1973 Fender Twin Reverb Speakers cutting out.

[keithb7]

Hi folks I have a 1973 MV Twin Reverb. I recently noticed the tone was a little less than I am used to. I stuck my head down and noticed only one speaker working.
I checked all connections with a DMM and all seem fine. I am running JBL K120 speakers with the quick spring push wire connectors. I tinned the speaker wires
to ensure they are solid. Continuity through all speaker connections to 1/4 jack on amp are good. Next I noticed that by switching the amp power off, and back
on, both speakers will work for sometimes 10 seconds, sometimes longer, some times less. Then 1 speaker only cuts out. Also odd is sometimes the left speaker will cut out, other times its the
right speaker that will cut out.

I pulled the chassis today and checked the back side of the jack, looks good and clean. All wires look good. Double checked all connections in the amp. All look good.
Before I try swapping speakers out, I thought I would measure the resistance on both sides of the windings of the OT, just incase
something showed up there. I unsoldered the connections to take ohm measurements. Input side between blue/brown wires I measure
73 ohms. Output side between green/black wires I measure 0.3 ohms. I am not familiar with what these winding measurements should be.
Before going further into speaker testing, do these OT winding measurements seem ok? Normal? Schematic here for convenience.

http://www.thevintagesound.com/ffg/s..._100_schem.jpg

Thanks in advance. Keith

[g1]

Yes, those OT resistances sound normal.
Try cutting off the tinned ends of your wires. The springs aren't really crimps, but sort of. Tinned wires should not be crimped. Put bare wires into crimps or spring terminals.
That's about the only thing I can think of that could make the random right/left cutting out.

[Randall]

Not likely your OT, since if the speakers are wired correctly in parallel any issues with the OT will effect both speakers, not one or the other. When one speaker has cut out, have you tried moving wires and connections to see if that changes anything? The part about it sometimes being one side and other times the other seems a bit fishy.

[Leo_Gnardo]

Often this kind of problem has to do with the speaker wiring - obviously - BUT in 40+ y/o aging JBL's I've found occasionally internal corrosion in the speaker push on connectors can stop the speaker from working. A complete careful disassembly/cleaning/reassembly of the connectors can restore them to reliably working condition. In some difficult cases I bypass the push ons and solder directly to the solder-lugs to which the tinsel lead-ins are attached.

[keithb7]

Thanks for your comments and experience. I will run a couple more checks and report back. I agree it's fishy that one or the other speaker cancels out. Scratching my head over that too as both speakers run together and share a connection to the OT.
Going to pull the JBLs and swap in some WGS G12Cs that I have.

[olddawg]

Maybe spray a little Caig into those spring terminals. Emphasis on a little. A syringe would be best. You don't want it migrating or getting on the cones.

[keithb7]

Put chassis back in and fired up the amp. Both JBL K120 speakers are working but crackling and vibration sounds going on.
Pulled JBLs and put in a pair of WGS G12C speakers. Powered up. Works perfect. No crackling. Both speaker working as they should.

I think I did these speakers in myself unfortunately. I transported the amp for about 8 mins in the trunk of a car. Laying flat on it's back. Cones up.
If I recall it was right after then that I noticed problems. I guess they were old and fragile? Hard way to learn a lesson. Oh well, rookie move.
A re-cone is in order.

If anyone has the patience to explain a little something to me, I'd appreciate it. When new speakers are shipped, they store them flat in a box, cones
up. Same position my JBLs were in when I put them in the trunk of the car. Why did this transport position ruin my K120's? I assume as mentioned earlier,
old and fragile? Thanks for your help.
Humbly, Keith

[52 Bill]

A re-cone is in order.

I think that you should look into some of the problems areas that were suggested before you look to having them reconed.

Or at least take them into a reconer that will look to see if the leads are the problem before reconing. We've all seen speakers with lead problems.

[Leo_Gnardo]

Why did this transport position ruin my K120's? I assume as mentioned earlier, old and fragile?

It was probably the best transport position anyway. On the new WGS, how do you know what position they were shipped in. "Every" is likely the answer. They get slung onto trucks & tumbled down conveyer belts. No shipper pays attention to "this side up" markings, and "Fragile" stickers are an invitation for the sorting crews to "KICK HERE".

This will also be the case if you ship your JBL's anywhere, very much including the return trip. I can go on about JBL's mangled in transit... but I'll spare you. Today. One good trick though, bolt each speaker to a piece of plywood say 3/8 or thicker if you must ship, and demand they bolt the repaired speakers on for the return trip. Obviously you're much better off carrying them to a recone shop and picking up too. Then there's only one person to blame if they get damaged, right?

