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Marshall amp hum
So I have a Marshall Super lead 100 with a hum issue. I could use another set of brains on this one, thanks for any help. I will describe what I have done in the way of troubleshooting but first the exact symptom.
A low hum not overly loud but very present when the standby is off and the volumes set to zero. Remove V1 no change, remove V2 the hum basically goes away save the low buzz sound emitting from the PT (talking tranny), remove V3 hum is still gone.
If you leave nothing plugged in but all tubes inserted and raise the (volume one or two) the hum sort of dissipates at higher volumes settings and gives way to the normal amp hiss you associate with Marshalls etc. For instance you can kind of find certain vol spots where the hum is mitigated to a degree, not totally, but some. To be clear this is not a hum that is overpowering when you are playing full out, but in idle (standby off) or with pots rolled down on the instrument the hum is there. It's more hum than my other Marshall amps of similar circuit topography.
What has been done so far to divide and conquer with no improvement save some less hiss/buzz via lead dress tweaking and all new filtering.
1. All filtering replaced power supply/screens/preamp cap under the board
2. All Tubes swapped out for another set
3. Bias supply caps replaced and bias diode was replaced.
4. V2 lead dress green preamp socket wire (coming from the two 470k) has been raised up so nothing is crossing it.
5. caps/resistors around V2 have been checked in circuit (the 470k, 100k, 820r) all read within tolerance.
6. On V7 filament leads I get 3.13v AC on both sides (@ 115v wall AC) to ground (note this measurement around V3--V1 does not stay as even and is more like 3.08 & 3.10 on the other side of the tube (pin 4/5 & 9) when reading from the preamp tubes.
7. No DC on the pots that I can see.
8. Voltage readings on the sockets are within spec.
9. No loose connections around sockets that I can find or on the pre-amp board.
10. Ground is tight on the speaker jack.
11. NFB is attached on the speaker jack.
Possible issues or solutions?
1. The bias ground (on board 10uf caps) goes to the buss wire on the pots, could this need to go to the bias winding ground tap lug (by the pt) instead?
2. Should I create an artificial heater ground via two 100ohm resistors to ground on the filament leads and loose the center tap?
3. Shotgun everything around V2 (mixer caps, resistors, cap)
4. Replace the two 10k 2w dropping resistors on the preamp?
5. something around the input jacks?
6. Bad vol pot???
Thanks for any feedback
A low hum not overly loud but very present when the standby is off and the volumes set to zero. Remove V1 no change, remove V2 the hum basically goes away save the low buzz sound emitting from the PT (talking tranny), remove V3 hum is still gone.
If you leave nothing plugged in but all tubes inserted and raise the (volume one or two) the hum sort of dissipates at higher volumes settings and gives way to the normal amp hiss you associate with Marshalls etc. For instance you can kind of find certain vol spots where the hum is mitigated to a degree, not totally, but some. To be clear this is not a hum that is overpowering when you are playing full out, but in idle (standby off) or with pots rolled down on the instrument the hum is there. It's more hum than my other Marshall amps of similar circuit topography.
What has been done so far to divide and conquer with no improvement save some less hiss/buzz via lead dress tweaking and all new filtering.
1. All filtering replaced power supply/screens/preamp cap under the board
2. All Tubes swapped out for another set
3. Bias supply caps replaced and bias diode was replaced.
4. V2 lead dress green preamp socket wire (coming from the two 470k) has been raised up so nothing is crossing it.
5. caps/resistors around V2 have been checked in circuit (the 470k, 100k, 820r) all read within tolerance.
6. On V7 filament leads I get 3.13v AC on both sides (@ 115v wall AC) to ground (note this measurement around V3--V1 does not stay as even and is more like 3.08 & 3.10 on the other side of the tube (pin 4/5 & 9) when reading from the preamp tubes.
7. No DC on the pots that I can see.
8. Voltage readings on the sockets are within spec.
9. No loose connections around sockets that I can find or on the pre-amp board.
10. Ground is tight on the speaker jack.
11. NFB is attached on the speaker jack.
Possible issues or solutions?
1. The bias ground (on board 10uf caps) goes to the buss wire on the pots, could this need to go to the bias winding ground tap lug (by the pt) instead?
2. Should I create an artificial heater ground via two 100ohm resistors to ground on the filament leads and loose the center tap?
3. Shotgun everything around V2 (mixer caps, resistors, cap)
4. Replace the two 10k 2w dropping resistors on the preamp?
5. something around the input jacks?
6. Bad vol pot???
Thanks for any feedback
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