Hi all,
I recently purchased a pair of Fostex T825 super tweeters, probably about 40 years old (originally from Fostex Laboratory series LS/3 monitors).
After a quick sweep the measurements lead me to believe one is faulty.
Does the lower trace show signs of a bad diaphragm? Or diaphragm not aligned/seated properly?
One sweep is with 4.7uF cap and another with 1.0uF cap.
Both measure 6.7ohms
Thankfully, the seller is willing to refund if I'm not satisfied.
Looking for some opinions before I bring them back.
Serial numbers are almost consecutive (11156, 11158).
I recently purchased a pair of Fostex T825 super tweeters, probably about 40 years old (originally from Fostex Laboratory series LS/3 monitors).
After a quick sweep the measurements lead me to believe one is faulty.
Does the lower trace show signs of a bad diaphragm? Or diaphragm not aligned/seated properly?
One sweep is with 4.7uF cap and another with 1.0uF cap.
Both measure 6.7ohms
Thankfully, the seller is willing to refund if I'm not satisfied.
Looking for some opinions before I bring them back.
Serial numbers are almost consecutive (11156, 11158).
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After trying a few more general Fostex searches I discovered this post:
https://www.diyaudio.com/community/threads/wanted-fostex-driver-parts-diaphragms.215673/post-7218835
Some helpful suggestions to think about.
https://www.diyaudio.com/community/threads/wanted-fostex-driver-parts-diaphragms.215673/post-7218835
Some helpful suggestions to think about.
Hi,
Here's some reference reading material for you.
See Fostex T-925 Recone ( re-diaphragm )
Replacement Diaphragm for Fostex T90A, T825A, T825En, T900A and T925A, 8 ohm
I don't know the difference between the original T825 and T825A models. They might be the same as the T-925 & T925A differences seen in the link above ( in the way and location the voice-coil leads dress-out ).
Solen should be able to order a pair of replacement diaphragms if you want to go that route ( originally they were aluminum diaphragms and then changed over to titanium many years back ).
PS: It would be handy if sellers actually measured their used wares before putting them up for sale ( wouldn't it ?? )
Here's some reference reading material for you.
See Fostex T-925 Recone ( re-diaphragm )
Replacement Diaphragm for Fostex T90A, T825A, T825En, T900A and T925A, 8 ohm
I don't know the difference between the original T825 and T825A models. They might be the same as the T-925 & T925A differences seen in the link above ( in the way and location the voice-coil leads dress-out ).
Solen should be able to order a pair of replacement diaphragms if you want to go that route ( originally they were aluminum diaphragms and then changed over to titanium many years back ).
PS: It would be handy if sellers actually measured their used wares before putting them up for sale ( wouldn't it ?? )
Thank you @EarlK !!!
That first link is excellent, can finally see how the diaphragm sits inside. Searches for these diaphragms resulted in the THLP link you shared.
Don't plan to pursue new diaphragms given the option for a full refund. I really wanted to like them as my first go experimenting with Fostex.
No idea either between T825A and T825, couldn't find that info. The original box and the driver itself indicate T825.
Read about these tweeters over the years and don't see them for sale often. Plan was to compare the T825 vs my JBL 2405H's, bringing them in around 10KHz for the top octave as shown with the 1.0uF cap. I enjoy comparing drivers/listening tests here at home.
The seller was very fair but I couldn't agree more, measuring beforehand would definitely be helpful for the buyer.
That first link is excellent, can finally see how the diaphragm sits inside. Searches for these diaphragms resulted in the THLP link you shared.
Don't plan to pursue new diaphragms given the option for a full refund. I really wanted to like them as my first go experimenting with Fostex.
No idea either between T825A and T825, couldn't find that info. The original box and the driver itself indicate T825.
Read about these tweeters over the years and don't see them for sale often. Plan was to compare the T825 vs my JBL 2405H's, bringing them in around 10KHz for the top octave as shown with the 1.0uF cap. I enjoy comparing drivers/listening tests here at home.
The seller was very fair but I couldn't agree more, measuring beforehand would definitely be helpful for the buyer.
