Guitar amplifiers usually consist of four sections, hence a systematic diagnosis of the same will help us fix it.
To repair your guitar amp:
- Check power supply and fuse; the remaining sections assume that the mains are permanently disconnected, except wherein a specific check is required.
- Power section: If your guitar amplifier doesn’t work completely, the problem may be the easiest to fix. Check the power supply section. The power supply check requires you to identify the DC (direct current) voltages. Locating the same is very simple. Search for a transformer--Big capacitor--Rectifier combination. This is usually located immediately after the power fuse on your circuit board. Test the output voltages, which are usually in the range of 12-0-12 V /0- 12V and 42-0-42 /0-80 V.
- The objective of checking the above voltage is simply to determine whether the fault lies in it or whether it can be safely eliminated in your systematic check. If the check gives no voltage, move to point 4 to 6. If the voltage checks are satisfactory, skip point 4 to 6.
- Now that we have determined that the fault lies in the voltage stage, we need to investigate this stage to successfully diagnose the fault. Look at the big capacitors - are there any signs of a 'burn -out' (characterized by charred appearance or flower pot bloom). Smell the section. (An expert is usually able to smell out the destroyed part by differentiating between 'burnt vinyl'- transformer;’ burnt electrolyte' - capacitor and burnt silicon - diode.)
- Check the transformer using a simple continuity test of both the 'Primary and secondary coils.' If the continuity is all right, apply mains power and test voltage across the secondary coils (N.B A/C settings of multimeter to be used). If this is satisfactory, unplug from Mains supply and move to point 6. If no mains supply is received in the secondary coil in spite of voltage supply being applied, have the transformer replaced.
- If the transformer has been safely eliminated as 'functional,' it is our endeavor to determine whether the rectifier is 'defunct.' Rectifiers are usually ‘bridge’ or ‘half wave’ and are served either in individual diodes or rectifier packs. To diagnose the rectifier pack, apply supply voltage to the input and measure output voltage. If rectifier is in the form of diodes, test transient characteristic of diode, i.e. simple: Apply continuity check on the two ends of the diode - is continuity met (y/n?) The opposite should be true when the polarity is reversed (by alternating ends against multimeter). If the rectifier/diode malfunctions, have it replaced.
- At this stage, we are confident that the power section is fully functional, and this is characterized by the supply of power to the circuit. We now proceed in the reverse direction to locate malfunction.
- Speaker check: The continuity of the speaker is checked with a multimeter. If speaker continuity is open, we have it replaced.
- We now are certain that the power section and end section are fully functional. We are down to the bare amplifier. Usually guitar amplifiers are designed as Cat B/C--we need to look for a balance of components across something called a 'Zero Point.' The 'Zero Point' is the point which feeds your guitar signal--this point is usually maintained at 0 V. The positive portion of your input voltage is fed through an appropriately biased power transistor and the negative portion through its pair.
- After you determine the balance of components, use a tester to gently and accurately touch on the Zero Point immediately before the final stage (power transistors near/connected to speaker). If a prominent earthing noise is heard on speakers, sigh as your expenses are curtailed.
- If the earthing noise is not prominent, you will have to test each transistor on this section for a transient characteristic check (as was performed in point 6).
- A power transistor usually consists of a metallic casing connected to the heatsink. Attached to this casing is a 'collector' node. Having determined the collector node, the continuity tests are performed to determine emitter and base (or Google the part number).
- The power transistor test is more stringent in terms of reverse bias testing, and a calculated decision needs to be taken depending on the power capabilities of the power supply unit and the circuit for handling the transient characteristic.
- If found faulty, the power transistors are replaced. If not faulty, the voltage amplification stage preceding the power amplification stage is determined.
- The voltage amplification stage usually consists of a physically smaller transistor, typically a CL or Ck 100. Test the Zero Point for 'earthing noise,' check bias resistors for appropriate values--smell for shorts or burn-outs.
- After we have eliminated the power and voltage amplification stages, all we have remaining is the pre -amp stage
- Determine the core of the pre-amp 'op-amp' or transistor; use zero point technique to determine fault section. Use multimeter to determine transient characteristic of semi-conductors and passive components. Also check board for cracks and breaks.