Fisher X-100-B popped four tubes

[Santalopian]

Hey all,

Great forum, been reading up on here for a while.

Have a Fisher X-100-B that was professionally restored about 12 months ago and gets 3-5 hours use per day, about five days a week. I warm it up for between 10-60 minutes before use and let it cool down after for the same amount, but usually longer. I use the AUX setting mostly, and hook up my Fiio X5 to it.

So, the other dat I forgot to turn it off and a house guest saw a huge arc during the night. Tried turning it on, no sound, but the four 7868's power tubes light up, as do V5 & V6 (voltage amp/phase inverter) preamp tubes.

Swapped the four ECC83's (V1-V4), amp turned on, got some static from the aux cable and then a huge arc or surge lit up V1-V4, and the right channel cut out.

Any ideas or thoughts on where to start? Thanks in advance.

 

[larryderouin]

Have the BIG CAN CAPS on top been replaced???? One of more of them may have shorted causing the flash. UNPLUG IT, and open up the bottom, and look for any arcing artifacts around the power supply area. Take some pics of the underneath (one pic overall, and then quarter the unit and pics of each quarter). DO NOT PLUG IT IN Until instructed to.

 

[thomo57]

I'd start by taking it back to the professional that restored it.

 

[Santalopian]

Have the BIG CAN CAPS on top been replaced???? One of more of them may have shorted causing the flash. UNPLUG IT, and open up the bottom, and look for any arcing artifacts around the power supply area. Take some pics of the underneath (one pic overall, and then quarter the unit and pics of each quarter). DO NOT PLUG IT IN Until instructed to.

I instructed him to replace everything that needed to be. I assumed those caps were replaced but I'll get some pictures and upload today, thanks for the reply.

 

[Santalopian]

 

[notdigital]

I don't see any undermounted caps to replace the 3 can caps in the unit. Unless the cans were cut and stuffed from above, your can caps are original

 

[Santalopian]

Hmmm good to know, surprised they weren't replaced as I asked him specifically to replace any caps needed and the selenium rectifier diode. He mentioned there were some upgrades already.

Okay, so now that I'm going to be in here - what exactly should I replace. I don't mind spending a bit to increase the sound quality.

OT: I've modified guitar pedals before and done a fair bit of soldering but would rather not take anything on that's dangerous per se. Thanks

 

[notdigital]

You basically have 4 choices re the cans:

1) You can identify which section(s) of the can is bad and under-mount a replacement and leave the remaining sections to blow out in the future and freak out another friend;

2) replace all the sections of all the cans with undermounted caps;

3) purchase replacement cans, pull out the old ones and pop in the replacements;

4) gut the old cans and stuff them with replacements.

Everyone has his preference depending on cost, time, aesthetics, etc. I like to stuff. It's cheaper, I know what I'm using, and it preserves the original look. Mind you, you're not really "increasing" the sound quality but rather restoring the piece to perform reliably as designed. There are a few things that are done to increase reliability but that's for another day. BTW, you already have one: a thermistor.

As far as danger is concerned I can tell you this: When I started at this restoration stuff just a few years back, I thought a tube is what tooth paste came in. Still, I was confident I could figure out whatever with the help of the good folk here particularly since I've always had tools around and I figured I'd just consider this new adventure an opportunity to get into Zen And The Art Of Fisher Maintenance. With helpful guidance and a healthy appreciation for the fact that electricity can really, really hurt, I set about restoring an 800-C that I found in the garbage. And that effort included stuffing the cans! It took some time because I was learning and I pretty much read every thread on AK related to the task at hand. So what am I saying? Your unit appears to be 90% done with the only major detail remaining being the cans. Read up on the options above and that will help you decide on a preference. You'll be done before you know it.

 

[larryderouin]

I'm wondering if he didn't have an output tube or two arc over. No screen stab or cathode resistors that I can see. Grid returns are 330K.

Santalopian; This unit uses 7868 output tubes, right? Can you tell me what the value of the 4 yellow caps in the upper right hand corner of the 1st pic. Either .047 or (I'm hoping .1uf)
I agree with Notdigital. It's about 90% done. There are some safety and tube longevity upgrades that should really be done, and it's mostly about $3.00 in resistors for the one's I'm thinking about.

 

[Santalopian]

notdigial & larryderouin: thank you kindly for the detailed replies. It is very much appreciated.

I'll probably gut and stuff and cans. I want to maintain the original look and sound while increasing reliability. If I'm able to increase the sound quality for a few more dollars, fine with me.

