Peavey KB/A15 repair Help

[webber]

I was wondering if someone could help me determine the problem.
I purchased this amp used, when I plugged my keyboard into it the 1st time, the sound was crackling and fading.
At this point I've checked all diodes D1-D6 which are good (~0.577-0.617),
I've replaced the TDA2040H and both JRC4560D
Still nothing came out of the speaker, I did have sound coming out of the earphone jack at first,
but after further testing its not there anymore. I've checked the Ohms accross the 3.2ohm speaker, the value
is all over the place. I momentarily touched a 9V battery accross the terminals and it popped(disconnected from the wires), I don't have another speaker to try... So, I don't know where I stand with all this testing
I do have the schematic and basic bench tools. Can someone offer some guidance as to what to do next?

[52 Bill]

Welcome to the place. It sounds like you've already eliminated a few things in your testing, like the speaker and chips.

The two most common problems that cause symptoms like the ones that you are describing are the FX loop jacks and the headphone jack speaker cutoff contact.

If your amp has a loop, test the FX loop jacks by inserting a spare cord from the send to the return jack. Did this restore the signal?

Check the contacts on the headphone jack that connect the signal output to the speaker. Are they closed and making good contact? Are the jack's pc board solder connections all solid?

And of course, because you have already swapped a number of parts, go back and double check all of your work to make sure that you have not accidentally added to the problem by creating a solder bridge or an open pc trace, etc.

[webber]

Thanks 52 Bill,
I double checked all solder joints and traces, all appear good, no shorts or broken traces were found.
So, I fed a tone into channel 1 and looked at it with the scope, followed thru the stages to pin1(input) of
the TDA2040H and it was present, then did the same with channel2 and it was present on pin1 also, so my JRC4560's are good. Looked at pin4(output) of TDA2040 in each case and there was nothing. Diodes D5 & D6 are good(0.562 & 0.577)
the speaker is disconnected at this point, do you think I have a bad TDA2040? since I'm not getting anything out?

[Enzo]

In amps that use them - and there are zillions - the TDA chip is the most likely thing to fail. Not that they are unreliable, just that they are the thing that faces the real world and can be stressed.

If you have signal at the input to the 2040, then everything before it is working. If it has both power rails present - does it? - then there ought to be signal at its output pin. if not, try a new IC.

[webber]

All conditions seem to be present, unless the output is shorted?
I suspected the TDA to be the problem in the beginning, I replaced it and when it still didn't work I suspected other components. But I guess I got a bad one. Anyways, I just noticed that the print says TDA2040H and I received a TDA2040
Is there a difference between the two? if there isn't then I just received a bad one and will replace it.
When the headphones are plugged in, I hear a pop then a hum, I never hear the tone, is there anything else
I should check to to make sure I don't have other bad components before I order replacements?
I've tested the old JRC4560DD and new JRC4560D and all seem to pass the signal up to pin1(input)
Also I'm not sure about the speaker and if I can find a replacement for it if its bad, is there a way I can try
another speaker with different impedance?

Thanks for the help guys,

[Enzo]

This circuit isn;t rocket science, the problem of no sound won't be due to wrong impedance speakers or some small voltage difference. If you connect a 3 ohm speaker where an 8 ohm speaker belongs, the amp would still work, it would simply overheat.

If the signal is making it as far as the input pin of the 2040, then all the op amps and stuff are working. If the output of the 2040 is "shorted" you;d probably be blowing fuses, although yu could probably turn off the power and use an ohm meter to see if that pin is shorted to something else. Disconnect the speaker so it doesn't confuse the readings.

A speaker works by putting a coil of wire in a magnetic field and applying a voltage to move the coil. But moving a wire in a magnetic field generates voltage too. That is how a microphone works. In fact, a speaker will work as a microphone, and while I don;t recommend it, if you apply a signal to a microphone, it will act as a tiny weak speaker. WHy am I rambling on about that? WHen you put your ohm meter on speaker terminals, any movement of the speaker cone will generate small voltages which will confuse your ohm meter. Set the speaker face down on the table and don;t touch it, now your reading should be more stable. The 9v battery test will tell you if the speaker works. WOn;t tell you how good it will sound, but it will tell yo9u it works. If the battery makes a pop, then any voltage getting to the speaker will make SOME sort of sound. Your problem is NO sound, not poor sound. SO the battery is all the test you need now.

GO to Mouser and look up TDA2040H and TDA2040V, and look at the pictures or data sheet. YOu will see the identical electronic component, but the legs on the V version are made to stick straight down into the board vertically, while the H version has legs bent to fit into the board with the part lying down horizontally. Thus V and H. As far as I know they only stock the V version anyway. Electrically it matters not.

[Enzo]

OK, I looked. This amp runs on a single supply, so there should be +25v or so on pin 5 of the 2040 and pin 3 is ground. Pin 1 is the input and should have signal AND it should be sitting at about +14v, and the output also sits at +14v. Not worried if it is off a couple volts, but we don;t want them at zero.

If there is power, please check, and signal reaches the input, then pin 4 should have output. If there is output at pin4, it must feed through a large cap to block the DC and then through the phones jack to the speaker. The phones jack has speaker cutout contacts, so if they don;t work, neither will the speaker.

The negative lead of the speaker, probably blue, should have continuity to ground through 0.1 ohm R38, make sire that resistor and circuit path are not open.

[webber]

Well after more testing and checking I found the problem.
as 52 Bill suspected "the headphone jack speaker cutoff contact" maybe this is why im my first post
I mentioned there was sound in the headphones at first then it wasn't there anymore.
I checked the output section, found 14.7V(pin1) and same on pin4(output) R38 and the caps were fine
and you were right too Enzo, "The phones jack has speaker cutout contacts, so if they don;t work, neither will the speaker"

The headphone contacts were dirty, I cleaned it and it works. only thing the speaker is noisy.
Maybe I'd like to replace it but don't know if I can find a 8" 3.2 Ohm speaker,
can I modify the circuit to put a different rating speaker?


I needed experienced guys to help me sort things out
Thanks for the help guys, really appreciate it

[52 Bill]

Glad to hear you got it figured out.

8" 3.2/4 ohm speakers are commonly used for Fender Champ replacement. Jensen and Weber make them as well as Eminence and probably a lot of other companies.

[Enzo]

And a 4 ohm speaker will work just fine, you wouldn;t hear a difference based upon impedance. This is a keyboard amp, so you would want a PA type speaker, a full range speaker, not a guitar speaker.

But what do you mean the speaker is noisy? If noise comes out while you are not playing, that is not the speakers fault.