Amp Help Please

[lpf3]

I have a Fender '65 Twin Re-issue that's 7 or 8 years old- When I flip the switches it doesn't turn on.

This came about suddenly; I was playing it with no problems & turned it off, came back about an hour later & now the red light doesn't come on - nothin'.

I checked the outlet- it's fine & the fuse appears intact. The cord & wires, as far as I can see (and feel) seem secure.

Not knowing anything about amps, what should I be looking for?

Thanks,

-lpf3

[sumgai]

lpf3,

First, fuses can look good, and still be bad. A multimeter is your only sure way to know what's what.

Second, is the pilot lamp burned out? You didn't say if you gave the amp time to warm up and do it's thing regardless of the pilot lamp's status.....

Third, the power switches in these amps can go bad, they aren't perfect either. Open it up, make sure the power-supply filter caps are discharged, then take a multimeter reading (Ohms) across the switch. (Cord unplugged, of course.) Wiggle the switch to make sure that the problem isn't mechanical.

Fourth and final (for me) potential problem will be one or more bad solder joints. Yes, they can work for years, and then give up the ghost overnight - I've had it happen to me, on my own gear. I'm sure you know what to do in this case, but if you're not sure, and the first three tests don't pan out, then get back to us here in the NutzHouse.

HTH



sumgai

[lpf3]

Thanks SG-

I've checked the fuse with the meter set to Ohms & the leads on either end of the fuse. For all values it reads 0- this shows continuity, yes?

I went out lookin' for a suitable fuse but it looks like I'm gonna have to get one from another city if it turns out I need one.

I haven't had a chance to open it up yet , I was going to rule out the easiest thing first.........


-lpf3

[sumgai]

lpf3,

Yes, 0Ω indicates a good fuse.

I take it that you've not checked the pilot light.... And I am being a bit presumptuous here, but have you looked at the back of your amp after perhaps 30 seconds, to see if any of the tubes are glowing? The pilot lamp is in parallel, so if it goes out, the tubes won't be affected.




sumgai

[lpf3]

have you looked at the back of your amp after perhaps 30 seconds, to see if any of the tubes are glowing? The pilot lamp is in parallel, so if it goes out, the tubes won't be affected.

That I did do, (first) hoping it was just the light, but that ain't it- still nothin'.

That rules out the easy stuff I'll get to opening it up in the next couple of days & get back to you.......


-lpf3

[lpf3]

So, moving right along-

I opened up the amp, everything appears to be secure in that nothing is loose, no wires hanging in the breeze & all solder joints appear to be intact.

I checked the switches with my meter set to Ohms- in every case it read OL. with the switch down & zero with the switch up; again I think this indicates continuity & my switches are probably good.

Next?

Thanks,

-lpf3


EDIT: I thought maybe the plug was bad but after jiggling the wires with the amp plugged in I am quite sure that I'm gettin' power to the switch.

BzzzzzzzzzT

[sumgai]

lpf3,

There's one more component to check first, before we suspect the transformer, or anything beyond that.

If you'll look at the schematic, which is found in the back of the Service Manual, see here:

www.schematicheaven.com/fenderamps/65_twin_reverb_%20manual.pdf

..... you'll note that there are not two but three doodads that control the AC between the power cord and the transformer. The switch and fuse are on the Black lead, and on the White lead you'll find a device we call a Thermistor. Its function is to sense heat, and change resistance accordingly. This is an attempt to prevent the amp from getting too hot (and burning up expensive components) - if the Thermistor gets too hot, it reduces the incoming voltage until the heat levels off at what Fender considers a safe point. But.... but if it fails for whatever reason, your whole amp becomes a door stop.

Don't bother with a resistance reading here, when it's not in use, the "amp off" value can be all over the map, depending on how it was made. Instead, simply measure the transformer's Black and White leads - if you get nothing, then this component is highly suspect.

The way to make sure, after the above test, is to simply short it - hook up a jumper wire across it, and re-test for AC at the Black/White wire pair. If the amp now works, then you know what to replace, doncha? And it isn't recommended that you bypass this thing forever, it's meant to keep your amp from burning up expensive components, should something go "just bad enough", but not so bad as to blow the fuse.

Now, once that's been replaced, turn your amp on and leave it at very low volume. Test a guitar through it. Still working? Good. Turn it up a bit, and re-test. Keep doing that until you get to ear-bleed territory. But hear me on this - if at any time the amp goes out to lunch, then the same Thermistor has probably blown again, and it's time to take the thing to a qualified service tech and have him/her figure out why it's blowing.

