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Fender Frontman 15G

Started by Blacklabel, July 22, 2007, 06:39:45 AM

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Blacklabel

Hi guys,
Im new to the forum and electronics so go easy on my knowledge...Ive got a Fender frontman 15G (the cheapo verson without reverb :'( ) now i was jamming with my Boss Metal zone Mt-2 pedal, running it through my amp listening with the head phones now i cranked the level and the gain/Distortion on the pedal and was really Squealing the thing doing some harmonics and stuff and after bout 20 seconds of playing my amp just turned its self off, i turned it off and on again but nothing....i tried my amp in another power point...but still nothing, i pulled the circuit board out and had a look and the fuse seems fine and i cant see anything blown. i read the other post on the frontman amp on this site about the rectifier diodes and filter caps could be the problem so i went and purchased a Multimeter to see if i could find the problem part but i got a reading from all the parts (i dont know if you can diagnose blown parts with a multimeter but for $12 thought id try it) anyway does anyone have any ideas on what could be wrong with my amp?

and also if it is the Electric Caps...there glued in with some sort of resin how do you remove it and do i need to re-resin it if i put new ones in....if you need pix just ask ill be glad to post.

teemuk

#1
First: Before you turn the amp ON again build yourself a current limiter out of a light bulb that plugs between the amp and the main receptacle. It will prevent any further destruction.

http://repairfaq.ece.drexel.edu/sam/tvfaq.htm#tvtslbt
http://www.diyguitarist.com/PDF_Files/AmpCurrentLimiter.pdf

You've seen the previous post about Fender Frontman. You also should have the schematic so what you can basically do is to use your multimeter to measure the DC voltages marked in the test points (power supply section) You can also measure the AC voltage of the power supply. Does the power LED light up?

If you need to trace the input signal, a computer may serve as a simple signal generator (i.e. if you have cooledit or similar software you can run a loop of continuos sinewave signal at the correct frequency and amplitude). Better measure the DC offset at the input before that, though (just to make sure you don't toast your soundcard).

You can use multimeter to test if parts are faulty. You should lift the component off the circuit to get proper readings though. Diode is busted if it conducts in both directions or none at all. Capacitors will give an odd, changing reading while your multimeter charges them (yep, it does that): Basically you'll see an increasing resistance until the cap is charged when it should have nearly an infinite resistance. This measurement is pretty tricky and usually only a good capacitor tester can provide trustworthy results. If you reverse the probes the reading will get interfered again. Practically, a short circuit (very low resistance that won't increase) means the cap is busted - you might have to wait for a while. Opamps should have the stated supply rail voltages (about +-16V), both inputs in the same voltage potential and DC offset of half the supply (typically zero) at both inputs and output. I posted links to some basic troubleshooting guides maybe about a month ago - you should find the concerned thread quite fast.

If all you can say is that your amp just went mute this about the only help I can give. I was about to suggest that you should measure if the thermal fuse of the power transformer is blown but the schematic shows that it is not used so forget it. Maybe you blew the power opamp, maybe some other opamp... Quite an achievement while using headphones - considering that, I would be inclined to suspect you have blown something from the preamp stage. Better just start probing than wait for us to guess what might be wrong. It's a simple circuit.

The resin on the caps is to hold them in place so that vibration cannot destroy their solder joints (those big caps are considerably heavy and can rattle themselves loose). A sharp knife might help in removing the resin. I recommend you re-resin either with hot glue or a drop of epoxy.

Blacklabel

Thanks ill check a few of them things now...and as for the question..No the led does not lite up, its as it its not plugged into the mains power... i will post when i check a few of these things. thanks ;D

teemuk

That would indicate trouble in the power supply. You likely find something fishy in there.

Blacklabel

So when you say power supply, i take it its the power pack...the big silver box that the mains power runs into then it plugs in to the switch and then to the main board...if it is can i test it myself?
thanks for all your help so far :)

teemuk

Power supply is the circuit that converts the mains AC to suitable DC the circuit can use. That covers transformer, diode rectifier and the filtering capacitors - plus all the neccessary fuses, switches, regulators etc.

Yes. You can test it yourself - if you know how but....

...Judging by your question you are pretty much inexperienced in this field. Are you sure your up to the task of starting to repair or troubleshoot the amp yet? Understanding how the circuit operates (and what the different components are) is pretty essential for that you know. Maybe you should "do your homework" first before you pop up the hood and start probing. It might save you from a lot of agony. If you don't know what to do - and how to do - you may damage the amp further or at worst electrocute yourself.

