Fender Solid State Amp Primary Fuse or more??

Stampede

Member
Messages
13
Hi All, I have an old Fender Stage Lead 212 that I got in a trade. I've never seen it working, but I want to try and get it back to life. The first thing I've noticed is the primary fuse is blown, I'm afraid this isn't a good sign. I'm wondering if there could be something deeper wrong with it that could be manifesting as the blown fuse. The fuse that was in the amp was a 3A 250V though a 3A 125V is recommended on the back.

So now I'm ordering some new fuses, in the mean time is there anything you guys think I should look into?

The transformer looks pretty grungy. Here's a link to a picture of it, it has what looks like liquid dripping and running down the side, but it's hardened. Maybe rosin or some part of it's construction over heated?

https://www.dropbox.com/s/1b083rys53dc32i/transformer.jpg?dl=0

Think it's worth replacing from the start? I don't know how easy it will to find a new one.

Thanks!
 

Kyle B

Member
Messages
5,074
IMO unless you fix it easily, not worth messing with. I see those Fender Stage Lead's selling for 'not much' all the time.

The stuff you see is probably varnish leftover from manufacturing....they dip the transformer and let it drip dry.

The transformer can be tested by opening the chassis, unplugging everything on the secondary (output) of the transformer, replace the fuse, turn the power on. If the fuse blows immediately, probably bad x-former. If it doesn't, then you start probing for AC voltages. Possible the transformer wires are soldered...that makes 'unplugging' it alot harder....

It's much more likely you have a blown power transistor or something else than a bad transformer.
 

VaughnC

Silver Supporting Member
Messages
17,852
Could be something as simple as a shorted rectifier diode or voltage regulator, or one of a dozen other possibilities. You'd have to eliminate the possibilities one at a time with logical testing rather than just replacing parts that might look bad. Sounds like you may be in over your head and it would be difficult to try and explain, via a forum medium like this, how to proceed. A tech could likely fix it in less than an hour if you think its worth putting money into. While it might be bad, power transformers tend to be fairly robust so that wouldn't be my first suspect. Good luck.
 
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Stampede

Member
Messages
13
Thanks for the feedback!
That's the consensus I'm getting, the transformer is probably fine. Doing a visual inspection of things yesterday it just stood out as probably the most funky looking transformer I've seen. And by funky I mean pure stank, drippy, gooey mess. lol, it looked like one of those, you know it probably wouldn't hurt to replace this, types of parts. But I'll leave it in there for now.

I order a whole bunch of primary fuses, and I'm going to give a go. Not looking to invest a fortune but it's a fun project to mess with. I've worked with some low-voltage electronics, and know some about electronics theory but guitar amp servicing is new to me. I'll whip out the multi-meter and dig in once those fuses come in. I appreciate the troubleshooting recommendation Kyle, and I will look for the most likely offenders shorted rectifier, voltage regulator, capacitors, etc. Thanks Vaughn.
 

J M Fahey

Member
Messages
2,437
You won't repair it by substituting a fresh fuse, there is some problem (usually blown/shorted power transistors) which killed the original one, and unless repaired will blow the new ones too.

That amp can be repaired, of course, but takes some skills .

Can you read a schematic, find and recognize parts on a PCB, successfully replace them and read and report voltage and resistance readings?

If so, we can help you; if not get a Tech.

FWIW it's a LOUD beefy amplifier, killer warm clean sound, and use your favorite pedal for dirt.
 

darkfenriz

Member
Messages
207
You won't repair it by substituting a fresh fuse, there is some problem (usually blown/shorted power transistors) which killed the original one, and unless repaired will blow the new ones too.

That amp can be repaired, of course, but takes some skills .

Can you read a schematic, find and recognize parts on a PCB, successfully replace them and read and report voltage and resistance readings?

If so, we can help you; if not get a Tech.