Punt! Your turn! ;D

HTH



sumgai

[sumgai]

lpf,

OK, you're correct, a reading of 0.00 shows continuity. Albeit, if this is through the transformer primary, I'd think there should be something over 0.00, at least a an ohm or two. But for now, let it ride......

Your transformer primary looks good for now, so set your meter on AC, and start checking the outputs (secondary windings). You should find about 6.3 volts or so on CP13 and CP14. If not, stop right there, you've got a problem right at the transformer. It may be nothing more than a solder joint, so touch 'em up with a hot iron, then re-test.

If there is 6.3vAC at those test points, then turn off the amp, and set your meter to resistance. Let the amp discharge it's caps (in the high voltage section), then we'll do a quick check on the filament wiring. Note on the schematic that there is a "Hum Balance" pot for the heaters. The wiper of that pot goes to ground. Hence, a fast way to see if the filaments are connected is to simply take a reading from any tube's filament pin to ground - if it's open (OL or OR on the meter), then you've got an open circuit..... each side of the filament string should read 40 to 70Ω to ground, meaning the pot is somewhere near the middle of its rotation. If those are both good, then check between the pot's terminals and the aforementioned CP13 and CP14. Somewhere in there you'll find your culprit. And until the filament voltage is restored, there's no point in going any further - high voltage on a cold tube gives you the same thing as no voltage at all - bupkis.

HTH




sumgai

edit: In checking to make sure my post posted, I no longer see lpf3's latest message..... lpf, did you accidentally (or on purpose) delete that thing? Staff, anyone?

further edit: No one knows what happened to the missing post. Hmmmm, perhaps Mr. Murphy came a calling..... Oh well, we'll just carry on.

[lpf3]

Gai-

lpf, did you accidentally (or on purpose) delete that thing? Staff, anyone?

Weren't me. You can call home and ask my wife! I did edit it but that was early on, awhile before you were online.

No matter- At least you saw it & I remember what I came up with - your latest set of instructions oughta occupy me for awhile.

Thanks-



-lpf3

[ijustwannastrat]

I hate to hijack, but lets hear about this amp? Do you just play it clean? Or do you put pedals ahead for some grit? Both?

I hope you can fix your amp.

[chuck]

the amp should sound sweet on its own when cranked ... all that overdriven tube compression ... yummy indeed

[sumgai]

Jul 8, 2010 23:20:30 GMT -5 lpf3 said:

...... You can call home and ask my wife!

So, are you coming out of the closet as a Redneck, Uneasy Rider?

[jcgss77]

Ok, I know that I am not quite an amp tech, but I don't remember anyone saying if the cord was tested for continuity. Did you check that, lpf3?

[lpf3]

Ok, I know that I am not quite an amp tech, but I don't remember anyone saying if the cord was tested for continuity. Did you check that, lpf3?

[a href=""][/a]


Yeah, double checked it. thanks.

;D ;D

-lpf3



[a href=""][/a]

[lpf3]

july 9, 2010, 9:05 pm, sumgaiwrote:

So, are you coming out of the closet as a Redneck, Uneasy Rider?

;D I figured that was too easy. Besides, what part of Telecaster and Twin Reverb gave it away? ;D

The truth is, due to the cynical nature of some of the musicians I've been hangin' with these days, I'm not prepared to make any such admissions. I haven't looked into the Legal aspects of living a Fiction-al life which is probably good since all I know about legal types is what I see on T.V. You know- Perry Mason, Matlock, Jaws....

So for now I am forced to scoot my boots in private. But, never one to dissapoint; Here are some cowboy lookin' redneck types who aren't quite as conservative as I .

(Gotta love Willie.........)


;D -lpf3

[sumgai]

lpf,

Don't you EVER do that to me again - ever!

And you owe me for a new monitor, and a mouthful's worth of milk and ChexMix cereal, all of which just suffered a not-too-pleasant collision.....

Smell the bull, indeed!






sumgai

[lpf3]

Mr. Sumgai, sir - you're the man.

I'm getting 6.5 V between CP13 & CP14. There is a fuse between the white and black wires that go from CP13 & CP14 to the heater; & this fuse reads OL.

A short jumper across this fuse & paydirt! The light came right on and the tubes they are a-glowin'. Looks like I'll have a Reverb amp in time for the Cowabunga Jam.

If we ever meet up in real life the beers are on newey me.

Thanks, man

-lpf3

[lpf3]

;D ;D ;D

Guess our posts crossed in the mail....... Glad ya liked it. If we do ever meet, if ya want, I'll buy ya some milk & cereal instead of those beers.



-lpf3

[sumgai]

lp,

You know, I like to carp about my old age and how I forget things all the time now, but I couldn't believe it about a fuse in the heater string. So I went back and looked.......... sure enough, the schematic doesn't show one, not at all! Nor does the service diagram show one - again, no fuse visible to the naked eye......

