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PostPosted: 23 Jul 2014, 23:45 
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Joined: 23 Jul 2014, 21:50
Posts: 3
Location: Missouri, USA
Hello everyone! I've browsed around a bit looking for some help on repairing some speakers that have gone kaput and this place looked really helpful! I've tried to repair this on my own almost a year back but was in a rush and found myself out of my depth in short order.

Here's the lowdown on my situation:
Klipsch ProMedia 2.1 computer speakers. Used them for the sound system at a gathering (mostly people hooking their phones up to play music from them) and everything was fine. Moved them back to my computer a few days later and found really weird sound coming from them. No sound on the left channel, and very weak/distorted sound in the right channel. The subwoofer performed as expected. I swapped the speakers around and got the same thing, ruling out a speaker issue.

I popped open the subwoofer and poked around until I found a toasted resistor on the High-Frequency amplifier board. Further poking around with a DMM found most of the transistors on the board had continuity across pins and a cap had a slightly puffy top. I was trying to get the speakers working for my upcoming wedding (sound system for a small venue on the cheeeeap) so I quickly bought new parts based on a schematic I found online and popped them in. I left the speakers and sub unplugged from the board and plugged it in. I heard a "things aren't going to work out for you" electrical buzz from the board for a few seconds before a loud pop and bad-news smoke started to rise. I quickly killed the power to it but clearly some serious damage had been done. With the wedding being 2 days away at that point, I stuffed it to the side and went about finding a different set of speakers. I'd like to now sit down and take the time to attempt a proper repair.

Part of the big problem with my original repair was that I couldn't find exact matches for replacement parts for two of the transistors. In my haste, I chose something I thought would be "close enough" and probably chose waaaaay incorrectly.

For reference for anyone willing to give this a go:
http://christopherbradshaw.net/The_Project_Bin/klipsch%20promedia%20subwoofer%20repair%20-%20page%201.html
That site has full schematics for the entire speaker system. It's been very useful and from everything I've checked so far, has been accurate.
It wasn't until earlier when I started revisiting this that I realized there are actually *two* high frequency amplifier boards, with only one represented in the schematic. They're basically identical only they had different transistors. I see now the Low Frequency board is on a different base, which I mistook the other HF board for. Now that I'm not all stressed out about time, I realize I may have gotten something mixed up.

As it stands now, one of the HF boards seems to be fine. I see no visible damage and everything seems to test as expected.
The other one is in bad shape.
Attachment:
IMG_0271.JPG


Attachment:
IMG_0275.JPG


Attachment:
IMG_0276.JPG

I'm only mostly embarrassed to show this...

Attached are some pictures of other places around the boards. I can add more later, but I'm limited to six attachments. Since I can't add anymore, here's a link to everything I've tested on the board now, in schematic form:
Image

Parts I replaced last time:
R17 - It was visibly burnt last time, but not as badly as this time... Replaced with 220ohm resistor.
R15 - Listed in the schematic as 6200ohm. That didn't match up with the original on the board. It was the same color-code as R15 on the working HF board which read 8200ohm. Previous one was reading 3030ohm. Replaced it with the 8200. It now reads 3050.
Q9 - Had continuity before. Exploded in great fashion this time.
Q10 - Has continuity across all pins, same as before.
Q7 - Continuity Across 2 of the 3 pins.
Q8 - Continuity Across 2 of the 3 pins.
C3 - Had a slightly puffy top so I replaced it. Appears to be normal now after the most recent damage.

Where I'm pretty sure I screwed up hard:
As listed in the schematic, Q7 is listed as 2SC2235 and Q8 as 2SA965. I couldn't find these in production anymore and I was in a big hurry so I substituted Q7 for KSC1845FTA https://www.mouser.com/Search/ProductDetail.aspx?R=KSC1845FTAvirtualkey51210000virtualkey512-KSC1845FTA
and Q8 for https://www.mouser.com/Search/ProductDetail.aspx?R=KSA916YTAvirtualkey51210000virtualkey512-KSA916YTA .
Why? It seemed like a good idea at the time and I was desperate to get the thing fixed. I couldn't find a replacement with identical specs so I went with the closest thing I could find that was in stock. Derp.

Also, the WORKING board has 2SC2235 and 2SA965. I'm not entirely sure what the original part numbers were on the dead board as I've taken those out and have surely thrown them out now. (The original repair attempt was over 8 months ago)

So... with all that said, is anyone willing to give this a shot? I'm clearly not well versed in proper electronics but I'm handy with a soldering iron and follow directions well. If someone could give me an idea of where to even start I'd really appreciate it! Even if it's just to help me find a suitable replacement for the original Q7 and Q8. I'll try to dig around and see if I can find anymore info on what the originals on this board may have been, but if anyone here knows what the circuit SHOULD be doing, that'd go for a lot.

Thanks!!