Before you issue a - very expensive - recone order, do as 52 Bill suggests, have 'em looked over by a pro & maybe you can avoid having to go the full process. My local quality recone guy takes pride in saving speakers without having to go whole hog unless it's absolutely necessary. OTOH you'll find lots who are just looking at the $$$ and will be happy to soak you. Also be advised JBL hasn't made recone kits for a long time so . . . "authentic JBL parts" recones cost stupid big money ($200 last I checked, likely more now as parts availability diminishes) and the other choice is "cheater cones."

[J M Fahey]

I tinned the speaker wires to ensure they are solid.

I **hope** you didn't tin the tinsel wires between frame mounted terminals (fixed) and speaker cone terminals (moving)

because if you did, you just killed them.

They are made out of **extra-flexible** twisted braid for an reason; if rigid speaker cone movement will crack or worse: tear them.


a Pro reconer can usually replace them without a full recone.

[keithb7]

No JM I did not tin those wires. Just the end of the speaker wires where they enter into the quick connects.
Someone else had mentioned the wires may be cut by the quick connects and recommended soldering them.
No improvement any way.

I will take my JBLs in to a local re-cone shop and see what they find. I inspected inside the quick connects with a flashlight and magnifier. Everything looks clean to me.

[g1]

No JM I did not tin those wires. Just the end of the speaker wires where they enter into the quick connects.
Someone else had mentioned the wires may be cut by the quick connects and recommended soldering them.
No improvement any way.

What I meant was to use bare wires into the quick connects, not tin them. It's not because they get cut. Crushing action will put resistance into solder. So when using crimps or quick connects, the wires should not be tinned, but bare wire. Leaving the wires bare will also increase the contact area, so you have better current capability. It's ok to add solder to a crimp after the crimping action.
Anyway, you have verified that it is not a problem with the connections. So perhaps there is connection issues at either end of the tinsel leads, either where they connect to the voice coils, or where they connect to the spring terminals. I think you inspected the terminals, so that leaves tinsel leads or voice coils.
This is an amp you have been using with no issues till now? It seems odd that a trip in the trunk would cause this, perhaps it is just coincidence? I would think a large bass transient would move the cones as much as some bumps in the road.

[keithb7]

Amp seemed trouble free before the trip in the trunk. it is odd and maybe just a coincidence. Here is a video. I know little to nothing about speakers but this has me thinking
there is a problem:

https://youtu.be/7f1ABE9iCqA

[Leo_Gnardo]

Amp seemed trouble free before the trip in the trunk. it is odd and maybe just a coincidence. Here is a video. I know little to nothing about speakers but this has me thinking
there is a problem:

https://youtu.be/7f1ABE9iCqA

Yes, voice coil rubs are also a problem with JBL. Two things contribute to this problem. 1: very narrow voice coil gap. B) magnet held in place by its own magnetic strength plus a couple dots of epoxy. If dropped or subject to excessive G force, the magnet may move. A little bit, you have VC rub. A lot, then voice coil is pinched in the gap and won't move at all.

Then again, after that ride in the trunk, there could be other items loose in the amp besides speakers. Some things like loose nuts & bolts, solder connections, cleaning up tube preamp pins (often corroded), cleaning pots, can be addressed in the home workshop.

Kamloops, way up in the Rockies. Beautiful views & all the clean air & sunshine you can stand I'm sure. And about a zillion miles/km from anything.... I'm guessing you have to go to Calgary to find any tech help.

[keithb7]

Yes, I am pretty spoiled here in Kamloops. There is a local tube amp tech. I get by mainly on my own resources these days. I have been reading, studying, practicing tube amp repairs and maintenance. When I am unsure about something I come here and always get great help. This forum has the friendliest, helpful professional members of any forum I have found. Thank you.

What do you folks think of these re-cone kits? My local speaker repairman suggested I buy a couple of kits and bring them in to him. He'll install them.

JBL-K120-Recone-Kit - Sound Speaker Repair

SS Audio Recone Kit for JBL D120, E120, K120, RK-JBLE120-8

Original JBL kits are quite rare and pricey I hear. I am a hobby player. I don't need to have Dick Dale or Jerry Garcia tone. I am ok with decent results. Will these be 75% of the original tone?
Probably impossible to answer that question. Any experience with these aftermarket re-cone kits? Thanks again. Keith