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Fisher Four-hundred Fotos?

Thanks for sharing the pictures and the discription of the project. I have a Sansui 1000A in the garage that I picked up at a garage sale for $5. It made music until I saw on of the 7591s' plates go red so I shut it down. I have wanted to rebuild it but have been hessitant, this thread makes me think I should give it a try. One can never have too many tubes!
 
Apparently the oil caps in the 1000A's are absolutely notorious, analog! They need to be changed; simple as that. Actually, and not entirely coincidentally, I picked up a 1000A a couple of weeks ago at the formerly-quarterly NEARC fleamarket -- possibly the last NEARC fleamarket :-( Pretty piece of hardware. This one's recapped.
 
I have heard the same thing, the original caps do not stand the test of time and there are a boat load of them. I think that is why it looks like such a scary task. I just need to dive in! I would like to get it working tho as it is a cool looking unit I am sure I could get a lot of help and advice from the folks here.
 
Excellent post! Good to know that the work that I've done on my 800B isn't wrong :) I look forward to seeing your next installment where you show us how to properly bias the amp (what to measure at what points).

Is there an easy way to see if the 1000v cap needs to be replaced? I also have one in my 800b but have left it alone since the sound is so warm.
 
Bias, you say? This is the most interesting part. The 400 uses fixed bias (as do its close relatives). Since you can't do much of anything to measure, or adjust, the bias in the stock machine, you'd better use a matched quad of output tubes. Their long-term health will depend in large part upon their biasing. Prevailing wisdom suggests something on the order of 0.30 mA current, or a bias voltage of ca. -20V (or so... this is off the top of my head).

In the stock 400, the four cathodes (pin 3) of the four output tubes are all grounded. If we replace the grounds with low-value resistors, we can now actually measure the bias current by measuring the voltage drop across the (known) resistance we've added. Adding cathode resistors, although controversial, might also add a "fuse" of sorts to protect the output transformers from a tube's shorting.
 
Thanks for posting these and sharing your experiences. I made a local copy of this thread (especially pics) so that I can draw on it for years to come. This is an incredible service to the community!

--gh
 
So here are the two pairs of 7868 tube sockets from below. All Pins 3 (cathodes) are tied to ground points on the chassis. You'll also see that there are 220 ohm resistors on two of the four sockets that are also tied to ground.

Fisher400two7868skts.jpg


Fisher400othertwo7868skts.jpg


There are also two 220 ohm resistors at the two 'ends' of the filament leads to the outputs... these need to end up going to ground. You need to make sure you don't unintentionally add your new cathode resistors (in series or parallel) when you snip the pin 3 grounds and insert a cathode resistor.
 
What we're going to do is replace the grounds on the cathodes with 10-ohm resistors. These should probably be metal film if you want them to serve double-duty as "fuses"; I have used carbon-film, though, in the two re-habs I've done.

Now that there is a finite resistance from cathode to ground, you can use Ohm's law to "measure" the actual cathode current for each tube. Since R = 10 ohms, if you measure the voltage drop (mV) across each of the cathode resistors you're going to add, and divide by 10 (ohms), you've measured the cathode current (mA). V = IR, and all that rot...
 
OK, so here's the undersides of BKville's 400 with the 10-ohm cathode resistors installed. Sorry the picture is not great. I 'circled' the 4 cathode resistors in red.

Fisher400outputswcathoderesistorshi.jpg


Here's a much better photo (tho' not such a great angle), again of BKville's 400 with the 4 Orange Drop coupling caps and 4 cathode resistors noted.

couplingcapsncathoderesistors.jpg
 
For the older 400 I just re-hab'bed, the voltage drop across the 4 cathode resistors were all in the 240-250 mV range (with the 3 k resistor in the bias supply), for a measured (!) bias current of 24-25 mA each. This is probably a little "cool" but maybe OK. I may need to "increase" (i.e., make more negative) the bias voltage a bit on this one for best sonic results... tbd.
 
Now, the best takeaway from all of these pictures is the pictures themselves. As I said before, I am NO EXPERT and my comments are provided strictly "as is"!

I do, however, hope that the photos are useful.

The wonderful thing about digital cameras and cheap mass storage is I can take a zillion pictures BEFORE I start snipping wires... I hope this means no more cases of "now, where does this yellow wire go?" :-)
 
... we'll make you sign a waver holding us all harmless if you 1) release the magic smoke from the electronic components and/or 2) electrocute or immolate yourself.
 
pen_man said:
Thanks for the photos and the write up. If I post images of my 400 would you assist me in a similar restoration as a forum project? I know that is asking alot, but you gave me some confidence in being able to see the project here on the forum. I think with a little guidance I could do the same.
Click to expand...
I would follow that thread with conviction and focused attention. You will get a lot more input, not just from one poster. Shoot, I'd even through some suggestions in, and they may or may not be accurate!

I would think that an acceptable ground rule for your request is that you promise to keep the photos on a server space that doesn't disappear as soon as you finish your project. In other words, a couple of years from now someone should be able to benefit from the posted photos. Shoot, googlepages gives you free 2gb with a gmail account (also free) - so you could document it there as well. Or, you could use blogger to document your rebuild, and cross post all of the images here - that's free and my photos from 2 years ago are still up...

--gh
 
FWIW, as a coda to part 1 of this thread, here's an "after" shot of the undersides of the second 400, which just went home to Worcester Friday night. I used Illinois caps on this one, because 1) they'd sounded so good in the 8B I did for the same owner (sourced from a VTV rebuild kit) and 2) they're axial (and very small), which made installation in the 400 a snap. I went for all 600V caps this time around.

P1010856.jpg


An aside: as it turned out, the FM in this 400 wasn't working at all. A tube tester and some patience revealed three bad tubes and one mis-substituted one! Four tubes later (sans alignment) it was working well in mono and stereo. A proper alignment is beyond my experience and my toolkit.
 
The double 1000 uF cap needs to be changed anyway. That looks like a silicon bridge; but it may be shorted. Smoke isn't a good sign. There may be something upstream that's amiss, and the poor rectifier may just be the weak spot.

EDIT: I cannot see whether something might be shorted on that bridge? Shorts (not designed by the original engineer) are not so good :-)
 
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