Hi Everyone
Looking for some wisdom... I have picked up a Fender 94 Twin Amp that has a problem in that when you switch it on theres an intermittent "scratchy" sound that does not shut up. I have done the standard isolation stuff and have traced the problem to what I believe to be the first stage of the "pre-amp out / power amp in" V7B on the schematic attached.
I have tried the standard things like swap tubes etc to see if that changes it and the bottom line is it does not.
I have also tried to bypass the V7 stage (dont need this facility anyway) by routing the circuit from the junction of R147 & R153 to C26 (PI input cap) and the good news was the scratchy sound stopped & overall tone improved (so there is obviously a problem somewhere in V7 but I got excessive humming which I could not trace or stop, so I have put the circuit back to how it was.
In addition when I plug directly into the "power amp in" socket the problem stops - but I have disconnected the pre-amp
Just to be clear when I remove all valves V1-V6 the problem is still there, when I pop V7 the problems stops. So from my reasoning it is something around V7B that appears wrong?
I have 2 thoughts:
a) Disconnect the V7 stages -and sort the humming problem
b) fix the original problem
The other thing I have noticed is the way the presence circuit is wired to the cathode of V7A amd someof the resistor arrangements around V7 cathodes and grids etc (bit foreign to me...)
Any advice greatfully accepted
Cheers
Sean
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Fender 94 Twin Amp - Problem
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Fender 94 Twin Amp - Problem
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Last edited by badfan on Fri Feb 25, 2011 5:39 am, edited 1 time in total.
Re: Fender 94 Twin Amp - Problem
Good luck on that one.
That is a very populated amp!
That is a very populated amp!
Tom
Don't let that smoke out!
Don't let that smoke out!
Noise?
If I am understanding you correctly, then you have a static sound, that is happening without any guitar signal? A crackling sound? When you jumped from the two resistors to the PI input cap, you skipped V7B. Since it is a cathode follower, there are no plate resistors, which usually would be the culprit. I would possibly change out C29. If it were leaky, it might cause some noise. You might also re-solder the few components around V7B also.
Oh, Also it could be the input cap to V7B, C69.
Oh, Also it could be the input cap to V7B, C69.
Sctratchy noise
Check the ground connection from the preamp board . same problem on mine (a 91). Re-soldered the ground connection joint and viola - quiet as a mouse 
Macshaft:)
Re: Fender 94 Twin Amp - Problem
Thanks Guys
I will try out the suggestions and let you know how I get on. Suffice to the layout and design on this one is a bit of a pig...
Cheers
Sean
________
Zx14 vs hayabusa
I will try out the suggestions and let you know how I get on. Suffice to the layout and design on this one is a bit of a pig...
Cheers
Sean
________
Zx14 vs hayabusa
Last edited by badfan on Fri Feb 25, 2011 5:39 am, edited 1 time in total.
Re: Fender 94 Twin Amp - Problem
Could be any number of things but a problem I've seen several times in that amp is the preamp supply caps coming loose on one side. I mean the big caps over by the inputs. I think there are two. It makes a static noise that is vibration sensitive and tends to be most audible when notes are trailing off. The first time it took me more than half a day to find it. A good thing to try is check the solder joints on the underside of the board with a good light and a magnifying glass while tweaking the parts a little.
Re: Fender 94 Twin Amp - Problem
Thanks for the help so far....
Where I am now is there are 2 distinct problems, the humming and the dodgy V7B stage.
I think I have found the problem of the humming. I have got the oscilloscope out and I am reading a small AC signal via the BIAS circuit being injected into pin 5 of the power tubes but i cant directly measure it via the DVM.
The hum is present with the PI tube out so that started me working back from the OT and that seems to be where it is coming from.
For now I would be happy to jump V7 as I dont use the functions and then sort the humming and voila I will be happy.
Is this normal for AC via the BIAS circuit or is this a definate fault?
Cheers
Sean
________
ULTIMATE FIGHTER
Where I am now is there are 2 distinct problems, the humming and the dodgy V7B stage.
I think I have found the problem of the humming. I have got the oscilloscope out and I am reading a small AC signal via the BIAS circuit being injected into pin 5 of the power tubes but i cant directly measure it via the DVM.
The hum is present with the PI tube out so that started me working back from the OT and that seems to be where it is coming from.
For now I would be happy to jump V7 as I dont use the functions and then sort the humming and voila I will be happy.
Is this normal for AC via the BIAS circuit or is this a definate fault?
Cheers
Sean
________
ULTIMATE FIGHTER
Last edited by badfan on Fri Feb 25, 2011 5:39 am, edited 1 time in total.
Re: Fender 94 Twin Amp - Problem
OK some futher info...
That scratchy sound is also still present when I take all pre-amp tubes so the problem must be in the BIAS,power amp section - right?
I just did'nt here that much before as it was a lot more prominant when the pre-amp was connected, now that I am using the scope I am seeing it when it happens on pin 5.
Cheers
Sean
________
Ford Taunus V4 Engine Specifications
That scratchy sound is also still present when I take all pre-amp tubes so the problem must be in the BIAS,power amp section - right?
I just did'nt here that much before as it was a lot more prominant when the pre-amp was connected, now that I am using the scope I am seeing it when it happens on pin 5.
Cheers
Sean
________
Ford Taunus V4 Engine Specifications