70s Ampeg V4B Blows Fuses

[ca7922303]

Are those 2 wires pinched to the chassis anywhere? Can you post a picture of the switch pointing to where those 2 wires are connected?

They are not pinched to the chassis.

[ca7922303]

R14/15 with C106/112 one leg lifted now read 3.3k in both positions of ultra lo switch.

Is it possible that these two(2) caps could be left out of circuit and amp operate correctly?

[ca7922303]

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

R14/15 with C106/112 one leg lifted now read 3.3k in both positions of ultra lo switch.

[ca7922303]

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

C106(0.208 nf), C112(0.198 nf), black probe to lifted leg - side, red probe to + solder joint on board, cap test on dmm.

[g1]

Wire from ultra low switch is soldered to + caps point C106/R107 and point C112/R108.

Are those 2 wires pinched to the chassis anywhere? Can you post a picture of the switch pointing to where those 2 wires are connected?

[ca7922303]

Wires show continuity, check that are soldered where should (on schematic).
Is the wire from ultra lo sw soldered to +caps point C106/R107 and point C112/R108, or soldered to -caps C106, C112 (gnd)

Wire from ultra low switch is soldered to + caps point C106/R107 and point C112/R108.

[vintagekiki]

Correction
mark C107 should be replaced with mark C112 .

[g1]

Please note that there's an error on the layout drawing for the polarity of C106 and C112. See red markings on this pic for corrections. The actual circuit board is probably marked correctly.

[vintagekiki]

Wires on ultra lo switch (4 on schematic), all show continuity. Will post measurements for c106/112 shortly. Have to lift one leg if each cap to measure correctly.

Wires show continuity, check that are soldered where should (on schematic).
Is the wire from ultra lo sw soldered to +caps point C106/R107 and point C112/R108, or soldered to -caps C106, C112 (gnd)

[ca7922303]

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

Wires on ultra lo switch (4 on schematic), all show continuity. Will post measurements for c106/112 shortly. Have to lift one leg if each cap to measure correctly.

[vintagekiki]

R14(0.8 ohms), R15(0.8ohms), R202(0.55k), R203(6.25k). R14/R15(3.08k) switching ultra lo.

R14(0.8 ohms) check (replace) C106 (6.8u)
R15(0.8 ohms) check (replace) C112 (6.8u)
Check that the wire from ultra lo sw is soldered correctly (+ caps C106, C112)
Measurements indicate that the wire from ultra lo sw is soldered incorrectly (to gnd), or the caps C106, C112 is bad.
Caps C106, C112 are located on the PCB below the ultra lo sw.
R202(0.55k) OK,
R203(6.25k) OK
R14/R15(3.08k) switching ultra lo OK

[ca7922303]

Is that one lug with no wire that's bent upward touching anything?

No, not touching anything.

[g1]

Photos of ultra lo switch

Is that one lug with no wire that's bent upward touching anything?

[ca7922303]

More a danger to the tubes V1 & V2. You need to find and deal with the problem. Can you get in close enough to the switch to post some pictures?
Also, I think you still have the issue where the heater voltage on the preamp tubes is lower than it is on the power tubes. (5.6VAC vs 6.3V). It should be the same for all tubes.

Photos of ultra lo switch

[g1]

Strong signal now thru speaker output. Greatly appreciate all the help. Is there any risk of blowing resistors if I switch ultra lo?

More a danger to the tubes V1 & V2. You need to find and deal with the problem. Can you get in close enough to the switch to post some pictures?
Also, I think you still have the issue where the heater voltage on the preamp tubes is lower than it is on the power tubes. (5.6VAC vs 6.3V). It should be the same for all tubes.