Altec 890A Bolero questions

[dyche01]

I just scored a pair of Altec Boleros. They are the 890A model, with the 3000-series horn and the 10" 406A driver. The drivers are in great shape; the cabinets are okay; and one grill cloth is ripped.





Interestingly, the big difference between the 406A drivers and the later 406Z's that are in my Maderas appears to be the basket. In the 406Z's the basket is heavy, cast aluminum while in the 406A the basket is much thinner, stamped aluminum.

 

[dyche01]

Now for my first big question. I want to rebuild the crossovers (labeled N-3000-G) but I cannot find any information about them online. The crossovers themselves are composed of one fixed resistor, one variable resistor, one capacitor, and one inductor. The inductor and capacitor have part numbers but no values stamped on them.



The capacitor is waxed-paper and looks like block of government cheese. The inductor is, I believe, iron core. I made a schematic by tracing the wires and, interestingly, the low frequency driver is fed straight from the input so that the roll off is purely mechanical.

 

[dyche01]

So here's my big question: what the hell are the values of the capacitor and inductor? I can't find a schematic for an N-3000-G crossover anywhere online and nobody has posted these numbers anywhere that I can see.

My first thought was to measure the capacitance but on my meter, one capacitor measures 5.3 µF and the other measures 22.5 µF.




Based on my Maderas, I was guessing that the capacitor should be 6 µF, but the right value depends on the inductor. If 6 µF is the right capacitance, then the inductor should be 0.5 mH, which sounds reasonable. If 20 µF is the right capacitance then the inductor would be 0.14 mH, which is also reasonable.

Anyone know either the inductance or capacitance used in this circuit?

 

[dyche01]

The closest I have come to finding an answer using Google is the image below, which shows an 890C and an 890A crossover side-by-side.

This version of the 890A crossover has an MPP capacitor instead of the waxed paper cap. The image is too pixelated to read the value but the size is consistent with the smaller capacitors in the 890C, which are 3 uF.

The higher the cap value the lower the crossover frequency so, putting in a capacitor that is too big could damage the sensitive tweeters. If nobody here has a definite answer, I might start with 3 uF.

 

[dyche01]

Well, here is another frustratingly unhelpful photo of a pair of N-3000-G crossovers from the internet. These use a silver (electrolytic?) capacitor but they are no help.

It is as if the photographer carefully arranged the wires to make sure and cover the value of the capacitor.

 

[Dr. Beaker]

I have a pair of N-3000-G crossovers. They each have film caps that are labeled as 3 uF. The inductor is not labeled, but that should not need to be replaced.

 

[dyche01]

Ah ha! Thanks!

I feel extremely validated, because I spent entirely too much time yesterday building tuned circuits and empirically determining that 3 uF has to be the right value for this crossover. I obviously never intended to replace the inductor, but knowing its inductance would tell me the right value for the capacitor. This is because the circuit connected to the tweeter is a simple second order low pass filter with a crossover frequency equal to one over the square root of the inductance times the capacitance (1/sqrt(LC)).

I decided to measure the inductance by making a tuned RLC circuit and measuring its resonant frequency. To do this I disconnected the inductor from the crossover circuit and hooked it in series with a capacitor of known value and an 8 Ohm power resistor. I then connected this circuit to the output of my Amber 50B amplifier and used an iPhone app to run sine waves of different frequencies through it.






This simple RLC circuit has a resonance frequency that is also equal to 1/sqrt(LC). To find the resonant frequency I plotted the RMS voltage across the 8 Ohm power resistor at different frequencies for both 3 uF and 6 uF capacitors. The 3 uF showed a resonance peak at 2800 Hz while the 6 uF cap peaked at 2000 Hz.



From these values I back calculated an inductance of 1.07 mH. I figured 2800 Hz is close enough to 3000 Hz so I popped in 3 uF Dayton caps.

 

[dyche01]

I also did one more bit of detective work last night that suggested 3 uF was probably the right value. When I pulled what I thought were waxed-paper capacitors out of the crossover and opened them up, it turned out that they were actually a pair of film capacitors wapped in waxed paper and shoved into a little cardboard box. The little film caps were also not marked (natch) and they measured all over the place, but their sizes suggested that that they might be a 1 uF cap in parallel with a 2 uF.