FWIW it's a LOUD beefy amplifier, killer warm clean sound, and use your favorite pedal for dirt.
In your view and experience - what makes it sound so good especially on clean?
Much of the electronics and components is common with roc-pro, performer and frontman series, which sound just as good IMO.
 

Stampede

Member
Messages
13
Hey darkfenriz, czesc, I read that a lot about this amp. I'm not sure how it compares parts wise to the amps you mentioned, but it seems to be what everyone's saying that this amp has some a great clean sound.

Hey Fahey, yea I can read schematics, find parts and do testing. That's not really a problem, I definitely appreciate your guys help in tracking down the gremlin hiding in my amp. Im going to take Kyle's suggestion and test the transformer power output, then when the fuses come in throw one in and just see what happens.

I think there is some non - fender wiring going on inside also lol, looks like she's had someone else tinkering inside. the power switch is splice in and wrapped in electrical tape. I'm going to get rid of that, it's at least tinned nicely but I rather get a direct connection (new wire) or at least heat shrink it. I'll post pictures later.
 

Stampede

Member
Messages
13
As promised, the overhead shot.



In the bottom right there is the squirrel-y action of spliced wire, and I forgot about the extra cap near the top right of the PCB. I didn't see that in the few schematics I have but it looks at least properly installed. It looks like the cap attaches to the "off" position of the groundswitch.
 

Stampede

Member
Messages
13
Good call, Usable Thought, I forgot all about light bulb limiters. Now I remember reading about them ages ago, I think I have all the necessary parts in my garage and basement anyway so I'll put one together. Great debug tool, Thanks!
 

J M Fahey

Member
Messages
2,437
In your view and experience - what makes it sound so good especially on clean?
Much of the electronics and components is common with roc-pro, performer and frontman series, which sound just as good IMO.
Yes, most late 15 or 20 years Fender amps have basically the same circuit, each designer has his own pet ideas and tends to use them all over the place; fine with me.
And yes, most of those other Fender amps also have quite good cleans, I never dissed them; the main difference I find is the speakers fitted.
Most lower price Fender use the same Eminence speaker, which is not bad considering the very very low OEM price, recognizable by the small very thin magnet (only 10mm, while standard is at least 14 or 15mm) which you find in "something 112", Performer, Roc Pro, M80, and a ton others, while Stage ones I saw use a quite heavier magnet one, and a heavier built voice coil, that alone makes them smoother, warmer and less fizzy.
The generic low cost Eminence speaker I mention is still being made by the truckload, of course, but is not usually available; it was once under the name Blue Tick Hound but was pulled afterwards.
The speaker makes a BIG difference in sound.

@ stampede:
thanks for the picture and you did a smart thing by posting it: it's not the Fender Stage we were all talking about but a much earlier one, probably early 80's and looks like the Made in Japan version (please check the back plate) : steel chassis (vs the modern aluminum one), transistors outside the main PCB, look like TO3 metallic case instead of modern square plastic ones, they should be mounted on actual finned heatsinks under the chassis instead of current alumin um solid bar bolted to chassis, etc.
Those old ones also had a smoother sound, and not the same generic circuit as those mentioned earlier.

Save some time and write Fender asking for the proper schematic, give them serial number and whatever is written in the back plate, it will probably be a scanned one rather than a real PDF, and post or link it here for suggestions.
Also build the lamp bulb limiter http://www.ssguitar.com/index.php?topic=2093.0
 

pdf64

Member
Messages
7,379
It looks as if the mains cable may only be 2 core, ungrounded?
Hopefully I'm wrong but if not, that's bad.
Equipment such as this require a safety ground.
Especially given the 'squirrel-y action of spliced wire' (mains cable?) and possibility of a PT fault (which, without a safety ground, may pull the chassis live).
 

