Fisher SA100 ceramic cap values not same for both channels"

heyraz

AK Subscriber
Subscriber
Referring to the Fisher SA100 schematic and parts list (the Sam's has an error), can anyone explain why C16 (channel A) and C17 (channel B)are not the same value?
According to the schematic and my SA100, the left and right channels are mirror images except for this one difference.

This is a link for the Fisher Statesman, which appears to have the same amplifier schematic (albeit drawn differently) than the Sams version:
http://www.fisherconsoles.com/servic...20s70 sm.pdf
The capacitors in question can be seen on the Amplifier Schematic-C16 (12pf) and C17(5pf).

The Sams version has many errors
http://www.quadesl.com/pdf/sa100_photofacts.pdf

Errors noted so far on the schematic:
R37 should be 47K ohm
R39 should be 2200 ohm
7189 plate voltage should be 380v (not 270v)

On the Sams schematic
Channel A C12 is correctly labeled 12pf
Channel B has an unlabeled capacitor in same position 5pf

I have been replacing the components one by one and have verified that the caps in question are different, Channel A 12pf, Channel B 5pf.

I'm just wondering why?
 
Heyraz -- The errors on the Sams SA-100 schematic are well known, and glaring. While some errors on the most intricate designs would be understandable, others simply leave you shaking your head. This is one of those occasions!

Regarding the caps in question, Fisher is rather famous for using different frequency tailoring components in one channel versus that of the other for a give design. This is because the value of these components are primarily determined by the layout and build of the unit. These components affect only the very highest reaches of audible frequencies, and are tailored to produce a similar response characteristics in each channel.

As an example, the Fisher 400 receiver -- because of it's layout and build -- uses greater lengths of shielded cable in one channel, versus the other. Shield cable has the effect of attenuating high frequency response, so at the input of the line stages, different value compensating caps are used in each channel to ultimately produce the same response from both channels in the final product.

The build of the SA-100 has one channel being directly driven from the input jack to the input of the first stage, while the other channel is driven from the jack through a length of shielded cable to its input stage. Also, the input tube of one channel is physically closer to one of its output tubes than in the other channel. Mechanical lay issues such as this can definitely affect the high frequency performance of two otherwise identical channels built into the same chassis. Therefore, it is very likely that they individually adjusted the cap in question in each channel to obtain the best compromise of frequency response, stability, and square wave performance in each channel after the final build was determined.

This effort is one of those little touches that helped to make Fisher what it was. Most other manufacturers would not (and did not) bother with such details.

I hope this helps!

Dave
 
Thanks Dave for pulling my chestnuts out of the fire. I figured if anyone here knew why, you would and I referred Rich here from fisher yahoo group.

Larry
 
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