Dave’s Amplifier Page
Peavey Mark IV Series Bass
I took on the task of repairing this bass amplifier when some friends at work told the owner about my electronic troubleshooting skills. I carried the monster home and immediately started searching the web for documentation, especially a schematic. The official Peavey site had the owner’s manual for download, but no schematics. I kept looking and realized that Peavey was a bit stingy compared to Fender about their amplifier schematics. I downloaded the owner’s manual and was pretty much able to determine from the information in it that all the preamp and tone shaping circuits were working okay.
I found a schematic for a Peavey Basic 50 and used it as a reference to look at the circuitry. That amp has a compression feature called DDT by Peavey and the amp I was working on had a ‘compression’ LED on the front panel. I was unable to cull enough information from the Basic 50 schematic to help me, but I became concerned about having to tackle any problems in the compressor section.
I was able to get a schematic from a place I found called Schematic Connection and $9 later had what I needed. The circuitry certainly looked familiar because of the similarity in the compressor section and also to some circuits I work on for Honeywell. Part of my responsibilities at work is to test and repair deflection amplifiers for CRT based cockpit displays. It is a solid state amplifier that converts low voltage AC signals into high current waveforms to drive the yoke on a CRT. The yoke is a coil of wire very much like a speaker in a guitar amp. The only difference was the deflection amps are high speed highly accurate power amps.
The power amp section of the Peavey was very similar to these so I just adopted my usual approach to troubleshooting one of these and checked the DC bias. I’ve learned that usually if an amp will correctly bias in the zero input DC mode, it will work in it’s normal AC mode. I discovered that the Peavey would not bias correctly. Everything was either pulled up to the positive rail or down to the negative one. This indicated to me that there was an open circuit somewhere which I quickly found. There was a 2K/5W open resistor that ties right off the positive rail to the rest of the amplifier. It’s companion resistor showed evidence of having been replaced so I doubled the wattage on the replacement part. Works great now. Total cost of repair $9.90