Electronics > Repair

Sinewave guru's? [amp repair] FIXED

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I'm trying to combine my two favorite hobby's. Playing music, and playing electrician... last one not as skillful though. Im trying to repair a 1970's vintage guitar amplifier. Everything seems to work great but the clean channel. The clean channel is only true clean, when input and output volume is really low. The distortion channel works like a charm.

I started out changing all the electrolytes, which took care of some "wobbeling" in the sound, caused by bad power supply smoothing caps.

It still distort on the clean channel way to soon. If i set the output volume to the same as the acoustic sound from my electric guitar, it starts distorting (pot at around 1-2 o'clock)... it shouldn't distort before pot at around 6-7 o'clock on the clean channel.

I measured the input in mV from my electric guitar, and created a sinewave at 440Hz at the same level, and feeded it in to the amp. The pot is about 2-3 o'clock, and the higher part of the sinewave go bananas. It has a huge spike on it, that has much more noise than the rest of the sinewave (might not be apparent by my poor mans mobilephone photo).

The lower signal is at the input of the amplifier, and the upper signal is at the output, connected across a power resistor at 8 ohms. If i turn the voloume i bit more down, the sinewaves at the top looks like titties  :-+

Are there any obvious thing causing this? (i've been so kind to supply the schematic for anyone interrested)

My next step is offcourse to backtrack with the oscilloscope and find a spot, where the extra peak disappeares, then try to figure out why the peaking starts after that exact part. But it would be awesome if a kind guru could help me out and point me in the right direction, instead of me woundering about in the dark  :-/O

What kind of amplifier is it? The obvious thing I would do is *carefully* probe my way through the signal path from input to power stage and determine where the distortion begins, this will narrow things down.

It's a Session Rockette 30 Watt's guitar amplifier.

Yeah that was the next thing i was going to do, but my time is limited untill after summer :-/

I am hoping someone out there know a lot about these kinds of things, and know what could cause this wierd spike...

That's a very strange fault you have there...  Is the amplifier a valve design, or transistor ! the fault is not true distortion, this would have a flat on the top positive cycles, or more normally negative cycles as well; this looks more like an odd frequency filter peaking, and could be just at this 440 Htz  frequency.

You state a distorted channel that works okay, is this an actual effects input, within the amplifier !

Also, remove the 8 Ohm Load and re-connect the speaker; then do the scope test measurement again ? has problem disappeared !! if not try this.

Try another frequency, and see if you get the same result; if this is not repeatable with another frequency, then it points to a resonance of some sort being generated, within the amplifier pre or output stage.

What is happening here, is the positive sine-wave suddenly rises in transition from the normal level, to peak then return back to normal; my thinking is resonance at this frequency ? or "instability", that might require further DC power rails capacitor decoupling replacement, or tracing through the circuit ideally.

If this turns out to be a frequency resonance, it might be narrowed down to areas around any transformers, inter-stages pre-post amplification, where capacitor de-coupling are across the transformer windings... at present that's just a guess.

Yes, a circuit would help, to see the design and type of amplifier.

Use your 'scope to look at the signal on the output of each opamp stage starting from the input.  That way you'll know where the distortion starts.

It's certainly an odd looking fault.  My first guess would be one of the opamps is bad.


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