Snagged a Fisher 400 - resto time!

Thumper

Thumper

Well-Known Member
Picked this up yesterday locally. S/N 66,xxx. Paid a good amount for it but I think it was fair, especially considering that I've now got it mostly apart and realized that it's almost completely original and untouched.

2017-06-27_12-01-10 by rickenbacker_man, on Flickr

The only deviations from stock that I see - there are two wires (one white one purple) that come off of the loudness switch that have been disconnected and have electrical tape on the end. Also, the cover on the back of the on/off switch is just kind of dangling there. Not sure if it was taken off or if it fell off. Maybe this pot and switch has been replaced at some time? Did the original pot have a loudness tap? That would explain the loose wires. I didn't take any close up shots yet but I can get some later.

IMG_20170626_2300117 by rickenbacker_man, on Flickr

Hoping the front panel and glass clean up nice, I haven't tried to clean them yet. The thing is DIRTY.

2017-06-27_12-03-45 by rickenbacker_man, on Flickr

Is this a legit Fisher case? I've seen another 400/500 in the same one, but it doesn't look like the usual ones with the metal screen on the upper back.

2017-06-27_12-04-26 by rickenbacker_man, on Flickr

The thing also had all these valet tags packed inside, kind of funny...
2017-06-27_12-05-47 by rickenbacker_man, on Flickr

All of the small tubes are Fisher branded, power tubes are GE's. I tested them on my B&K 747B, two are at about 75% and two are at about 25%. I may snag another used pair off ebay and do the IBAM board. I will also be contacting Jim McShane today to inquire about a full rebuild kit.

Also - the bonehead seller had turned it on. I assume without speakers connected. He took pictures and I guess the positive is that it powered up and didn't release the magic smoke, but one of the pics showed one of the power tubes redplating.

Good fun!
 
Great find Thumper! Hopefully, the AC power switch is still good, but unfortunately, it sounds like the loudness switch was being used as an AC power switch due to its failure. New ones are available.

Good luck with your 400!

Dave
 
Hey Dave,
I think the pot and switch are a replacement unit. The pot doesn't have loudness taps, so the wires going to the switch were simply taped off. Not that big of a deal to me as I never use the loudness switch anyway. I'll be installing inrush limiters to hopefully get some long life out of the switch that's in there.
Rob
 
I went and ordered Hayseed cans since I liked the looks of them (even though they're $$$). Also went with a metalbone sonicap kit as well. Both kits are on the way. I've got knob brights and LED's on the way too.

In the meantime I cleaned up the faceplate with magic eraser (came out great!) and the dial glass with nothing more than a blue scott towel and hot breath. That took about about 45 minutes and with lots of nervousness after reading about guys wiping the letters off... Came out looking great though! Both the faceplate and glass were covered in some sort of brown goo - I'm pretty sure it wasn't nicotine so I'm guessing just 50+ years of finger oils, cooking oils, dead skin cells and whatever else...

After I get both cap kits installed I'll be putting together a mouser order for the bias/balance board, a couple of CL-80's, a new power cord and whatever else I find along the way that needs to be replaced.

BTW here's a close up of the volume pot/switch. Definitely looks to be a replacement, yes? No loudness taps.

2017-06-30_12-28-44 by rickenbacker_man, on Flickr
 
The only one I have is R50-160-166a from a 59T. The values are correct and the switch is good, but the shaft is too short for the 400. Theoretically you could swap the front cover with the shaft and thefront channel wiper on it and go for it. Then you'd have a loudness tap, switch, and could wire up the loudness switch again, just to keep it accurate.

If you have a set of calipers you take the O.D. of the shaft, and the I.D. of the hole. Then find one that matches or is slightly larger. You can trim these to fit. (talking about these) https://www.mcmaster.com/#plastic-grommet-bushings/=18apxj6 with the angled cut in them. Use the U.S. measurements as the metrics will be harder to fit in the hole.

My oldest son is a Detective in the P.D. there.
 
Thanks Larry, I might take you up on that if this one dies, but it's still alive at the moment. I have the old pot/switch from my Scott lk48b kicking around too. No idea if the values are the same tho.

So I did some no power tube tests the other night and noticed that V8 thru V10 weren't lit. Bias voltage was like -6v and 8-10 heaters were sitting at about -9. Hayseed cans and metalbone's kit both came today and I didn't get that far, but, ... BIAS!

No idea if this will show up, on my phone..
https://flic.kr/p/VK9Chm
 
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BIAS POWER SUPPLY NEEDS Rebuilt. Hopefully with the Hayseed Can's you got the underside Bias Dual Cap. Replace the Rectifier, and the two .01 black beauty's at the V.D. cap. Plus the diodes.
 
This is the pic I unsuccessfully tried sharing the other night:

2017-07-02_12-38-55 by rickenbacker_man, on Flickr

Not the prettiest, but bias caps only installed using metalbones kit. I don't plan on touching the rectifier unless you guys suggest otherwise, it looks like it's silicon. Diodes will go and black beauties will go, but I have a question there - the "death cap" is the one on the primary side of the PT, right? Shouldn't this be an XY rated cap? Metalbone's kit came with films of some sort.

