I found a nice little guitar amp in one of my favourite second hand electronic dealer which is a Crate TD 35 watts, a 35watts amp based on two transistors and a tube pre-amp which is why I was unable to resist !
The amp came with a huge buzz when powered on and looked like it passed the last two years in a wet basement so I thought it would be my first project to post on this site !
Step 1: Dissambly of the board
The first step was to track back the HUM in the speaker. After the dissambly of the main PCB board, the problem was easy to find :
- First some rust around the tube connector due to too much time spent in the garage !
- Secondly, a capacitor clearly rusted and dead in the PSU to change, they are easily distinguishable they are near the diode bridge which were also dead and had to be replaced
When the diodes and the capacitor had been replaced with some test capacitor, the HUM was still present when I plug the power back and 5 seconds after the transistor (3rd photo) began to smoke really hard.
So third step : a pair of transistor was replaced, they came by pair and the one in question was a TIP120, very common in amplifier, coupled with a TIP125. So I replaced both of them with a pair of TIP121/TIP126 along with a really burned resistor and ...... tada ! Clear sound when plug back !
To improve the sound a little and prevent another explosion of the capacitor, the two 3300uf 35v capacitors were replaced with 2 Nichicon Golf plated 6600uf 35v and their way now !