Summit Audio EQP-200A

This EQ unit came to me in peices from a local music store. A tech had previously tried to fix it but had not finished the job. It's a high-end, analog mastering eq that has tube and solid state circuitry. I found a piece of tape with "bad regulator" written on it so thats where I started my investigation.

It was in pieces when I recieved it and all the screws were nowhere to be found.

All the fuses had been blown and the main fuse was missing its socket so I found a replacement for it.

I removed all the voltage regulators from the PCB to test them on a breadbord. I first tested for shorts with a multimeter and then applied DC with a power supply to check their function. I found that both the 24V and -24V regulators were bad, one shorted out completely.

Here's the regulator board with the new replacements. I noticed that the .5A fuses after the regulators had been replaced with 2A fuses. This explains why the regulators fried since they are only rated for 1.5A. I moved on to look for shorts downstream of the regulators.

The regulators supplied an array of op-amps custom made by Summit and potted in epoxy. I assume this is why the unit was shelved for so long. One of the op-amps was shorted across the rails and had a loose heatsink so I removed it.

I jumped the fuses with an ammeter and turned the unit on slowly with a variac. It was working on one channel and drawing less than 500 mA at 117VAC so I knew I had found the short.

After contacting Summit about the issue, they sold me a replacement part that is the modern equivalent of the one that I removed. Instead of a heatsink, it had a metal housing. You can see from this shot that there are two transistors peaking out of the epoxy soup. Some kind of class A amplifier secret recipe.

In addition to the op amp and fuse holder, I got some new screws for the chassis cover.

New op-amp in place

At this point, I wanted to test the unit but the internal fuses were 8GA size which I learned is very uncommon for the .5A rating. I couldn't even find any on mouser for a reasonable price so I improvised with some alligator clips just to test it. Both channels worked and it was drawing a normal ammount of current.

The output on the scope

I heard some noise from the rotary switches so I cleaned them up with contact cleaner.

Santa Cruz Electronics came through for the fuses. They don't stock this fuse but they keep a drawer filled with old stock so I was happy to be able to find these.

New fuses in place

New screws in place

Back in one piece. I tested one more time with audio before returning it to the owner.