first fisher 400 rebuild

Regarding the orange wire.....previous owner probably wanted to reduce stress on the double pole power switch...maybe the main side of the switch failed and he decided to use the convenience outlet side to power the unit....and relegated the outlets to continuous status...?
 
folks, i have a couple of questions based on what different akers have posted above, and in my fish, fish, fish thread. i just got my own 400 (don't know the serial number, but it's about 27K), and am getting everything ready to get it working.

sgmlaw, you said to replace c83 with a y-rated AC suppressor cap. i was going to replace it with a solen PPM, .01uf/1Kv. will that work?

dadbar, you suggested getting a 5k/2W pot to adjust the bias on the 7868s. should i get 4 and wire them in all at once, then adjust, or do them all with 1 pot, replacing the old 5.6k resistor with the correct one that keeps the four tubes balanced and running cool? there's no relationship between each tube that would be altered by the series changeover of resistors, i assume. this would be done after all the rest of the components have been changed.

thanks,
bill
 
sgmlaw, you said to replace c83 with a y-rated AC suppressor cap. i was going to replace it with a solen PPM, .01uf/1Kv. will that work?

thanks,
bill

Bill,

NO, that Solen won't work in that application. While the PPM is AC rated to 330v, it does not have the transient capability required to satisfy UL1414, and is not Class Y rated for line to ground use.

The UL standards are not meant to bust your chomps, but to keep you safe. A Class Y capacitor is designed to handle line transients in the several Kv range, and to fail open.

Use one of these:

http://www.vishay.com/docs/23102/440l.pdf

Available here:

http://www.mouser.com/ProductDetail...=sGAEpiMZZMt1mVBmZSXTPLKdrmN0rpoA4nlS1ux5iM4=

And I'd be surprised of many of the kits include the correct capacitor for this application.
 
excellent, thanks!! that's good to know; i wrote out my replacement list from the schematic, rather than seeing it in person. i've yet to spend time under the hood, and am trying to get the parts together before i start to do anything.
 
Sometimes (actually most of the time) it's better to get under the hood and VERIFY the actual components prior to ordering new ones. That way you have a list that you compare with what's actually under the hood and make changes. Like some of the techs' have said, manufacturing line changes didn't ALWAY's make it to the manuals, and mid line changes were common without notifications to engineering for book changes, or changes were made in the book, but the line continued for a while until the old parts were used up.

Larry
 
OK, Today I finally got to do some work underneath the fisher. Here are some pictures. If anyone sees any mistakes in my wiring, please correct me, because I don't want to ruin this thing.


Here is a shot of the new rectifier bridge I got from radio shack. It is bolted the the side of the chassis using one of the holes and bolts from the original selenium job.

DSC_0391.jpg


Here shows the new bridge, and two new 1000uf caps (negative ends are facing the bridge

DSC_0390.jpg


Here are the two 1000uf caps mounted in the original clamp. I inserted a neoprene rubber shim around the inside of the clamp. That made the new caps fit in there nice and tight and keeps them away fromt the chassis.

DSC_0393-1.jpg


A couple shots of the added 10 ohm resistors between pin 3 of the outputs and ground.

DSC_0396.jpg


DSC_0397.jpg




I also replaced the 100uf 25V electrolytic C74, with a 100uf 35V. This OK?

Lastly I bought a 10uf 50V electolytic to replace the 8uf 50V (C55). When I went to install it I realized it's non-polarized. Can anyone tell me if a 10uf 50V non-polarized lytic will work ok in the C55 position????

Thank you to everyone for your input!!
 
Looks good. The 100uf/35V in lieu of a 100uf/25V is FINE. All you are doing is adding a little headroom in the max voltage.

I haven't got the schematics near me but I think the 10uf non polarized may be ok in lieu of the 8uf polarized.

Larry
 
polishing knob brights??

