Fisher 800B - Getting Back on Line

I'm new to the forum and reading through past threads got me interested in digging out my old Fisher 800B receiver. I bought the unit used back in the mid-80s when I first got the tube bug and it ended up being drug all over the west with me when I was going to grad school and working a bunch of different field jobs. The receiver has been put away for some time and I was wondering what the best advice is for getting it back into play and for doing some basic modifications. I'm not a serious electrical technician but I do like the sound and looks of my old Fisher, Dynaco and Heathkit tube gear and appreciate any advice I may get. Many thanks!

GENERAL AND TUBE INSPECTION.

1st off do a visual inspection of the top of the chassis for loose parts. Do the same underneath, looking for burnt parts, loose connections, etc. If everything looks ok, close it up. Take out the tubes, marking each one to the Tube layout plan in the manual. Very important that each tube goes back in it's original socket. Test the tubes to make sure they are ok. Pay particular attention for shorted tubes, gassy tubes, or tubes with a white getter (white getters indicate loss of vacuum and are an automatic FILE 13 candidate.) Replace any bad tubes with same tube #. You have your choice of Domestic, Europen, or Chinese(last resort in most cases). They do not have to be "FISHER" branded although all FISHER branded tubes will usually be a higher ended brand like Mullard, Telefunken, Brimar. It your wallet won't carry a big load of tubes, go used domestic or new Russian. Tubes in the Signal path (non tuner) can be rolled (swapped) to change sound quality or increase or decrease tonal effects. But mark the tubes so they go in the same socket each time.

1st POWER UP!

SPEAKERS or DUMMY LOADS (8oHm 50W resistors) are required for ANY POWER UP. Lack of a load on the output transformers can/will burn out the transformers, and cause other damage to the output tubes, and associated circuitry.

Don't Plug it in to a wall socket and just turn it on. Get a variac, and bring up the voltage in stages, checking for smoking parts, dead tubes, etc. Start at 40V and leave it for an hour like that. What you are doing is "reforming the capacitors" so they can at least work during testing. Raise the voltage after an hour by 10 volts, doing this until you get to 100V. After 100V @ 1 hour without problems go ahead and plug in to wall socket. Watch for tubes "redplating", can capacitors getting hot, or warmer than radiant heating from the transformers, smoking resistors, etc. If you see any of this at any time, shut it down NOW! Then fix the problem.

Read any and all threads on the 500-B or 800-B in this and the TUBE Forum. Lot's of good information. The 500-B/800-B are the same except the 800-B has an AM tuner in it that the 500-B doesn't.

Ask questions if u run into problems.

Larry

Your detailed instructions are greatly appreciated. I'm hoping to get it up and running and pair it with a set of XP-7 "project" speakers I recently picked up. Again, many thanks - Jeff

Larry and others in here know this gear inside and out, I'm a novice and they have been a great help to me..

Many thanks. This is good stuff and good stuff to hear!

PS - Just snagged a variac.

Maybe outside and 1/2 inside, but not inside out. I'm a noob also compared to a lot of the guys on here. as a for instance; I wanted subs when I was in the Navy, it was one of the overriding items I wanted to do from about 6 yrs old. Hyperbaric chamber @ 100ft and clogged sinus' denied me Subs. So I volunteer on one in Baltimore for 15 yrs now. A couple of the guys took me thru a qualification program for that class, and I did the walkthru. The guy who signed my qual card/certificate retired when I was in, and he didn't pull any punches during the walkthru. I passed. It don't mean anything officially as far as the Navy is concerned but it helps when I need to work on something technical on the boat. And I'm one of very few people in the world, who are qualified in "Tench class Museum Submarines". Same thing with tube gear in a sense. I dig in and learn a good portion of it on my own. The math for figuring out different things lets me down. I could never wrap myself around math with letters and such. So I'm hindered on that. Also not knowing how the test gear works doesn't help either.

So I'll give as much as I can.

Larry

Nothing wrong with the empirical approach at all!

I refurbed my bone stock original 800B (even had ALL the original worn out tubes) recently (see http://www.audiokarma.org/forums/showthread.php?t=511382)
I purchased a kit from Jim McShane and replaced all the important caps as well as all tubes that tested bad. I used a dim bulb tester that I built and a variac purchased off of ebay for $50.
If you are handy with a soldering iron, you can complete one of the rebuild kits. Jim's is top quality parts, but you have to be able to follow the schematic closely, as the parts are not a one-for-one swap. There are other kits that have more detailed instructions if you don't have much experience working on electronic gear.
I'm listening to the 800B right now, and it has been a very rewarding experience doing it myself, and I know I will be enjoying it for years to come.

I found your 800B rebuild thread and it is a great read. Is the contact info for the rebuild kit suppliers available on AK? Many thanks.

