klem kedidelhopper wrote:
On Apr 12, 6:30 pm, "David Farber" wrote:
David Farber wrote:
"Gareth Magennis" wrote in message
...
"N_Cook" wrote in ...
Gareth Magennis wrote in message
...
"David Farber" wrote in message
...
This Bose CD-3000 tabletop player was brought to my attention
because intermittently there were popping/static noise was coming
out of the speakers. It was quite loud. When I powered it up via
the 12 volts input, the problem didn't seem so noticeable. I took
it to the shop, opened it up, and noticed there were two car
stereo-type speaker amplifier chips powering the speakers. Using
these two datasheets,
http://www.datasheetcatalog.org/data...cs/1512.pdfand
http://pdf1.alldatasheet.com/datashe...TMICROELECTRON...
7375.html I
was able to figure out that the TDA7375 powers the left and right
speaker and the TDA7396 powers the subwoofer. I checked the
prices of replacing these parts and each one was less than $10.
Since the pc board where they resided was difficult to access
and make tests while powered on, I decided to replace the stereo
amp chip. It didn't help. I figured then it had to
be the other chip. I replaced the other chip with no improvement.
Then I
figured out a way to remove all the pc boards and cables and
power it up
outside the unit where I could make some meaningful tests. First
of all,
there was some small dc offset going to the subwoofer. I soldered
a 330 ohm 0.5 watt resistor across the subwoofer output terminal
so it would have some load. It was about 340mv. Not great but
certainly passable. Then after about 15 seconds, the dc shot up
to 12 volts and my bench meter showed a spike in current. This
voltage swing delay occurs every time I power it on. If I just
disconnect the subwoofer, the left and right speaker outputs are
fine. A schematic would be of immense help here or perhaps
someone has had some experience with this unit. Any and all
suggestions are welcome. Thanks for your reply.
--
David Farber
Los Osos, CA
Could this be yet another "conductive glue" problem?
(See the "TDA7295" thread here, first post 18/3/13, last post
confirming this problem 29/03/13)
Gareth.
Either that or a failing minor supply to an op-amp farther back,
eg a small
cap on one rail, is my guess. Either way requires tracing back from
the input/s to the output device
If this unit normally waits 15 secs or so before coming off
Standby, it would support the conductive glue theory, as this is
perhaps when the Standby voltage gets into an IC pin via the glue,
and causes the output to DC.
A brand new TDA device should not have 300mV of DC anyway, this
must be a clue, there is a leak somewhere.
Gareth.
I was leaning toward a conductive glue problem because there are a
few wires that are fed from the amp board to the main board that are
protected by a flimsy sleeve that is covered with what looks like a
foam material. It was sticky to the touch. I cleaned it off from the
parts of the pc board that it was touching but it made no
difference.
Then I replaced the two input caps going to pins 1 and 2 of the TDA.
The originals were .47uF film type. I replaced them with .1uF
ceramic type just to see if it made any difference. It did. Noise
and popping stopped. Voltage swings stopped. And I even tested it
for ten whole minutes. I'll give it a full burn in tomorrow but
usually it wouldn't hold steady even for one minute, especially
powered by AC. I think the AC was more of a headache because it put
a higher supply voltage (15VDC) than my bench supply which I only
powered up to 12VDC. Maybe not. Anyway, I'll replace it with the
original valued capacitors if this problem stays away for good. By
the way, the reference numbers of the caps are C62 and C64.
Thanks for all your suggestions.
I powered it back on this afternoon. It lasted for about 30 seconds
before
it acted up again. Back to the drawing board for me. I'm going to
see if I
can figure out a way to just power up the TDA chip by bypassing the
cpu
power up signal with everything else disconnected and then see what
the amp
output does. It would be nice to know if the problem is on the amp
board or
coming from somewhere else. Would I need to put some dc on the two
signal
input caps to bias this
correctly?http://www.datasheetcatalog.org/data...onics/1512.pdf
--
David Farber
Los Osos, CA
Hi David
I looked at the "standard test and application circuit" in the PDF
link you sent and although there is no schematic of the inside of the
chip the test circuit shows a 2200mfd electrolytic in series with each
speaker. If your radio indeed has those then in theory there should be
no way to read DC on the speaker unless the cap was leaking.
(I'm assuming it's only one speaker you're having this problem on). In
any case until you do resolve this I would temporarily put put a
quarter or half amp 3AG type fuse in series with the speaker. 12VDC
should be capable of wiping out a voice coil. I lost a nice 12" Jensen
once when a 60 watt amp I was working on decided to unload 60VDC onto
it. Now my bench speakers are fused. Some lessons come hard. Lenny
Hi Lenny,
I believe you're looking the link to the quad power amp datasheet (TDA7375)
which turned out not to be the source of the dc output. That chip powers the
left and right speakers. The problem output was coming from the TDA7396
which powers the small subwoofer. I didn't test this with the speaker
connected. I used a load resistor to monitor the dc output.
Just to follow up on the progress since my last post, I isolated the
amplifier board and hooked up a 14 volt source to power the amp and a 5 volt
source to pin 8 to turn the amp on. I didn't hook up any input to the amp. I
just sat around and watched the dc level out. It was stable without any
popping.
Then, I followed the input wires back to the main pc board which is home to
the AM/FM tuner, preamp, and everything else except the display board. I was
led to pin 1 of a TL0748CN. The data sheet is here.
http://www.datasheetcatalog.org/data...onics/2297.pdf
I resoldered the connections in the vicinity, hooked everything back up
again, and so far the popping and dc shifts are gone. It was playing for an
hour today. The problem is, I don't want to just let it sit and play if I
can't be within listening range for the reasons you stated earlier. I don't
want dc going into the speaker or to damage any other components.
Thanks for your reply.
--
David Farber
Los Osos, CA