darkfenriz

Member
Messages
207
Yes, most late 15 or 20 years Fender amps have basically the same circuit, each designer has his own pet ideas and tends to use them all over the place; fine with me.
And yes, most of those other Fender amps also have quite good cleans, I never dissed them; the main difference I find is the speakers fitted.
Most lower price Fender use the same Eminence speaker, which is not bad considering the very very low OEM price, recognizable by the small very thin magnet (only 10mm, while standard is at least 14 or 15mm) which you find in "something 112", Performer, Roc Pro, M80, and a ton others, while Stage ones I saw use a quite heavier magnet one, and a heavier built voice coil, that alone makes them smoother, warmer and less fizzy.
The generic low cost Eminence speaker I mention is still being made by the truckload, of course, but is not usually available; it was once under the name Blue Tick Hound but was pulled afterwards.
The speaker makes a BIG difference in sound.
Thanks for sharing that. Interesting indeed. I agree the cheapest 12" Fender speaker is not that bad, possibly on par with cheapest Celestions and Peavey blue marvel (???) and the likes.

Another thing I always suspected is the strange op-amp based input stage they all share - it is not any sort of textbook circuit afaik, I think it is a form of bootstrapping and what strikes me is the extremely high input impedance it shows to the guitar input.
 

darkfenriz

Member
Messages
207
Stampede - czesc. Good luck with bringing it back to life. If you can provide more detailed photos we may be able to stop the failure.
 

J M Fahey

Member
Messages
2,437
Thanks for sharing that. Interesting indeed. I agree the cheapest 12" Fender speaker is not that bad, possibly on par with cheapest Celestions and Peavey blue marvel (???) and the likes.

Another thing I always suspected is the strange op-amp based input stage they all share - it is not any sort of textbook circuit afaik, I think it is a form of bootstrapping and what strikes me is the extremely high input impedance it shows to the guitar input.
It's a high pass filter, never calculated or simulated it but probably cuts everything below , say, 80Hz or 60 Hz .

I don't like it very much because guitar wiring, microphones, whether it's active or passive, change its parameters, They should have added a buffer between guitar and filter, but hey, it's there so we must accept it.
 

Stampede

Member
Messages
13
JM Fahey, you're right indeed there are some monster heat fins on the bottom of the chassis. I am sending the request to Fender, I have 2 other schematics I've sourced but they're just slightly different. Hopefully they can provide the original.

pdf64, I double checked last night it is in fact a 3 prong cord and it appears to be connected properly inside the chassis. So one less thing to worry about :)

I gathered all the parts for the light bulb limiter and did the research on assembling it, so I'll put that together tonight. The fuses came in so I'm just about ready to test. I think I'll fix of the splices before I do anything though, just to get it heat shrinked and looking pretty.
 

VaughnC

Silver Supporting Member
Messages
17,852
If you know how, the first thing I'd do is a junction check of the rectifier diodes.
 

Stampede

Member
Messages
13
Sorry for the long delay, I've been prepping for a move amongst other things but I finally got a chance to finish the basic prep work.

Light Bulb Limiter


ReSoldered - ReWrapped


Hey Vaughn you mentioned checking the rectifier diodes, do you have any links to guides or how-to's? I'm wondering what kind of results I should be getting so I know my diodes are all square.

Thanks!
 

UsableThought

Member
Messages
1,650
Hey Vaughn you mentioned checking the rectifier diodes, do you have any links to guides or how-to's? I'm wondering what kind of results I should be getting so I know my diodes are all square.
I'm not Vaughn, but checking diodes is easy. They should be checked out-of-circuit, so just disconnect at whatever end is convenient. Purpose of checking is to see if a diode that failed closed (conducts both ways) is causing the fuse to blow. Usually diodes fail open, not closed, or so they say; but closed is possible. If the diodes test OK, you can cross them off your list of possible causes.

Just Google and you'll get lots of how-to's - e.g.
http://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/meter-check-of-a-diode/

These days even relatively cheap DMM's will have a diode check function, which makes it even easier & also gives you a better reading of forward voltage. But in your case just using the ohmmeter function ought to be enough to spot a bad diode. If you get low resistance both ways, that's shorted. Note the polarity of your meter leads matters - the article above explains this.
 
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