Service Shortcuts – November 1932

November 24, 2015 1 comment

Radio Retailing brings you some helpful service shortcuts:

Tube Numbering That Means Something

The RMA is seriously considering a new three-character system of marking radio receiving tubes which may shortly come into general use in place of the present meaningless type numbers. Easily grasped, the system makes it possible to roughly identify all tube types by their designations without resorting to complicated charts. A detailed description of the plan follows.

The first numeral is to be definitely related to filament, or heater, voltage.

0 to 2 volts: 1
2.1 to 2.9: 2
3.0 to 3.9: 3
4.0 to 4.9: 4
5.0 to 5.9: 5
6.0 to 6.9: 6

The second character is to be a letter, arbitrarily assigned to distinguish tubes having the same filament voltage and same number of elements from each other. The first tube of a given type to be marketed will be classified “A”, the second “B” and so on. Thus a 24 would be a 2A5 while the later 35 would be a 2B5.

The third character is to correspond with the number of usable elements having external connections.

Following is a table which shows old and new designating numbers of standard receiving tubes.

Do not use this chart as a tube substitution reference.

(It wasn’t adopted quite as described, though. For instance, a “6E5” tube – understood in the above chart to be a #42 power pentode, is actually an eye tube. Do not use the above chart as a tube substitution reference.)

Practical 175 Kc. Oscillator

Here is a circuit diagram of a 175 kc. oscillator used in my shop. It employs a type 30 tube and is completely encased in an old brass shield taken from a battery radio. Batteries could have been external, with the leads to the oscillator shielded. 22 1/2 volts is sufficient plate potential for a strong signal.

Practical 175 Kc. Oscillator Fig. 1

The main oscillator coil, L2, is wound in the slot of a wooden form having the dimensions shown in the attached drawing. It should consist of exactly 196 turns of No. 21 s.c.e. wire wound 14 turns to the layer, tapped at the 98th turn. The pickup coil, L1, is 14 turns of No. 21 s.c.e. wire basket-weave wound to 4 1/2 inches in diameter, squared off and fitted around the main coil. A coil wound haphazard fashion in the hand, securely tied with string, will do if basket-weaving forms are not available.

Practical 175 Kc. Oscillator Fig 2

To calibrate the device I selected a superheterodyne known to have accurate dial calibration. The oscillator was connected to the input circuit of this set and the receiver tuned to exactly 875 kc. Then the oscillator dial was rotated until it produced the loudest possible signal.

The device may of course be used to generate signals within the broadcast band of frequencies by selecting the proper harmonics.

A Replacement Mercury Switch

Several makes of automatic record-changing phonographs employ a liquid mercury switch of the tilting variety as part of the changing mechanism. These frequently become cracked, allowing air to seep through the glass and foul the mercury.

To replace these switches where the time required to secure delivery from the factory cannot be spared secure a small pillbottle having the same dimensions as the original switch tube, a rubber stopper and two ordinary sewing needles. Fill the bottle about 1/4 full of mercury, insert the stopper as tightly as possible and then push the needles through it as far apart as possible. Solder flexible leads to the protruding eyes of the needles and then coat bottle and needles with molten wax.

Fasten the completed unit to the switch tipping frame in the same manner as the original mercury unit.

Knob-Removing Cord

To remove contrary tuning and adjusting knobs of the slip-on type take a piece of heavy cord about 9 in. long and tie the ends together, forming a loop. Now tie a knot approximately in the center of the loop, thus forming the cord into two loops.

Slip one loop behind the knob and the other on your fingers and pull. The knob will come away without damage to either finger-nails or cabinet.

High D.C. Voltage from a 6-Volt Battery

An old B-eliminator may be quickly and cheaply converted into a device for obtaining high d.c. potentials from a 6-volt storage battery. The same device also provides high a.c. potentials which are useful in the shop.

Disconnect the 5-volt filament winding which normally heats the 280 rectifier and short the two socket filament terminals together as shown in the accompanying diagram. Leave the high positive lead to the filter circuit untouched. Plug a BR rectifier into this socket.

High D.C. Voltage from a 6-Volt Battery

Now, connect a high-frequency buzzer, the contacts of which are shunted by a 1. mfd. high-voltage condenser, in series with the 5-volt winding and a storage battery. Thus connected the 5-volt winding becomes the primary winding while the original primary is left open. High-voltage d.c. may not be obtained from the output circuit of the device while high-voltage a.c. is present across the original primary leads.

Using a Majestic eliminator and the primary of a Ford spark-coil for an interrupter the output will be approximately 190 volts d.c. Adjustment of the vibrator points is not unduly critical. Current output is largely determined by the character of the buzzer used.

