Fisher 500-C vintage stereo receiver Reviewing & Testing Vintage Audio Components
The biggest challenge in reviewing and testing vintage audio equipment is knowing the condition and history of the review sample. Seemingly original and "museum-condition" units may have had repairs or modifications that are not readily visible.
For this review, I listened to three of five 500-C units with both vintage and modern loudspeakers to give the reader a broad usage range. The older speakers were Bozaks, which were initially designed for and matched with 1950s and 1960s tube amplifiers that ranged in output from 14 to 75Wpc, and thus were perfect matches for a Fisher 500-C. The modern speakers were the 96dB-sensitivity Classic Audio Studio Standard II, a three-way design featuring state-of-the-art TAD compression drivers and 15" woofers housed in cabinets that at least looked vintage.
There is no established industry standard for evaluating vintage audio equipment. For this review, I wanted to focus my subjective auditioning on Fisher 500-Cs in general, not one particular unit. For bench testing, I submitted a sample that would be representative of something any consumer might buy—a second-hand unit with unknown history that has been looked over and repaired by a technician, but not modified. I broke down the definition of vintage unit condition into three categories: 1) original condition unknown, 2) repaired to functional condition, and 3) modified to or above original spec. The last category is broad, and open to almost everything from replacing the output coupling capacitors with super-exotics to replacing each and every electrolytic cap and original resistor. And who's to say which modern capacitor sounds like the original? Even here, personal taste and preference are parts of the equation.
To approximate a malfunctioning 500-C bought on eBay and repaired by a local shop for a modest fee, I submitted for measurement a 500-C that had been repaired to basic functionality. Please note that I did not change the output coupling caps. The originals still worked, and produced acceptable sound. I use a number of different brands of coupling capacitors, including the Sprague Orange Drop 716P, for what I feel to be their sharp yet musical sound.
I evaluated the following units sonically and averaged the review results to account for unit-to-unit variation: serial numbers 58022S, 62946T, and 70630W (the first unit was also bench-tested). Each had a RadioShack silicon bridge rectifier (part #276-1152) installed to replace the Fisher's original, failure-prone selenium rectifier. All units were biased by changing the bias-supply shunt resistor (R131 in the schematic) to achieve 34–35 milliamps of current through the output tubes. This subjective target is a good compromise for the EH7591 reissue tubes. A 10 ohm, 0.5W resistor was added between the cathode and ground of each output tube socket to protect the output tubes and transformers, and to allow convenient calculating of each tube's bias current.
Because of time and scheduling constraints, only unit 58022S was tested. Its hum problem was repaired by replacing the large dual 1000µF bias-filter capacitor with two individual 1000µF or 1500µF electrolytic capacitors. Everything else was left in its original state. The phono stage in the unit submitted to JA for testing turned out to have problems with its RIAA equalization and low-frequency overload. Perhaps this was due to a parts failure or to specifications particular to that production run. My comments on the phono stage in this review were not based on this unit.
In addition to the above changes, unit 70630W had a 5 ohm, 5W resistor added in series with resistor R142 to change its effective value from 15 to 20 ohms. This was done to slightly reduce the DC filament voltage on four of the 12AX7 tubes (including the phono tubes). This small adjustment was made to allow for today's wall voltages.—Peter Breuninger