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Author Topic: SOLVED - Looking for diagnostic help but it's not a (guitar) amp  (Read 347 times)

Den.

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SOLVED - Looking for diagnostic help but it's not a (guitar) amp
« on: November 05, 2018, 10:06:40 PM »

Hi, I've been lurking here for a few years and have enjoyed reading the repair topics and learning a little about solid-state devices. But now I find I need diagnostic help for a problem that doesn't involve a guitar amplifier. I'm hoping that someone can point me in the right direction with this non-amp problem but I will understand if this isn't the proper place to post. I just had my little powered bookshelf computer speakers go kaput. Suddenly and with no prior warning they started squealing and buzzing. And they do this even after unplugging them from the computer. The 15vac power supply tests OK and I don't see any obvious damage on either side of the board. I am going to include a link to a short video that resides on Google Drive so you can hear what I'm talking about.
https://drive.google.com/open?id=1mw1AzFlJktfG7W-b3fgpAxluKt4s-HGD
« Last Edit: November 17, 2018, 11:12:29 PM by Den. »
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Cpt. FixIt

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Re: Looking for diagnostic help but it's not an amp
« Reply #1 on: November 06, 2018, 02:23:25 AM »

That IS an amplifier problem, isn't it? And it looks like a chip amp, so I don't think you've come to the wrong place.
What is the device on the heatsink? Would be helpful to have a reference schematic from the manufacturers datasheet.
Does the device still pass signal or is the buzz the only sound it makes?
The sound in the video would suggest a PS problem, sounds like 120Hz from a bridge rectifier to me.
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Den.

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Re: Looking for diagnostic help but it's not an amp
« Reply #2 on: November 06, 2018, 11:41:11 AM »


...Does the device still pass signal or is the buzz the only sound it makes?What is the device on the heatsink? Would be helpful to have a reference schematic from the manufacturers datasheet...

The buzz is the only sound it makes and it is not controllable with the volume knob. On a whim I just measured 6.4vdc on the speaker output with the speaker disconnected.
Searching the web I couldn't find the schematic but I did find that the device on the heatsink is an output chip; an A2007A power amp IC. I found that info on this page that describes some of the components used:

https://www.staff.science.uu.nl/~tel00101/FotoAlbum/RadioCorner/Sets/HK195.htm
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g1

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Re: Looking for diagnostic help but it's not an amp
« Reply #3 on: November 06, 2018, 02:13:36 PM »

Just so there is no confusion, that is a TDA2007A.
Datasheet attached.
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Cpt. FixIt

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Re: Looking for diagnostic help but it's not an amp
« Reply #4 on: November 06, 2018, 03:09:25 PM »

Indeed, a TDA2007A, and the SGS-Thompson is the only available datasheet as it seems. If you take a look at the reference schematic on page 4, you can see an electrolytic cap C3 for ripple rejection. I would take a look at that one first. The DC at the speaker seems odd, but it could just be a weird multimeter readout of the buzz.
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gbono

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Re: Looking for diagnostic help but it's not an amp
« Reply #5 on: November 06, 2018, 03:25:20 PM »

TDA2007A discontinued July 29, 2009. Hope that isn't the issue.
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Den.

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Re: Looking for diagnostic help but it's not an amp
« Reply #6 on: November 06, 2018, 05:45:50 PM »

...If you take a look at the reference schematic on page 4, you can see an electrolytic cap C3 for ripple rejection. I would take a look at that one first...

I found the 22uf C3 and I de-soldered it and inspected it. No sign of leakage or bulges. I replaced it with another 22uf anyway then tested again...no change.
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g1

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #7 on: November 06, 2018, 08:21:05 PM »

The DC at the output will be making the hum.  And it will probably ruin the speaker if it is not yet damaged, so don't connect the speaker until you get rid of the DC on the output.
Either the power chip is blown, or it is getting DC on it's input.
Measure DC volts at all 9 pins and post your results.
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Jazz P Bass

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #8 on: November 07, 2018, 02:00:36 AM »

If it does turn out to be the TDA2007A, I have had real good luck with this seller (they are located in England).
https://www.littlediode.com/components/TDA2007A.html?gclid=EAIaIQobChMIjoihq9rB3gIVxSaGCh0rCAsiEAkYCiABEgK7WvD_BwE
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phatt

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #9 on: November 07, 2018, 07:09:36 AM »

I might sound like a party pooper but seriously a lot of computer speakers are dirt cheap and it may end up cheaper to buy new or if you are like me,
I frequent a lot of 2nd Hand op-shops and I often see them going for a few bucks.
I replace these things with second hand units and keep the broken stuff as it's a source of spare parts. I see 3 pots, a dozen green caps, an opamp,, handy stuff to have when you want to whip up a test circuit. 8|
Unless those speakers hold some sentimental value it's hardly worth the effort.
Just my view, Phil. ;)
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Den.

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #10 on: November 07, 2018, 05:30:19 PM »

Measure DC volts at all 9 pins and post your results...

I also measured the ACv in case they mean anything. The converter output (AC to AC) measures 17.2 VAC

PIN     1      2       3       4       5       6      7       8       9
           
DCv    1     0.7    8.2     0.7     1       0     6.7   14.5   6.7   

ACv   1.6    1      17.2   0.9    1.4      0     14     31    14.1
.
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Den.

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #11 on: November 07, 2018, 05:54:15 PM »

Unless those speakers hold some sentimental value it's hardly worth the effort

But, I know for a fact that every time I save some money and fix it myself an Angel gets it's wings.

Quote
...and keep the broken stuff as it's a source of spare parts...

Yeah, me too...
« Last Edit: November 07, 2018, 05:57:46 PM by Den. »
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g1

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #12 on: November 08, 2018, 11:15:34 PM »

Not sure if this little amp is driving one speaker (mono bridged) or 2 (stereo)?
That will determine which configuration of the output IC they are using.
In stereo configuration, they use a cap at each output to couple the speakers (pg.4)
In mono mode, the speaker is between the 2 outputs, and no output caps are used. (pg.5)

In either mode, the inputs have caps to block DC, so the DC you are seeing at pins 1 & 5 make me think the chip is likely blown.
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Den.

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #13 on: November 14, 2018, 05:27:59 PM »

Here's an update. I ran across an HK195 repair video on YouTube where a fellow had a problem similar to mine. He found that the 3300uf 25v cap was leaking and replacing it solved the problem. My 3300uf 25v cap showed no signs of bulging or leaking, so I unsoldered and removed it for inspection. It looked like there was a small amount of residue on one of the legs right where it exits the base. I didn't have a 3300uf 25v in my parts stash so I replaced it with a 2200uf 35v. That solved the problem but I'm wondering if using that wrong spec cap can do any damage before the 3300uf cap I ordered is delivered and installed? Thoughts?
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g1

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Re: Looking for diagnostic help but it's not a (guitar) amp
« Reply #14 on: November 15, 2018, 01:41:41 PM »

  It will not cause any problem, maybe a touch more hum than the stock value cap.  I doubt it would be noticeable, maybe you would notice with low frequencies at high volumes.
No problem using it until you get the correct part.
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