first fisher 400 rebuild

bobbyonions

AK Member
Hi folks, I recently picked up an early version Fisher 400 serial # 16289E. It has cleaned up very nicely and I want to start a rebuild.
I've found alot of usefull info here concerning rebuilding the 400. I was hoping some of you could give me some support if I get stuck or confused as I go along. I've read some conflicting info here, partially because there are a few different schematics with different info. I have yet to find a schematic for the units under 20000.

Here's the basic background on this unit:

It has the early-style wood cab in nice shape (missing the metal grill and wooden rail-type feet). It has all 9 w. german 12ax7's, six of them diamond bottoms, and the three in the mpx section don't have diamonds( still w. germany). Unit came with only three good 7868's various brands. This is the version with the selenium bridge and the 5.6k resistor in the bias supply.
I've done a slow power with a variac, and verified good output on both channels. I noticed a little red plating which doesn't surprise me. I've had it powered up for max of 5 minutes or so, and don't plan to power up again until I rebuild the bias supply.
I have the new bridge rectifier and the two 1000uf caps and various resistor values to begin the work. So far I read that I need to replace the bridge, the two 1000uf caps. I’ve read to change the 5.6k resistor to 2.7K. I also read a post saying to change the 15ohm power resister to 20ohm (something to do with adjusting voltages on the heater to the phono section 12ax7’s). I’m not sure about the resistor changes.
Any help or advise will be greatly appreciated. I will update as I go, and post photos if anyone is interested. Thanks.
 
I'll be watching this thread because of my latest acquisition as well! good luck with everything. Can't wait to have daily listener fisher!
:D
 
I have the new bridge rectifier and the two 1000uf caps and various resistor values to begin the work. So far I read that I need to replace the bridge, the two 1000uf caps. I’ve read to change the 5.6k resistor to 2.7K. I also read a post saying to change the 15ohm power resister to 20ohm (something to do with adjusting voltages on the heater to the phono section 12ax7’s). I’m not sure about the resistor changes.
Any help or advise will be greatly appreciated. I will update as I go, and post photos if anyone is interested. Thanks.

You have many of the key points covered. Get the selenium bridge out of there. Halve that dropping resistor on the bias supply. That will do a huge favor to the outputs. Avoid using a MOX - they sound awful in this application. You don't need to mess with the power resistor. The increased bias supply voltage (even at today's higher mains voltages) is still within normal heater parameters. The affected heaters are wired in series.

Here's the rest:

Replace ALL the small under chassis axial electrolytics. All of them. That includes the ones everyone ignores, such as C74. Don't forget C210 and C55 either. Otherwise, you will be tortured with little problems later.

Unless you are having a problem with the MPX, you can leave the other caps there alone, or change them out to alter sonics or improve long term reliability. I wouldn't mess with the de-emphasis network. Don't tear out the chokes. Don't play with the diode packs.

Replace the main coupling caps, C62-65. Use polyesters to maintain original voicing. Or you can try PIOs or polyprops for kicks. I prefer polyesters or PIOs over modern designer labels. Soviet K40s are very good. Heavily overrate voltages if PIOs are used. Absolutely no leakage whatsoever is allowed in these positions. I don't alter capacitance values as some do. If the bias leakage fears of the new production outputs spook you, then substitute 301kohms for C62-65. That will roughly maintain the R-C filter corner. The cooler bias supply voltage will do the rest against redplating.

Install 10 ohm 1/4 watt metal film resistors on the cathode supplies to the outputs (pin 3). These act as "fuses" to perhaps someday save an OPT and allow individual output bias measurement. Use metal films, NOT carbon or MOX types.

Replace both diodes in the doubler circuit.

Replace C83 with a Y-rated AC suppressor cap. I'd do the same with C80, but you can get by with a 1.6kv/ac rated orange drop there. Remember, these two are AC applications; don't rely on DC part ratings. In lieu of C83, you can remove it and R118 and replace the cord set with a grounded one.

C48 and 49 are optional to replace. I usually do so to tweak the sound in conjunction with the main stages. Same with C19 and 20. If you spin vinyl and want a higher-quality preamp performance, replace C10 and 11 with films (as Fisher did with their separates).

If the main filters are running cool, and the idle current draw is acceptable, I leave them in. Unless abused, they're much hardier than people think, even at their present age. But you can replace them if you have fears. There is a ham guy custom rolling new cans if necessary.

That's the bulk of it, and will get the unit running safely and dependably in most instances.

Good luck!
 
You have many of the key points covered. Get the selenium bridge out of there. Halve that dropping resistor on the bias supply. That will do a huge favor to the outputs. Avoid using a MOX - they sound awful in this application. You don't need to mess with the power resistor. The increased bias supply voltage (even at today's higher mains voltages) is still within normal heater parameters. The affected heaters are wired in series.

Here's the rest:......

