhere and modified, or could have been a state authorized radio modified to tune a different frequency range.

It looks like two new coils are in place, which could correspond to an antenna and oscillator coil. These would definitely need to be replaced to change the tuning range of the radio. They could be coils to adjust an RF tuning range for two stages of an RF amplifier, though. It’s tough to tell from the circuit board the principle of operation of this radio – it could be a superheterodyne receiver, which would offer a bit better performance and would be able to tune the FM signals as well as AM/Shortwave, but it could also be a tuned radio frequency detector which would be able to pick up the AM and Shortwave bands only in most circumstances.

The superhet would leak a low-level oscillator signal that could potentially be picked up by counter-intelligence agents to determine if someone was listening to a prohibited broadcast (although the average law enforcement officer in North Korea might not be sophisticated enough to operate direction finding equipment to find such a receiver) but would be able to pick up FM transmissions as well. The TRF type radio wouldn’t leak a local oscillator signal but might not be able to effectively decode an FM transmission depending on its construction. Really, it’s likely there are a variety of underground radios with different capabilities depending on what can be scrapped together at the time.

This all goes to show that hacking circuits is more than just a hobby – it can have real, life-changing implications for people in other situations using the same skills. That’s pretty powerful.

For more about this particular radio, visit the Original Article at NK News.

For more about underground information access in North Korea in general, The Atlantic has a great piece in their April 2011 issue.

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Now for Sale: Westinghouse H-126 Little Jewel “Refrigerator” Capacitor Replacement Kits

July 2, 2014 Leave a comment

Rain City Audio is proud to announce a new product offering: capacitor repair kits for the Westinghouse H-126 Little Jewel / “Refrigerator” Antique Radio!

This repair kit contains the most commonly needed parts to refurbish your Westinghouse H-126 Little Jewel / Refrigerator radio. You’ll receive 13 modern, high quality replacement film and electrolytic capacitors to bring your radio up to full performance. Take the guesswork out of fixing your own collectible model of Little Jewel, and save the hassle of using a parts site like Digi-Key,  and order a kitted set of parts that’s ready to replace.

Included in this kit:

1 x 0.001 uF (You’ll receive 0.001 uF)
1 x 0.2 uF (You’ll receive 0.22 uF)
2 x 0.04 uF (You’ll receive 0.047 uF)
1 x 0.005 uF (You’ll receive 0.0047 uF)
1 x 0.025 uF (You’ll receive 0.022 uF)
2 x 0.01 uF (You’ll receive 0.01 uF)
1 x 0.1 uF (You’ll receive 0.1 uF)
1 x 0.1 uF (You’ll receive an X1Y2 Safety Capacitor to replace the across-the-line RFI suppression capacitor.)
1 x 20 uF 50V Electrolytic (You’ll receive 22 uF 50V)
2 x 50 uF 450V Electrolytic (You’ll receive 47 uF 450V)

Westinghouse H-126 Little Jewel/Fridge Capacitor Kit – $39.00 with Free Shipping!


1936 GE Model A-52 Antique Radio Repaired!

November 22, 2013 Leave a comment

A change of pace from the Bose equalizers and hi-fi I’ve been working on a lot of lately, I had the pleasure of working on a 1936 GE Model A-52 antique radio.

This is a nicely designed and straightforward table radio with 5 tubes, AM and one Shortwave band. Back in the ’30s, RCA and GE shared chassis and designs quite closely and it’s no surprise this one uses all RCA metal tubes, 6A8 6K7 6Q7 6F6 5Z4.

This radio had been serviced in the past but was due for another go-around. Most of the capacitors had been replaced in the ’70s or ’80s, although there were a few that still needed to be replaced. I swapped the 4 capacitors which were definitely in need of replacement, but the other units tested fine and are recent enough I’m not too worried about them.

The radio power switch, though, had been bypassed. The radio’s owner reported the switch was sparking in the back. I tracked one down after several weeks and was able to get it installed and it functioned perfectly after that.

The radio’s alignment was already spot-on so no adjustments needed there. I re-assembled the radio and let it play for several hours of burn-in testing before sending it back to it’s home where it will continue to play beautifully for years to come.

If you’re in Eastern Washington near the Seattle Metro Area, I can help bring your antique or vintage radio back to life. Please inquire, or visit the portfolio of my other work.

Bose 901 Series I Equalizer #47431 Repaired!

November 20, 2013 Leave a comment

Once in a rare while I’ll run across an old Bose Active Equalizer that I buy for myself, but they never last too long – I get a lot of requests to purchase a complete Bose 901 Series I equalizer to go with a set of speakers which long since lost the matching controller. I did just recently have one in stock and it went quickly; once they’re purchased I repair them on demand before sending them along.

This one’s all original as far as I can see. It was reported to have a dead channel when I purchased it, and the resistors had certainly drifted out of their tolerances with age. The case was in decent shape for being 40+ years old, too, although the light doesn’t quite catch it all very well.

Component replacement was pretty straightforward.

Some Bose 901 Series I and Series II equalizers used BC239C-labeled transistors, others used 2N5088s. They’re nearly identical – indeed, the rest of the circuit is identical – but they have a slightly different gain spec. Practically, this just translates into a slight difference in the volume control on your receiver – the generated curve is the same and both are identically factory specification compliant. When I need to replace transistors, I use all 2N5088s – but in this case, all transistors were good, so no replacement necessary! The neon bulb was flickering, though, so I replaced it with a brand new NE-2A and current limiting resistor. Then, a good solid control cleaning so all the switches moved freely and made good contact.

This one went to its permanent home next week where it should perform for many years to come! With precision metal foil resistors and new electrolytic and film capacitors, not to mention the very light duty cycle experienced by the equalizer (which draws only 1.5W total power consumption), mean it will be a long time until this needs service again.

I can repair your Bose 901 Series I, Series II, Series III or Series IV Active Equalizer for a low flat-rate with some optional upgrades. Most every one of the Series I equalizers needs to be reconditioned at this point. The majority of Series II do as well, and even the later series are coming up with defective capacitors and op-amps more regularly.

Coming Soon: Restuffed Historical Capacitors for Radio Restoration

November 13, 2013 1 comment

I’m happy to announce that starting soon, I’ll be offering professionally restuffed vintage capacitors for historically accurate repair and restoration of your antique radio! In my rebuild process, the old capacitor is carefully stripped of the old wax coating and the innards carefully removed. The body of the capacitor is lightly cleaned and a new high voltage axial lead film capacitor is installed in the cylinder. The ends are then filled with a medium density clay filler to provide stability, followed by a fresh coat of bee’s wax. The end product is nearly indistinguishable from one in original condition, perfect for performing a historically accurate repair of a valuable antique radio.

If you’re interested, leave a comment! Pricing is expected to be $3-5 per capacitor depending on type and value.

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!

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.


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