Fisher SA-100

adnick said:
Yea, I’m muddying the water (-;

Back to the SA-100, and I’ll ask this question again to regain focus.

Anyone have experience using the center channel? If so how was it implemented and what kind of results ensued?


VR

Andy
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Dave might stop by with a comment, but my understanding (came from Dave's post(s)) is that Fisher implemented two basic designs for the center channel. (1) A difference signal between the two channels (not a great result as it would cancel similar signals between channels - meaning true mono would give essentially zero output.) (2) A summation signal between the two channels (an apparent improvement from the previous design). I am not sure, but if the SA-100 is an older design and maybe it uses the difference signal, but I'm guessing.

Many don't use the center channel. It was created at a time in stereo history when stereo was young and the split between the channels wasn't always done well. I think the concept was to fill a gap between the channels for (sloppy channel assignment). In fact some hobbyists remove the center channel circuitry because the way the circuitry was designed causes a funny ground arrangement where the speaker common isn't at chassis ground potential. I have a Fisher KX-100 and Fisher KX-200 that both have this funny business and I have to use the 4 ohm tap on the speaker terminals as a ground for turntable connections. I don't like it but haven't removed it yet. I work around it just fine by using the 4 ohm tap (or no dedicated ground from the TT at all) and so I have no urgency to remove it but still I wish it wasn't there. I seem to remember a thread where Dave showed how to remove it from one unit and it changed quite a bit in the output and feedback circuitry on the amp.
 
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Tim D said:
Dave might stop by with a comment, but my understanding (came from Dave's post(s)) is that Fisher implemented two basic designs for the center channel. (1) A difference signal between the two channels (not a great result as it would cancel similar signals between channels - meaning true mono would give essentially zero output. (2) A summation signal between the two channels (an apparent improvement from the previous design). I am not sure, but if the SA-100 is an older design and maybe it uses the difference signal, but I'm guessing.

Many don't use the center channel. It was created at a time in stereo history when stereo was young and the split between the channels wasn't always done well. I think the concept was to fill a gap between the channels for (sloppy channel assignment). In fact some hobbyists remove the center channel circuitry because the way the circuitry was designed causes a funny ground arrangement where the speaker common isn't at chassis ground potential. I have a Fisher KX-100 and Fisher KX-200 that both have this funny business and I have to use the 4 ohm tap on the speaker terminals as a ground for turntable connections. I don't like it but haven't removed it yet. I work around it just fine by using the 4 ohm tap (or no dedicated ground from the TT at all) and so I have no urgency to remove it but still I wish it wasn't there. I seem to remember a thread where Dave showed how to remove it from one unit and it changed quite a bit in the output and feedback circuitry on the amp.
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Tim:

Excellent explanation, thanks for taking the time....

VR
Andy
 
I have not ever used the center channel taps on any of my Fisher equipment so I can't add anything, but I know someone that uses it for a powered sub.
 
Starting the re-cap process, wouldn’t ya know the .022 on hand are 200v and the original specs are 250v )-:

Had enough Orange Drop .047 to do a complete replacement, had 2 Russian .047 built decided not to mix them...

Also waiting 50uf caps, had some 47uf however, 1 of the 3 are a different brand so that wasn’t my favorite option.


Noticed that the high watt resistors on the sides were right against one another and not placed over the side vent holes, so I rearranged them, may not make much difference but, every little bit has to help some.
E43C24E3-568D-4AA0-B1A6-9B16C1CB45F6.jpeg
 
Learning a lesson the hard way/relearning, or getting too focused on the task at hand....

My revelation: Make sure I’m using, and paying attention, to the correct schematic (-:

Doing the normal type research of reading all the threads that reference the SA 100 and of course there’s lots of great information and ideas....

Was using this one: https://audiokarma.org/forums/index.php?threads/here-we-go-again-fisher-sa-100.678922/page-2 “Raz” made some great changes to his SA100 and my attention was drawn to the discussion about voltages of the 7247 tubes because I was also checking voltages.

“Notdigital” Had listed his findings along with the specified voltages....My 7247 pin 1 voltages were also “high” compared to the specifications listed in the thread.