Thanks sgmlaw!! Very infomative response. I will refer to that info repeatedly until I can digest all of it. The schematic that I have is for units starting at 20000, so I don't know if it's the right one yet. I've yet to study it closely and compare to the unit.
I took some before photos today and hope to dig in tomorrow by putting in a new bridge rectifier, and both 1000uf 35V lytics, and replace the 5.6k resistor with a 2.7K. I'd kind of like to power up the unit after each phase of the rebuild to check progress. The problem is, I only have three 7868's and I read that powering up without tubes is a bad idea. Anyone know about this?
I will try to get some before pictures up here very soon. How do you insert photo's in the body of these posts? Insert image icon up top? Any recomendations on pixel size that's good?

KN8243, are you planning on doing the work on yours too?
 
I'd kind of like to power up the unit after each phase of the rebuild to check progress. The problem is, I only have three 7868's and I read that powering up without tubes is a bad idea. Anyone know about this?

There is usually no harm in energizing the unit without tubing, provided that mains voltages are not too high. You can run into problems, however, if you energize the instrument with less than all the outputs installed, and an adequate output load attached. For restoration, I typically remove all the tubes until the job is done and all voltages check out. It's also a good idea to have a few dead tubes around in various sizes that you can use for socket contact alignment when soldering in new part leads.

You might be better off in tackling each restoration step before moving on to the next, and then waiting until completion until re-energizing the unit. Every time you energize, you recharge the big filters, which then have to discharged to safely work around again. You are also energizing parts that might fail before their turn for replacement, causing further problems.
 
sgmlaw: Couple questions for ya. See below. Based on 20001-29999 manual.
Larry

Don't forget C210 and C55 either. Otherwise, you will be tortured with little problems later.

.C210??????

Replace the main coupling caps, C62-65. Use polyesters to maintain original voicing. Or you can try PIOs or polyprops for kicks. I prefer polyesters or PIOs over modern designer labels. Soviet K40s are very good. Heavily overrate voltages if PIOs are used. Absolutely no leakage whatsoever is allowed in these positions. I don't alter capacitance values as some do. If the bias leakage fears of the new production outputs spook you, then substitute 301kohms for C62-65. That will roughly maintain the R-C filter corner. The cooler bias supply voltage will do the rest against redplating.
You mean R98,99,100,101? the 330KoHm Resistors?

Install 10 ohm 1/4 watt metal film resistors on the cathode supplies to the outputs (pin 3). These act as "fuses" to perhaps someday save an OPT and allow individual output bias measurement. Use metal films, NOT carbon or MOX types.
V14 has a resistor on PIN3 already. It will need a separate ground. It will throw off your readings if you leave it on the pin.
Replace both diodes in the doubler circuit.

Replace C83 with a Y-rated AC suppressor cap. I'd do the same with C80, but you can get by with a 1.6kv/ac rated orange drop there. Remember, these two are AC applications; don't rely on DC part ratings. In lieu of C83, you can remove it and R118 and replace the cord set with a grounded one.

C48 and 49 are optional to replace. I usually do so to tweak the sound in conjunction with the main stages. Same with C19 and 20. If you spin vinyl and want a higher-quality preamp performance, replace C10 and 11 with films (as Fisher did with their separates).

If the main filters are running cool, and the idle current draw is acceptable, I leave them in. Unless abused, they're much hardier than people think, even at their present age. But you can replace them if you have fears. There is a ham guy custom rolling new cans if necessary.

That's the bulk of it, and will get the unit running safely and dependably in most instances.

Good luck!
 
C210 is the 1uf in the MPX subchassis. It routinely fails.

Yes, I was referring to R98-101.

I have never encountered a stock 400 with a resistor factory-fitted to V14's cathode.
 
Here's a pic of mine as received from previous owner. Looks original to me, solder was factory looking and resistor looked to be original compared to others on board. Not trying to argue the point, just saying. I've seen same basic thing on my Sansui 1000A.

V-14pin3-4.jpg



C210.. OK. Forgot about the MPX Subchassis.

R99,,,,,..... Just wanted to clarify, as it threw me there for a minute or so.
 
Here's a pic of mine as received from previous owner. Looks original to me, solder was factory looking and resistor looked to be original compared to others on board. Not trying to argue the point, just saying. I've seen same basic thing on my Sansui 1000A.
. . .

That resistor (running off of pin 4), and its companion on the other socket (running from pin 5), are 220 ohm bleed off resistors for the heater circuit. They have nothing to do with the cathodes, but are occasionally tied to ground at pin 3 instead of the nearby lug (depending on the assembler).

Obviously, when performing circuit changes, such as fusing the cathodes in this instance, one needs to maintain the path of surrounding and/or connected components. At this stage, I tend to ignore the obvious. But perhaps it should be restated for the newbies.
 
That resistor (running off of pin 4), and its companion on the other socket (running from pin 5), are 220 ohm bleed off resistors for the heater circuit. They have nothing to do with the cathodes, but are occasionally tied to ground at pin 3 instead of the nearby lug (depending on the assembler).

Obviously, when performing circuit changes, such as fusing the cathodes in this instance, one needs to maintain the path of surrounding and/or connected components. At this stage, I tend to ignore the obvious. But perhaps it should be restated for the newbies.

On my unit, the 220ohm resistor on pin 5 goes to pin 3 before ground.

