Fender 112 se

[robgrif]

Hi all have a Fender 112 SE seems to function well however the drive channel seems to be bleeding over to clean channel .
When clean channel is selected volume at 0 you can hear the drive channel .if drive channel pre gain is turned to 0 all is well turn it all the way up and it comes back
What would be causing this .Is it a sign of something beginning to fail.At loud volumes this is present but cannot really be heard.
any thoughts or comments would be greatly appreciated.
thanks

[Jazz P Bass]

My guess, and it is simply that, is you may have a jfet that is leaking.
The jfets are used extensively for switching the channel circuits.
There is more than one involved in the switching so it will take some troubleshooting skills to verify exactly what the real problem is & what is causing it.

[robgrif]

fender 112 se

My guess, and it is simply that, is you may have a jfet that is leaking.
The jfets are used extensively for switching the channel circuits.
There is more than one involved in the switching so it will take some troubleshooting skills to verify exactly what the real problem is & what is causing it.

thanks for response. i will attempt to figure this out with that suggestion in mind .jfet as switch i guess i need to figure out which one shuts off the drive channel signal . does it shut it off or just send the signal to ground?

[Enzo]

I'd agree, see if one of the JFETs Q12,13,14 is remaining conductive when it shouldn;t.

[Jazz P Bass]

Note: Jfets out of the box are normally 'ON' from D to S. (low resistance)
It takes a Gate signal to turn it OFF.

So, when the device is called to be turned off, if you have the correct gate signal & the Drain to Source is not going high resistance, then that device is suspect.

The first thing to check is the normally ON resistance.
It should be a low reading.

[robgrif]

I'd agree, see if one of the JFETs Q12,13,14 is remaining conductive when it shouldn;t.

Thanks Enzo for your response. that narrowed my search down quickly. Looking at board there is a solder bridge q12 between C and E .I am guessing that this was not intentional. Looks like it came from factory like that. What do you think. Would that cause the problem .
thanks for help

[robgrif]

Thank you for response and useful information. I was looking at board and noticed the solder bridge on q12.thinking it might be the problem. Meter reads short c. To e thanks again.

[Jazz P Bass]

Q12 is a JFet type J111.
The pins are marked L to R with the flat face towards you: D S G. (drain, source, gate)
With no gate signal, D to S should be shorted.

[robgrif]

Fender se 112

Q12 is a JFet type J111.
The pins are marked L to R with the flat face towards you: D S G. (drain, source, gate)
With no gate signal, D to S should be shorted.

Thanks again sorry for pin confusion. There was solder bridging two terminals on q12 so I removed the bit connecting the two pins and resoldered JFet. However now it seems the amp emits 60 cycle hum at speaker no dc present and no signal from inputs is coming through
Seems I am going backwards in my attempt.not sure what happened will keep trying

[J M Fahey]

Hi all have a Fender 112 SE seems to function well however the drive channel seems to be bleeding over to clean channel .
When clean channel is selected volume at 0 you can hear the drive channel .if drive channel pre gain is turned to 0 all is well turn it all the way up and it comes back

Not so sure you have a problem.
If you can only hear it in a bedroom, at home, but no, say, in a garage with your friends, then it's not a problem.

What would be causing this .Is it a sign of something beginning to fail.

Definitely no.
If you have a slight bleeding, it was there from day 1.
Transistor parts do not "wear out", they either work or die.

At loud volumes this is present but cannot really be heard.

I thought so.

FWIW and before wasting time analyzing possible causes, remember that FETs are NOT repeat NOT zero ohms when on, but a much higher and significative value, think 50 ohms or more.
Datasheet shows "30 to 50 ohms" but the curves on Fig 8 actually show a higher spread: 20 yo 80 ohms.

So when it's "grounding" something, it's not actually achieving it.

You will always have *some* residual audio, in this case the distorted signal.

designers are no fools, know their trade, so if distortion feedthrough is unbearable they will work to lower it, but if it can't be heard undr normal conditions (who plays with clean volume on 0 , max distortion set but distortion channel OFF?) they package the amp and sell it.

Very popular CMOS switches (CD40xx) , which by the way is what Fender uss now, also have even higher residual resistance.

Only guaranteed 0 ohms switch is a relay, because it's metal to metal contact, but they are expensive, harder to drive, can click and pop, so they are usd in more expensive stuff, such as more expensive Fenders, Mesa Boogie, high gain Peavey, etc.
And they bring their own bag of problems.

[robgrif]

correction

Thanks again sorry for pin confusion. There was solder bridging two terminals on q12 so I removed the bit connecting the two pins and resoldered JFet. However now it seems the amp emits 60 cycle hum at speaker no dc present and no signal from inputs is coming through
Seems I am going backwards in my attempt.not sure what happened will keep trying

ok seems just the clean channel has no output
power amp in is fine also
distortion channel is fine
not sure what to look for .think q12 might be bad will take board out again and test it

[robgrif]

Fender 112se

ok seems just the clean channel has no output
power amp in is fine also
distortion channel is fine
not sure what to look for .think q12 might be bad will take board out again and test it

Hi all been checking for source of problem but no success. Voltage on all op amps reads 45 volts
Test procedure described on schematic has different values. Not sure what that means.The only problem seems to be the clean channel. It has a very faint output of input signal. Drive channel works fine and switching between channels works as do the led indicator lights for each channel.
Does anyone have any thoughts on root cause of this problem. If it is an op-amp which one would be the clean channel amp. Btw initial problem was bleed through of drive channel to clean channel. Only attempt to correct was to clean solder between pins of q12.
Thanks in advance for any suggestions.

[J M Fahey]

In a hurry now going to the Dentist (ouch !!!) , will reread at night and try to suggest something.

Meanwhile, do you have any (working) small amp with you?
A battery powered thingie would be best but any typical 10/15W beginner's type will do.
The idea is turning it into a signal tracer to follow audio along the (complex) path from input jack to speaker to see where it stops.
If you dare, you can build a small signal tracer with an LM386, a 9V battery and a pair earphones.

Plan B: if you have an MP3 player you can build a signal injector to do about the same, only you use your own amp as tracer and work backwards.

Plan C: if you dare, you can turn a notebook into a reasonably useful oscilloscope, good enough to see waveforms and of course, whether they are there or not.

[52 Bill]

In order for the clean channel to play, three fets need to be turned off Q12, Q13 and Q14. Q12 and Q14 turn off the signal from the distortion channel, leaving Q13 that un-grounds the clean channel signal.

Seeing that the distortion channel is now turning off, check Q13 or temporarily remove it and see if the clean channel comes back again. If it still doesn't work, half of IC U1 is the clean channel amp. Test it or replace it and see what you get.

[Jazz P Bass]

To add:
Think of the Jfets as a switch.
They are normally ON. (D to S conducts)

To turn them Off requires a negative voltage on the Gate pin.

In Clean mode, there is -15Vdc on all of the gates (Q12, 13 & 14)
Therefore, the three Jfets are Off.
They do not conduct D to S.

Now in Drive mode, the negative Gate voltage goes positive, turning the Jfets back to a normally closed condition.
As 52 Bill stated, Q12 & Q14 allow the Dirty channels functions.
Clean mode disallows the Dirty functions.

Q13 is used to short Clean mode signal to ground when Dirty is engaged.
So if Q13 is stuck On, there goes your signal.

Remember that Clean mode turns all of the Jfets Off. (-15 Vdc at the gates)
Dirty mode simply lets the Jfets go back to being in there normally closed state.