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Wherever you go – you’re headed for FUN with Motorola Portables

October 1, 2014 Leave a comment


No matter where you’re going, you’re headed for more Fun with a Motorola Portable! They play anywhere – on their own batteries or on house current. You’ll like the dependable performance that comes from the ruggedly built chassis and shock-resistant tubes. And you’ll be proud of the beautiful styling of the sturdy, weather-resistant cases. Make your choice the choice of millions…Motorola!


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Mastech Digital Multimeter MS8261 Unboxing

September 30, 2014 2 comments

I ended up needing another digital multimeter to make a couple of voltage measurements at the same time, and bought another of the Mastech MS8261 multimeters I recommended in my Getting Started with Basic Tools post from a bit ago. Now I’ve got two! They’re very nice units for the price, I highly recommend them. It takes voltage measurements to 750V AC or 1000V DC with suitable probes and can also measure current, basic capacitance, frequency of slower signals, temperature from a K-type thermocouple, continuity, and some diode and transistor tests.

There’s three included AAA batteries, the multifunction socket for adapting to the thermocouple and transistors, a set of probes, and a manual which surprisingly was printed only in English and was very detailed in the functionality of the meter.

These are low-end batteries, but the meter doesn’t draw much current. I expect they’ll last at least a year.

The light is a nice touch. Tap it once to trigger the backlight, which stays on for about 5 seconds before fading out. No need to worry about draining the battery by leaving the light on all the time.

Measuring the internal resistance of the leads. This can be compensated for by keeping the leads touched together, then tapping the “REL” button to zero the meter. It’s useful for getting a more accurate read on low-value resistors, like cathode resistors in an amplifier or measuring the difference from a voltage reference.

These are solid little meters. Not pictured, but it agrees with my Keithley 6.5-digit meter on all ranges tested, so it seems they come with a decent factory calibration. They’re not expensive, either – they’re only $24.99 each. I own two of these and the only times I need a bit more precision – adjusting the bias on highly sensitive stereo amplifiers – I use my Keithley. Otherwise, this does everything I want it to and it’s much more portable than a benchtop unit.

Mastech AC/DC Auto/Manual Range Digital Multimeter – MS8268

New Toy: HP 130C Oscilloscope

September 29, 2014 Leave a comment

I’ve been looking for a nice balanced input oscilloscope to use as a bench audio scope for a bit, and I do enjoy some vintage test equipment…so I recently ended up with an HP 130C oscilloscope.

This early ’60s tube scope from HP is the third revision of their first oscilloscope line, the 130 series, which began with the 130A. The 130C has a much more serviceable construction and a variety of improvements, including a no parallax CRT for more accurate readings. It’s often regarded as one of the finest oscilloscopes HP has ever produced for the care that went into its design. It’s not the most full-featured by any means, but it was top of the line in its day and still offers extremely sensitive inputs for the smallest signal measurements.

This one was being used at HP Microwave Semiconductor in California until 1982 when it was put into storage.

Being in use into the ’80s, most of the paper capacitors which would have failed have been replaced. There’s only one 0.1 uF capacitor in place which needs to be swapped out, and the electrolytic capacitors, and it will be ready for a first power-up.

This one will be a great audio scope. It has a built-in X-Y mode and very, very sensitive – 200 uV/div, which is 20x more sensitive than any of my other oscilloscopes for detecting the smallest amounts of distortion and high frequency noise on an audio signal. There’s a working condition HP 130C on eBay right now which displays a trace for an example of what they look like. It’s a very classy instrument!

Should be a fun project. I’ll post here when it’s ready to go!

Steampunk USB Vacuum Tube Flash Drives

September 26, 2014 Leave a comment

I ran across these very cool steampunk-inspired USB flash drives drives built around a vacuum tube while looking for some tube shields. They look like they’re really well built and are a clever use of an old vacuum tube. I have a jar of dead 7- and 9-pin miniature tubes that I give away for art projects, and this is a perfect example of how electronic waste can be recycled into something cool.



They come in 8-64GB sizes and 4 color choices. Looks like they’d make a good gift, too. They’re for sale from eBay vendor slavatech, and ship for free worldwide.

Sony TA-4650 VFET Stereo Integrated Amplifier Check-Up

September 24, 2014 Leave a comment

This repair cross-posted from Rain City Audio:

I recently had this Sony TA-4650 in the shop for a check-up. It was reportedly giving static in one channel. That’s trouble on an amp like this – Sony used an ambitious new technology, the VFET, a new design of vertically oriented FET final amplifier. These were known as having very low output impedance and low distortion. The TA-4650 puts out 30W/channel into an 8 ohm load and is specified for < 0.05% THD.

