Fender Frontman 15B (BASS) schematic needed - model without Ext Speaker socket!

[chazpope]

Hi

I have this amp and none of the schematics I have found match it.
All schematics (incl from fender.com) show an EXT speaker and my amp does not have one - instead it has an AUX phono input.

The amp works but has an 'added annoying crackle' - seems to happen even when it is turned on and another amp is played next to it ..

Thanks!

[Enzo]

Turn it on, ball up your fist, and whack the top of it. Does it react? It ought to ignore your whack. if it crackles or does something in response, you have a loose connection somewhere, likely cracked solder. If that is the case, use a wooden chopstick or other insulated thing to poke at each part on the board, looking for something that seems to react of cause the noise. Also, make sure all the control shaft nuts are snug if it has them. Loose hardware on controls, jacks, even transformers can cause noises.

I don't have any FM15B in my file with me, maybe on paper at the shop.

But CALL Fender support and ask for it. Have your serial number when you call, since it can help determine the version. Also, look on the rear panel for country of origin. The Indonesian ones are not the same schematic as the Mexican ones for example.

[chazpope]

Hi, I had already re-soldered it it seemed to make no difference, but I will do that again next week just to be sure. No it does not react on knocking. Last night I plugged my bass through a preamp into the AUX in socket and although it was not completely gone, there was considerably less crackle. The crackle seems to be amplified by the MID knob so it must come from before that. I was planning to isolate which bit of the circuit causes it but thought to source the correct schematic first. I wonder if a dodgy limiter CA3080 can cause it as I have had before with another Fender bass amp, but then the sizzle seems to be volume related, I wonder could it be a half-bad front opamp.

So next I will ask Fender for diagram and report on progress.

[J M Fahey]

Try it with another speaker, frayed tinsel wires in a speaker, doubly so if a Bass one (more movement) are an often unnoticed source of mystery crackle.
Another similar source may be weak/rusty female spade connectors at the speaker wires or in general any poor contact connector mating surface.
Which in a general term includes soldered connections and even poor leg connections inside a cap, one particular example being electrolytic caps b ecause at some point there is an aluminum<>something else connection and those are always crimped; aluminum can not be soldered by us mortals.
Any and all of them can usually be tested by the chopstick system.
Just use a new dry chopstick because Soy Sauce (or fermented fish head one) is real salty and electrically conductive.

[vintagekiki]

All schematics (incl from fender.com) show an EXT speaker and my amp does not have one - instead it has an AUX phono input.

Your amp is Fender Frontman 25B, with tape (phono) sockets and without ext speaker socket

[chazpope]

Ok thanks all, I have found the problem - the 2 screws that fix the heat sink plate to the chassis were completely missing (maybe never installed). Although there was electrical connection between these two via other circuits, I noticed a slight buzz when they were separated at the screw spot. And I guess at high volume vibrations and large currents some buzz-loop was formed (and amplified) at the spot. I may be wrong of course about all this ...

Now the amp works OK (as much as such an amp can work for bass - in your bedroom only I did see on the web someone install a plank inside to seal the speaker space completely and reduce the farting-out - I may try that at some point and keep and use the amp for 'chamber' gigs.

On the original issue of diagram though - no luck - Fender did send me the same diagram I had already - the one with speaker out and without aux in. I did trace the aux in through a cap to one of the OP amp legs as expected so it must be a late mod that Fender forgot about

[chazpope]

No sorry it is not so, I read the model from the back plus yours has even more outs

[J M Fahey]

Can you please post the one they sent you?
Good to have in the library anyway.
Thanks.

PD: and if you are still intrigued about yours (I am ), take a couple pictures: back panel, specially the model number, the front panel, maybe a gut shot.
Does the PCB have any visible label?
The PT?
Modern employees may only know the latest chinese ones, but I remember some 80's FM (some sold under the sub brand Squire) , which were made in Japan or Taiwan, and were discrete power amp, not an IC.
Any parts date codes in transistors, pots or electrolytics?

Are they Japan/TW/Korea/China?

Just for kics, a little detective work.