Fender 212r repair: distorted sound output and bump on power on

[vincenzodesanctis]

Ok Mick, you mean AC I think. One pin across R109 and R112 (http://s2.postimg.org/s0d2envi1/probe.jpg) and the other pin where? Sorry for my low knoledge

[Mick Bailey]

You need to measure the DC voltage across each resistor individually. Your probe is correctly positioned, the other probe needs to go to the other end of R109 and take that reading, then move it to R112 and measure that.

[vincenzodesanctis]

I don't know if it's normal but the VDCs across the resistors are zero. No VDC. But when I play guitars I can hear the distorted sound. Maybe my multimeter fails.

[vincenzodesanctis]

it's just for my understanding...I have an oscilloscope, but I don't know how to use it well.
Suppose I want to test TP18, how to put the oscilloscope's pin and the oscilloscope's alligator?
May I attach the alligator to the chassis?
Please see my image example http://s13.postimg.org/9t7qcqtzr/oscilloscope_test.jpg to see if it's correct.

And, in case I want to use multimeter to test VAC on TP18, can I connect the black pin to the ground and the red pin to the TP18? I don't understand if the black pin goes to ground or some other way.

[Mick Bailey]

Zero voltage could be the reason the sound is distorted, as the amp may not be correctly biased. If this is the case you'll get crossover distortion. The output transistors need to be tuned on ever so slightly at rest to overcome the voltage drop that's present with any semiconductor junction. You can check the voltage across the Base-Emitter of Q20 & Q21. It should be more than 0.6v or so. My guess is that it will be less.

Take care not to short out the transistor with your meter probes. You can also trace back those connections on the schematic to a more convenient point. For example, you could measure Q20 Base-emitter voltage by measuring between R106 (where it connects to Q18) and R109 (where it connects to Q20).

If you've got any doubts about your meter, you need to check it out and make sure you're confident with the readings.

Edit, you posted as I was typing and eating a sandwich. A scope makes for much easier diagnosis. Do you have the means to generate a sine wave?

Just a thought - double check the solder joints on R102 and R104.

[vincenzodesanctis]

Thank you Mick. Later I will check the Q20 and Q21 voltage.
Regarding the scope I know the meaning of the sine wave, but I don't know exactly where to put the pin and the alligator and if it's usefull for my purpose.

[Mick Bailey]

Your sine wave source connects to the input (the schematic shows 5mv) You can download free waveform generators, or just get hold of a 1Khz test file to play on a laptop or pad. Make sure the amp is on the clean channel, reverb off.

Remove the speaker and connect the ground clip of your scope to the amp ground (chassis) ans the scope probe to the + speaker output that comes off TP29. Adjust your scope to get a stable waveform displayed.

We're interested in the shape of the waveform - whether one half is missing, or it's clipped, or if there's a notch along the zero axis. Any of these will cause a distorted output. If you could post a picture of the scope trace that would help quite a bit.

[vincenzodesanctis]

my sweep generator is the Escort EFG-3210. It's a simple 2Mhz square wave, triangle wave and sine wave. This is the picture http://i202.photobucket.com/albums/a...PART_K/K07.jpg
I think it's enough as sine wave generator.
It has a lot of buttons and functions the I don't know understand, for example sweep rate (from 0.5hz to 50Hz) and sweep internal or external, amplitude, duty, DC offset... The only things that I know are how to select the wave, how to select the range (for example 1khz as you wrote) and how to adjust the frequenzy from 0.2 to 2.0. I suppose the "aplitude" is the decibel or the voltage...maybe...
This is the detail http://www.mondovi.polito.it/docenti...ElnMondovi.pdf

[vincenzodesanctis]

I bought the oscilloscope and the sweep sine gnerator 20 years ago and I never used them

[Mick Bailey]

You just need a straight sine wave - no sweep, no DC offset. Amplitude is output level. Connect it straight to your scope and you'll see the waveform.

[Jazz P Bass]

That generator looks fine.

A few of the functions are for Radio.

Keep it simple.

Range, internal sweep & signal amplitude.
Amplitude is the one to watch.
A typical microphone input is 25 mv's.
While a guitar typically is 100mv's.
(at least those are the voltages that I use for test purposes.)

Use the black lead of the scope as the zero volt reference (ground).
Probe with the other laed.

If you are not yet comfortable with using the equipment, set up the generator & hook up the scope to the amp output.

Fiddle with the scope until you see what the separate controls do.
Usually all you need to know is how to set the heighth & the width of the trace so as to capture whatever frequency & output amplitude you are dealing with.

Easy peezy.

[vincenzodesanctis]

when I return back to home this evening I will test and I will put the result! Thank you guys!

[vincenzodesanctis]

aaaaaaaaahhhhhhhh R102 is broken!!!
On scheme R102 is a Fu47. I don't know what means Fu. Is it a normal 1/4w resistor? it's light blue. I have one 47ohm but it's normal 1/4w, is it good?

[vincenzodesanctis]

maybe it's a normal 1/4w metal film resistor. If I replace it a caron film one? is it the same?

[g1]

Fu means fusible. It is designed to act as a fuse when necessary without the chance of fire.