Reverse log C100K poti replacement in a Fender Frontmann 25r if you don't have a C type??

[rock soderstrom]

Hi guys, I'm fixing an amp for a mate. He needs it urgently. It's a Fender Frontman 25R and the level pot was dead. To replace it you have to unscrew all the other pots, the treble pot (R29) has crossed the Jordan during that as well, the quality is extremely poor, the amp is a piece of junk, but my jazz buddy swears by it and needs a quick fix.

I have a lot of pots to choose from, but I don't have a C100K type for the tone control.

What can I replace it with, is there a combination of a log or lin type pot and a paralell resistor that comes close to the characteristic curve of a C100K, can I somehow fake that?

I have a similar C10K type, how would it probably behave?

Any help is appreciated, potentiometer curves are not really my thing....

 

[rock soderstrom]

Done. The EQ regulates rather in homeopathic doses, no big difference between 100K C log or lin IMHO.

 

[fazer]

Was going to say that’s a typical fender tweed deluxe treble tone circuit. Linear was what was used. Should just be a different setting for treble on the knob.

Good job

 

[CJ]

this town is weird, none of the gigging musicians have backup equipment,
i guess they hawked them all for gas/food/drugs/waterpump
all the closet musicians have 20 amps and 20 guitars ready to go,
so "i need it yesterday" is heard often around this shop
usually give them a loner and they end up buying it which is an old trick.

 

[CJ]

ok so ten minutes after i post, here comes a fender frontman whith someone who has a gig this thursday wtf?

makes a sound like a blown spk but spks are good, any ideas?

thank God no chipamp, screw you tda7924!

 

[rock soderstrom]

this town is weird, none of the gigging musicians have backup equipment,
i guess they hawked them all for gas/food/drugs/waterpump
all the closet musicians have 20 amps and 20 guitars ready to go,

You have a point...and I wouldn't want to play one single gig with a Fender Frontman 25r. Sound is a matter of taste but the build quality is really horrible, I'm still shocked.

No matter what you touch inside, it literally crumbles. The pots, supposedly from Alps are really bad and completely unstable. I have compared them with normal Alps Potis, extreme quality difference, one wonders where Fender has bought this crap.

Funny are also these solder joints on the blade connectors from the power transformer to the PCB. If you want to pull them off, you pull the connector out of the solder. I have not seen anything like this before.



Even the much cheaper "Fame" replica of this amp is way better quality. Both come from China.

 

[rock soderstrom]

ok so ten minutes after i post, here comes a fender frontman whith someone who has a gig this thursday wtf?

makes a sound like a blown spk but spks are good, any ideas?

thank God no chipamp, screw you tda7924!

LOL, check the spade connectors transformer - PCB

 

[rock soderstrom]

The Frontman 25r is good looking, reverb is a real mini spring pan with nice sound. It is very portable but build quality overall is really bad, I would not buy it, even when super cheap.

 

[rock soderstrom]

makes a sound like a blown spk but spks are good, any ideas?

Contact problems at those mini switches or scratchy pots?

 

[CJ]

sprayed all those yes they were acting up,

just replaced u6 tl072 diff pair driver in the output section,

 

[heatwalk]

does it pass clean out of the FX loop?

 

[CJ]

seems to be ok with the opamp replacement,

had clean signal out of the preamp jack, yes.

this isolated it to the output section,

weird thing is that it started working ok then went back to chopping half the waveform, like one side of the opamp latched up,

so we are nervous about the repair as i have never seen an intermittent opamp before

best thing to do is test it with guitar for a few days, yay! i love this job.

 

[Whoops]

the amp is a piece of junk

I confirm that

 

[CJ]

yeah pretty harsh amp,



weird, the problem of the distorted chopped off negative half of the waveform comes and goes,

tried new filter caps all around, no change,

but when i put a voltmeter lead on the diode d24 the waveform snaps back to normal,

then it comes back,

blue arrow is the point that i touch with the meter to get the good waveform back,

good thing i did not send this amp out!

and then go to the show this thursday and watch the amp fail,

thats the problem with this job, i don't dare go to any local shows,

thinking an .01 cap across r76 might be the ticket as this circuit is unstable

edit: no good, amp went into osc.
so that node is very touchy.

 

[jmgoins]

I'm a noobie, so apologies for asking obvious questions, but are you sure it isn't a cold or cracked solder connection in the area?

 

[CJ]

It's possible but when I press on the board or hit components with a chop stick the amp stays the same.


I started a new thread on this in the Drawing Room and the puzzle remains unsolved.