73 Fender Silverface Twin Reverb Noise New Findings & Questions

[gtrplayr1976]

So to update on the issue(s) I'm having with this SF Twin : The Vibrato channel has a crackly ,poppy sound...like rice crispies. I have changed all of the plate resistors, the caps were replaced about 3 years ago, I replaced the volume pot, and a bunch of other resistors in that circuit. Some things I noticed yesterday : As I increase the volume, treble,or bass I have a dc voltage that increases on the center tap of the bass ,and treble ,and the right tap of the volume up to 130 vdc . It also show on the .1 cap in the tone circuit ? I noticed the Vibrato channel Bass control is wired a little different than that of the Normal channel. The center ,and right tabs are jumpered ,and the Volume has no ground as the Normal channel does. When I put the test lead on the center tab of the volume it has a loud hum that sounds like a bad ground. It doesn't do this on the Normal channel. According to the schematic these should be wired the same. Am I correct on this ?

Thanks a heap

[gtrplayr1976]

bump... This sound clip may ,or may not help.

[acorkos]

if it were mine, I would replace the .1uF cap and rewire the pots as per the original ab763 layout, and see if the noise goes away.

http://www.thevintagesound.com/ffg/s...763_layout.gif

[g1]

Sounds like the .1 cap may be leaky and need replacement as acorkos mentioned.
The jumper on the bass pot should not matter as the right tab is unused, some people jump the unused tab to the wiper.
Not sure what you mean about the volume pot ground, the tab should be soldered to the case of the pot, that is the ground. Does the normal channel have an extra ground wire connected? Or is one of the tabs on channel 2 volume pot not connected to anything? If that were the case, I don't think the ch.2 volume control would work.

[gtrplayr1976]

acorkos , That layout is different. I haven't found one that matches this amp nor a schematic that totally matches.

g-one , The volume pot on the normal channel has a ground from the tab to the brass plate. The Vibrato channel does not ,but it has the wire from pin 8 V2 ,25/25 cap ,828 resistor connected to the left tab. None of the tabs are grounded to the pot. First picture is the normal channel, second is vibrato. I'll try to get a better pic of the second.

[acorkos]

it has the wire from pin 8 V2 ,25/25 cap ,828 resistor connected to the left tab. None of the tabs are grounded to the pot.

that wire must be grounded, along with the left lug of the volume pot. the missing ground may be the source of your voltage leak

[52 Bill]

Better photos will help, but to me it looks like there has been a lot of added wiring, so I would think that may be the reason that the circuit doesn't match any schematic, and the reason you have hum.

Show some better photos and we can all see what has been changed.

[gtrplayr1976]

that wire must be grounded, along with the left lug of the volume pot. the missing ground may be the source of your voltage leak

I added a ground yesterday. It didn't help.

[gtrplayr1976]

Better photos will help, but to me it looks like there has been a lot of added wiring, so I would think that may be the reason that the circuit doesn't match any schematic, and the reason you have hum.

Show some better photos and we can all see what has been changed.

I have not added any wiring myself ,and according to the owner no one has worked on it before me. I had the same thought when I saw some things in there that didn't match any schematic ,or layout.
These are about the best pics I have for now.

[52 Bill]

The Vibrato channel does not ,but it has the wire from pin 8 V2 ,25/25 cap ,828 resistor connected to the left tab. None of the tabs are grounded to the pot.

If you follow the wire from the left tab back to the fiberboard, where it connects to the 820 ohm resistor and the 25uF cap, there is also a black wire that connects to the brass grounding plate, or at least there should be. Take a meter reading from chassis to the left side pot tab. What resistance do you get?

[Tom Phillips]

... The volume pot on the normal channel has a ground from the tab to the brass plate. The Vibrato channel does not ,but it has the wire from pin 8 V2 ,25/25 cap ,828 resistor connected to the left tab...

I believe that you are misinterpreting the wiring of the coax cable shield connected to the left lug of the volume pot. Note that there is another white wire connected to that lug which heads off to the left and out of the picture. I'll bet it heads to a ground point. Use your Ohm meter to measure continuity from the left pot lug to the chassis. I'll bet that you find they are connected. Note that the coax has two connections at the pot but only the center wire is connected at the V2 tube socket. That is normal stock wiring.

By the time your posted photos were taken the amp already had extensive component replacement. It has the typical messy silverface wiring which looks pretty much stock. I see also that the ground connection under the Normal channel pot has been messed with. Who knows what mistakes may have crept in during past work.

[g1]

g-one , The volume pot on the normal channel has a ground from the tab to the brass plate. The Vibrato channel does not ,but it has the wire from pin 8 V2 ,25/25 cap ,828 resistor connected to the left tab. None of the tabs are grounded to the pot. First picture is the normal channel, second is vibrato. I'll try to get a better pic of the second.

No. The wire from pin 8 of V2 goes to the other side of the 820R/25uf. The left tab of the volume pot goes to the ground side of the 820R/25uf, which is connected by the black wire to ground as 52Bill stated.
The white wires from the left tab of both volume pots go to the left tabs of the mid controls. That is how the mid pots are grounded.
Hawe you tried replacing that .1 cap yet?

[gtrplayr1976]

I am out of town right now. I will get back on this on Monday. g-one , I replaced the cap with no change. Thank you all so much for the help !

[gtrplayr1976]

Bill, You are correct . There is a wire to the brass plate, so it is grounded. I didn't look close enough the first time. Resistance is 0.

[g1]

So, from what I gather so far, you have crackling noise in vibrato channel only, and DC voltage (up to 130VDC) on the volume pot. Is this correct? Are the tone pots acting like they should?
You changed the .1 cap in the tone circuit for Ch.2. That leaves the .047 or the 250pf in the ch.2 tone circuit as the only places for the DC to be leaking through. Try replacing them.