Repairing an Adcom GFP-565 Preamplifier

My dad's stereo setup is probably what got me interested in electronics to begin with. All those exposed wires, vibrating speaker diaphragms, display panel lights, turning records, oh dear I'm going on like it's the latest SI Swimsuit issue, I've got major problems. Moving on, We recently took this wonderful preamp out of storage, and no sound came out. Naturally, I will put my degree to work in a practical manner (this is most rare) and diagnose, then repair the problem. I will attempt to demonstrate what I think is a logical, methodical debugging protocol.

The Adcom GFP-565 stereo preamplifier debuted around 1990 and was billed as the "first affordable stereophile preamp" retailing for $900 (though clearly "affordable" is a subjective term, being $1543 adjusted for inflation). A modest design, it has three line-level inputs, a phono input, an a tape dubbing loop. It also has tone controls (which can be bypassed) and no remote control. The construction, inside and out is fantastic.

On the PCB, you can see that every square inch not used for signals forms a ground plane. Some will argue this is not a good idea, clearly Adcom disagreed.

You can also see those silver and blue ribbons that look like ladders. Those are the coolest switches I have seen. What they are are switches which are electrical at one end, and mechanical at the other. A common problem with preamplifiers is that the input selector switches are very early in the signal path, right after the input jacks, which are typically on the back of the chassis. The switches are usually conveniently located on the front. The problem then is that it is bad practice to route the electrical signals to the front, especially because at that stage they are very low level. A standard practice is to have rotary switches with very long shafts, that extend from the back of the case to the front. In this preamp, the knobs at the front cause the metal ribbon to move within the blue plastic sleeve, repositioning the switch. In spite of this complicated mechanical system, the switches have a very firm, positive feel. Here is another view of the switches.

The power supply is a discrete linear circuit, with a nicely shielded transformer.

The PCB underside is unsurprising. This is a two-layer PCB.

The most important first step, and this will save hours of labor, is to try and find the circuit schematic. Fortunately, on some questionable Russian webserver (thanks Google!) I obtained, by first uploading some schematics of Carver power amps, a decent schematic of this preamp.

If the schematic is hard to read, click the image for the full resolution.

So looking at the schematic, we make these deductions.

1. If there is no sound from either channel, probably either the power supply is out, or the output relay circuity is blown. That is because the L/R channels don't share any ICs, they are independent paths.
2. If there is sound from one channel not the other, most likely the ouput transistors of one of the power op-amps has failed. These are the Adcom 1A op amps, they are mounted on a heat sink.
3. The only switch besides the input selector that is in the signal path is the tone control. This is a likely point of failure.

Here is my test setup. I have a scope, multimeter (Not the fanciest one, but one I got from Radio Shack recently because I didn't have one at home. Plus, I love analog multimeters.) and a signal source, aka. iPod. Since I am not doing any quantitative measurements aside from seeing if there is a signal or not, looking at an audio wave bouncing around on a scope is more than than a boing ol' sine wave.

Looking for a problem is best done with a "binary partition search" or so I think it's called. Basically, you keep dividing your circuit in two, to narrow down the problem location.

So first, I tested the supplies. Both the +/-18V and +/-15V regulated supplies were functioning correctly. So now I want to probe the midpoint of the circuit, and I choose that to be the "Bypass" output. There is still no signal. So, I need to go back one. To see if the input selectors are working, I probed R205/R206, and there was a signal. This means there is a problem within the first buffer stage, off the volume/balance control. The outputs of IC201/IC202, the volume control buffer, were also dead. That means we've isolated the problem to be in the balance or volume control pots. Looking at the output of the balance pot, there was no signal. Bingo, the problem was within the balance pot. The pot was made by Alps, a well respected maker of precision resistors. How both wipers of the pot died at the same time is beyond me, but after 20 years of faithful service I suppose it's allowed. Anyway, fixing that and the amp works good as new. I have summarized the steps of my debugging process in the marked-up schematic.