advertisement


The Stirling Broadcast "Repair Kit" LS3/5A

Yank

Yank

Bulbous Also Tapered
I have a pair of Stirling Broadcast "Repair Kit" LS3/5As that were damaged by a lightning strike some years ago. The woofers were exposed to DC and the coils are melted. They've just been sitting in a corner gathering dust all this time. These were designed by Derek Hughes. I was thinking of parting them out, my first step was to see if the crossovers were (1) still good and (2) useful for any other sort of LS3/5A-ish project. So I did some measurements:

L1 = 1.12mH, 0.45 ohms
L2 = 0.65mh, 0.3 ohms
L3 = 0.212mH, 0.16 ohms
L4 = 0.055mH, 0.1 ohms

DrqD01K.jpg


All the yellow capacitors are 6.8 uF, all the brown ones are 2.2uF.

Tracing out the circuit, the LF network is 1.1mH in parallel with 2.2uF and two 33 ohm resistors, then 2x 6.8uF to ground through a 1.5 ohm resistor, then 0.65mH in series and another 2x 6.8uF to ground. The HF is 2.2uF in series with 0.212mH to ground, followed by the adjustable L-pad, then 2x 2.2uF in series with the 0.055mH to ground. I have not quite determined the function of R13, a 9.1 ohm resistor, there is a felt pad between the crossover board and the back of the tweeter that is stuck to the board with some very sticky adhesive.

So these are unlike any of the other LS3/5As and unlikely to be of use to anyone. I have therefore decided to replace the Monacor woofers and use these speakers for TV. The cabinets are not pretty, having been knocked over by the cat a few times too many. The only parts of any value are the very nice Scanspeak tweeters.

Maybe next week I'll dive into the Harbeth SHL5+ and attempt to dissect their crossovers.
 
Would love to see pics of your cabinets Yank. That provenance might add to their desirability. Not many owners can claim cat attacks and a lightning strike. I’ve always believed bad luck comes in threes. Move them upstairs in case a Mack truck comes through the wall. 🙂
 
The cabinets are just the Moldovan boxes sold on eBay. I'd bin them if it weren't for the sunk cost fallacy.

I also have a pair of Rogers LS7Ts with one damaged cabinet, I was thinking about concocting BC-1 - sized boxes for the LS7 woofers and use the Scan tweeters from the Stirlings. But it'd be a lot easier to just get the Stirlings working again.
 
Looking at the schematic for the 11-ohm LS3/5A, I may have to reconsider my assumptions about the positions of some of the components:

GZHv8uY.jpg


The Stirlings appear to follow this same scheme, even down to the part numbers, except that there is no C9 and it's just R13 across the tweeter instead of a Zobel. Only the component values have been changed...
 
Here's an internet picture of the 11-ohm crossover, it looks like the same circuit board:

rogers-9.jpg
 
One odd feature I forgot to mention is that the Monacor woofer used by Stirling is a dual voice coil unit. They are connected in parallel.
 
ToTo Man said:
Is that the SPH-135TC?
Click to expand...

No model number is printed on Stirling's "modified" version, but I believe that is correct. I need to find a pair.

I'm guessing their "modification" is removing the "Monacor" sticker, and soldering on a pair of jumper wires connecting the two voice coils in parallel.
 
Yank said:
Looking at the schematic for the 11-ohm LS3/5A, I may have to reconsider my assumptions about the positions of some of the components:

GZHv8uY.jpg


The Stirlings appear to follow this same scheme, even down to the part numbers, except that there is no C9 and it's just R13 across the tweeter instead of a Zobel. Only the component values have been changed...
Click to expand...

oYkytgt.jpg
 
Finding a pair of the Monacor SPH-135TC has been surprisingly difficult. I first emailed Stirling, who couldn't even be bothered to respond. There is no US retail outlet for Monacor, so I had to look to UK and EU sources, and most of the ones who still listed these specific units had no inventory. I finally found Willy's HiFi, Southend-On-Sea, and now have replacement woofers on the way to me.
 
Yank said:
I first emailed Stirling, who couldn't even be bothered to respond.
Click to expand...
I have spoken to DS recently, he's a very nice and helpful person when you do get hold of him. I have found email communication a bit of a challenge, I think he favours phone calls over email.
 
I received the Monacor woofers today, and have installed them. There are limits to the testing I can do at this time of night, but both are passing signal. The one I photographed and measured seems to be working fine. The second speaker, which I didn't disassemble until tonight, saw considerably more heat than the first one - the inductors L1 and L2 both look pretty bad. I may be imagining things, but maybe the assembled speaker sounds more "shouty" than the other one. I'll probably have to replace the inductors. I hope Falcon can match them.
 
Placed order for inductors with Falcon, who claimed they wind to order and to expect 5-7 days before the items ship. They shipped within hours of the order being placed. :)
 
The inductors from Falcon arrived in today's mail.

Here is the damage:

DvyOBhH.jpg


Compare to the photo at the top of this thread...

That's what you get when you apply 40V DC to a speaker for several hours.

And this is what I received from Falcon:

H1OQb9I.jpg


The 1.2 mH chokes are going to be a challenge to fit.

I ordered two sets of inductors to (1) meet Falcon's minimum order requirements, and (B) to make the speakers match. I got the resistors "just in case".
 
This is the only way I could think to make it fit:

lDcjWpx.jpg


Certainly not fit for shipping. I've made a sloppy job of it, and must wait for glue to dry before I can do a final assembly and see if it works properly.
 
You must log in or register to reply here.


advertisement


Back
Top