I powered it up on the variac with these new bias caps and bias voltages look good now: -17V to power tubes and -24V to preamp heaters. I was a little nervous as my kill-a-watt (everybody needs one of these, seriously) said it was pulling 0.57A with no power tubes installed. Thinking that was a bit high, I put in the LED's which came today and it dropped to 0.54A. Then I google-fu'd and read that a 12AX7 pulls about 150mA, so 0.5A doesn't seem all that out of line. I still plan on doing the full electrical resto, but I'm half tempted to stick this worn out set of GE 7868's in there and see what happens. Thoughts?
 
Yeah the "death cap is off the primary. And it should be replaced with a X-Y rated cap. But you can get by temporarily with the metalbone cap. Might as well put in a new diode bridge while you are in there for peace of mind.

Larry
 
It's aliiiiive!

I had some issues over the past few days - something metallic was rattling around inside of the PT and it started blowing fuses. I pulled the endbells and two pieces of broken bolt came floating out. WTF? Anyway, all is good now. Bias supply caps, doubler cap and diodes were replaced and since all voltages at the power tubes looked good, after first popping the cleaned up glass and faceplate on (after installing my new LED's), I stuck the power tubes in and fired it up. Since I haven't yet cleaned any of the controls, I had to fiddle around to get sound out of both channels but eventually got there using my XM as a source.

Putting along into my bench speakers (Minimus 7's):

2017-07-07_10-49-20 by rickenbacker_man, on Flickr

2017-07-07_10-49-54 by rickenbacker_man, on Flickr

No redplating of any sign and it actually sounds REALLY good considering the resto is only like 10% done. Before sticking the 7868's in I decided to retest them on my 747B and got some different reading this time - 70% on three and about 15% on one. Not sure what that's about, dirty pins maybe? Anyway, one side is noticeably louder than the other so I'm off to ebay to see if I can find another good used GE 7868.

Control cleaning and installation of the new quad cap is up next - I noticed it was running a bit warm. I've got new knob brights on the way too.

rufler - regarding tuner shaft bushings - I need the plastic snap-in one that goes through the front panel behind the faceplate. It's so worn out that the shaft flops all over the dang place.
 
Thumper; Put in those 10 ohm 1/4w cathode resistors so you can measure the actual cathode current on each tube. Use this and the plate voltage to calculate wattage on each tube. You'll find your weak tube on the side that is lower in volume. I would highly recommend building an IBAM or IBBA board if you are out for another QUAD of GE's as they will not be matched. The IBAM/IBBA boards will allow you to individually set each tube to a current draw that when converted to watts is between 70%-80% of maximum dissipation of 19W. You could then check the ma on each of them and match the best of the 7 or 8. Set the IBAM for 33ma on each tube, then check the bias voltages. The one's with the highest numbers (away from ZERO) will be your best one's. They should be within a volt or so on the bias voltage. But if they aren't don't worry about it. Don't set the bias by the voltage however, use the current measurement. This is the best indicator of tube strength and longevity. Current draw is indicative of how well the tube is working.
 
Thanks Larry, yeah, along with a full recap I plan on doing the cathode resistors and IBBA board in the very near future - along with some or all of Dave's other mods - I just couldn't resist the urge to give her a test run last night!

Since I have three GE's that test good I want to try and get one more off of the auction site - I thought about the EH's but I read all about the bigger pins and I want to be able to run NOS or OS tubes in it.
 
Update - I snagged a good used GE 7868. Finished installing the hayseed cans and most of metalbone's kit (still have to do a couple more coupling caps and the FM section). Installed the 10 ohm cathode resistors and took some measurements last night:

V14 - 0.285V
V15 - 0.276V
V16 - 0.251V
V17 - 0.201V

Even as-is, it sounds fantastic! Next step will be to finish installing the rest of the metalbone kit, and work up a parts list and order parts for Dave's IBBA board.

2017-07-17_12-02-03 by rickenbacker_man, on Flickr
 
I would suggest the IBAM or IBBA boards to even out those tubes. Even at that the highest running tubes are at the low low end of normal, The other two are running flat out COLD, especially V17. Swap the pair's and see if that tube comes up a little. What's the pin 9, pin1, pin2 voltages on each tube with them reading the above on PIN3?
 
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Larry,

I swapped V14 and V17 since they were the furthest apart. Pin 9 (plate? - my 7868 datasheet is barely legible) voltages are all 412V and pin 1 (screen?) are all 371V. Here are my new bias measurements and pin 2 measurements:

V14 0.194 -15.7
V15 0.270 -16.0
V16 0.240 -16.0
V17 0.282 -16.0

V14 looks iffy but it still sounds great! While I'm researching IBBA parts I'm still curious about these 330k screen resistors I've read about - the original skit doesn't even show any screen resistors?
 
The 330K resistors are the Grid return resistors. R-117-118-119-120. Change them to 220K or 200K and then change the coupling caps to whatever the metalbone kits sent you. Probably .1uf.
The screen resistors(100ohm) are an add on to alleviate or at worst minimize arcing of the tubes. It's more prevalent on the Russian tubes BUT it's a good idea on all tubes. Go into the Stickies Subforum and find the Thread "Common FISHER PARTS". There is a post on the 3rd or 4th page IIRC with pics on how to add them on 7591 sockets and also the 7868 sockets.
 
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