I would've had to have the unit on my bench to see that one coming. Sorry 'bout that, fellow Nut!

Now to figure out why that fuse blew. Perhaps it was a one-time thing, possibly heat. Perhaps it was a weak fuse to begin with. Or a tube (which one?) could have an intermittent short in its filament, or....... I even saw, once, a tube who's filament would open up when hot, and close back together when it cooled down. That made the tube go on/off/on/off/on/off..... which of course caused the main transformer to supply more amperage and usual, and that evenually drove the main fuse to blow. I spotted that one visually, but only after staring the tubes in the face for several minutes. Pure luck, finding and figuring out that one. But then again, the check was the same at the end of the job.

If this doesn't happen again, then I'd say keep on truckin'. If it does happen in a week, or even a month, then try swapping out the power tubes, they're the most likely to cause this kind of thing, because they draw beaucoup more current than the pre-amp tubes. Also, you might consider taking a very close look along the heater string wiring for any potential shorts to unwanted contacts. As noted, most of the time a simple visual inspection solves the problem.

HTH



sumgai

[lpf3]

Ok, I've found a place to get the correct fuses locally- I oughta be up & runnin' in the next few days.

I would've had to have the unit on my bench to see that one coming. Sorry 'bout that, fellow Nut!

Aw, not even. I'm stoked about how much I learned on this project & without your help I wouldn't have known where to start. ( I won't waste another karma point on you, you'll have to settle for Thanks ;D) And like you said that fuse wasn't on the schematic anyway......

Now to figure out why that fuse blew. Perhaps it was a one-time thing, possibly heat. Perhaps it was a weak fuse to begin with. Or a tube (which one?) could have an intermittent short in its filament,

I played the amp for several minutes with the fuse bypassed just to check it out (OK, and to be loud, too. ) and I did notice that one tube wasn't glowing quite as brightly as the others. Not much of a difference, but noticeable. Also- I don't know if this means anything- after a few minutes the tubes were extremely hot. Now, I've never paid attention to how hot the tubes get before so I have nothing to compare it to, but they were extremely hot to the touch.

After I get the new fuse I'll check it out a little more & take it from there.......


-lpf3

[sumgai]

lp,

Find another Twin, and compare the heat from your power tubes against that other amp - that's the best solution I can offer on the subject of heat. There might be some small variation, but in general they all get up to about the same temp.... or at least that's what they're supposed to do.

If a single tube is overheating, you'll usually notice that it's glowing like orange, as well as brighter. If it's the whole set of 'em, then you either rely on memory (of better times), or compare them to something else that is known to be working correctly.

And if you're lucky, and have that second Twin in your grubby paws (and not at the local Guitar Center), you can just swap the two sets of tubes, after letting them cool down, of course. This let's you determine for certain that it's either the set of tubes, or else it's the amp itself that's causing the issue.

HTH



sumga

[lpf3]

OK.......

I put the new fuse in & fired 'er up. All systems go! I left it on for a couple of hours, with no known issues. I made a closer visual check of the wiring & connections, etc. It all seems good.

Unfortunately I don't have another tube amp to compare it to so going to Guitar Center is exactly what I 'll have to do, & hope to remember how hot the tubes seem. I did find that my tubes didn't change in temperature or brightness in all that time, so I'm not too worried about it right now.

I think we dunnit, Sensei - anything else I should check out as long as it's apart?


-lpf3

[chuck]

do your power tubes glow a pretty blue or a bright orange ?

[cynical1]

So, are you coming out of the closet as a Redneck, Uneasy Rider?

The truth is, due to the cynical nature of some of the musicians I've been hangin' with these days, I'm not prepared to make any such admissions. I haven't looked into the Legal aspects of living a Fiction-al life which is probably good since all I know about legal types is what I see on T.V. You know- Perry Mason, Matlock, Jaws....

This wouldn't be a thinly veiled reference in the making, would it? That's OK, as long as you keep getting your shots and continue your therapy I'm fine...

HTC1

[lpf3]

This wouldn't be a thinly veiled reference in the making, would it? That's OK, as long as you keep getting your shots and continue your therapy I'm fine...

Huh? More shots!?!?

-lpf3

[sumgai]

Jul 15, 2010 19:17:34 GMT -5 lpf3 said:

This wouldn't be a thinly veiled reference in the making, would it? That's OK, as long as you keep getting your shots and continue your therapy I'm fine...

Huh? More shots!?!?

He meant as in, from your friendly, local, neighborhood therapist. You know, the guy down at the corner Spiritual Emporium who can slide a shot across the bar to you....