Fisher 480a questions

Thocom

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This should be chocked full of dumb questions so please bear with me.. And sorry in advance.

I'm trying to get a 1963 console back up and running after letting my ADD get the best of me. I had powered up the 480A with a Variac, it ran fine for a while, then while looking at the tubes and noticing one brighter than the rest, I decided to touch it.. Bad move, it shorted out internally than actually cracked. I turned it off and unplugged it within a few seconds. I had also turned the bias knob about a MM prior to this (I was cleaning it with my finger to actually just see what it was). Add another variable: I had just sprayed deOxit into the socket a half hour prior to any of this.

I had another EL84 that I put into the socket the next day and turned it back on. Everthing appeared to power on correctly, but with no sound.. Nothing, not even a hum.
I had another thread on this that some fine folks gave me some advice, but decided to start a new thread on just troubleshooting it. I started replacing the caps, I did the black cardboard 100uF (C15) because I had one laying around, the rest of the caps I'm hoping to pick up at the antique radio show/swap meet/Flea Market that is happening this Friday and Saturday at Kutztown PA.

Does this sound like I may have fried the output transformer? (I would be really bummed if I have).
I tested the OT by just checking continuity between PIN 3 on the sockets, it's good, also tested the two silver diodes, power is going in the right direction. but can anyone tell me a good way to test to see if I freid her? Can I do this on a bench without speakers hooked up? Guessing I should pull the tubes to test trans? Better to use a variac at low V to do this? Do I need to pull the wires?
I guess what I'm looking for is a link to a step my step method..
I'm very new to tubes and Fisher but I really want to learn. Thanks in advance to anyone who can help me out!
Here's a link to the thread I started the other day on this:

http://www.audiokarma.org/forums/showthread.php?t=591480

-Tom
 
You need to check the voltages. I'd power it up with a variac and a "poor mans variac" (google it for plans, theory and explanation. Don't worry, it's not expensive or complicated, but should be in your toolbox for exactly this kind of thing.
 
TC -- Very doubtful that this event fried your OPTs - at least not BOTH of them for no sound at all. What is possible, is that some residual deOxit got on the base of the tube and caused it to crack under heat -- but that's just a guess of course.

To (first) determine if the problem lies in the power amplifier chassis, or the preamp chassis, trying sending a known good signal into the power amp chassis from a different source other than the companion preamp (but it still must remain connected). If you get sound, the problem is in the preamp, if not it is in the power amplifier chassis. Let us know, and then we can go from there.....

Dave
 
Thanks

TC -- Very doubtful that this event fried your OPTs - at least not BOTH of them for no sound at all. What is possible, is that some residual deOxit got on the base of the tube and caused it to crack under heat -- but that's just a guess of course.

To (first) determine if the problem lies in the power amplifier chassis, or the preamp chassis, trying sending a known good signal into the power amp chassis from a different source other than the companion preamp (but it still must remain connected). If you get sound, the problem is in the preamp, if not it is in the power amplifier chassis. Let us know, and then we can go from there.....

Dave

Cool, thanks Dave! It's the first thing I will do in the morning.. Before I leave for the Swap meet.

-Tom
 
Sounds more likely you lost a screen resistor or something else in the supply. Also possible the cathode resistor went open circuit if its common to both channels.

Maybe I'll run into you at Kutztown. I'll be the overweight guy in the tye-dye and the gingy goatee.
 
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Dave,
As I mentioned, I have four of those chassis. The first one I got cheap and fell in love. The next one also came cheap, only with great tubes (GE7189A). The next two were also cheap, too cheap to pass up. Point is I'm familiar with that chassis and it was the last amp I had on my personal stereo till you helped me repair my SA100. The other three chassis became "test beds" allowing me to try different components to gauge their effect against the first unmodified amp. By unmodified, I mean original caps. All of them were modified to work as Stand Alone amps.
A couple of thoughts come to mind.
1. My guess is the whole thing started with a cracked output tube. The same thing happened to me with a quad of Amperex tubes I purchased. The crack was clearly evident in the posted photo but it took a few on-off cycles for the crack to open up and let the atmosphere in. I saw it glow and go, the next day the silver getter became a white flake as designed.
2. "No Sound". There's a good chance the speaker selector plug became disengaged or possibly removed while he was cleaning the sockets. If it's still connected to the control unit, there's a cutoff circuit included in the turntable molex that shuts the thing down when the record's over. Many variables are unaddressed. It would be easier to diagnose if it were already converted to a Stand Alone amp.
3. The most disturbing variable is that he replaced the first capacitor in the voltage multiplier circuit. That's a problem I ran into on the last unit I worked on-I couldn't get the voltages right (compared to the unmodified unit). That first cap is covered in cardboard and mounted on an insulator because the can is energized. If he installed it incorrectly it could be a problem. Also-more than likely the new cardboard covered cap and the old multisection cap (that makes up the second half of the series capacitor circuit) are most likely not electrically equal. I doubt he knew to install balancing resistors across each capacitor to make sure they operated correctly.

