Bose CD-3000 tablle radio/CD player sending dc to speaker.

[David Farber]

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,
www.datasheetcatalog.org/datasheet/stmicroelectronics/1512.pdf and
http://pdf1.alldatasheet.com/datasheet-pdf/view/25150/STMICROELECTRONICS/TDA7375.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.

 

[N_Cook]

http://pdf1.alldatasheet.com/datasheet-pdf/view/25150/STMICROELECTRONICS/TDA
7375.html I

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

 

[David Farber]

"N_Cook" wrote in message

http://pdf1.alldatasheet.com/datasheet-pdf/view/25150/STMICROELECTRONICS/TDA
7375.html I

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.

 

[David Farber]

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/datasheet/stmicroelectronics/1512.pdf

 

[klem kedidelhopper]

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/datasheet/stmicroelectronics/1512.pdf

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

 

[William Sommerwerck]

I lost a nice 12" Jensen when a 60W amp unloaded

60V DC onto it. Now my bench speakers are fused.

As was the Jensen. <ahem>

 

[[email protected]]

According to that datasheet the only thing that can put DC on the output isthe input. If the input is capacitively coupled it would seem to have to be a defective IC but you have changed it which pretty much rules that out. So now what ?

I say when shit like this happens look at the PC board. If there are no setting resistors for the DC level on the input, any leakage path will do it.

I called Jethro bodine to cypher this out and we got 26 dB gain. well 20 dBis about 100, plus 6 makes it about 400. (approximately, fuckya about precision). The input impedance is 60 K, and since it a semiconductor and we ain't talking Mhz that means the input resistance is 60 K.

Let's get conservative on the gain. It is say, 200. That means for a six volt output it would take what ? You have 14 volts on the board, a leakage resistance of six megohms could result in 1/whatever at the input. Multiply it back up by the gain and you got volts, not millivolts.

Such leakage paths have many causes. Sometimes customers spill something init like soda, and after years it starts becoming conductive. Another big one is electrolyte leaking from the caps. This is another insidious one because sometimes you cannot see it and it (or the soda, or bigscreen CRT coolant) gets more conductive as heat and voltage is applied, and sometimes it reverses when turned and cooled off.

Tell ya what, take a 10 K resaistor between pins 1 & 2 of that chip and seewhat happens.

Either that, or take the thing apart and clean the PC board with acetome and alcohol. (acetone first, then HOT water, and then alcohol)

If you replaced the coupling caps, it has to be some kind of leakage. thereis no bootstrap or any of that shit here. This is simply two OPAMPs with common feedback, there ain't no more.

 

[David Farber]

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/datasheet/stmicroelectronics/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]

According to that datasheet the only thing that can put DC on the
output is the input. If the input is capacitively coupled it would
seem to have to be a defective IC but you have changed it which
pretty much rules that out. So now what ?

I say when shit like this happens look at the PC board. If there are
no setting resistors for the DC level on the input, any leakage path
will do it.

I called Jethro bodine to cypher this out and we got 26 dB gain. well
20 dB is about 100, plus 6 makes it about 400. (approximately, fuckya
about precision). The input impedance is 60 K, and since it a
semiconductor and we ain't talking Mhz that means the input
resistance is 60 K.

Let's get conservative on the gain. It is say, 200. That means for a
six volt output it would take what ? You have 14 volts on the board,
a leakage resistance of six megohms could result in 1/whatever at the
input. Multiply it back up by the gain and you got volts, not
millivolts.

Such leakage paths have many causes. Sometimes customers spill
something in it like soda, and after years it starts becoming
conductive. Another big one is electrolyte leaking from the caps.
This is another insidious one because sometimes you cannot see it and
it (or the soda, or bigscreen CRT coolant) gets more conductive as
heat and voltage is applied, and sometimes it reverses when turned
and cooled off.

Tell ya what, take a 10 K resaistor between pins 1 & 2 of that chip
and see what happens.

Either that, or take the thing apart and clean the PC board with
acetome and alcohol. (acetone first, then HOT water, and then
alcohol)

If you replaced the coupling caps, it has to be some kind of leakage.
there is no bootstrap or any of that shit here. This is simply two
OPAMPs with common feedback, there ain't no more.

The owner does live about a mile from the Pacific Ocean so I considered that
there could be damage from the salt air but there was no evidence of any
corrosion. The telescoping antenna does have some slight signs of pitting
though.

I explained in a response to Lenny (aka klem kedidelhopper) how I resoldered
some connections in the preamp which feeds the main amp. So far things are
looking good. If things go haywire again, I'll try that 10k resistor
experiment.

Thanks for your reply.