I did some initial checks with the oscilloscope, signal generator and distortion analyzer and wasn’t able to reproduce the static issue; it sounded crisp and clean and I didn’t notice anything out of the ordinary monitoring the signal path with the scope.

It’s a pretty easy to service design, but fortunately, none of the boards really needed to come out. Some of the controls were a bit scratchy, though, and the pushbuttons made nasty pops when pressed – one even tripped the speaker protection relays. Definitely time for a thorough cleaning with control cleaner, solvent and lubricant. The pushbutton ganged switches, volume and balance controls are all open frame and can get grimy; the treble and bass controls are sealed and can’t get clogged up.

Afterwards, all functions operated without any nasty pops on any control. I tweaked the power supply reference voltage and the bias on both channels:

THD was a little out of spec on one channel, but still < 0.15% at rated power – among the best rated amplifiers of the day even still. The other channel measured 0.073% THD at rated power but I’m getting close to the noise floor of my test setup. The meter can measure down to 0.0045% THD, but the internal reference – while very good – still contains 0.063%. THD. This amplifier passes a listening test on my personal K-Horns, too, and is ready to go home.

Quick and easy! It’s a very clear, crisp, neutral receiver that really lets the source material shine through.

Rain City Audio Stereo Repair

NordeMende Globetraveler 7/601 Rebuild

September 23, 2014 Leave a comment

This repair is cross-posted from Rain City Audio:

This rare and valuable example of late 1960s German engineering came through my shop recently, the NordeMende Globetraveler 7/601. Marketed as the Globetrotter in Europe, RCA already marketed a radio called the Globetrotter in the United States and so the NordeMende was re-branded Globetraveler when sold over here. While it came in working on its original components, there were a few issues. First, the alignment was a bit off. As a battery radio, the dial lamps are only on when the lamp button is pressed; that switch was stuck with the lamps always on. The bass and treble controls were a bit gummed up, too, and it sounded muddy.

Since this is a rare, intricate and valuable piece it deserved a full overhaul, complete with a precision alignment of every band – and there’s a lot of them: FM, AM, Longwave, and 11 distinct spread shortwave bands. Talk about a lot of tweaking!

Those problems would’ve been extremely tough to fix, but fortunately the owner had a donor parts set in even worse shape he was able to send along as well. Between the two poorly working radios, there’s enough to make one good one!

These early PCBs are extremely fragile and even with lab-grade rework tools can still be damaged. Fortunately, this set used all axial electrolytic capacitors, so I was able to clip the leads and use the old leads as mounting parts. A few locations I was able to de-solder and re-solder directly to the PCB, but most were axially mounted.

You can see in the lower left corner of the MAWIM capacitor that it’s started to leak a bit.

Capacitors replaced, the audio quality cleaned up very nicely. Then, on to tackling the other issues. After unsuccessfully trying to clean the switch with several different methods, the only way to swap them out was to disassemble the entire front dial.

The controls in question are the lower two on the right:

I carefully removed first the lower, then the upper, keeping track of the lead arrangements.

I installed a new resistor to replace the one whose leads I’d cut.

Then it was time to reinstall the new controls in reverse:

Now all the functions work perfectly! I used some rubbing alcohol to clean a few switch contacts which were acting up but otherwise it was in good shape and ready to be aligned.

First up is a bias adjustment:

Followed by 8 AM adjustments:

Longwave adjustments:

Shortwave bias adjustment:

Followed by no fewer than 34 separate adjustments for each shortwave band. I used a new precision signal generator for the alignment:

Using the scope and generator together, it was easy to check the alignment out. I’m not going to post every band here, but due to the sharp tuning, it was very important to get these just right.

Repeat 3 dozen times, and the alignment is complete! Very, very fortunately none of the trimmers or coils were damaged so it was long but uneventful. Then, with it working perfectly, back in the case:

With all new parts and a precision alignment on all bands, this should play well for a long time. It’s pretty sensitive, too, although shortwave stations are pretty tough to pick up these days so I’m unsure how the real-world performance will be. It sounds great, and pulls in a ton of AM and FM stations though!

Rain City Audio Vintage Radio Repair

Vintage Engineering is Beautiful

September 22, 2014 Leave a comment

Varistor from Harmony H500 Guitar Amplifier


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