So many variables-I wish he addressed them one at a time and checked the voltages against a schematic before he replaced that first cap.

Rich
 
Rich -- All excellent points. I too have had an original Fisher 7189 crack off at the base. It wasn't operating at the time, so no light show, but an instantly ruined tube none the less. I've also had it happen to a new manufacture Gold Lion EL84, so it must be a manufacturing defect rather than a time issue. Since it happened right after the OP used some deOxit, I was throwing that possibility in the ring as well. But in any event, such occurrences don't typically take out other components unless they are weak. It is entirely possible that the light show could have taken out a weak screen dropping resistor, as the light show itself was likely an event that involved the screen grid, and the power resistors Fisher used in those circuits do not have a good long term reputation over time.

I agree that a no sound of any kind scenario is often ultimately a speaker connection issue. Even just an operating output stage will usually cause some indication in a speaker when connected if listening carefully. The OP is lucky to have someone like you who has direct experience with these units to know where such a break could possibly occur. Schematics show connections, but not real world issues that develop with a given unit over time.

As for the V-doubler circuit, typically, incorrectly installed or defective components will either simply not let it work at all (both caps open), work to some degree but with much hum (one cap good, one open), or blow the fuse (either/both cap shorted, or installed backwards). Since the OP has not indicated a blown fuse in this process, the last scenario can generally be ruled out, and we have no sound at all to know if the second scenario is in play. The first scenario would of course cause a no sound condition.

Balancing resistors are only required when caps are wired in series to obtain a higher voltage rating in a stand alone configuration. The resistors act to equalize the DC voltage drop across the two caps. However, while the caps are connected in series in a doubler circuit, such resistors are not necessary to equalize the voltage, since the HV winding and diodes take care of that issue automatically in such circuits.

Also, using unequal value cap sections in a doubler configuration is not really a problem, as long as the smallest section has adequate storage capacity for the task at hand. A common example of this is all stereo Fisher receivers with SS power supplies. These units all employ V-doubler rectifier circuits, but use highly unequal caps in the V-doubler circuit, because the bottom circuit of the doubler configuration is also serving to supply the tuner sections as well as the audio section, where as the top circuit of the doubler is only serving the audio section. To have adequate capacity with the additional load of the tuner section, the bottom V-doubler cap is doubled in size over that of the top cap. No equalizing resistors are required, again because of the natural equalizing action of the doubler circuit itself. Even doubler circuits that do not draw any current from the doubler center tap will show only a slight difference in voltage between the two doubler sections (only a volt or two) if the doubler capacitance values used are quite different in value -- as long as the smaller of the two caps is of at least minimum storage capability for the load the doubler is supplying.

No sound issues are pretty easy to chase down, and I'm sure that with your added direct hands on experience, the OP will find it in no time!

Dave
 
Cap is still there

Dave,
As I mentioned, I have four of those chassis. The first one I got cheap and fell in love. The next one also came cheap, only with great tubes (GE7189A). The next two were also cheap, too cheap to pass up. Point is I'm familiar with that chassis and it was the last amp I had on my personal stereo till you helped me repair my SA100. The other three chassis became "test beds" allowing me to try different components to gauge their effect against the first unmodified amp. By unmodified, I mean original caps. All of them were modified to work as Stand Alone amps.
A couple of thoughts come to mind.
1. My guess is the whole thing started with a cracked output tube. The same thing happened to me with a quad of Amperex tubes I purchased. The crack was clearly evident in the posted photo but it took a few on-off cycles for the crack to open up and let the atmosphere in. I saw it glow and go, the next day the silver getter became a white flake as designed.
2. "No Sound". There's a good chance the speaker selector plug became disengaged or possibly removed while he was cleaning the sockets. If it's still connected to the control unit, there's a cutoff circuit included in the turntable molex that shuts the thing down when the record's over. Many variables are unaddressed. It would be easier to diagnose if it were already converted to a Stand Alone amp.
3. The most disturbing variable is that he replaced the first capacitor in the voltage multiplier circuit. That's a problem I ran into on the last unit I worked on-I couldn't get the voltages right (compared to the unmodified unit). That first cap is covered in cardboard and mounted on an insulator because the can is energized. If he installed it incorrectly it could be a problem. Also-more than likely the new cardboard covered cap and the old multisection cap (that makes up the second half of the series capacitor circuit) are most likely not electrically equal. I doubt he knew to install balancing resistors across each capacitor to make sure they operated correctly.