Thanks larryderouin , for pointing out the potential issue. It seems that it would be a common mistake for beginners (me), to not move that resistor directly to ground. I was aware of it from reading other 400 restore threads but it doesn't hurt anything to point out the obvious. Thanks guys.

I made sure I drained all the caps today and while the iron was heating up, I realized one of my 1000uf caps for the bias supply was actually a 2200uf(was in the wrong bin at the shack). Im not going to tear into it until I have everthing and R shack is closed today (LaBOR DAY) so will have to wait. I'll try to get some before photos up here soon.
 
"I’ve read to change the 5.6k resistor to 2.7K."

I found that changing the 5.6K resistor to 2.7K was too big of a change. With my 400 the 2.7K was a good value to start with but biased the outputs at around 12 ma. I tried several different resistor values until my 7868's were running at around 30ma. If I remember correctly I ended up with a resistor value of 3.9K. My 400 sounded a bit thin with the outputs running 12ma current but sounds great at 30ma. The power transformer and output transformers run very cool with the outputs biased at 30ma.
 
"I’ve read to change the 5.6k resistor to 2.7K."

I found that changing the 5.6K resistor to 2.7K was too big of a change. With my 400 the 2.7K was a good value to start with but biased the outputs at around 12 ma. I tried several different resistor values until my 7868's were running at around 30ma. If I remember correctly I ended up with a resistor value of 3.9K. My 400 sounded a bit thin with the outputs running 12ma current but sounds great at 30ma. The power transformer and output transformers run very cool with the outputs biased at 30ma.

Thanks for that info dsndblm. I'll keep that in mind.

Here are a couple pictures of the unit and case. Everything looks original under the hood except for one thing, (see last photo)

DSC_0371.jpg

I have one of the missing knob brights so I'm lookin for one.

DSC_0372-1.jpg


DSC_0381.jpg


In the photo below you can see that there is a trio of twisted wires that exit a coiled steel jacket (cloth insulated orange, white, and a brown one) These come from the on/off switch. As they exit the steel jacket in the picture you can see that the orange one is snipped. It looks like it originally went to the hot or ground lug of the convenience outlets. Now they got that wire at the outlet running under the fuse holder then connecting to the terminal strip. Anyone have a clue why this was done? Did they make it so the convenience outlets are allways hot even with the power switch off? I hope I explained that well you all to see what I'm talkin about below.

DSC_0386.jpg
 
Here's a better picture of what I was talking concerning the snipped wire, On the left you can see the wires exiting the steel jacket. Also adding a full underneath shot.

DSC_0387.jpg

DSC_0389.jpg
 
Last edited:
bobby,

here's the info you requested, and the pics.

each rail is 1/2" thick where it attaches to the case. it reduces to 7/16" on each of the bearing surfaces (the bottom of the feet). the angle is approximately 10 degrees, tilting outwards from the bottom of the feet.

the legs are 11/16" tall (may have been 3/4", now worn), and the rest of the rail is 5/16" tall. the back siderail legs are 1 11/32" from back to front. the front siderail legs are 1 3/4" measured to the tip of the mitre joint. the front rail legs are 1 1/2" measured to the tip of the mitre joint.

again, measured from the tip of the mitre joint, the front rail is 18" wide, and each siderail is 14 1/8" back to front (flush with the back of the case). the front rail is positioned 3/8" from the lower front lip of the face frame, and 1/4" from the line of the face frame attachment to the case. the side rail is positioned 7/16" inboard of the side of the case.

i think that's it. let me know if anything is unclear, or if you need other measurements.
 

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Thanks for all the info on the cabinet legs madwing. That was exactly the detailed information I need to consruct some reproduction feet for this thing.
 
Replacement screens are on the auction site, number 250321349927. I bought one of his for my Fisher, it fits perfectly, and looks awesome. Make sure you give him the proper dimensions, apparently there were two different sizes.
 
Replacement screens are on the auction site, number 250321349927. I bought one of his for my Fisher, it fits perfectly, and looks awesome. Make sure you give him the proper dimensions, apparently there were two different sizes.

Thanks for the heads up on those. Those are the nicest looking repros I've seen so far. I'm gonna talk to a metal-worker friend first. If he stocks a similar material, I'll ask him to cut and bend me one. Buy local!!!
 
bobbyonions,
That's an older style cabinet. Be sure to provide the screen seller with that information. Some of the older cabinets have deeper cutouts from the back so they could vent the output tubes when they use to be in front of the output transformers instead of behind the output transformers. If you dig around, you'll find that some 500-Bs had tubes in front of (forward, closer to the front panel) the output trannies. I don't know about 800Bs, so I can't verify that.
 
bobbyonions,
That's an older style cabinet. Be sure to provide the screen seller with that information. Some of the older cabinets have deeper cutouts from the back so they could vent the output tubes when they use to be in front of the output transformers instead of behind the output transformers. If you dig around, you'll find that some 500-Bs had tubes in front of (forward, closer to the front panel) the output trannies. I don't know about 800Bs, so I can't verify that.
Yes, some 800Bs ARE like that as well. (not all of them though). I own one that is. It does not have a cabinet because it was mounted in a console -- so, I cannot provide a comparison for the cabinet opening. The chassis on mine is spotless thanks to that console.
 
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