Problem with JBL PSWD-112 amp (1 Viewer)

[Rick Guynn]

I decided to post this here instead of in the speakers area since it is more of a technical nature.

I am hoping maybe someone else might have some experience with this and be able to narrow my search a bit. I am a novice at electronics (I can read schematics, know the basics and have the tools) but I appreciate any help I can get.

Anyway, my amp recently exhibited a problem of appearing to become 'less sensitive' on the input (low-level). It would take quite a bit to kick it on, and then it would turn off as soon as it lost sufficient signal instead of waiting the 15 mins it is supposed to.

So I got the service manual, which had a service bulletin in it regarding cold solder on the primary 'amp module'. The bulletin merely called for desoldering and resoldering all (28) of the connections.

I performed the soldering job, after which time the sub worked.. sort of. Now it would not turn off, but appeared to be producing sound as normal. So I decided to let that go.

A couple of days later I decided to try and set my speaker levels. I ran a signal through the sub (50 Hz I think) and it began clipping (I didn't really have it up that loud) badly.

Now it will not play anything without apparently going into overload. The signal LED on the front panel flickers between green and red and the sub 'sounds' like it is clipping.

I have no idea what I could have done to exacerbate the amp problem.... anyone have any siggestions? I have a scope and amp planning on trying to trace the signal through the circuit this weekend, but it would be nice to have something to look out for.

Thanks in advance.

Rick Guynn

 

[ColinM]

Did you get this thing new?

Just curious....

 

[Rick Guynn]

Yes, I've had it for about 2 years now. Funny thing is that the original I bought quit working a few weeks after I bought it. JBL replaced it with a refurb, which as it turns out was one of the 'newer' revised models. Of course, I bought the original before all of the 'bad amp' stories surfaced....

It has worked great for the past two years and then started this problem a few weeks ago. I bought a used plate amp that I am going to mount in it's own enclosure to power the sub until I either repair the original amp or decide it's hopeless.

Luckily the service manual was free (pdf online even) and pretty detailed with schematics. Being a novice at electronics, I do not have the experience to immediately narrow the problem down to a particular area, so I am going to have to brute force it. Just hoping someone else might be able to narrow it a bit for me..

 

[Jeff Rosz]

hello rick,

would you mind posting that link to the service manual .pdf file for me please. i'll take a gander at it. also is there a number printed on the "amp module", if so, can you post that too? i hate "amp modules", they are a regular pita, but if its a common module they are cheap in most cases to replace.

anyway it sounds like you are on the right track by signal tracing with the scope. pay particular attention to any transistors(if it has any...see next paragraph) before the "amp module". they are commonly called the "pre-driver" stage composed of usually two stages/transistors and the "driver" stage, usually one stage/transistor. to spot the divisions between stages, look for a capacitor in series with the base of a transistor.

this "amp module" is quite probably the pre- and driver stages on one chip and is hard to tell if its the problem without replacing it. if so(meaning you dont see any of the traditional amp stages in the schematic), its very important to find the part number printed on the module.

 

[Rick Guynn]

Thanks for the reply Jeff. Getting to the Service manual online is kinda weird. The actual link is this:
Link Removed
It's a relatively large file (around 4 megs). The thing is, you may not be able to access that directory unless you are already viewing the JBL costumer service web site, which is:
http://www.jbl.com/home/product_supp...rodId=PSW-D112
Once you are at the address immediately above, paste the first address I listed into the address bar and hit return.
If you leave off the filename and just go to:
Link Removed
You can see all of the service manuals that they have online.
If you still have problems accessing, let me know and I can extract the scematics to jpg files and e-mail them to you.
Now, the amp module is called an S53AMI, and they do not include a schematic for the internals of this unit.
Thanks again Jeff
Rick

 

[Jeff Rosz]

hello rick,
ok, sorry it took me so long.
i'm looking at the POWER AMP schematic/circuit board. the part labeled U1, a TL 064. its made by texas instruments. its a common jfet op amp. it is the main part in the muting and signal LED section of the schematic. this part probably costs 10 cents or less in bulk or about a buck-fifty at ratshack or your local parts house. just ask the guy to cross reference it to the manufacturer they carry, all clones of it should be direct drop-in replacements. it is a 14 pin integrated circuit. while you are at the parts house, go ahead and get a 14 pin IC socket and solder that to the pcb in place of the old TL064, then plug the replacement IC into it. its much easier to just replace the IC and see if that fixes the problem than to try to troubleshoot or signal trace the whole circuit. remember, its on the POWER AMP board, not the preamp board. hope this helps, good luck.
see the TL064 op amp here.
edit: oh ya, there is/should be a 3 wire cable from the signal LED on the panel to the power amp board via a 3 pin molex connector. make sure its getting good connection. inspect the male and female pins/sockets inside the plastic molex housing to see if anything is afoul.

 

[Rick Guynn]

Thanks alot Jeff! I'll let you know how it turns out!

-Rick

 

[Jeff Rosz]

heya rick,

ive been away for a while (i work on "the road")...any progress?

 

[Rick Guynn]

Well, I replaced the IC by itself and that didn't work :frowning: So just for grins I replaced Q1 (the JFET in the mute circuit). Still no dice.
On top of the problem (it seems to have gone back to the original problem of requiring high input signal and shutiing off immediately after loss of signal) it had, it now seems to have a high-pitched squeal that is present regardless of a signal or whether active. I only noticed that after replacing Q1, so I may put the original part back in to see if it makes that go away.
At this point, I can ony figure that it must be a capacitor in the mute circuit somewhere. And I am not sure if signal tracing is going to easily reveal this or not.
So that's where it is.. unfortunately I had only enough time last weekend to change the components (company in town). I *did* rig it up so I could use an external amp easily, so I am not toally subless at the moment.
I will try and do some tracing this weekend if I can scrape out the time...