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

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.

input,

speaker

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Then

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

input,

speaker

they

decided

Then

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?

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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

As was the Jensen.

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 b e a defective IC but you have changed it which pretty much rules that out. So now what ?

I say when s*it like this happens look at the PC board. If there are no set ting 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 prec ision). The input impedance is 60 K, and since it a semiconductor and we ai n'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 vo lt output it would take what ? You have 14 volts on the board, a leakage re sistance of six megohms could result in 1/whatever at the input. Multiply i t 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 o ne is electrolyte leaking from the caps. This is another insidious one beca use sometimes you cannot see it and it (or the soda, or bigscreen CRT coola nt) gets more conductive as heat and voltage is applied, and sometimes it r everses 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 an d 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 s*it here. This is simply two OPAMPs with c ommon feedback, there ain't no more.

correctly?

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.

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.

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.