Larry: I'll get those values to you. Thanks for the suggestions and drawing it out for me.

7868 tubes, correct.

 

[larryderouin]

V1 thru V4 are part of the bias circuit (heaters only). If one of your 7868's had a short screen to grid, or grid to cathode, it could very well have shot thru the bias circuit and flashed all of the small signal tubes heaters.(V1 thru V4.) So the whole bias circuit is suspect too. That's 1 of 2 things I can see that would cause all 4 small signal tubes to flash over at the same time. Pull one leg of each of the following resistors out of circuit and test with an ohm meter. R57 thru R60 (330K 1/4w.) R71 (560ohm 5W), R69 (3.3K 1/4W), R76 (820ohm 1/4W), R70 (BIAS POT 5K), R61,62,63,64 (1K 1/4w). Take V1 thru V4, plus V7 thru V10(output tubes) and have all 8 tested for shorts. Mark them with position as you pull them. The resistors usually have a 10% tolerance but anything over 5% replace.

On V1 thru V4, The Anodes are fed from 2 different sources. V1 & V2 it's cap C3 Section "C". On V3 & V4 it's cap C3 Section "B". It's also possible that you had a cascading failure of C3. If that's the case V1 thru V4 are most likely only good for .22long rifle plinking from a wood fence.

Do you have a schematic for the X-100-B. There are 2 versions of the manual and will be keyed to your serial #. Both versions are on this page at Fisherconsoles.com. Get the one that your serial # falls into. http://www.fisherconsoles.com/service%20owner%20manual%20list.html

Notdigital has upped his game. Now he thinks a tube is what Rogaine or Poli-Grip comes in..... (Just pingin on ya George!)

 

[Santalopian]

Will do as you recommend within the next few days and report back, thanks again.

The yellow caps are .047k, not 0.1 uf. Will replace them. Should I do the other two that are to the left of them as well, see pic.

Also, how do I tell I have a thermistor; notdigital pointed it out.

 

[Dneprrider]

"Also, how do I tell I have a thermistor; notdigital pointed it out"

It is the black disc component in the bottom right side of your main picture. I cropped it for display here.

 

[thornev]

The picture is too small to tell, but yes, it looks like a thermistor that slows down the ramp up of electricity when you first turn on the unit. Is there a number on it like CL-80? CL-something is usually what's on it and it should be connected to a wire that is one of the electric leads to the on/off switch.

 

[larryderouin]

Wait a minute. This is Cathode biased so disregard the change for the output coupling caps (4 in the corner). AND DISREGARD the 330K to 220K resistor change. Max value for the 7868 grid resistors in Cathode bias is 1 megohm. So no change needed there. Picture in post #9 EDITED to reflect this. I apologize for the bad gouge.

Thermistor is there. It's fine as is for location. You might want to replace it due to the arcing event probably shorted it inside. Quick way to check, involves disconnecting one lead, taking a reading cold using alligator or mini clips on your meter, then directing the blast from a hair dryer on hot temp, fast fan and watch for resistance change. If it changes it's good, if no change it's bad.

Like Notdigital said earlier it's about 90% done. Do ALL the CAN CAPS, have ALL the tubes checked for shorts (not whitey tighty's or boxer), replace any that are bad or marginal. Build yourself a DBT (Dim Bulb Tester), and borrow a variac if you can. Ask over on the tube forum and here if anyone active lives within 100 miles of you with a tube tester. If there is a music store close that does Guitar tube amp repairs they might test the tubes for you.

 

[Santalopian]

Wait a minute. This is Cathode biased so disregard the change for the output coupling caps (4 in the corner). AND DISREGARD the 330K to 220K resistor change. Max value for the 7868 grid resistors in Cathode bias is 1 megohm. So no change needed there. Picture in post #9 EDITED to reflect this. I apologize for the bad gouge.

Larry: question for you. You mentioned to disregard the 330k to 220k resistor change, but if I replace my 7868 tubes with new production EH, I actually should have to change to the 220k resistors, correct?

Otherwise, if I go with NOS, I can keep the 330k, or replace with new 330k, or at least that is my understanding after speaking with Jim McShane.

 

[gadget73]

Do V1 - V4 light? If they do, the output stage is conducting current. If not, either somehow all four output tubes have gone TU or there is some other problem causing them to not work. An open in any of those tubes will make none of them work and that will shut down the amp too. Also worth testing the tubes that came out. I had a heater-cathode short on a 12ax7 tube in my TA-600 that caused the output tubes to start running away. I caught it before anything bad happened, but if I had let it go it probably would have fried something.