The two recommended suppliers are:

Jim McShane - jimmcshane@prodigy.net

Metalbone - (google this one.)

Jim's kits come with the electrolytic caps, but they are not a 1 to 1 replacement. I believe the Metalbone kit has everything but the large electrolytic can caps. You could also combine the Metalbone kit and order custom can caps from Hayseed Hamfest.

It really depends on your level of experience - I've done a couple of simpler tube amp rebuilds before, but doing the Fisher 800 was a bit more complex. At any rate, I am happy that I went with Jim's kit. My 800B sounds fantastic.

Thanks much!

I snagged a Variac off ebay and it showed up today. I'm going power up the amp tonight following the above recommendations. Wish me luck.

Make sure you've got speaker's or dummy loads connected when you do. I'd recommend a Dim Bulb Tester in series with the variac. 100W bulb to start.

Larry

quadaptor said:

I snagged a Variac off ebay and it showed up today. I'm going power up the amp tonight following the above recommendations. Wish me luck.

Click to expand...

Glad to hear you got the Variac. If the amp is totally stock, you should be OK with powering it up slowly, but you would protect yourself even more if you put together a simple dim bulb tester. It really saved my bacon during the rebuild - I had something wired wrong twice and caught it with the tester. One time that the variac helped as well - I had something miswired and as I slowly applied voltage, I saw one of the larger resistors begin to smoke. Was able to back off and fix before doing any real damage.

How do you assemble a dim bulb tester - assuming you are not referring to the guy doing the work (just kidding). Thanks to all again.


PS - the unit is totally stock, but I'm sure its time for a cap replacement. I'd like to start with the can caps as supplied by Hayseed Hamfest.

A dim bulb tester is very easy to make. It lets you check for direct shorts when powering up your gear with the variac. A trip to the hardware store for a dual socket A/C wall outlet, a simple on/off light switch, a standard bulb socket (or even easier, get a light bulb to outlet adapter, a wall box to mount this stuff in, a wall plate to make it look nice and a A/C power cord. Also pick up a 100w bulb (I also picked up a 200w bulb).

I can't recall the build thread right now, but will check later and add a link. Very simple and will prevent damaging the unit if you are going to be changing out parts.

Also, you may want to consider restuffing your original can caps with new capacitors. New caps are much smaller and you can pry open most old can caps, rip out the old innards and stuff it with the same value new caps. Suggest getting radial electrolytic caps to make the restuffing job easier.

I'm not a wiz at this stuff, but does the dim bulb tester act like a fuse?

Quad -- not only does it act like a fuse, but a fuse and an indicator as well. Without a variac, it is normal for the bulb in a DBT to initially glow brightly when the unit is first turned on. This is due to the heavier-than-normal in rush of current caused by cold heaters in the tubes, and the charging current required by the electrolytic caps in the power supply. But rather quickly, as the heaters warm and the caps charge, the bulb should reduce to a very dim condition, indicating that current flow has dropped to normal.

If a fault condition exists however, the bulb -- which normally goes dim in just a couple of seconds -- will remain bright, indicating a problem exists, and, by taking up most of the current flow in that condition, prevents further damage inside the unit.

When used with a variac however, a DBT will still act as a fuse and indicator, but in that scenario with a fault condition, the bulb will simply glow brighter and brighter as the variac is ultimately brought up to full AC line voltage. With a no fault condition, the bulb will just come up to a dim glow at full AC line voltage, since the slow ramp up by the variac inherently limits the normal turn on in rush of current.

I hope this helps!

Dave

Dim Bulb Tester

Here's a pic of the dim bulb tester that I built:



It's from my 800B rebuild thread.

Here's a basic wiring diagram for a dim bulb tester:



This diagram is from the Antique Radios Forums here:
http://antiqueradios.forum5.info/t486-dim-bulb-testers-what-wattage-bulbs-to-have-on-hand

Start with a 100 watt bulb, but just be aware that the 800B requires a lot of power - you will have to switch to a 200w bulb to get it to pass enough juice to power the 800B's 24 tubes and to experience a somewhat dim bulb. Even with a 200w bulb, I found the 800B would not fully energize enough to bring in FM signals strongly. But it serves it's primary purpose - to limit the amount of current you are drawing to make sure you don't accidently fry a valuable component that is not easily replaceable.

The procedure I followed was to do component replacement (ie: replace coupling caps) then do a slow power-up with the 800B plugged into the dim bulb tester, which was in turn plugged into the variac. Slowly apply power with the variac while watching the bulb. If you get the unit powered up to 115v successfully, then you shut the power off on the variac, wait a little while and then go on to the next component replacement. Be mindful of residual power in the electrolytic caps. I usually gave it a ten minute rest between power down and going back in, especially if working in the power cap sections.