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PostPosted: 25 Jul 2014, 10:33 
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Joined: 23 Jul 2014, 21:50
Posts: 3
Location: Missouri, USA
So after a little more research, I found the site with the source of the schematics, found here:
http://www.thompdale.com/bash_amplifier/2-1/2-1_bash_amp.htm
He gives a great run-through of all of the systems.

Seems like both of the high frequency boards are identical in the component department. Each just drives the left and right speakers independently. This takes away my fear of "Oh no, I've thrown away the originals!"
And, after further digging around on the internets, I've found another channel for getting the proper replacement transistors. At this point, considering the damage done to the board, I'm planning on replacing a majority of the components just to be on the safe side.

My question now is, assuming I get the board properly repaired and running correctly, should I be checking something else that would have caused this mess to begin with? Specifically is there some part in the signal sending system I should check to make sure it's not surging before I put in the repaired board, power it up and end up in the same boat?

Thanks!


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PostPosted: 12 Aug 2014, 10:54 
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Joined: 23 Jul 2014, 21:50
Posts: 3
Location: Missouri, USA
Just to close this out for anyone that happens across this in the future:
I was able to get the amp up and running again by replacing the following:

C3 - 47uf, 25v
R14 - (was close to R17 when it burned up, replaced just in case even though it tested good)
R15 - 8.2k Resistor though it's listed as 6.2k on the schematic. Cross check it with what's on the other board as earlier versions may have a 6.2k.
R17 - 220ohm 1/8watt
Q6 - 2N3904
Q7 - 2SC2235 <--
Q8 - 2SA965 <-- Q7 and Q8 I had to get on eBay, they're obsolete now.
Q9 - TIP41C
Q10 - TIP42C

It was important that I took the transistors out of circuit to test them individually as Q6 didn't appear to be defective while still soldered in. It didn't have any connection between its emitter and base at all.
Only downside is that once I got the amp fixed I found out that it took out my left-channel woofer with it. I've ordered a replacement for that and there seem to be plenty to choose from on eBay. Seems to be a common issue for the 2.1's if the amp board blows. I tested the amp with some other speakers and they work great with the new parts in place. I'm looking forward to having my good computer speakers back!


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PostPosted: 03 Jun 2016, 11:43 
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Joined: 21 Mar 2013, 19:45
Posts: 2
I'm repairing a friend's Promedia 2.1 system and have *EXACTLY* the same failure, same parts toasted, same 3" speaker in one of the satellites blown.

Like you, I located the two predriver transistors on eBay.

Previously to this failure, the two MOSFETs on the Class D amp that drives the sub had blown.

My friend has bad luck with his Promedia 2.1! I was amazed at how low-tech the design is. Also, the volume control is in the gain feedback loop of an opamp, rather than a voltage divider in front of a fixed-gain opamp circuit. That's pretty primitive.


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PostPosted: 03 Jun 2016, 13:23 
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Joined: 04 Jun 2008, 20:59
Posts: 3814
Location: Arizona, USA
Hi, I missed the original postings...but when I looked as the schematic I was going to suggest replacing Q6. It is in a place that could seriously alter the bias on the output stages and you had already replaced all them. I hate this sort of repair as there are often more bad components than good ones left. Sansui amps had a tendency to wipe out the entire section...all the semiconductors in the channel when certain sorts of minor failures occurred.

Good listening
Bruce

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PostPosted: 11 Jun 2018, 16:34 
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Joined: 11 Jun 2018, 16:21
Posts: 1
Hello all,

joined this board to solicit a little help. :)
my promedia 2.1, old version.. the version with the hefty transformer, not the new chip amp version gave up the ghost.. and i did more damage int he process of trouble shooting.

its been working 10 years straight with it never having been turned off and for about 8 months i had to stow it away.
i connected everything back a few days ago and theres was a little sizzling sound and then nothing.
so basic trouble shooting, cheked connections, swapped speakers around etc... another sizzle and then nothing.
at which point i turned it all off and took it apart.
everything looked fine, no charred bit.. nothing

checked voltage at satellite speaker output and right showed 24V dc... (what the heck..).. which explains the sizzle i heard... twice! i blew out the woofer voice coils

left channel showed 0 volts...

a little googling and it seems that the transistors (?) on the right channel has shorted in the "open" position" and sent 24 volts to the satellite. shouldnt have the fuses on the HF board prevented the satellites speaker from blowing? what is the point of those 2 fuses on the 2 HF boards then? they are the originals, theyve never been changed,.

and that seems to align with the repair that i see here....

was wondering.. with a voltmeter in a working set, what are the voltages i should see at the speaker output terminals?

im still debating on weather i should try to repair this as i now need 2 new woofers. (the tweeters tested fine. ). fortunately i already have 2 3 inch neodymium tang band drivers from another project that fit right in... what i dont want is to blow them as well with a botched repair.

im guessing, following some of the info here and swapping those parts out will fix this....

or is it just easier and more cost effective to turn the sub into a footstool and get a new set when it goes for sale at 99$ at best buy....

any and all help is much appreciated.


regards,
asmd


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