Hi, AE13:
Kudos to you for snagging a pair of rare Fostex drivers!
), but if it was me, I'd open them up for a look-see... or perhaps you've already done that? In view of the fact that they are basically functional, I wouldn't give up on them easily!
Hope you keep us updated!
Wilf
Kudos to you for snagging a pair of rare Fostex drivers!
So: did you give them a listen? Is the difference you measured clearly audible and disturbing? Not telling you what to do; (also don't know how much $$ you sunk into this!
), but if it was me, I'd open them up for a look-see... or perhaps you've already done that? In view of the fact that they are basically functional, I wouldn't give up on them easily!Indeed! Sadly, and with no disrespect intended: in my experience many/most sellers don't have the expertise to give that kind of useful information; and if they do, they are probably building that into their price... then there's the ones that are seemingly intentionally vague in their assessment of their items' functional condition: I've learned to accept that for what it is, and bid accordingly...
Hope you keep us updated!
Wilf
@w_oswald I did listen to a few songs using left channel only, the good T825 sounds "different" compared to the JBL 2405H. Not as dynamic, definitely not as efficient but to my ears bit more refined. I wouldn't claim one is better than the other. Both good in different ways.
The bad T825 sounds filtered, attenuated and less refined compared to the good T825. To be fair, should've listened first, measured after but the 6dB dip around 12kHz is audible.
If I was you, I would open them too Clearly you have experience given your suggestions in that post I linked above. The only diaphragm I replaced was a JBL 2445 from original to aquaplas (2450SL) and it was straightforward. My hesitation is ruining the back decal/plate and therefore losing my option for refund... which after further review ... I suspect the bad one was previously serviced. The decal/plate appears to show signs of a removal. The top part of the decal looks chipped like it was lifted before. Now the decal plate is firmly (glued) in place with no play, the good T825 decal has a bit of a loose feel when I tap it.
I wonder if there is two different diaphragms and hence the different response.
Or maybe a repair job that wasn't executed properly.
What do you think about the top of the decal being chipped off?
The bad T825 sounds filtered, attenuated and less refined compared to the good T825. To be fair, should've listened first, measured after but the 6dB dip around 12kHz is audible.
If I was you, I would open them too
Clearly you have experience given your suggestions in that post I linked above. The only diaphragm I replaced was a JBL 2445 from original to aquaplas (2450SL) and it was straightforward. My hesitation is ruining the back decal/plate and therefore losing my option for refund... which after further review ... I suspect the bad one was previously serviced. The decal/plate appears to show signs of a removal. The top part of the decal looks chipped like it was lifted before. Now the decal plate is firmly (glued) in place with no play, the good T825 decal has a bit of a loose feel when I tap it.I wonder if there is two different diaphragms and hence the different response.
Or maybe a repair job that wasn't executed properly.
What do you think about the top of the decal being chipped off?
You very well may be right in suspecting that the unit pictured has been worked on. I guess you have to decide for yourself whether to tear into it or not. I've found that with patience, brushing some solvent - acetone has proven successful for me - into the gap around the edge of the decal will soften the adhesive to allow it to be gently lifted without bending it. (Obviously not on this particular model...)
I've found that dried-up ferro-fluid in the voice coil gap can severely attenuate the output of a tweeter or compression driver. Seeing that yours are effectively twins, that's not likely the issue here. As you suspect: maybe some carelessly-done repair is to blame...
Hmmm: as you're comparing them to JBL 2405H tweeters, I would've thought you'd be after the Fostex T845 units. (Yeah, I know: pretty much unobtanium...) I recently ordered myself a pair of these (new) from AliExpress: Haven't yet had the chance to seriously evaluate them, but they look pretty nice!
Best regards, Wilf.
I've found that dried-up ferro-fluid in the voice coil gap can severely attenuate the output of a tweeter or compression driver. Seeing that yours are effectively twins, that's not likely the issue here. As you suspect: maybe some carelessly-done repair is to blame...
