Thursday, June 5, 2014

The Latest Find: Outdoor Party Speaker Pair

Well, I have been on the look-out for highly efficient speakers that will play at reasonably high levels outside.  While I was mowing my lawn a few days ago, I noticed a yard sale in progress just up the street at a neighbor's house that had recently sold.  Somewhat impatiently, I finished mowing the lawn and then went inside to get my camera just in case anything of interest turned up at the sale.

Instead of taking pictures, I wound up acquiring, through an exchange of equal value, a pair of owner-designed and built bass-reflex cabinets.  Inside each cabinet, I found an Electro Voice SP12B, a Calrad CN-2 crossover, and an Atlas Sound HR-3 horn tweeter.



 Atlas HR-3 - Calrad CN-2 - Electro Voice SP12B
 Atlas HR-3 - Calrad CN-2 - Electro Voice SP12B

The Atlas HR-3 tweeter appears to date the bass-reflex project at sometime after 1958.  Here is some advertising from the magazine Audio, February 1958:


Atlas Sound HR-3 Horn Tweeter (Audio February 1958)
Atlas Sound HR-3 Horn Tweeter (Audio February 1958)
 
The Atlas HR-3 tweeters in these cabinets look like this:


Atlas Sound HR-3 Horn Tweeter 2
Atlas Sound HR-3 Horn Tweeter 1


Atlas Sound HR-3 Horn Tweeter 2
Atlas Sound HR-3 Horn Tweeter 2
Here are images of the Calrad CN-2 crossover:


Calrad CN-2 crossover label
Calrad CN-2 crossover label


Calrad CN-2 crossover schematic
Calrad CN-2 crossover schematic
The Calrad CN-2 pictured is a series dividing network.  As you may already know, this means that the woofer and tweeter are wired in series.  I verified this by drilling out the four rivets holding one of them together and peering inside at the innards.  The ferrite ring core inductor and leads are potted into the case so you cannot see them.  The inductor is tapped and there are two capacitors so that you can change from 2500Hz to 3500Hz by shorting or un-shorting a pair of screw terminals.  They are found configured at 2500Hz in this instance.

The SP12B woofers are the same as you will find in a previous post, Corner Speaker Pair with ElectroVoice SP12B Speakers.   Here is some advertising from Hi-Fi Stereo Review, February, 1962, courtesy of http://vintagevacuumaudio.com/:


SP12B Ad in Hi-Fi Stereo Review February 1962 - p1
SP12B Ad in Hi-Fi Stereo Review February 1962 - p1


SP12B Ad in Hi-Fi Stereo Review February 1962 - p2
SP12B Ad in Hi-Fi Stereo Review February 1962 - p2
Here are images of the interior of each cabinet:


Bass-Reflex Cabinet Interior 1
Bass-Reflex Cabinet Interior 1


Bass-Reflex Cabinet Interior 2
Bass-Reflex Cabinet Interior 2
And here they are set up outside for listening tests:


Bass-Reflex Pair Outside For Testing
Bass-Reflex Pair Outside For Testing


These have a very lively sound and will definitely make great Party Speakers.  I moved these with the wheelbarrow pictured.  I am thinking of adding the Jensen tweeters found in the Magnavox speakers featured in the post,  More on Plywood Horns and Poor Man's Horn Driver, to the Corner Speaker Pair with ElectroVoice SP12B Speakers.  

Saturday, April 26, 2014

Mike Matthews Dirt Road Special - Speaker Upgrade

Several months ago, I posted about a repair on the Mike Matthews Dirt Road Special.  A few days ago, I had the good fortune to run across an exact replacement, electronically and acoustically, for the Celestion 12 Inch original speaker for this amplifier.  For example, the new Fender 12 Inch replacement speaker has almost exactly the same free-air resonance frequency.  The two sound nearly identical under a "tap" test.  By removing the GE (antique) temporary replacement and installing the new Fender, it is now possible to re-install the original power supply transformer to get the original 70 Volt DC power supply level.  The small stone phase shift effect is almost infinitely adjustable with the full 70 Volt supply.  Here is the Mike Matthews Dirt Road Special with new Fender replacement speaker:


Mike Matthews Dirt Road Special with Fender Speaker
Mike Matthews Dirt Road Special with Fender Speaker



Wednesday, October 16, 2013

Corner Speaker Pair with ElectroVoice SP12B Speakers

This pair of corner speakers arrived with a little dust but otherwise in almost new condition.  Here is how they looked when I first got them:

Corner Speaker Pair with ElectroVoice SP12B Drivers
Corner Speaker Pair with ElectroVoice SP12B Drivers
Of course, I took them apart to see what was inside and discovered SP12B Drivers in pristine new condition.  EV manufactured several versions of these over the years.  Here are some photos of the ones in these boxes:


ElectroVoice SP12B Speaker
ElectroVoice SP12B Speaker

ElectroVoice SP12B Speaker Label
ElectroVoice SP12B Speaker Label


ElectroVoice SP12B Speaker Cone and Whizzer
ElectroVoice SP12B Speaker Cone and Whizzer

ElectroVoice SP12B Speaker Voice Coil Dust Cover
ElectroVoice SP12B Speaker Voice Coil Dust Cover


ElectroVoice SP12B Speaker Terminals
ElectroVoice SP12B Speaker Terminals
 
I weighed one of them on a spring scale:


ElectroVoice SP12B Speaker Weighs 12 1/2 Pounds
ElectroVoice SP12B Speaker Weighs 12 1/2 Pounds

Here is what the back of the cabinets looks like:

Corner Speaker (Back)
Corner Speaker (Back)

The terminals on the back look like this:

Corner Speaker Terminals
Corner Speaker Terminals

To sound best, they should be positioned in a corner:


Corner Placement For Best Sound
Corner Placement For Best Sound

These are among the best sounding speakers I have heard.

The EH Dirt Road Special is Back for repairs

Well, quite a lot has happened since my last post on the great little amplifier, the Electro Harmonix Mike Matthews Dirt Road Special.  I replaced the original 70 VAC secondary transformer with a 12 VAC transformer.  My laborious temporary speaker repair did not hold.  I have replaced the original Celestion speaker with a vintage replacement speaker that I have had for quite a long time. 

Here is a photo of the 12 VAC transformer installed in the amplifier:

12 VAC Transformer in Dirt Road Special
12 VAC Transformer in Dirt Road Special

I got the transformer at Radio Shack.  It has a three Ampere rating.   You can see that I have used heat-shrink tubing over the splices I made in the primary leads.  Here is a photo of the amp with the replacement speaker installed:

Electro Harmonix Mike Matthews Dirt Road Special Amp with GE speaker
Electro Harmonix Mike Matthews Dirt Road Special Amp with GE speaker

This is an early 1950s speaker I inherited from a guy who used to hold "record hops" in his barn a few miles from here.  He had several of these speakers, each in a plywood open back box hanging from a barn beam.  He ran speaker wires to them from a record player on which he played mostly 45 rpm records of tunes that were popular at the time.  At any rate, I have been trying to find out more about the speaker for quite a long time.  It has an Alnico magnet and is similar in appearance to a GE model S-1201D speaker illustrated in the following scan of page 7 of the February 1952 issue of Audio Engineering magazine.

GE S-1201D Ad in Audio Engineering February 1952
GE S-1201D Ad in Audio Engineering February 1952
 
The speaker still has its original felt dust cap.  The speaker is very efficient and sounds great in this amplifier.  It ought to.  If you apply the inflation calculator at http://www.westegg.com/inflation/ this speaker would have cost $1279.50 in 2012!

Saturday, February 16, 2013

Beware Of Old Gear!

To all engineers and technicians out there:  Beware of old gear that appears to be in working condition!  Last year, a friend dropped off an Electro-Harmonix Mike Matthews Dirt Road Special here.  It was actually working but there was extreme crackling noise in the volume pot.  I was thoroughly impressed by the sound of the built-in "Small Stone Phaser".  The label on the highly regarded British made Celestion Speaker was visible through a bass port on the back panel.  Here are some pictures of the amplifier.
Electro-Harmonix Mike Matthews Dirt Road Special

Top view

To clean the pots, all of which were quite noisy, I decided to remove the back panel and take a look inside.  This is when things began to get interesting.  I had noticed that the Philips screw fastener heads visible on the exterior were considerably rusty.  However, I was surprised to find that the three wood screws along the bottom edge of the back panel were securely rusted into the wood.  One of the three would not even begin to turn.  I gently tapped on each of them with a Philips driver bit and eventually removed two of them but the third did not respond.  Finally, after stripping the screw head, I had to drill the head of the screw away so that only the shaft remained.  Then I could remove the back panel.  Here are all of the wood screws and a couple of machine screws removed later.

