fisher 800c tube problems

have bought a recapped fisher 800c and this has happened twice.... I was using JJ tubes in my fisher and one of the tubes red platted and I turned it off and turned it back on and then it stopped doing that... Unfort when it red platted it caused that tube to go bad.... I've heard many people having problems with jj tubes so I switched to electroharmonix. I have been using them for 2-3 months now and they sound better then the JJ tubes... Unfort the samething happened. I was listening to music for about 4 hours straight using my phono and then all of suddenly I heard static comming through like a radiostation sound almost and I looked at my reciever and I saw that tube sparking inside it.... What is going on? How do I fix this? I think that tube socket always seems alittle brighter to me compared to the other tubes too. This problem is always happening in the same socket.. I touched both speaker output transformers and 1 was more hot compared to the other one.

this is what the tech already did to the fisher before I bought it

New SDS Power Supply Capacitor Board Installed
New Silicon Rectrifiers New Orange Drop Type Coupling Capacitors
All Switches & Controls cleaned (DeOxit)

Just because a tech did the work, doesn't mean it's been done correctly, or Well.

Get the pictures of the undersides, concentrating on the area around the tube that's redplating, the socket itself from the top. Is insertion of a tube into this socket easier than the others???? If so, tighten up on the socket pins. Other possibilities are a cold solder joint for that one tube, wrong value parts.


We've all done it at one time or another.

Get the pictures of underneath and we'll take a look and see what we can see about getting you back to listening.

Larry

Thanks I never repaired a reciever on my own before... I usually bring it to a tech but its so expensive... I really want to learn.. If I open it up is anyone willing to help me step by step on what to do?

Yep.
Take out the EH output tubes. Mark what sockets they were in just in case the one that's given the light show is bad.
Take the unit out of the cabinet (if it has one). Flip it over.
Unscrew the four side bolts on the chassis bottom and loosen the two in back. Take pictures of the area near the output tube sockets.

Post the pictures with questions you have.

ta for the steps x THe reciever is pretty heavy is it safe to turn it upside down with all the weight on the face plate and transformers?

Its the outer edge one. (for top view)

higher resolution

We need to see the underside. What we need to see really is the left rear quarter under the chassis. That'll show us the adjoining circuitry and coupling capacitors.
Red plating often happens because of faulty coupling capacitors.

Alright

d 2

http://togermano.com/temp/fisher/ bigger pics here

The output coupling caps are those four nut brown pieces in the middle of the picture showing the full rear quarter.
This looks like a comprehensive update.
I might respectfully suggest that you verify that those output coupling caps have a value check. Those cost about a quarter each and are not a good choice sonically, though they should be passing AC properly and blocking DC. Not being able to block DC is typically the kind of problem you'll find.

A couple suggestions:
1. Check the value of those coupling caps. They should be .047uf 600 volt or two .047 400V and two .047 600V.
2. Using the search tool on the upper right while you're in the top level of the fisher forum, search on the word fusistor and read all the posts.

audiodon said:

The output coupling caps are those four nut brown pieces in the middle of the picture showing the full rear quarter.
This looks like a comprehensive update.
I might respectfully suggest that you verify that those output coupling caps have a value check. Those cost about a quarter each and are not a good choice sonically, though they should be passing AC properly and blocking DC. Not being able to block DC is typically the kind of problem you'll find.

A couple suggestions:
1. Check the value of those coupling caps. They should be .047uf 600 volt or two .047 400V and two .047 600V.
2. Using the search tool on the upper right while you're in the top level of the fisher forum, search on the word fusistor and read all the posts.

Click to expand...

those caps are suspectes...
just because they are new that don't mena they are still good after a short period....
replace them....

I have a cheap meter how do I go about testing it?

are you guys talking about the ones with the blue wire?

Where do I buy such caps as well?

To keep the EH's &(and the ocassional JJ) with the 800C, the coupling cap value needs to come up to .1uf, and the grid resistors, down to 200K vs. 330K. If the OP was using OS tubes I'd agree on the .047uf, but he's using EH7591's and JJ's at one point. If he's going to continue to use EH'7591's he has to take the Grid resistor value DOWN to 220K at least. There is a capacitor/resistor timing balance I don't understand, needless to say, it's been explained that as one value goes down, the other has to go up. In this case the grid resistors drop from 330K to 200K, and the capacitors go up to .1uf / 600V across the board.

This is what may have arced that 7591 and killed the JJ's. The original VALUE's for the grid resistors/coupling caps. Note that this also helps the original tubes longevity.

I'd be leery of that outboard socket. It's showing signs of heat stress, and microscopic cracks can be used as cross paths.


Here's what i'd do to make this one stable.
1.) replace the 4 coupling caps with .1uf/630V caps (OP Choice of cap)
2.) replace 300k grid resistors with 200K R 1/2Watt.
3.) install 10oHm 1/4W resistors to ground @ pin 5 on each output tube.
4.) install 100oHm 1/4W screen stabilization resistors to jumper @ pin 4&8 on each tube.
5.) install 10K variable resistor (pot) in BIAS POWER lead to facillitate some adjustment in the Bias circuit. IBAM should follow soon.

Also go over the whole BIAS Circuit to make sure whomever did whatever mods to this unit, didn't gaff it.


Tonyg88. Put atleast a county and State in your profile so we know where your are. You may get lucky and find a FISHER guy close to your location.

Larry

Yes. The Orange colored caps with The Blue lead covering, should be replaced with a different value.

Your choice. Parts express, Mouser, Digikey, Newark, or any number of Film capacitor resellers. Stay away from Ebay when ordering caps. I buy only from reputable Brick and Mortar stores, that have a high turnover in that I can get pretty fresh caps.

You will also be changing the 4 resistors with the ORANGE/ORANGE/YELLOW/Gold Stripe. Those are 330,000 oHm resistors with a variance of 10%. You want to replace them with 200,000 oHm resistors with a vairance of at least 10%. 5% is better. (Ratshack has them in 1/4 and 1/2Watt.). 1/4watt is fine.

http://myfreewares.weebly.com/resistor-colour-code-solver.html One of the best Resistor Color Code Solver's out there. You can also use it for the old flat mica capacitors also. Download this as you'll need it.

Larry

Than you. Can you guys give me links or brand names to buy from that are good quallity? I wanna buy the best quallity I can as if I buy a crappy quallity one and I dont like it i'll spend 2x as much... You say 1/4 watt is fine... DOes that mean 1/2 watt is better?

There is also other orange caps in the front of the reciever..... pic here

http://togermano.com/temp/fisher/20120406_142403.jpg