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 !
Step 2: Remodelling of the Case
After the PCB was repaired, I passed to the cabinet which needed a little refreshment !
I followed a really good tutorial on the internet to re-tolex a guitar cabinet :
Actually I did not find Tolex so I went for a really good vinyl found in a drapery, 1m x 1/2m for 20$ and I went for a off-white vinyl which can be very good associated with the black parts. So in order, the steps I followed were the following :
- remove the present carpet
- sand just a little the wood to obtain a porous surface
- make 2 pieces of vinyl : one for the top with the two sides and one for the bottom
I used like prescribe in the tutorial a water-base cement (9$) which is really good to glue this vinyl on wood and non-toxic and easy to apply with a disposable paintbrush (1$).
I used some rubber-band to let the vinyl dry in place but the cement is really fast to glue.
Step 3: Re-installation of the Parts
After the vinyl was completely dry, the parts were washed and the rush around the speaker was removed with a steel wool. All the screws were replaced with new ones to give a small metal aspect which is nice.
Step 4: Re-soldering of the Main Board and Finish
Next part is the resoldering of the capacitors. I purchased some high quality caps with higher value to obtain a better sound. For 15$, I received 2x 6800uf 35v Nichicon Gold Tune (special audio) instead of the two generic 3300uf in place.
First thought : really nice and beautiful !
Second thought : a little bit high..... I needed some larger than higher caps but that is the disadvantage of order online ! Anyway, I did not want to send them back so I made some adjustments in the case (Picture 3) but they worth it !
And ...voila !! The pcb is done !
The first tests were good for the sound but... too bad.. the amp gets radio frequency ! So I am headed back to test the grounding connections !
Step 5: Tests of the Ground
To remove the radio interference I first thought of the rust around the tube connection so I completely desolder it and clean it up with some iron brush and a good electronic contact cleaner. Tighten a little bit the connections (where you plug the tube) to obtain a better contact : that was not the source of radio interferences but still useful !
Next step : the input jacks seemed to have taken a lot of humidity and small water spots were visible on the metallic parts so I desolder them and made some bridges to not break the ground connections and try to reconnect the whole thing ... And Great ! No radio stations anymore ! So I am going to order new ones and see if the radio disappears !