C56 Dorian repair

[dbullis]

Hello everyone,

I'm new to vintage JBLs and am very anxious to get started on my new hobby! For starters, a good friend inherited a pair of 1967 vintage C56 Dorians. They sound great! We haven't opened them up yet to inspect the condition of the drivers, but I'd like to. On one of the speakers, it's obvious that the upper end isn't ringing out, i.e. tweeter non-functioning? What could be the possible cause? That said, what drivers are in the C56s and what should I look out for? If we can't get someone to repair them, we may have to do it ourselves. We don't have any technical literature for the speakers, so any advice about them here would be great!

Thanks a bunch!
Daryl

[Zilch]

Hi, Daryl, and welcome!

C56 Dorian was an enclosure, suitable for installation of a variety of drivers and systems, most popularly, the LE14A woofer and either LE20 (S12 system, LX8 crossover) or LE175DLH (S1 system, LX10 crossover) tweeter.

The LE14A's will likely need new foam surrounds if they still have the original ivory-colored Lansaloy, which will have hardened and cracked over time, seriously compromising the bass output.

Poor high frequency performance is likely because the L-Pads in the crossovers are dirty or oxidized.

You need to determine which drivers are in them. The crossover type will tell much of the tale, but pics of the drivers from the front would be very helpful.

These will sound very nice when brought back to standard performance.

[Audiobeer]

I'd turn the high frequency l-pad back and forth on the speaker having problems. Make sure the volume is down low. If it kicks in clean it.

[dbullis]

It's been well over a year since my friend's problem was discovered with the L14's in his Dorian set. He took it to a repair shop that specializes in JBL repairs and after much hemming and hawing, the repairman went ahead with a reconing. But curiously enough, he sent the following message to my friend that I'm not sure how to interpret:

"Close inspection of your LE14 showed classic signs of DC burn. I tell you this because it is almost ALWAYS caused by the amp malfunctioning and passing DC current to the speaker. The crossover prevents the tweeter from seeing this, but, most low freq. transducers have no protection from this malady. If you plan to use the same amp that was on the speaker when it went down I strongly suggest you have said amp looked at by a highly qualified tech before you use it."

My question is, how can the technician "see" the so-called DC burn? I have never heard of that. My buddy uses a Denon DRA-545R amp which seems to work fine otherwise. If this DC burn is a reality, could it have happened with the previous owner who owned them for 30 years before my buddy got them?

Please someone, explain!

Thanks a bunch!

Daryl

[doodlebug]

What the tech is trying to say - and he's a good guy for raising the issue, BTW - is that at some point in the past an amplifier was used that was not working well.

Specifically, this amp had a DC voltage that appeared across the positive and negative speaker connections. When a speaker was connected, this DC voltage caused a certain amount of current to flow through the voice coil, causing the woofer to be pulled in or out, depending on the polarity of this DC voltage. The problem is/was that the current flowing through the voice coil, heated the wire which was attached to the voice coil bobbin.

When a sufficient amount of heat was developed, the bobbin will be deformed. In a worse case scenario, the bobbin is completely 'cooked' and will fall apart. There will be scraping noises when the woofer's cone is deflected by hand. This is the bobbin rubbing in the gap in the magnet. I've even seen examples where the bobbin separated from the woofer cone - essentially cooked the glue to where it let go.

Just to be safe, it isn't a bad idea to have your friend's amp checked out.

Hope that helps - and have your friend thank that tech, too.

Cheers,

David

[dbullis]

This has been a long saga with my buddy's JBLs, but after the tech fixed the one speaker with a reconing and fixed the damage from the DC burn, we thought that was it. But my friend didn't bring the other one in for the tech to look at and now he realizes that the other one had the original problem with the lack of high freq. So now that is in the shop with the first one that was repaired and the tech is saying that it is indeed the crossover, but his solution is to replace the capacitors. Would that take care of the problem and what relation does that have to the so-called L-pads?

Thanks for any answers in advance.

Daryl

p.s. I am beyond myself with anticipation in hearing these JBLs bellowing out at full strength!

[subwoof]

The tech you mentioned called me last night to verify the value of that cap ( 6uF / LX2a ). I saw your dorians in his shop in syracuse last week....:o)

I am also restoring (4) of these that have the "potato masher" HF and like yours, nearly all of those old crossovers have bad caps. They are old paper types and tend to dry out and fail with time. This is no different that taking the 1967 chevy he inherited also and replacing the radiator hoses. Things get old.

It's also possible that the amplifier burn he saw ( and showed me ) was caused by the speaker cone being frozen "backwards" due to the hardened surround.

When a tech sees a voice coil burn only on one end, it's assumed that the amplifier went DC, pushed the coil all the way in ( or out ) and cooked it *in a short amount of time*.

Now that exact SAME burn / cook can happen when the surrounds go stiff and force the coil into that same extreme position and the power being fed to the woofer goes to the whole voice coil BUT BUT part of the coil is in the gap ( and has a little heatsinking ) and the other part is hanging in the air and gets cooked *over a long period of time*.

They look the same when autopsied but the JBL service manual doesn't explain this "sceneario" to the techs.

Of course it was also common to use amplifiers that weren't as stable as today ( flame linear, SAE, etc ). One sneeze and you had a smoke machine.

sub

ps - that is a very nice woofer / but if you feel the need for speed, build a B380 sub to go with them.