Rusted screw fasteners
What I noticed right away was that the amp must have been in a flood because, not only were the screws rusted but also the back of the speaker was rusted with much of the paint flaking away.


Anyway, the pot interiors were accessible for spraying with contact cleaner without any further disassembly.  With the pots cleaned, I applied power and there was no sound at all from the speaker!  I unsoldered one of the speaker terminals and found that the speaker's impedance was an open circuit!  Next, I removed the speaker and installed another twelve inch speaker.  There was a steady hum from the amplifier but otherwise no sound!  After finding a schematic of the output circuit online, the next thing I did was to remove the amplifier chassis, unsolder the two power output transistors and find that the PNP transistor (TO3 case) had a collector-to-emitter short.  The NPN transistor pn juncitons tested fine with no shorts.  Here is a photo with the PNP transistor removed:

PNP Transistor removed
Here is another close-up of the board for reference.

Printed on the board is the number EH1313 B

I tried checking the connections to the speaker's voice coil and could find nothing amiss.  I finally decided to remove the cone/surround/spider/voice coil assembly to try to get a look at the voice coil because it appeared that the coil wire must have parted somewhere in the coil.  In addition, the coil was rubbing against something in the gap and not moving freely.  With the cone assembly removed from the basket, I noticed that there was really heavy rusting around the spider edge and the spider glue had disintegrated into small fragments, some of which had fallen into the voice coil gap.

Rusting on speaker basket

Rust and glue particles in voice coil gap

At some point, with the cone assembly removed, I measured a voice coil DC impedance of 7.2 Ohm.  This was surprising.  I decided to reinstall the cone assembly after cleaning the rust and clearing the debris from the voice coil gap.

Close examination of the voice coil showed a little scraping but did not reveal any obvious broken wire in the coil.
Scraping on voice coil

Scraping on voice coil

After thorough cleaning with vacuum and fine tweezers, I temporarily covered the gap with black electrical tape to keep filings from entering the gap while cleaning loose rust away.

Loose rust cleaned away

With the rust cleaned out, it was possible to reinstall the cone assembly and align the coil in the gap.

Speaker cone assembly glued and aligned with paper strips


Close-up

However, after the glue set, the voice coil had opened up again and the carefully restored speaker did not work! Here is a picture of the setup showing a voice coil open circuit on the meter.

Speaker still does not work!

The only way I could get the open voice coil to close was to insert a short wood brace across the diameter of the coil former.  As soon as I discovered this, I positioned the brace for maximum effect and glued the ends to the former.  Here is a photo showing the speaker working with an AC voltage applied to the terminals.

Celestion speaker temporary repair with wood brace

With the speaker working, if only temporarily, I decided to order a matched PNP-NPN transistor pair to replace both output transistors.  I selected transistors with robust specifications, including maximum ratings not likely to be exceeded in this amplifier design.

Here is a picture with the new transistors installed.

Amp with new output transistors installed

I powered this up using a Variac autotransformer.  Everything is working fine with the Variac high enough to make 14 Volts DC from the power supply.  With a full 120 VAC applied, the power supply voltage would be much higher at about 70 Volts.  My friend is suggesting that his potential customer might be interested in doing some studio work with this amp.  If that is the case, I may decide to change the power transformer to one with 12 VAC or even 9 VAC secondary to reduce the power and protect the original speaker, as repaired, from too much power from the amp.  This amp is really loud with only 14 Volts.  In the event someone installs a new speaker, the original transformer should be used.  On the other hand, the amp should work great powered with a 12Volt car battery instead of the AC power supply!  I will post further photos soon...

For more on this, check out the latest post at EH is back for repairs.