3.8 Volt Pilots

Number 13 Mazda focussing 3.8-volt flashlight lamps work out fine where 2.5-volt pilot lights repeatedly burn out due to excess voltag.e They are also handy when 2.5-volt bulbs are not immediately available.

Try the number 13, too, in Sparton sets using 3-volt filament type 485 tubes.

Curing Cone Rattles

If shellacking a cone and centering its apex fails to remove objectionable rattle press the rubber of a lead-pencil firmly to the extreme edge of the cone where it is clamped or glued in place. Try the pencil pressure on top, bottom and sides, being careful not to press so hard that the cone is damaged, until a point is reached where the rattle ceases.

If the cone is glued, smear shellac heavily between frame and cone at this point and let it try. If it is clamped, loosen the clamp enough to permit the insertion of a small soft-wood shim.

Another Well-Equipped Test Bench

Bill Garlitzs of Coraopolis, Pa., sends us this photo of one of his two excellently equipped test benches. Note the set analyzer, tube checker, two oscillators, output meter, battery charger and complement of tools.

Another Well-Equipped Test Bench November 1932

Bose® 901 Series I Active Equalizer #51616 Repaired

November 23, 2015 Leave a comment

From the Rain City Audio Repair Blog:

Another Bose® 901 Series I Active Equalizer came through the shop for an overhaul! This one was from the very end of the Series I production run, serial #51616.

This one appeared to be all original. The owner reported it was working mostly but was due for a tune-up, so it received the standard service!

Fully restored with all new Nichicon Fine Gold capacitors, precision resistors and film capacitors this equalizer is going to last for a long time to come.

Are Kids the Future of Ham Radio?

November 22, 2015 1 comment

Bob K0NR has an alternate take on how to keep ham radio alive as many of its practitioners age up:

For whatever reason, it seems that most people find themselves in a situation as an adult that causes them to say “I want to get my ham radio license.” When asked why they want to get their ham license, the top response is always emergency/disaster communications, followed by backcountry communications, pursuing electronics as a hobby and learning about radio communications. I suspect that starting to be established in a community and having some disposable income also play a role.

My hypothesis is that the most effective way of growing a vibrant ham radio community is to target adults ages 25 to 40.

He’s got some data to back it up from the tech license classes he teaches:


Hard to argue with those numbers. I passed my tech (and general) at about 25, but it was definitely for those reasons: emergency communications, backcountry communications (such as while skiing), and relating to my existing electronics hobby. In high school and college, I had way too much going on – not to mention not as developed of an interest in electronics in the first place – to pay any attention to ham radio.

Not that I’m doing much better now, my only operation so far has been a handful of sessions on 70cm Simplex with cheap HTs to communicate with another vehicle in something of a convoy situation.

Are you a ham radio operator? What do you think?

[The K0NR Radio Site]

Electronics Repair Equipment Survey

November 21, 2015 Leave a comment

Do you play with electronics as a hobby, are you a freelance repair technician, or do you work for an established repair shop? Take the Electronics Workbench Equipment Survey! Share a little bit about what equipment you regularly use to get the job done, and I’ll aggregate the results after the end of the year and share a bit about what type of gear people are working with.

Electronics Workbench Equipment Survey – Google Forms


November 20, 2015 2 comments



Same great content, and all your old links will still work – starting in 2016, you’ll see more content here around all types of vintage electronics, hobbyist projects, and other resources in addition to KF7LZE’s original content. Thanks for visiting!

Categories: Commentary

Harman-Kardon Festival D-1000 Tube Receiver Repair

November 17, 2015 3 comments

All vintage gear has a story to tell, but something really rare and interesting recently came across the bench: the Harman-Kardon Festival D-1000. While it looks somewhat unassuming, a fairly straightforward 15-tube receiver with push-pull 6L6GC outputs and an AM and FM tuner, it’s definitely more than meets the eye: the Festival D-1000 was the very first commercially produced integrated stereo receiver! Produced in 1954, the concept of combining a pre-amp, power amplifier, and tuner into one as our modern receivers due had been mulled about for a while but it proved a significant engineering challenge. In 1954, Harman Kardon changed all that with their release of the Festival D-1000 receiver.

Harman-Kardon did have to make some design compromises to fit everything into this package, so while it’s a solid performer, if one were looking the absolute best sound at the time it would still be separates.

This one came from a storage unit, and it was due for an overhaul before being put into service. First up was testing all the tubes – and they all passed! The 5881 outputs were replaced the 6L6GCs, but they’re interchangeable.

