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Coyote
Advanced Member
Username: Coyote
Post Number: 813
Registered: 11-2004
| | Posted on Wednesday, July 05, 2006 - 8:57 am: | 
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Hey, while this is and isn't an amp question, I know there are a lot of you out there that have a greater deal of electronic experience than I.
Here's the ?,
I have a Fender Frontman 15 watt practice amp that blows its fuse as soon as power is applied.
Now, here is what started it, don't laugh.
We, (my boys and I) were testing a old EV sub-woofer out as my oldest boy wanted to use it for a bass cabinet. It had an old 15" speaker in it and the only amp besides my 66 Fender Twin we had that had a speaker out jack was the small Fender, so we hooked it up to it to see if the thing worked. Well, it didn't make a sound, so I shut the amp off, opened the front of the cabinet to gain access to the back side of the speaker and found the speaker un-hooked. Hooked it back up, turned the amp back on, and with my son play'n through the amp, I began fiddle'n around with the speaker since there was still no sound. I found that if I gently pushed on the cone it would start producing sound and then stop. I finally figured out the voice coil was corroded and it felt like it was moving in sand instead of smoothly like it should. Needless to say the speaker is no good and got replaced with a Emenence Kilomax 15". Well, bout that time I saw a flash in the lil amp and it quit. Fuse. Ok, no problem, I'll get another fuse, of correct type and replace it and it will be ok, right? Nope, new fuse blew immediately.
Ok, so, the amp is not worth take'n to someone to repair as it would probably cost about the same as just buy'n a new amp. The lil amp hasn't been used much and doesnt get used much if at all, but would still like to repair it myself, but don't know where to begin.
Here is a link to the schematic found on line at Fender's web site:
http://www.fender.com/support/amp_schematics/pdfs/Frontman_15G_Schematic_68F.pdf
Here is the manual also:
http://www.fender.com/support/manuals/pdfs/manuals_elec/guitarpdf/Frontman.pdf
This is the 15G model.
Any help of where to start looking, testing is appreciated.
I've got the amp apart, and have visually inspected the componets and do not see any thing obviously burned or anything. I'm suspecting maybe the transformer, or the output Transistor maybe, but dont know how or if they can be tested. |
Jameslarson
Intermediate Member
Username: Jameslarson
Post Number: 239
Registered: 4-2006
| | Posted on Wednesday, July 05, 2006 - 11:58 am: |
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I don't know about the amp, but I can tell you that an Eminence Kilomax speaker is one incredible speaker. Especially an 18". |
Coyote
Advanced Member
Username: Coyote
Post Number: 814
Registered: 11-2004
| | Posted on Wednesday, July 05, 2006 - 2:45 pm: |
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Yea, the 15" does an incredible job hooked up to a Fender KXR 200...
and I still have another sitting around waiting for a cabinet.... |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1877
Registered: 9-2004
| | Posted on Wednesday, July 05, 2006 - 5:58 pm: |
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i can make a couple of guesses that may help, but your best bet is to send a PM or an email to Chad, as he is "the man" when it comes to audio gear, and has lots of experience.
im thinking that the EV speaker, being blown, may have had an impedance way off from what the amp is supposed to handle, and the amp was trying to output to it, and that weakened the final amp transistor/IC, and when you hooked up a good speaker to it, that was it, and it gave up the ghost.
i think when the fuse keeps blowing it means that the transistor is shorted internally, and you should be able to test that with a voltmeter if it can test transistors.
gotta remove it from the chassis to test it.
good luck, nice to hear you and the kids are jammin'!
matt |
Chad
Advanced Member
Username: Chad
Post Number: 958
Registered: 11-2004
| | Posted on Wednesday, July 05, 2006 - 10:37 pm: |
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Heh I have repaired a Ka-Zillion of these things  I was a fender service center for 5 years. What was wrong with the EV is that it had been bottomed out and knocked the last few windings of the voice coil off and broke it OR it was rubbing badly from coil expansion. Moving it around completed the connection and brought the amp in and out of load. Normally this would be fine for a differential solid state audio amp but these use a TDA device that is heavily servo controlled. They freak out when load is applied and un applied at a high rate of speed.
U4 is a TDA 2050 they are available pretty cheap. Replace it and I'm 99% sure you will be back in action again On disassembly you may find the one in there with a crater in it, they go in flames when they go, usually.
Also, if you are running the kilomax hard you may want to model it in software for the enclosure it is in. They want a BIG enclosure and can be strange ducks at times
WinISD is good free software.