While I'm waiting for the rest of my parts for the re-build, I thought I'd do some cosmetic restoration. Some of the knobs brights are oxidized in areas. I was wondering if these polish out. Does anyone know if they are laquered? Thanks. Dave.
 
dadbar, you suggested getting a 5k/2W pot to adjust the bias on the 7868s. should i get 4 and wire them in all at once, then adjust, or do them all with 1 pot, replacing the old 5.6k resistor with the correct one that keeps the four tubes balanced and running cool? there's no relationship between each tube that would be altered by the series changeover of resistors, i assume. this would be done after all the rest of the components have been changed.

thanks,
bill


Madwing- someone recently posted a scheme with trimmers to adjust all 4 tubes...I forget who that was. If you buy 4 of the 5K/2W pots you will have to find space for them all (they're pretty big). I am just using one pot with a matched quad which works fine for my own lazy needs. The pot and exterior test point lets me spot check things every now and then to make sure nothing has drifted. Once a year I open up the 400 and check the bias current on all 4 tubes to make sure they are still fairly well matched..
 
thanks, dadbar. how do you use just one pot to test the bias on all four tubes?

and larry, i plan on making the final list of parts with my 400 nude on the table before me, checking each part for what's there and making adjustments to my list...
 
Here's a thread on biasing a 400. It starts to really heat up on page 4, but there's a ton of good information about 400 reconditioning in general.
http://www.audiokarma.org/forums/showthread.php?t=140313

Let me state up front that I've never gone this far in biasing. I simply put in 10 ohm resistors off the 7868 cathode, and then select and move tubes around until I have two relatively matched push-pull pairs. But I saved the thread in case I'd like to try this some day.
 
Individual bias pots

Here's the schematic by Drew for individiual pots. It's worth it if you're up to it.
 

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Here's the schematic by Drew for individiual pots. It's worth it if you're up to it.

I know people like this mod, and it is convenient. But it's a lot more to go wrong, in the last place you want anything to go wrong.

This thing with the 200kohm grid resistors . . . you've got to double the coupling capacitance to maintain the corner frequency. 301kohm meets the spec for that output family, and without the R-C issues. If a new production bottle red plates at 301kohms from grid leakage, it isn't fit to be in the socket.
 
I know people like this mod, and it is convenient. But it's a lot more to go wrong, in the last place you want anything to go wrong.

This thing with the 200kohm grid resistors . . . you've got to double the coupling capacitance to maintain the corner frequency. 301kohm meets the spec for that output family, and without the R-C issues. If a new production bottle red plates at 301kohms from grid leakage, it isn't fit to be in the socket.

The RCA manual does say 300k (max) is good for a 7868 with fixed bias, so the initial design is valid. If the current tubes are up to the RCA spec for 7868, there shouldn't be any problem. Have there been grid current problems with the currently available tubes that moving to a lower resistor will solve?

I do like having the four bias pots though. I just don't know if I should bother changing the resistor from 300k.
 
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The RCA manual does say 300k (max) is good for a 7868 with fixed bias, so the initial design is valid. If the current tubes are up to the RCA spec for 7868, there shouldn't be any problem. Have there been grid current problems with the currently available tubes that moving to a lower resistor will solve?

I do like having the four bias pots though. I just don't know if I should bother changing the resistor from 300k.

Well . . . Fisher put 330kohms in there, which is about 10% over the spec.

The old production handle it just fine. And I will tell you, in my experience, the new production handle it just fine, too -- provided the overall core bias voltage is adjusted at R102. But for folks who get spooked, 301kohms is the spec, and that only raises the R-C cutoff frequency by 1hz.

Where it gets dicey is where you keep a hot original bias to begin with, and the grid leakage on some new production starts a cascade effect into run away, and a red plate.

I just don't see the need to put a 200kohms (or less according to some) grid resistor in there with any output, old or new. Then you have a cascade effect with the mods. A 0.1 uf PIO is BIG.
 
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