Underneath it looks like it had never been serviced before:

Spot-checking the resistors, the sample I selected were all within their tolerance. Harman-Kardon must have used a good quality resistor with a protective coating – it’s quite unusual to find a unit this old where the resistors check out. So it was on to capacitors:

Adding some terminal strips for the power supply capacitors:

Component replacement complete!

Next up was an alignment. The FM reception distortion started off a bit above 0.5%, which is definitely pretty far out of adjustment; it finished out around 0.05%. Much better!

After following the factory alignment instructions, FM cleaned up very nicely, and AM was much improved!

All fixed up, this receiver will be a fantastic addition to any hi-fi collection, and will be a great performer for years to come.

Totem Acoustic ARC Digital HD Amplifier on Kickstarter

November 16, 2015 Leave a comment

Totem Acoustic sent me some information about a very interesting new hifi amplifier they’re launching on Kickstarter. Based out of Montreal, they’re an established brand making hifi speakers optimized for desktop and streaming applications, and are coming out with a new PC- and device-centric power amplifier to complete the line-up – the ARC Digital HD Amplifier.


I’m really attracted to the minimalist design and small form factor. My current home theater receiver is only ever used with a stereo pair of speakers and a computer audio source anyway. This is the future!

The ARC Digital HD Amplifier has no external controls and only a USB input or Bluetooth connection on the rear (along with power, speaker outputs, and subwoofer line outputs.) Volume is controlled by your source and the amplifiers enters and leaves stand-by based on whether there’s a source connected, and you can apply equalizer curves via Bluetooth with a free controller app for Android and iOS.


That sleek little package hides an interesting Class D amplifier pushing 2x60W with <0.05% THD @ 1 kHz (20 Hz-20 kHz AES17 measurement bandwidth, the real deal) and integral 24-Bit/192 kHz USB DAC and aptX Bluetooth DAC. At 60W per channel, it’s got plenty of power for most decently efficient speakers in normal sized rooms. It’s going to go great with my Klipsch KG4s, at 94 dB 1W/1m efficiency, and have plenty of headroom to spare.

This looks like a really worthwhile product, and a great first step into the realm of high-end audio if you’re interested in such things. Especially if, like me, you primarily listen to digital audio at home.

Check it out on Kickstarter, it looks like a worthwhile power amplifier if you’re looking for something with clean, understated aesthetics to pair primarily with a PC or stream from mobile devices. I’ve backed this project and if it’s successful, I’ll follow up with an unboxing in the summer of 2016!

Totem Acoustic full press release below:

Montreal, Quebec, Canada, November 11, 2015 –

Totem Acoustic from Montreal, Canada, just launched its first Kickstarter campaign to fund production of the first series of its new ARC amplifier. Since Totem is known primarily in the HiFi technology scene for its quality high-end speakers, the ARC represents a new adventure into the realm of amplifiers.

The ARC is a game-changer in the well established HiFi world, altering every perspective by supplanting the need for equipment typically used today. Adjusting to the modern taste prevalent in the compact and digital world, the ARC delivers high performance for a moderate price. As a state-of-the-art Class D amplifier, the ARC operates silently without the need of a noisy cooling fan. All you hear is music.

The ARC is ultra-compact and space-friendly. Being the size of a book, it can be placed anywhere. It has no buttons or knobs and turns on automatically when connected. With a total power of 120W, it runs at high efficiency thereby providing large amounts of audio output power.

The ARC is a fully-integrated amplifier system. Amplification is provided by the latest in Class-D amplifier technology. It is designed using digital inputs: USB Audio 2.0 and Bluetooth aptX Audio. The USB audio connection delivers professional audio quality sound, with 24-bit resolution and sampling rates up to 192 kHz. The Bluetooth connection can provide a “CD-quality” connection via aptX.  The free and optional ARC App allows for optimal sound control.

Today most of the music we listen to is digital; it’s on our phones, our laptops or streamed from the web. But, being passionate about music, we feel a level of authenticity is missing. There is a disconnect, a real gap, between high quality audio systems and digital music, and that is why we’ve created the ARC. The ARC was made to bridge the gap between fine sound and the world of digital music explains Robert Valtierra, inventor of the ARC amplifier.

About Totem Acoustic

Created in 1987 by sound sage Vince Bruzzese, Totem’s mission is to develop loudspeakers capable of reproducing a truly musical and moving performance. The goal is to constantly provide designs that are both affordable and real “soul movers” for all music lovers.

For more information:

Gabriel Bull
Director of Technologies at Totem Acoustic


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