So many variables-I wish he addressed them one at a time and checked the voltages against a schematic before he replaced that first cap.

Rich

I can easily reinstall the old cap. I replaced it under the chassis and left the original mounted in place.
Oh yeah, and I'm standing right here...:)
 

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To be clear

I'm not sure if I was clear on what I posted regarding replacing the cap, that wasn't replaced until AFTER the meltdown so that can be removed from the equation.
You are right though, I probably should not have added any more variables to this issue by doing that and I plan to put it back the way I found it before powering up. (I haven't powered it up with the new cap yet).

Thanks again,
Tom
 
This is a dumb question to ask, but did you re-connect the RCA inputs after you had the chassis out to work on the cap?

Those sockets are loose and require retentioning. You should be able to test voltages without the tubes though-that's the first place I would start.
Whenever I had one tube fail or redplate, it occurred while I was listening and always occurred with a mismatched quad. The sound would distort and fade to zero, but never did that take out both channels.

The 480A and 481A chassis were identical except for two coupling caps. You can download a pdf of the schematic here:

There's a thread somewhere around here that suggests using an unused output tube socket pin as a tie point to install a resistor. Don't do it because the 7189 tube is internally wired differently than el84.

http://www.fisherconsoles.com/schematics/custom electra VII e49 schematic.jpg

http://www.fisherconsoles.com/

When I get a chance this weekend I'll shoot pictures and upload them with an explanation of how to convert the amp to Stand Alone if you're interested.

Rich

PS-Dave, thanks for the explanation about the caps in series. When I'm done with the SA300B I have a few thoughts about the 480A/481A that I wanted to run by you. Rich
 
Thanks

This is a dumb question to ask, but did you re-connect the RCA inputs after you had the chassis out to work on the cap?

Those sockets are loose and require retentioning. You should be able to test voltages without the tubes though-that's the first place I would start.
Whenever I had one tube fail or redplate, it occurred while I was listening and always occurred with a mismatched quad. The sound would distort and fade to zero, but never did that take out both channels.

The 480A and 481A chassis were identical except for two coupling caps. You can download a pdf of the schematic here:

There's a thread somewhere around here that suggests using an unused output tube socket pin as a tie point to install a resistor. Don't do it because the 7189 tube is internally wired differently than el84.

http://www.fisherconsoles.com/schematics/custom electra VII e49 schematic.jpg

http://www.fisherconsoles.com/

When I get a chance this weekend I'll shoot pictures and upload them with an explanation of how to convert the amp to Stand Alone if you're interested.

Rich

PS-Dave, thanks for the explanation about the caps in series. When I'm done with the SA300B I have a few thoughts about the 480A/481A that I wanted to run by you. Rich

No, I haven't put the chassis back in yet. I got the schematic, thanks. I am going to have some time tonight to get into it.


I've been at the antique radio swap meet today all day, and WOW there's a lot of stuff there...but NO fisher El84 tubes lol.. But they did have all the caps I needed.

They had boat loads of old fisher amps that people were going crazy over! By 11 AM people were walking out with arm loads of them.. There were still some mono blocks left.
 
Blue tye dye

Sounds more likely you lost a screen resistor or something else in the supply. Also possible the cathode resistor went open circuit if its common to both channels.

Maybe I'll run into you at Kutztown. I'll be the overweight guy in the tye-dye and the gingy goatee.

Was your tye dye blue? It was the only one I saw..
 
Ahh haa

This is a dumb question to ask, but did you re-connect the RCA inputs after you had the chassis out to work on the cap?

Those sockets are loose and require retentioning. You should be able to test voltages without the tubes though-that's the first place I would start.
Whenever I had one tube fail or redplate, it occurred while I was listening and always occurred with a mismatched quad. The sound would distort and fade to zero, but never did that take out both channels.

The 480A and 481A chassis were identical except for two coupling caps. You can download a pdf of the schematic here:

There's a thread somewhere around here that suggests using an unused output tube socket pin as a tie point to install a resistor. Don't do it because the 7189 tube is internally wired differently than el84.

http://www.fisherconsoles.com/schematics/custom electra VII e49 schematic.jpg

http://www.fisherconsoles.com/

When I get a chance this weekend I'll shoot pictures and upload them with an explanation of how to convert the amp to Stand Alone if you're interested.

Rich

PS-Dave, thanks for the explanation about the caps in series. When I'm done with the SA300B I have a few thoughts about the 480A/481A that I wanted to run by you. Rich


You may be on to something Rich.