Had completed most of my recap, so set the bias according the manual , set up the signal generator and oscilloscope and the output wave looked good....

Thinking, pondering....decided to look at the schematic again, had originally made sure the schematic I had matched my amps serial number, and it was right there on paper, pin 1 7247 190v

Boy did I feel foolish )-; Don’t even want to talk about the time wasted...

Probably not an issue for most, but thought I’d share non the less...

VR
Andy.
 
Have issues with my internet connection, strange going on...

Anyway, wanted to try and update my SA100 progress:

Waiting on the new Hayseed cap can, the old one tests fine, just changing out of an abundance of caution.

Changed out the power supply and bias caps, most caps sans the can have been changed at this point.

Changed and spaced out several of the high watt resistors, believe separating them pays off in cooling.

Originally planned on running it sans the bottom cover, then decided to install it with spacers, around a 1/4” gap all the way round.

Installed some wooden feet, planned on making feet similar to the original feet on a 400C wooden case, just make them a bit taller. For more cooling believe hollow aluminum feet would act as heat sinks....

Speaking of cooling, this one has been running for a few hours and doesn’t seem to be any hotter than my FM1.

However, the side underneath the output transformers is hot but there is a lot of resistors on that side, in fact I’m gonna install a set of tall aluminum feet.

Really have to say, these are truly fantastic amps...

VR
Andy



33AE3CEB-99BD-4E8A-96F7-1BAB247AD335.jpegC0087E5F-0B51-4D27-A499-F03AEEB8EBED.jpeg44F08740-E348-4F3F-8838-69B560376032.jpeg
 
Hopefully in a week or two I’ll have this paired with a 400C thanks to fellow member “Greg Nal”....

Going to implement Dave G’s 400C mods, so that should be fun.

A couple of photos of what’s coming:

0990190E-0256-4C70-ADD2-073618E3A478.jpeg 0959175D-79A3-4A3E-98FA-4475DB6DEB2B.jpeg
 
That gold faceplate looks nice. Many Fishers with that type of faceplate don't survive in that condition.
 
Thought it looked good also, found the thread with detailed pictures and measurements of an original wooden case, so planning on building my own.
 
RS Steve said:
Mine is all snuggled in its wood case, just have not set the system up yet, no room! :dunno:

View attachment 1937543
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LOL, understand that, I’m probably gonna sell one or more of my other amps/preamps...

That looks fantastic, don’t recall, did you make any of the changes Dave G recommends?

I’ve just got too many projects )-: would like to get the case done but it’s going to be a while
 
adnick said:
LOL, understand that, I’m probably gonna sell one or more of my other amps/preamps...

That looks fantastic, don’t recall, did you make any of the changes Dave G recommends?

I’ve just got too many projects )-: would like to get the case done but it’s going to be a while
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The person I got it from did Dave's mods except for the phono section and loudness circuit. I will get around to that at some point.
 
Now I recall that you told me that, my bad...

This preamp has had considerable work done but the selenium rectifier was left in? Not something I’ve encountered before, most of the time the selenium is the first thing to go.

My plan is to make Dave’s changes that are clear to me, full wave bridge rectifier, channel A cable routings, tone controls, then hope the other changes will become more clear to me as things progress along....
 
arts said:
The Sovtek EL84M (or whatever the cyrillic-writing Russian military version is) would likely be the best choice for this amp.

I would not trust any of the electrolytics unless you have the ability to test them for leakage at their actual operating voltages.
Lots of caps test good for ESR and leakage at a few volts,but when you apply the many-hundreds-of-volts they actually operate at in circuit, pop goes the weasel:eek:
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I've got a Lafayette LR-800 that was originally designed for 7189As that I bought from a shop. They put CBS EL84s in one of which eventually cooked for obvious reasons I dropped in a set of heavier duty EL84Ms and haven't had a single problem since.
 
OutlawSun said:
I've got a Lafayette LR-800 that was originally designed for 7189As that I bought from a shop. They put CBS EL84s in one of which eventually cooked I dropped in a set of heavier duty EL84Ms and haven't had a single problem since.
Click to expand...

Currently running Sovtek el84ms and it sounds really good, but have also ordered some military 84m’s from Russia— just wanted to a little comparison....
 
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