I'd personally want to know exactly what the current state is before replacing tubes or other parts. Also worth asking the person that rebuilt it if the can cap was restuffed.

 

[Santalopian]

Took it to a tech to look it over and he said that it's one of the output tubes that's causing the arc and that I should replace all four (obviously). Didn't get the exact reason why but he claimed it worked when he tested it but it will keep arc'ing if I use it a lot before rolling the tubes.

 

[larryderouin]

Santalopian. The Tube Data Sheet for 7868's is essentially the same as 7591's in that they are for all intents and purposes the same tube in different packages. Think of a FORD Crown Vic and a Mercury Marquis/Marauder. Same car different badge.

Anyway. With CATHODE BIAS the #1 grid maximum Circuit Resistance is 1.0 megohm. (Sylvania lists this as "SELF BIAS")
With FIXED BIAS the #1 Grid Maximum Circuit Resistance is 0.3megohm (300K ohms). This is one of the reasons why on units with Fixed bias the Grid return resistors must be changed to a level that will both keep the tube happy and your ears happy at the same time. CATHODE BIAS UNITS NEED NO CHANGE to the Grid Return resistors or coupling caps!

Install or have the tech install a Screen Resistor Network (100ohm resistors) on Each tube. This will help greatly with regard to arcing. Show this article to your Tech. http://www.tronola.com/html/maximize_tube_life.html
For a Visual on the Network, See the COMMON PARTS FOR FISHERS Thread in the FISHER STICKIES. POST #40 on page 2. http://audiokarma.org/forums/index.php?threads/most-common-parts-needed-for-fishers.327561/page-2#post-6480170 You might want to read the whole thread tho.

 

[Santalopian]

Do V1 - V4 light? If they do, the output stage is conducting current. If not, either somehow all four output tubes have gone TU or there is some other problem causing them to not work. An open in any of those tubes will make none of them work and that will shut down the amp too. Also worth testing the tubes that came out. I had a heater-cathode short on a 12ax7 tube in my TA-600 that caused the output tubes to start running away. I caught it before anything bad happened, but if I had let it go it probably would have fried something.

I'd personally want to know exactly what the current state is before replacing tubes or other parts. Also worth asking the person that rebuilt it if the can cap was restuffed.

I'll check soonish if they still light. I doubt the can caps were restuffed, my tech didn't do it and said they seemed original.

Santalopian. The Tube Data Sheet for 7868's is essentially the same as 7591's in that they are for all intents and purposes the same tube in different packages. Think of a FORD Crown Vic and a Mercury Marquis/Marauder. Same car different badge.

Anyway. With CATHODE BIAS the #1 grid maximum Circuit Resistance is 1.0 megohm. (Sylvania lists this as "SELF BIAS")
With FIXED BIAS the #1 Grid Maximum Circuit Resistance is 0.3megohm (300K ohms). This is one of the reasons why on units with Fixed bias the Grid return resistors must be changed to a level that will both keep the tube happy and your ears happy at the same time. CATHODE BIAS UNITS NEED NO CHANGE to the Grid Return resistors or coupling caps!

Install or have the tech install a Screen Resistor Network (100ohm resistors) on Each tube. This will help greatly with regard to arcing. Show this article to your Tech. http://www.tronola.com/html/maximize_tube_life.html
For a Visual on the Network, See the COMMON PARTS FOR FISHERS Thread in the FISHER STICKIES. POST #40 on page 2. http://audiokarma.org/forums/index.php?threads/most-common-parts-needed-for-fishers.327561/page-2#post-6480170 You might want to read the whole thread tho.

Gotcha - will check the tube data sheet(s).

Interesting that these cathode bias units need no change to the grid return resistors or coupling caps.

Jim McShane recommended to change the grid resistors to 200/220k when going with new tubes and also replacing the .047 coupling caps with 0.1 uf Nichicon polypropylene capacitors to avoid loss of deep bass.

Does it hurt to change them regardless?

I'll take a look at the articles you posted especially the "visual of the network" as I really need to know what has been replaced and what needs to be still.

It seems like I have Orange Caps and IC MPW caps (output coupling caps I believe) coming off the preamp tubes but I don't know what it originally looked like; these would likely have been replaced correct?

Thanks again gents