813 B Low Pass Crossover

[David Ketley]

Just removed the crossovers to have them upgraded with new capacitors and better wiring but I find on the two rear units with the coils on that there are 2 different coil sizes though capacitor wise they look the same. Any idea what’s going on I Presume one is from another model ? I will try and attach pictures. Although the capacitors look different one is placed on the back owing to the larger coil and one capacitor looks like a replacement although its the right value.
Look forward to your comments.
Thanks Dave

[eso]

Do you have a copy of the schematic? I'd check the parts values against the schematics for starters. For the coils you'd need to disconnect them and measure their inductance and DCR.

While I couldn't say for sure, I would not be surprised if there were times when parts would be used from different sources that would look somewhat different.

If you don't have a schematic and can't find one in the archives here of JBL pro, let me know. I'm sure I have one but I'd have to dig around for it.

eso

[David Ketley]

Thanks ESO I just don’t have the test equipment to check things out but John at J.S. Audio here in the UK is an X Musical Fidelity engineer and modds there products so I’m sure he will sort them out for me. Managed to locate the circuit diagram so will send with the units.
I also want to eliminate the fuses and the bulbs out of the system and see if there is any way of replacing the potts with a fixed resister? There is also a big difference in the external wiring, one using thin wire, the other much more substantial. I was hearing the difference between these speakers but put it down to old age now I realise there are actual differences in specifications even if only the external wiring.
Dave

[eso]

I also want to eliminate the fuses and the bulbs out of the system and see if there is any way of replacing the potts with a fixed resister?

The bulbs are a part of the circuit. I'd be careful with that. Also, remember that these were designed as high powered studio monitors where the install would be EQd using test tones and room EQs.

Before tearing into original equipment you might consider just building ground up to experiment with. This site could be helpful:

http://web.mac.com/donaldpatten/iWeb/X-Over/X-Over.html

I have some original Urei crossovers I've found that I am using to reverse engineer new ones, but I personally wouldn't do any more to them than replace perts that need replacing to keep them in spec.

my 2¢

eso

[David Ketley]

Thanks for the information ESO. I think we have realised since 1984 that even the cabling and connectors can make a difference to the sound. Although in the studio its possible to tweak the output to give a flat response in the home one listens to all manner of music recorded on vastly different equipment, balance etc. Then there are the other variables such as furnishings and even the number of people in the room. All one can do is to get a sound that one is happy with then listen to some music and not get too hooked up on the actual "quality" of the sound whist maintaining the integrity of the original design.
I was tempted to replace the bass units with some more modern ones as they seem only a cheap pressed steel chassis but was scolded as it would substantially alter the sound. So I’ve ended up paying over £100 just for a new surround on one speaker! (Eminence recommended the Kappa Pro 15LF )
Let’s hope I can hear and like the new capacitors and cables.
Thanks
Dave