Just thinking aloud:
1) cone tweeters DO need at least a series cap.
2) original caps may have been removed by somebody who failed to replace them.
Worst is that at a few milliwatt level, say transistor radio connected to cabinet just to check they "work", you WILL hear Bass coming from woofer, treble coming from Tweeter and assume "everything is fine"
Of course, once you apply 10W or so, Tweeter voice coil catches fire, but that would not be apparent in a quick test.
3) A FEW cheap tweeters come with a cheap capacitor already connected at Factory, just to make inexperienced owner life easier.
Many of those were not intended as "replacements" (where some kind of crossover would be present) but as "add ons" to , say, console/gramophone cabinets usually sporting a single speaker, with or without whizzer cone.
I have seen some with the capacitor visible, and one or two with cap inside, between closed frame and cone.
The way to check that is to measure DCR, it will show "open", yet touching terminals with a battery (1.5V is low enough to damage nothing) they will click, specially if you keep inverting polarity each time (so you fully charge/discharge cap).
Now IF you read something around 4 to 8 ohm, by all means add a crossover cap, 2,7uF mentioned by Galu is fine.
EDIT: more common than I thought, there even is a tutorial for them=
https://soundcertified.com/how-to-wire-tweeters-built-in-crossover-to-amp/
1) cone tweeters DO need at least a series cap.
2) original caps may have been removed by somebody who failed to replace them.
Worst is that at a few milliwatt level, say transistor radio connected to cabinet just to check they "work", you WILL hear Bass coming from woofer, treble coming from Tweeter and assume "everything is fine"
Of course, once you apply 10W or so, Tweeter voice coil catches fire, but that would not be apparent in a quick test.
3) A FEW cheap tweeters come with a cheap capacitor already connected at Factory, just to make inexperienced owner life easier.
Many of those were not intended as "replacements" (where some kind of crossover would be present) but as "add ons" to , say, console/gramophone cabinets usually sporting a single speaker, with or without whizzer cone.
I have seen some with the capacitor visible, and one or two with cap inside, between closed frame and cone.
The way to check that is to measure DCR, it will show "open", yet touching terminals with a battery (1.5V is low enough to damage nothing) they will click, specially if you keep inverting polarity each time (so you fully charge/discharge cap).
Now IF you read something around 4 to 8 ohm, by all means add a crossover cap, 2,7uF mentioned by Galu is fine.
EDIT: more common than I thought, there even is a tutorial for them=
https://soundcertified.com/how-to-wire-tweeters-built-in-crossover-to-amp/
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Do you also expect the cone mass to change significantly?
If it's mainly the compliance that changes, the behaviour at low frequencies will change somewhat, around and below the woofer's resonant frequency. As long as the cone mass of the woofers doesn't change much, their sensitivity shouldn't change much either.
Hi Galu & Chris.
Great detective work.
The caps were well camouflaged, sitting behind wiring & terminal block on the back of the terminal plate.
2.7uF deteriorated to 3.2uF & 3.5uF (on old tester).
I've now refoamed the woofers & replaced the caps with Teds.
2 x 2uF in parallel to make it 4uF to each tweeter.
I hope 1.3uF variation doesn't adversly affect them.
The Jensens are sealed boxes but only had lining around back, top & bottom of the boxes.
I stuffed them with cotton/wool wadding.
Sound reproduction was very acceptable.
I'll also freshen up the matt black on the baffles & re glue some of the vinyl wrap which has lifted.
I only have one more hurdle to overcome.
One of the tweeters has had the fine wires from its terminal severed from the driver (see photos).
I think this is a very delicate repair job as the wires are very fine.
Don't know how to tackle it?
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Don't you mean Ted's?
4 uF is a little too far from the target 2.7 uF for comfort. It would be kinder to the tweeter to go with a single 2 uF.
So, the tweeter terminals have been competely ripped off!
From your photos, I can't really see how you're going to get access to the tweeter wires in order to solder on extra lengths of fine copper wire. If it is possible to access the wires, you will have to carefully scrape the enamel off the ends before they will accept solder.
Good call.
I haven't soldered the caps in place, so will easily be able to reduce down to 2uF.
If you look carefully in the photo you can just see the end of one of the wires, with dried silicone around it.
If I could move the grill away I might have access from that side.
Don't know if the grill is just glued in position or fastened some other way.
I'd like to prise it away but not damage it?
I haven't soldered the caps in place, so will easily be able to reduce down to 2uF.
If you look carefully in the photo you can just see the end of one of the wires, with dried silicone around it.
If I could move the grill away I might have access from that side.
Don't know if the grill is just glued in position or fastened some other way.
I'd like to prise it away but not damage it?
The Monacor HT-22/8 tweeter is a likely replacement. Although inexpensive, I suspect you won't think your restoration will justify their cost!
https://www.amazon.co.uk/Monacor-HT-22-Cone-Tweeter-Ohm/dp/B002FB1FYW
https://www.amazon.co.uk/Monacor-HT-22-Cone-Tweeter-Ohm/dp/B002FB1FYW
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I'll work on the measurement.
I've been open minded about repairing the wires.
Motto: Hard to Toss.
I'll have a shot.
Apart from making sure there's no protective coating, is there a secret to soldering very fine wires?
The Monacor's look very similar, but the diameter is a little out.
Monacors = 90mm to 63mm diameter rebate.
Existing = 107mm to 87mm diameter rebate.
If there is, I'm not telling you!
You can always fashion an adaptor plate as shown in the attachment.
Attachments
I'll go for micro surgery.So, the tweeter terminals have been competely ripped off!
I can try to twirl some cat6 wire onto the hair like wires which feed the tweeter voice coil and then attempt solder.
Worried heat might melt the copper??
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I never gave up on these.
The golden rule, assess before restoring.
I worked backwards.
Turned out one of the tweeters had the terminal ripped off and fine wires leading to the voice coil torn.
Cut a long story short, I performed micro surgery today.
Using strands of some very fine copper wiring I was cable to extend the voice coil wiring by soldering extensions.
Had to fool around several times.
The wire was so fine the solder just didn't want to adhere.
In the end success.
After stuffing the boxes with wool/cotton material, re foaming the surrounds on the woofers, replacing the 2.7μF old caps with Teds 2μF caps, and resoldering the tweeter voicecoil wiring, finally the Jensens work.
These have a voice of their own.
Very pleasing & listenable.
Thanks to all here at diy.