Don't get me wrong: I don't mind helping and I understand that everyone is a beginner at some point but there seems to be an obvious limit in your skills. I just think you're not ready yet to mess with the circuit. If you really are unsure what the power supply is then you definitely need to do some research and study electronics more. Once you have a basic idea how the circuit works you may find the help you get a lot more helpful; at this point it likely just confuses you, or at worst makes you do stupid things that can harm the amp or you. I don't want to take responsibility of that. You should know your skills and whether they need improving.

Anyway, I hope you interprete these words as encouragement. Electronics is a nice hobby but just like any hobby it involves some work in order to progress and get better in it.

Blacklabel

Yep, no problems i see where your coming from and yes i appreciate your concern, I have done modding to Wah pedals and stuff like that but its my first time using Schematics and following circuits but i understand the principles on Circuits and how they work but im not 100% the terminology and i wouldn't work on mains power sorta stuff like power packs and that until im fully ready but like fuses and diodes, resistors and stuff like that im fine with, but as i said originally i am new to electronics and i appreciate everything your helping me with so far and i am still testing the circuits...but i have found a problem with one of the diodes..i get a reading with it both ways i test it so ill replace that one and see how i go...once again thanks foe your help so far. :)

syndromet

That might solve your problem

nice

Hi! I hope this will help you!
Schematic
:tu:

nice!

XinTX

Well I took a look at my deceased 15G yesterday.  The area around the power indicator LED (designated LD3 on the schematic on the left side about midway down) appears to be slightly discolored.  I used a 9V batter to apply power directly to the LED.  Tried it both directions and it still lights.  Is this correct?

The issue I have with the amp is that it blows the 500mA fuse immediately on power up.  So I'm searching for what must be a dead short.  But to date I haven't found it. 

Blacklabel

Hey, about the led..i tested mine today, and it only lights up the one way... now i even tested the smaller 2 leds in the preamp and they are the same...hope that helps with yours

Blacklabel

Hi, I Replaced the 2 Main Elec Capacitiors..and the Diodes that ground straight after them....and it still wont turn on..anyone got any ideas on what might be wrong...thanks

XinTX

Don't know.  Mine won't turn on because it blows the 500 mA fuse about as quick as a flash bulb when I try to power it up.  I know of several of these failed Frontman 15's.  I'm convinced they're truly a POS amp. 

LJ King

Quote from: Blacklabel on August 07, 2007, 06:11:12 AM
Hi, I Replaced the 2 Main Elec Capacitiors..and the Diodes that ground straight after them....and it still wont turn on..anyone got any ideas on what might be wrong...thanks

Remote debugging is difficult at best - even with pictures (hint).

Hopefully you used the proper zeners and not regular diodes, and installed them with proper polarity.

What voltage readings are you getting at the replaced caps?

R.G.

You got some good advice early on that you haven't taken yet.

You can't debug fuse blowing problems very effectively if you only get 35 milliseconds of test time before the next one blows.
QuoteFirst: Before you turn the amp ON again build yourself a current limiter out of a light bulb that plugs between the amp and the main receptacle. It will prevent any further destruction.

http://repairfaq.ece.drexel.edu/sam/tvfaq.htm#tvtslbt
http://www.diyguitarist.com/PDF_Files/AmpCurrentLimiter.pdf
Go do it. There's a cleaned-up version of this at GEO (http://www.geofex.com) that is a little safer wiring as well.

But do it. That will let you quit tossing in fresh parts to wherever someone thinks you MIGHT have a problem.

There is a disciplined way to go about this.
1. Get the fuse blowing to stop so you can test voltages.
2. Test the power supply and get it running correctly, not blowing fuses
3. Remove the limiter and test individual pieces.

What you're doing in asking "Anybody know what it might be?" is asking to be told to replace a whole chain of expensive parts in hopes of eventually replacing the one or two failing ones by random luck.

Do the limiter, then get back here and we'll go to the next step. I've helped lots of people get running again with remote debugging. Maybe we can help you too.
QuoteDon't know.  Mine won't turn on because it blows the 500 mA fuse about as quick as a flash bulb when I try to power it up.  I know of several of these failed Frontman 15's.  I'm convinced they're truly a POS amp.
Well, there are several obvious design flaws I see in the schemo.