Fender Bassman 1968 silverface

[thebassman]

Hi Greg,
Thanks for getting back to me
Yes my channel 1 still has scratchy pots, thats the volume, bass, mid and treble. The channel 1 volume only makes the scatchyness louder.
There is a constant back ground noise when the pots are not being turned and it does this whether an instrument is plugged in or not.
The back ground noise is not a hum, more of a hissy, crackly white noise.
I have switched pre-amp tube with the good channel 2 tube, but it does not help.

I will experiment with the DMM and search for DC voltage at the weekend.

martin

[soundmasterg]

Ok, the fact that the volume makes the scratchiness louder means that the scratchiness is coming in the circuit before the volume control. The fact that it is more of a scratchy sound and not really a hum would lead me to believe it is either DC on the pot, a loose connection, or an oscillation of some sort. The oscillation is quite hard to find without a signal generator and oscilloscope, so I hope it is one of the others. Let me know what you find out.

Greg

[thebassman]

Hi greg,
i have been checking my amp circuit for DC voltage on the pots and i found lots, here are my results. I have taken the readings from all the pots testing from pin 1 to pin 3 from left to right facing the front of the amp :
V1: 1=0, 2=0, 3=0
T: 1=222.2, 2=119.7, 3=77.0
M: 1=(no connection), 2=1.8, 3=0
B: 1=(no connection), 2=77, 3=0

V2: 1=0, 2=0, 3=0
T: 1=0.2, 2=0.18, 3=0.07
M: 1=(no connection), 2=0, 3=0
B: 1=(no connection), 2=0.08, 3=0

MV: 1=0.1, 2=0.04, 3=0

As you can see the problem seems to be coming from input 1. I have checked the voltages up to the caps and the other side of the caps and it seems to be the same. The circuit then goes back to the pre-amp tube V1B. From here on i get a bit lost as to where the circuit goes or starts.
Can you help me analyse the circit diagram please as i am not great at reading them.
cheers
marin

[johnallg]

To me it looks like the small capacitor at the top of the treble pot right below the 7025 (300pf?? hard to read) is shorted.

[soundmasterg]

Good call John!

Sorry for the delay getting back to you....had to put a car back together after the clutch went out.

The problem is that the 300 pf (picofarad) capacitor has shorted out and is letting all the DC pass it onto the treble pot. Since one leg of the treble pot is connected to the bass pot, that is why the one leg has the 77 volts on it also. If you replace this cap with a new 300pf ceramic cap, you should be good to go. Some people like to use a silver mica cap in place of the ceramic for a smoother sound, but it is up to you. Some silver mica caps fail at higher voltages, so a ceramic is certainly a safer bet. These caps are cheap.....around 50 cents each usually. Antique Electronics carries them in silver mica, and has a 330pf in ceramic disk, which is close enough. (see links) Replace the cap, and then check for voltages on the pots again and you should have almost nothing, and the noises should be gone.

Greg

http://www.tubesandmore.com/scripts/fox ... 13_CERAMIC

http://www.tubesandmore.com/scripts/fox ... =12_SILVER

[thebassman]

Thats it guys, you have hit the nail on the head .

I changed the suspect cap last night and it sorted it, luckily i had a spare silver mica cap. I think i will replace it with a ceramic one sometine soon.
I am going out tonight for a band rehearsal, so i should be able to give it a good blast.

Thanks very much for all your help over the last few months.

I am a very happy man.

[soundmasterg]

You're welcome! You'll find the silver mica will sound smoother with less piercing highs, but sometimes they are missing a certain aggression that the ceramic have. The silver mica can also sometimes not hold up well to the voltage despite having a high enough rating. There are tricks to get around this, but as long as it works for you, that is the key! Glad it fixed the issue....remember the solution and the troubleshooting process for the future and you'll thank yourself.

Greg

[thebassman]

Hi guys

I have been using my bassman now for a couple of full blown rehearsals now and it sounds really nice, watch those guys with bassman effects pedals drool .

The only weird thing is that when i try to switch the amp on, it doesn't come on. I sometimes have to flick the power switch about three or four times before the power light comes on. It's a bit of a nervous moment. Any ideas?

[johnallg]

Two quick ideas - flaky switch, or the fuse holder is flaky or loose. Just guessing.