Hmmm: as you're comparing them to JBL 2405H tweeters, I would've thought you'd be after the Fostex T845 units. (Yeah, I know: pretty much unobtanium...) I recently ordered myself a pair of these (new) from AliExpress: Haven't yet had the chance to seriously evaluate them, but they look pretty nice!
Best regards, Wilf.
Thanks Wilf, appreciate your insight and knowledge. I decided to return the T825's for a full refund. Troubleshooting and repairing the driver is not something I wanted to take on given the price paid, nor do I have the experience to evaluate it properly. Yourself and EarlK reaffirmed my initial thoughts ... disassembly required and troubleshoot further. The info and suggestions made sense but I felt a full refund was better for me in this particular case.
Those tweeters you posted ^ look similar to the Beyma CP21. They do look nice, hopefully they sound nice too. Curious to know your thoughts after you give them a fair listening test.
I like the JBL 2405H's but also enjoy trying out different options. My experience is very limited with super tweeters. I prefer my system with them rather than without.
Those tweeters you posted ^ look similar to the Beyma CP21. They do look nice, hopefully they sound nice too. Curious to know your thoughts after you give them a fair listening test.
I like the JBL 2405H's but also enjoy trying out different options. My experience is very limited with super tweeters. I prefer my system with them rather than without.
I think you made the right decision returning them. I owned a pair for many years and they matched to better than 1dB over most of their operating range. I no longer have response measurements for them or I would share. I found them a bit laid back and soft was crossing them around 8kHz, not the most extended HF driver I have owned.
They were replaced with a pair of Faital Pro HF10AK on STH100 horns shortly after I switched to Yuichi A-290 clones with TD-4001.
The 2405 and 077 (slightly different materials) are both quite good IMLE.
They were replaced with a pair of Faital Pro HF10AK on STH100 horns shortly after I switched to Yuichi A-290 clones with TD-4001.
The 2405 and 077 (slightly different materials) are both quite good IMLE.
@kevinkr I remember reading your posts when searching the Fostex T825 tweeters.
The Faital Pro HF10AK is something I'd like to try ... same with TD-4001. You have some nice drivers.
My system is an ongoing work in progress, learn a lot from experimenting. This DIY journey has lead me to understand I prefer the sound of large horns. The 2405H handles the top octave nicely where RCF ND350 begins to roll naturally starting around 10kHz on the big radial horn. Took some playing around to time align/find the correct height for the 2405H to integrate properly with the 1" horn below. Here is some pictures of a DIY Macgyver stand that achieves the necessary height I needed for nice flat response between the two drivers. Bookshelf L-brackets, patent pending
Personally don't like using EQ to lift the natural roll-off of compression drivers, sounds a bit off to these ears. Obviously more so with 2" vs 1" but I find the JBL 2405H is effortless in top octave. The complete system sounds better overall with 2405H compared to without, even the midrange and bass is improved for some reason.
The Faital Pro HF10AK is something I'd like to try ... same with TD-4001. You have some nice drivers.
My system is an ongoing work in progress, learn a lot from experimenting. This DIY journey has lead me to understand I prefer the sound of large horns. The 2405H handles the top octave nicely where RCF ND350 begins to roll naturally starting around 10kHz on the big radial horn. Took some playing around to time align/find the correct height for the 2405H to integrate properly with the 1" horn below. Here is some pictures of a DIY Macgyver stand that achieves the necessary height I needed for nice flat response between the two drivers. Bookshelf L-brackets, patent pending
Personally don't like using EQ to lift the natural roll-off of compression drivers, sounds a bit off to these ears. Obviously more so with 2" vs 1" but I find the JBL 2405H is effortless in top octave. The complete system sounds better overall with 2405H compared to without, even the midrange and bass is improved for some reason.
Yes obviously i should have read that you returned them too
Yes it is fairly common with low power handling, vintage alnico magnet drivers. When the problem is difference in output without other anomalies, that is usually the issue.
Not uncommon with TAD's , old JBL Alnicos etc.
If you read up on the JBL 1500AL series, you can find out how they addressed the issue in that design also to avoid the issue.
And yes they can be serviced and recharged in general.
The video shows the process in a simple and straightforward way, by Upland loudspeaker Service (@edgewound).