4A-32B Loudspeaker: the story of a repair

Posted: 5/5/2020 6:51:40 PM

From: Theremin Motherland

Joined: 11/13/2005

By the will of fate (hmm, thanks to my mother-in-law and her son who began to throw out old things) I got a pair of vintage audio systems.

The labels on the top say "FORMANTA M50-2, loudspeaker, serial number..." and "Warning! Don't attache to AC power plug".

Such systems were often used in cinemas of small towns and in village clubs.


This is a bass reflex enclosure with two "4A-32B" drivers in parallel.
The driver labels declare "P=25W, R=16 ohm, Resonance Freq=(missing), s/n, year of production".

OK, what the net will tell about it?

(or in Russian)

This is a legendary full-range speaker which has its own fans and opponents.
It was produced by several factories in the USSR and has a lot of modification. The special pictogram on label says us that driver was produced in Samarkand factory in Uzbekistan, the ex-republic of USSR. 

My interest is to use these drivers with amp contained theremins.

After checking:
1-st driver is OK
2-nd and 3-rd - coil is OK, needs in recentering,
4-th is fully crashed.

4-th is my coveted patient!

Posted: 5/6/2020 7:29:26 PM

From: Theremin Motherland

Joined: 11/13/2005

I have a strong feeling that this speaker was once beeng repaired  (oblique winding, traces of glue, reduced number of turns).

Note to previous post:
"4A-32" is very loved by tube oriented guitarists who like to build DIY combos.

First thing is gluing the pieces of main cone. I used a synthetic rubber glue for obvious seam and acetone diluted epoxy for impregnation and strengthening of suspicious places.

Posted: 5/7/2020 7:08:32 PM

From: Theremin Motherland

Joined: 11/13/2005

Now it's time for the voice coil.

Measured DC resistance of other speakers is around 12 ohm.
Suspect that 16 ohm (declared) is a resistance in working (heated) condition plus some frequency-averaged reactivity.
(Indirectly this is followed from symbol "Z" instead of "R")

Data taken from  http://ldsound.ru/4a-32-lomo/ :

Frame: aluminium foil 0.1 mm
Wire diameter: 0.18 mm
Winding: 130 turns, 2 layers
Inner diameter: 44.5 mm
Outer diameter: 45.6 mm

I did not find 0.18 mm wire, only 0.2 mm, so to maintain resistance, the number of turns will have to be slightly increased. This will increase the length of winding that is the pros because of better speaker linearity at hight amplitudes.

Where to get the aluminium foil 0.1 mm? -- the beverage can. To prevent a short-circuit turn, the aluminium frame should be gaped.

Also, to shape the voice coil I machined a special teflon mandrel (44.5 mm diameter)

Everything is ready!

Posted: 5/7/2020 10:32:15 PM

From: Northern NJ, USA

Joined: 2/17/2012

Nice ILYA!  You're making me feel bad about tossing our center channel speakers out (due to foam surround rot).  Replaced them with a couple of new speakers that didn't really fit the enclosure, had to rasp wood for hours, and even now the faceplate and grill are bulging.

Are you using magnet wire with high temperature insulation?  When I buy wire for coils on eBay I get the stuff that you can melt the insulation off with a soldering iron, which makes tinning the ends much easier.  But I would think you would want the highest melting temperature insulation you could find for a speaker voice coil (particularly one in a PA speaker).

I build a pair of speakers a long time ago that used concentric drivers and had a horn pipe going through the center of the magnet pole piece.  I could see lots of peaks and dips in the frequency response, and they didn't sound anywhere near as good as even low end bookshelf speakers I've built since - but they could get pretty loud!  Big diameters mean "beamy" mids, so the sweet spot for listening may be fairly narrow.  It's always a shock seeing the inside of a PA speaker, so little side reinforcement or cross bracing.

IMO the use of the term "soundstage" when used in speakers and headphones reviews is almost always a danger sign! :-)

Good luck!

Posted: 5/8/2020 1:40:57 PM

From: Theremin Motherland

Joined: 11/13/2005

"Are you using magnet wire with high temperature insulation?"

Almost all the wire types produced in Russia have a heat-resistant insulation layer. The self melting insulation (instant solder wire) are exotic here, so I don't even bother.

Having read the forums, I'm more worried about a wire-to-frame glue. Folks complain that an ordinary epoxy doesn't work so well here. But it is cheap and affordable. As I don't plan to use the speaker at 100% power, I’ll probably use the epoxy.

Posted: 5/8/2020 1:52:47 PM

From: Theremin Motherland

Joined: 11/13/2005

Forgot to mention that I plan to use materials that will be at hand at home or regular job.

Posted: 5/11/2020 5:56:17 PM

From: Theremin Motherland

Joined: 11/13/2005

The structure is as follows:
1. Aluminum foil
2. Tissue paper (1.1 turns), epoxy impregnated
3. 75 + 70 turns (2 layers), epoxy impregnated

About 1 hour of work.

Posted: 5/11/2020 5:58:23 PM

From: Theremin Motherland

Joined: 11/13/2005

After solidification of epoxy, I was puzzled how to remove this puff cake from  teflon mandrel. At first I tried to do it with mechanical force, but nothing worked.The second attempt with heating the outer surface was more successful – I shifted the coil by 5 mm, but that’s all it’s been possible to achieve.

okay, let's go the opposite way – put the coil into freezer. After hour, the mandrel fell out of the coil by itself!!

Posted: 5/13/2020 4:14:03 PM

From: Theremin Motherland

Joined: 11/13/2005

Voice coil: just cropping and fitting:

Next two steps:
1. Gluing the spider to voice coil (epoxy)
2. Gluing the perimeter of spider to basket (rubber glue)
I performed both of these procedures by placing the voice coil in magnetic gap and centering the coil using strips of paper:

Posted: 5/13/2020 4:18:38 PM

From: Theremin Motherland

Joined: 11/13/2005

In parallel with the previous procedures, I painted the diaphragm black (spray). Corrugated ring area was protected by mask (paper ring) – later a raw rubber solution was applied to this area.

The basket was also been painted:

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