fender frontman 25r no sound from speaker

[goontron]

i have a frontman 25r that has no sound from the speaker, ext speaker jack, or headphones jack but if i put a very high frequency (21470 Hz) into the input jack i can hear it on the reverb if its turned up. the electrolytics are fine so what should i check next?

[budm]

Re: fender frontman 25r no sound from speaker

Here is the SCH

[goontron]

Re: fender frontman 25r no sound from speaker

thanks for the SCH budm it looks like probing at R14 for audio might be a good start, still pretty stuck on this one
edit: it looks as if Q1 is pulling it down because on one side of R14 theres audio but on the other nothing but R14 ohms out good, does this sound like a possibility?

[budm]

Re: fender frontman 25r no sound from speaker

R14 is for the normal level signal that signal is switched in and out by the Gain selector switch, on normal mode Q1 is turn off (High resistance) so the output signal from OPAMP U1-B will flow through R14, R15 to the input U3-A pin2. At the same time Q2 is also off (high ressistance) which will block the high-gain audio stage signal from feeding U2-B, so there will be no output from U2-B to feed U3-A. When switch set to high gain, Q1 is on so it grounds out the output signal of U1-B, at the same time the high gain signal from the output of U2-A will now able to flow through the low resistance of Q2 to the input of U2-B and its output will now be applied to the input of U3-A.
SO, at this point, do you get low signal from both settings? if you do then we need to look at the output of U3-A and after that signal path.
Also, if you turn up the REVERB level and shake the cabinet, do you hear real loud reverb spring, it sounds like thunder?

[Longbow]

Re: fender frontman 25r no sound from speaker

no sound from the speaker, what should i check next?

The usual plan is to get out your meter and confirm the (+/-) 27v, 15v and 13v. What made you think of plugging a 21KHz signal into the input?

[budm]

Re: fender frontman 25r no sound from speaker

If you do not have the scope, then I would use 500 Hz tone about 100mV feeding the input, then set the DMM to AC volt and just probe and see if you will see any output from the Opamp all the way to the R55. You can vary the volume to see the signal goes up and down also.

[goontron]

Re: fender frontman 25r no sound from speaker

i have 15v on each winding of the transformer and tapping the reverb does nothing but today it made the .8watt speaker plugged into the external speaker jack catch fire in my hand after i taped the board and it started buzzing and popping.

[goontron]

Re: fender frontman 25r no sound from speaker

ok so i taped on resistor 76 and it tried to work but at the same time i flexed the board and it went dead this time tapping didnt help

[budm]

Re: fender frontman 25r no sound from speaker

If you can tap on the circuit and causes the buzz and popping sound then inspect the board for bad joints, at the same time check the DC voltage at the speaker out put to make sure it does not have large dc voltage on the output.

[goontron]

Re: fender frontman 25r no sound from speaker

24 volts at the speaker, is that bad?

[goontron]

Re: fender frontman 25r no sound from speaker

ok the TDA1514A is very bad it just smoked without blowing the fuse and a new revision wont work of the TDA1514A wont work so im just going to throw this one out.

[Longbow]

Re: fender frontman 25r no sound from speaker

Well, just as a retrospective I think that the use of a multimeter and a little circuit contemplation might have saved this unit from a flameout. The output IC is/was definitely shorted as the 24 vdc on the output confirms. Since the power IC is direct coupled, a working amplifier would have 0 vdc on the output.

One thing to keep in mind when working with power output stages is that a failed transistor or IC usually takes a number of other parts in the circuit with it. Just replacing one of the faulty parts results in more smoke. We've all been there.

[goontron]

Re: fender frontman 25r no sound from speaker

the TDA1514A is the output IC and the new revision of the IC scaters plastic if you try it. Everything else is fine but i dont want to try and mod it to handle the new IC.