Here is what I did:

I put the Amp back in, and tried it. This time I could hear a VERY low signal out of one speaker. (Super low, dog hearing low).
I unplugged the preamp RCA jacks and plugged a CD player directly into them. No volume control.. BOTH CHANNELS WORKED!! Albeit, they sounded bad. Lots of distortion, and crackling. I unplugged it immediately. But I am guessing this is a good sign? As far as the amp goes that is.
I had put in a magnavox El84 into the socket that had the Roman candle tube.. Is that ok? Do I need to match them? I have 2 Maggie's I can use..
 
TC -- that test was basically what I was suggesting you do back in post #3, so with that done, it would imply problems on both chassis, as if the power amp chassis was working up to snuff (assuming everything is connected correctly), then you should have had a good clear sound through the speakers. The fact that is wasn't implies problems, and the fact that you can't get anything at all through the preamp chassis implies problems with it as well -- although the problem could be one problem that is interrelated to both chassis. With that in mind, have you tested the 12AX7 tubes on the preamp chassis? If they are bad, the power amp chassis will not work properly.

Let us know!

Dave
 
Yep-tubes need to be matched. If you take note of the plate voltages, you can see why this is more of a 7189 amp than an el84 amp. Anyway, if the tubes are horribly mismatched, one tube will draw more current at the expense of the other.
The SA100 has individual bias controls for each channel as well as DC balance controls to match current drew between the sets within each channel. It's a nice feature and allows you to adjust for tubes that are slightly mismatched. I thought about adding that feature to the 481A, but didn't want to incur the added expense, especially if I'm not going to keep it. Splitting the bias control into left and right channels isn't too big of a deal though and not terribly expensive or time consuming if you're already under the hood. It's something to consider. If you do so, then you'll only need matched sets of two, not a matched set of four.
It might be worth your while if you're just getting into the hobby and want to learn new stuff. I think splitting the bias control into two channels will pay off in the long run.
 
Thanks again and again:)

I really appreciate all your advice guys. Yes, I'm new to the hobby, although I've been an audiophile for years and I'm actually IN radio.. My background is RF then moved on to Sat Radio where I've been now for 8 years..Which is funny, because I am just learning this stuff at 45 years old. And by the way... I'm hooked!! I have some good solid state stuff (Krell Showcase Amp and Pre, parasound 5 channel amp, with Yamaha A1000) but after hearing a good tube amp with a good turntable I was floored and now my man cave is filled with tube gear set up a small home lab and now waiting for me to learn more so I can work on it.
I've watched every youtube video of working safely (lol, don't wanna die).

Back to this fisher... Next up, I'm going to make sure all tubes are ok, and will post.

Thanks again!

-Tom
 
Quick question

I'm heading back to the radio swap meet. I'm going to pick up some EL84 tubes. I could not find any "Fisher" tubes yesterday.. Whats a good alternative? There's tons of new old stock there as well as new stock.

Thanks!
Tom
 
I like GE, Sylvania, RCA(EARLY RCA). Most of the Manufacturers used a certain brand of tube(say GE for example), and had them branded with their name on it. You would need to look for the EIA code Number on the tube.
 
7189 tubes are best suited for that amp-they're rated for higher voltages. GE-7189A tube is highly regarded, I have a few quads. Many other companies re-branded tubes manufactured by GE, but you can always tell a true GE tube because GE etched its logo and dots on the glass. Look at some pictures on ebay and you'll see what I mean.

For EL84/6BQ5 tubes (lower voltage rating than 7189 but will work)-Sylvania Black Plates seem to command a very high price on ebay. I have two quads that I haven't listened to yet. Before I try the Sylvania's in my SA100- I wanted to get familiar with my GE 7189A's (very even sounding, nothing seems to phase them) and then my Amperex 6BQ5's (very clear, brighter than my GE's and a really fun tube to listen to). I might try the Sylvania's later today before I sit down with a friend for a listening session (and scotch, of course).

In your amp's chassis the Amperex tubes are amazing! Blew my Aragon 2002 MKII out of the water. Much better than NOS RCA, Magnavox and Sovtek. My 12" 45RPM copy of Creedence's "Susie Q" put out by MOV sounds like Fogerty is in the room, no kidding, everybody says the same thing. I have three friends that want to buy that amp from me now (that's why I have 4 of them).

If you want to learn more about tube amps-pick up a copy of "Tube Amplifiers" by Morgan Jones. Really good, chock full of theory, easy to understand. He also put out a companion book "Building Tube Amplifiers" that has more to do with technique and layout and testing. Very good book-one of my favorites.
 
Picked up some tubes

I picked up some Mullard el84s for a decent price. New in box for $8 each.

I am guessing I need to straighten out the problem with this amp before modifying to a stand alone.. Time to start reading.

Going to hit Barnes and Noble tonight and look for the tube amp book your referring to, probably going to end up on Amazon though..

-Tom
 
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