1969 Fender Champ Amp repair

[Steelwitch]

Just got in an all original, never touched 1969 champ. I put a 3 prong cord in here and noticed while testing that the treble and bass controls don’t make any kind of drastic difference at all. The treble control kind of has an effect on signal, but much lower than it should, and bass control is not affecting signal at all!

I tested the pot already and it looks good, wiring looks ok, the 250pf coupling cap tests ok. What else should I look for? Any tricks for tracing a problem like this? Thanks!

[eschertron]

Could someone have lifted the tone stack? Maybe a bad solder connection on one end of the 'mid' resistor to ground?

[Justin Thomas]

Has it been bypassed in some way? Like, not done properly? So maybe the xontrols have some very minimal effect but not enough? Are all of the grounds solid? Give them a respectable tug - sometimes they LOOK solid but aren't & the wire just pops out of the solder blob.

Justin

Or what eschertron said!

[Steelwitch]

All grounds and wiring are solid. This amp is super quiet and functioning exactly like it should except this

[Leo_Gnardo]

Two things to offer: 1: There's not much bass available anyway from an 8 inch speaker especially in an open back cab, even moreso because the Champ's output transformer is extremely bass deficient. You may be able to measure about 5 watts clean at 500 Hz but barely 1 watt at 100 Hz. and B) The EQ stack has a 15K fixed resistor which sets the midrange. You may reduce the value of that resistor, creating a deeper midrange dip and thereby make your treble & bass controls operate in a more obvious manner. Consider the standard Fender T/B eq employs a 6K8 resistor in this position. And Fender's TMB eq's in black & silver amps of that period use a 10K pot. You could try tacking a parallel resistor onto that 15K until you get the response you like, or substitute a 10K pot. For some customers who want a mid control I steal input #2 and mount the pot in that hole. Have fun!

[Justin Thomas]

There's not much bass available, but the controls should have much more than a barely noticeable effect, though...

Is it possible there's a broken lead inside a component that isn't visible? I've had both caps & resistors pull that stunt before...

Justin

[Steelwitch]

I poked around at the circuit on a scope and all connections were definitely good, nothing loose, nothing microphonic. All controls are being looked at on a scope so speaker is not really a factor. I’m considering clipping in new tone stack caps and see if they’ve failed in some way..

[Chuck H]

I poked around at the circuit on a scope and all connections were definitely good, nothing loose, nothing microphonic. All controls are being looked at on a scope so speaker is not really a factor. I’m considering clipping in new tone stack caps and see if they’ve failed in some way..

I think you should locate the cause of the problem before you start soldering anything. JM2C

Have you scoped the signal coming right out of the tone stack? This would be the only place that a profound difference would be visible on a scope.

If the tone stack is actually not functioning then it it is either because the tone stack ground is bad or because the circuit has been altered to bypass it. If any of the tone stack caps were bad you would be having different symptoms than what you describe. You have looked but not seen. There IS a problem other than bad caps.

[Chuck H]

Are all of the grounds solid? Give them a respectable tug - sometimes they LOOK solid but aren't & the wire just pops out of the solder blob.

So... Now we have the Enzo WHACK and the Justin TUG.

It's starting to get a little man-love weird around here

Jusrin

[nickb]

So... Now we have the Enzo WHACK and the Justin TUG.

It's starting to get a little man-love weird around here

Jusrin

Yes, but Justin's is respectable.

[Steelwitch]

I’m trying to avoid soldering anything! I just put a 1k sine wave through with the scope on the volume pot. Same result. The signal looks very pure but is not affected by the bass control at all. I could use a tip or two on seeing frequency response for an amp like this

[Steelwitch]

Just to confirm that It’s not actually affecting signal. At least to my ears through a speaker, it really didn’t.

[Leo_Gnardo]

I’m trying to avoid soldering anything! I just put a 1k sine wave through with the scope on the volume pot. Same result. The signal looks very pure but is not affected by the bass control at all. I could use a tip or two on seeing frequency response for an amp like this

Who would expect a 1000 Hz signal to be much affected by a BASS control? Try 100 Hz.

[Steelwitch]

That makes sense! Maybe it is working.. dang that’s pretty obvious. I will report back. Lol I feel like a lot of my posts here are really basic but I appreciate all of your help! I’m still getting a feel for how to test the right way. Y’all are awesome with the info

[Chuck H]

I usually test with an 800Hz signal and IME the classic Fender tone stack absolutely should affect the signal in a noteworthy way at that frequency. I wouldn't expect NO affect at 1000Hz. So I think it's probable that the OP's assessment is correct. But...

What is the input signals amplitude? If testing with a very high input signal and keeping the volume knob low that may moot some affect from the tone tack. 80mV to 100mV would be a good input signal for testing.