Chad |
Coyote
Advanced Member
Username: Coyote
Post Number: 816
Registered: 11-2004
| | Posted on Thursday, July 06, 2006 - 5:50 pm: |
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KV, well, the amp blew before the Kilomax was installed. The original EV speaker blew it. But your guess as to what happened seems to be on track with what Chad is say'n. And yes, we are try'n to jam, but our music preferences are a bit different, they like the new metal stuff mostly and a bit of classic and I'm a blues, classic rock guy, so we can at least jam on some classic till I show them just how much fun play'n some blues can be.
Chad, great news from you my friend. I knew you or KV would be my answer men, thats why I posted it here. Thats what makes this forum the most awesomess forum on the net. Lots of great tech people that know more than just radios. The Kilomax is mounted in a LARGE sub-woofer cabinet and is now being driven by a Fender KXR200 amp that was removed from its original combo cabinet and made into just a head. It kicks.....
Thanks guys, I knew I could count on all of you...
Stu |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1885
Registered: 9-2004
| | Posted on Thursday, July 06, 2006 - 6:25 pm: |
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you mean i was on the right track? WOO HOO!
way to go chad, dont ever change man! (said in your best "one too many" accent)
good luck stu, and as for jammin' wit da yoots;
check out a band called "shinedown". they are pretty popular these days and still have a sort of classic rock feel. great singer!
should be pretty easy to find some tabs online and impress the heck out of the kids with your hippness.
later guys!
matt |
Coyote
Advanced Member
Username: Coyote
Post Number: 819
Registered: 11-2004
| | Posted on Friday, July 07, 2006 - 8:31 am: |
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Matt, will do. Can't say I've heard of them or them... the name does sound familiar. I'll look into it.
Thanks.
yoots...  ..taken from My Cousin Vinny??..
Stu |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1896
Registered: 9-2004
| | Posted on Sunday, July 09, 2006 - 2:14 am: |
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yeah, thats where i got it. LOL
have fun!
matt |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 3071
Registered: 7-2004
| | Posted on Sunday, July 09, 2006 - 8:36 am: |
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i knew what a 'yoot' was....i'm just a stones throw from brooklyn & got a bit of it in my blood. use guys jus gotta lern how ta tawk rite.
vinny wasn't funny, lisa was. even italians from brooklyn make funna her  |
Coyote
Advanced Member
Username: Coyote
Post Number: 849
Registered: 11-2004
| | Posted on Friday, August 11, 2006 - 6:57 pm: |
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Ok, Chad, KV, anyone,
Replaced the transistor, amp will now come on without blowing the fuse, BUT, now it has a loud hum to it and no signal/sound out when instrument plugged in. Any suggestions? Cap replacement?
Thanks for the help ahead of time...
Stu |
Chad
Senior Member
Username: Chad
Post Number: 1029
Registered: 11-2004
| | Posted on Sunday, August 13, 2006 - 10:52 pm: |
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Does the speaker push forward/backwards and want to stay there?
Check quick, this is DC and bad for the speaker.
Better off disconnecting the driver and checking for DC offset at the speaker output. It's not a tube amp, you are safe doing this.
Chad |
Coyote
Advanced Member
Username: Coyote
Post Number: 853
Registered: 11-2004
| | Posted on Monday, August 14, 2006 - 8:59 am: |
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Chad, no, the speaker moves back a forth while its humming, or at least thats what it looked like, I'll look closer this evening to make sure.
Thanks |
Chad
Senior Member
Username: Chad
Post Number: 1032
Registered: 11-2004
| | Posted on Monday, August 14, 2006 - 6:46 pm: |
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put your finger on it to be sure
chad |
Coyote
Advanced Member
Username: Coyote
Post Number: 858
Registered: 11-2004
| | Posted on Wednesday, August 16, 2006 - 8:26 am: |
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Chad, I finally checked on this, and it does seem that the speaker does push forward and stay there.
Now, how do I check the DC offset? Or what else blew/fried?
Thanks again...
Stu |
Chad
Senior Member
Username: Chad
Post Number: 1038
Registered: 11-2004
| | Posted on Wednesday, August 16, 2006 - 11:22 pm: |
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Check to make sure both rails are there, you should have + and - voltage, I can't remember the amount but it's not huge. Also check the low voltage for the rails, this can be found on an Op Amp on pins 4 and 8. The dot is one, count down to 4 then the one diagnonal is 8 (not across from 4) Lossof one rail can cause this, If you have lost LV look into the regulator Zeners (I think that one has them). Let me know what you find. I'd pop the final out before testing, again, they are servo controlled and will do ANYTHING to stay what they "think" is linear, even blow up. If there was an insulator on the heatsink (again can't remember) ii is still in place an undamaged correct?
Chad |
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