 

[gdmoore28]

Enjoying the thread. Your detective work is inspiring. Looks like you've found the correct value. Every reason in the world to be super cautious with those 3000 series tweets, too.

GeeDeeEmm

 

[dyche01]

Enjoying the thread. Your detective work is inspiring. Looks like you've found the correct value. Every reason in the world to be super cautious with those 3000 series tweets, too.

GeeDeeEmm

Thanks! And you are exactly right, part of the reason for being extra careful with this project is to avoid frying the delicate 3000-G tweeters.

 

[dyche01]

Did a few final tweaks before closing up the cabinets. Since the later versions of this speaker, as well as the Madera, used a driver with a much heavier, cast basket, I figured there would be some advantage in damping the thin, stamped-metal baskets on the driver and passive radiator. I remember this making a difference in my old Polk Monitor 7's. I used the last of my leftover Dynamat to cover both drivers and radiators.


I also scraped off the dust of the decayed foam gaskets and sealed up the cabinets with Mortite. I gave them a few hours listening last night and completely fell in love with their sound. When I first tested them before buying, they sounded very dark and boomy. Now, after cleaning, recapping, dampinging, and sealing they sound gorgeous. I worried that crossing over so high might leave a hole in the upper midrange, but there is no hole I can detect. The highs are airy and the lows are rich and full-bodied. Listening to Debussy's 'Nocturnes' I felt like the speakers had disappeared and I was floating above the orchestra. I understand why their sound has been described as 'technicolor'.

 

[dyche01]

There are a lot of hints that these are early production 890A's, including the plywood baffle and the sketchy capacitors. The serial numbers are also quite low: 141 and 146. I looked through the Lansing Heritage website but couldn't find any information on serial numbers for this model. Anyone here have an idea of the date that these numbers would correspond to?

 

[mars_volta]

Had a pair of those when I was younger. Would like to hear them now.

 

[gdmoore28]

Fantastic. How about posting a video of them in action in the thread that GD70 started recently? I'd love to hear them, and so far Altec has not been represented there.

GeeDeeEmm

 

[dyche01]

Fantastic. How about posting a video of them in action in the thread that GD70 started recently? I'd love to hear them, and so far Altec has not been represented there.

GeeDeeEmm

Will do!

 

[malden]

I just scored a pair of Altec Boleros. They are the 890A model, with the 3000-series horn and the 10" 406A driver. The drivers are in great shape; the cabinets are okay; and one grill cloth is ripped.

Interestingly, the big difference between the 406A drivers and the later 406Z's that are in my Maderas appears to be the basket. In the 406Z's the basket is heavy, cast aluminum while in the 406A the basket is much thinner, stamped aluminum.

I wasn't aware of it until you pointed this out. I have a pair of Bolero 890C as well as a pair of the Madera and they both have cast aluminum, not stamped steel baskets. They also have "Foster" style horns rather than the mini 811B style.

 

[dyche01]

I finally started working on the Bolero cabinets. They were in pretty good shape, but one had a corner ding and both had a little water damage that caused the veneer to separate along one edge. There were also a couple of small gouges on the bottom and side of one speaker. I consolidated the edge separation and flaky MDF with wood glue. I know some people use epoxy for this, but what do the woodworking experts in the crowd recommend?



After the glue dried I filled the voids with walnut-colored wood filler. I didn't try to match the color exactly, because I like the slightly distressed look of the wood filler. In the past I would have sanded everything down and applied a completely new finish, but after reading several speaker restoration threads in this forum I decided to try Howards Restore-a-Finish and Feed-n-Wax.

I applied the Restore-a-finish with 000 steel wool and it worked amazingly well. After letting the cabinets dry for a day, I applied a couple of coats of Feed-n-Wax. The walnut cabinets now look great, the wood grain looks really deep and rich but the old battle scars are still detectable.

 

[dyche01]

Fantastic. How about posting a video of them in action in the thread that GD70 started recently? I'd love to hear them, and so far Altec has not been represented there.

GeeDeeEmm

Thanks for the suggestion. I posted a video there and figured it wouldn't hurt to cross-post it here:

 

[CopperWizard]

Cool.

 

[Sam Cogley]

Be gentle to those 3000 tweeters. Unless whoever bought the Micro-Mike equipment starts repairing them, they're currently non-repairable.