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General electronics help


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Hey guys,

I've got this audio board i need to repair. Since I don't have a logic probe yet, nor leads for my scope, so kinda working blind.

 

Since it's such a simple circuit board, I've kinda just been replacing parts hoping for a result. So far it's improved nothing :) I've done the main caps and pair of transistors, and the high wattage resistors. These are all in the power supply section (30vac -> 12vdc)

 

Using a meter, the power is good, I've got 11.9vdc.

 

There are many capacitors labelled "104" and nothing else. I've got a heap of "104m"'caps in my collection, are these compatible?

 

http://img.tapatalk.com/d/14/02/27/uqyryjy9.jpg

http://img.tapatalk.com/d/14/02/27/y8agaryz.jpg

 

If they are, great, if not, are they likely to fail?

 

Failing all that, what else should I be looking at?

 

The basics of the board is 4 20-30v AC inputs, feed a "activate" pin on 4 555 timer ics. Each has a small trimpot to set it's frequency, thus making different "beeps/tones".

 

The bit I'm not 100% on is what happens after the 555s. Obviously they're combined into a single signal and fed to the speaker. The small ICs are hex gates mainly it seems.

 

Given it doesn't matter which input is used, and the power supply seems fine, I'm going to guess the power supply section is ok, and the 555 part and below is ok. This means it's in the middle bit. Any ideas where to look?http://img.tapatalk.com/d/14/02/27/ty4ugaha.jpg

http://img.tapatalk.com/d/14/02/27/ydyravub.jpg

http://img.tapatalk.com/d/14/02/27/ga2ajaga.jpg

 

Whole board:

http://img.tapatalk.com/d/14/02/27/3urapyjy.jpg

http://img.tapatalk.com/d/14/02/27/y7y5ytan.jpg

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The 104 disks you are looking at are ceramic capacitors, virtually bullet proof and highly unlikely to fail.

 

The most likely failure based on part type would be the tantalum capacitors, the blue teardrop looking capacitors. They do not age gracefully.

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Ahh ok, cool, thanks for the tip, I'll leave the ceramics alone.

 

Of course, the suspect ones are the ones I don't have spares for, looks like I'm off to Jaycar tomorrow :)

 

They're labelled +1 35v .. Is +1 all I need to find a suitable replacement?

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With a speaker connected and AC power connected, do you get any hiss at all from the speaker?

Next would check the two output transistors.

Oh don't forget the speaker it's self? Just suggestions, I don't know what you know or have checked.

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As soon as my probes turn up from china, I will ;)

 

Can get a set of leads shipped for like $4, just gotta be patient :)

 

In the interim, I'm using the fish-in-a-barrel technique of troubleshooting.luckily, until now, I've had heaps of compatible parts. Unluckily, none have improved it yet :)

 

I'll get some of the blue caps tomorrow, and if that doesn't fix it, I'll wait for the CRO leads and test it out. Beyond that, it Can only really be the gate ics tho I reckon, or the volume pot itself. Not much else left that could affect all the channels at once.

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With a speaker connected and AC power connected, do you get any hiss at all from the speaker?

Next would check the two output transistors.

Oh don't forget the speaker it's self? Just suggestions, I don't know what you know or have checked.

 

I haven't directly tested the speaker, but it does give a "pop" when the plug is plugged in. Now you mention it tho, no buzz! I'm sure there used to be an idle buzz. I've replaced the pair of transistors already, no change.

 

There is a single transistor at the top that seems to be power supply related, that's worn clean off, haven't replaced it but the cd seems stable and correct, so I'll leave it for now.

 

I'm thinking it's gotta be either the blue cap as mentioned, the volume pot itself, or one of the 3 gate ics. There isn't much else that is common to all channels and isn't powers supply related.

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I would use a metal jewellers screwdriver and touch it on the wire link running in between the two output transistors and see if it buzzes or pops, from what I can see that is the driver input for the amp stage. that will tell you the output is ok.

The blue caps shouldn't be a problem unless one is shorted. But then you would loose your 12volts or at least one of the four tones.

You may even get the buzz if you touch the volume pot.

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Ok, no buzz from touching pot, no sounds when touching transistor legs. Pot tests 0 - 03 ohm in both direction, so seems legit. I've bought new ics for the logic gates, but this is starting to look like a "amp" issue, despite the transistors being replaced. Replaced the tant cap near the top too, no change

 

Haven't tested speaker yet, will do that next :)

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