Tricky Troubleshooting API 550A

Hi,
I've got a vintage API 550a with a curious problem. I'm getting an intermittent fluttering/crackle in the output regardless of setting. If I remember correctly, it is there even in bypass, which isn't that surprising as it isn't a true bypass. I initially suspected a bad cap or two and replaced a couple suspicious looking electrolytics with no improvement.

At this point, I suspect that it's an op amp issue. Before I yank the opamps, can anyone suggest anything else to try first? It's the kind of problem that won't show up on the AP (at least as I'm testing it) and is therefore a bit tricky to determine if it has been fixed. I sure notice it when it's patched into a vocal chain!!!

My next step is to power the unit up and trace from input to output with the scope... just not sure if I'll be able to visually see the problem noise well enough to locate where it's originating from.

Ian

check the switches and solder joints

Good call... I have noticed that "excercising" the filter switch can sometimes alleviate the problem momentarily.

Just adding a possible solution to this thread:

A flutter/crackle/wind noise sound coming from the 550B can be (and in my experience most often is) the 2501 Buffer amps. There are 5 of these in the unit - 3 on one board and 2 on the other. They are about 1/2" x 1" little circuit boards that are soldered in via 5 pins. They are typically labelled "2501" on the backside.

If the 2501 amps are covered in a black epoxy in your unit, the issue is usually that the epoxy has started conducting and is causing a short circuit across the amp, causing the noise. There is a way to test and see if this has happened: take an ohmmeter and measure across pins 1 (the single pin farthest away from the other 4 pins) and pin 2. In a working buffer amp, the measurement should be higher than 2.2 Meg ohms. I've seen good ones read as high as 32 Meg ohms. You should also get the same measurement across pins 1 and 5, as well.

If your measurements are below 2.2 Meg ohms, replace that buffer amp.

You can order these for $8 each + shipping from API directly:
https://service.apiaudio.com/parts
An alternative solution to try (if you live somewhere that can't get the API replacements) is to soak the buffer amps in a bath of acetone for a few days and use a metal dental pick to scrape off the black epoxy. Try getting it out as much as possible, with the goal of exposing the 3 tiny solder joints on each of the 3 black transistors. BE VERY CAREFUL TO NOT PUT PRESSURE OR SCRAPE AT THE JOINTS OF THE 5 LONG LEGS!!! IF THESE ARE LOOSENED, THEY STOP MAKING CONTACT AND NEED TO BE CAREFULLY RESOLDERED OR THE BUFFER AMP WILL BE DEAD!! Just be careful and scrape AWAY from the 5 long legs. It's a time consuming process, but can solve your problem in a pinch. You may need to do multiple soaks.

I fixed 7 EQ's by soaking the buffer amps and fixed an 8th by just replacing the buffer amps.

Good luck!
Omar - Browner Sound, Hialeah, FL