Here are my first steps in the repairs this week:
- Search Google [or your favorite search engine] for published schematics.
- Start at the wall and work your way in.
- Check solder connections and part integrity all the way through. Broken jacks, resistors, solder joints, etc. could lead you to the source of disaster.
- Look for burn marks or abnormal-looking solder connections. [Solder can and occasionally will defy gravity.]
- Blown fuses could indicate shorts across the power supply rails. There should be significant resistance between ground and positive/negative supplies.
This week I was given the opportunity to repair two items, three if you count an item that was just given to me. The first item was a guitar amplifier. It was a solid-state amp made by Fender [Frontman 15G].
The transformer on the amplifier was burned out, and I had to find out why. I tested many of the components to make sure nothing had shorted out, and I didn’t find anything. I finally tested the power connections to the output amp IC, and found that it was only 1.8 ohms. The voltage was -24 and +24 giving us 48 possible volts. The current draw was an estimated 26 amps!!! No wonder it died. The great part about this repair is that I had an identical amplifier that was also broken, but the TDA2050 and power transformer worked in it. After taking the other amp apart I noticed that every potentiometer and the input jack had been severed from the circuit board.
I replaced the tranny and amp IC in the other amplifier and it worked like a champ. For the other amplifier, I am going to install new potentiometers and put in a transformer that will output about 88% of the original transformer. Finding a 32 VCT transformer at a reasonable price is not possible, so I’m going to use a 28 VCT. Sadly, it will cost about half of what the amp is worth to fix it, so it isn’t worth selling it, but I’m going to keep it for myself and sell another one of my amps.
The other item that was brought to me was a clamp meter. It just beeped constantly no matter what setting it was on. I took it apart and found that something was clamped that was beyond the capability of the meter. There were surface mount parts that had moved because the solder melted. The SMT resistors were between 20% and 30% lower than their published values. I would have been completely happy trying to fix it, but I don’t have any tools to work on surface mount components.
Even if I had the tools and parts, the labor alone would have probably exceeded the cost of a comparable new meter. [The meter brought to me was missing the battery compartment cover, and the owner was using tape to hold the battery in. Buying a new meter would be a great idea.]