70s Ampeg V4B Blows Fuses

[ca7922303]

Remove C106 and then re-check V1 pin 3 voltage.

Cut one(1) leg of C106, V1 pin#3(11mv).

[ca7922303]

Cut one(1) leg of C106, V1 pin#3(11mv).

Will cut other leg if necessary. Will have to remove board to get to other leg.

[g1]

Also, found this wire loose. Could it causing some of the issues with preamp?

Look at the center lug of the treble pot. Is there anything connected there? That is where this wire should go.

[ca7922303]

Check preamp continued from #257

It is assumed that you have previously checked tubes V1, V2, V3 on another amp and that they are as correct as possible (usable)
Check the shielded connection cable from the external amp jack (J4 or J5 V4B) to the input jack external amplifier.

- Measure the voltage at pin 1 (V3).
- Measure the voltage at pin 10 (V201).
- Does the loud hum change its tone color with regulation bass, mid and treble (VR105, VR104 and VR103) pots.
What do you hear on the external amplifier?

If the signal described during check preamp (#257) does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted replace V3 (12DW7) first (tube V1 and V2 for now is OK). If you do not have 12DW7 for temporarily check, can be used 12AX7.
What do you hear on the external amplifier?

If and in addition to replace V3 (12DW7) signal does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted, replace the compactron tube V201 (6K11).
What do you hear on the external amplifier?

If the signal still does not pass to the external amp jack (J4 or J5 V4B) check (replace)
- C7 (.1), C9 (.47), C8 (.01);
- C205 (.1), C201, C202 (.01);
And that's all.

C7(.048), C9(.66), C8(10.75), C205(11.03), C201(9.8), C202(.03) all measurements are n f, set on cap test dmm.

[ca7922303]

Check preamp continued from #257

It is assumed that you have previously checked tubes V1, V2, V3 on another amp and that they are as correct as possible (usable)
Check the shielded connection cable from the external amp jack (J4 or J5 V4B) to the input jack external amplifier.

- Measure the voltage at pin 1 (V3).
- Measure the voltage at pin 10 (V201).
- Does the loud hum change its tone color with regulation bass, mid and treble (VR105, VR104 and VR103) pots.
What do you hear on the external amplifier?

If the signal described during check preamp (#257) does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted replace V3 (12DW7) first (tube V1 and V2 for now is OK). If you do not have 12DW7 for temporarily check, can be used 12AX7.
What do you hear on the external amplifier?

If and in addition to replace V3 (12DW7) signal does not pass to the external amp jack (J4 or J5 V4B) or is weak or distorted, replace the compactron tube V201 (6K11).
What do you hear on the external amplifier?

If the signal still does not pass to the external amp jack (J4 or J5 V4B) check (replace)
- C7 (.1), C9 (.47), C8 (.01);
- C205 (.1), C201, C202 (.01);
And that's all.

Tone color changes with mid-range control pit and rocker switch only.

[vintagekiki]

Tubes V1, V2, V3, V4, V201 still not lit, heater voltage (6.3VAC) somewhere is interrupted.
Check do you have on preamp PCB, voltage (6.3VAC) on pin4&5 and pin 9 (preamp tubes), which comes from pin2 and pin7 tube V5 (6L6GC) (PCB red right)
Check do you have on EQ PCB, voltage (6.3VAC) on pin1 and pin12 tube V201 (6K11) which comes from from preamp PCB pin4&5 and pin 9 (PCB red left)
When all the tubes are lit, you can perform the required VDC measurements.

https://ampeg.com/support/files/Schematics/V%20Series/V-4B%20(1971)/V4B%20Schematics.pdf

[ca7922303]

C7(.048), C9(.66), C8(10.75), C205(11.03), C201(9.8), C202(.03) all measurements are n f, set on cap test dmm.

Do any of these caps require replacing due to these measurements?

[vintagekiki]

Do any of these caps require replacing due to these measurements?

Provided that you accurately measured, any caps which deviating from the schematic value */- 10%

When all tubes filaments are not illuminated (lit), all measurements are unusable.
When all tubes are lit and heat up, you continue with measurements. see #276
thanks

[ca7922303]

Tubes V1, V2, V3, V4, V201 still not lit, heater voltage (6.3VAC) somewhere is interrupted.
Check do you have on preamp PCB, voltage (6.3VAC) on pin4&5 and pin 9 (preamp tubes), which comes from pin2 and pin7 tube V5 (6L6GC) (PCB red right)
Check do you have on EQ PCB, voltage (6.3VAC) on pin1 and pin12 tube V201 (6K11) which comes from from preamp PCB pin4&5 and pin 9 (PCB red left)
When all the tubes are lit, you can perform the required VDC measurements.

https://ampeg.com/support/files/Schematics/V%20Series/V-4B%20(1971)/V4B%20Schematics.pdf

Can you explain what you mean by (red right) and red left)?

[vintagekiki]

Can you explain what you mean by (red right) and red left)?

Look at the attachment #276 and everything will be clear to you what is left, what is right and what is red color.

Check do you have on preamp PCB, voltage (6.3VAC) on pin4&5 and pin 9 (preamp tubes), which comes from pin2 and pin7 tube V5 (6L6GC) (PCB red right)
Check do you have on EQ PCB, voltage (6.3VAC) on pin1 and pin12 tube V201 (6K11) which comes from from preamp PCB pin4&5 and pin 9 (PCB red left)

[g1]

Did you connect that wire to the treble control yet? If not, the preamp can not work. So no point in checking for bad capacitors etc. until you connect that wire.
See red arrow on picture below. It looks like the ground (shield) wire of the cable is still connected, but it is shown in the drawing as the dotted line. VR103 is the treble control.

And when you measure caps with your meter, one leg must be disconnected or the measurement is meaningless.
Once you connect that wire and fix the intermittent heaters (bad connection), the amp will probably work. Then you can look at other issues as required.

[ca7922303]

Did you connect that wire to the treble control yet? If not, the preamp can not work. So no point in checking for bad capacitors etc. until you connect that wire.
See red arrow on picture below. It looks like the ground (shield) wire of the cable is still connected, but it is shown in the drawing as the dotted line. VR103 is the treble control.

And when you measure caps with your meter, one leg must be disconnected or the measurement is meaningless.
Once you connect that wire and fix the intermittent heaters (bad connection), the amp will probably work. Then you can look at other issues as required.

Yes, I was able to reconnect the wire to board which has a trace to center lug of vr103.

[ca7922303]

Tubes V1, V2, V3, V4, V201 still not lit, heater voltage (6.3VAC) somewhere is interrupted.
Check do you have on preamp PCB, voltage (6.3VAC) on pin4&5 and pin 9 (preamp tubes), which comes from pin2 and pin7 tube V5 (6L6GC) (PCB red right)
Check do you have on EQ PCB, voltage (6.3VAC) on pin1 and pin12 tube V201 (6K11) which comes from from preamp PCB pin4&5 and pin 9 (PCB red left)
When all the tubes are lit, you can perform the required VDC measurements.

https://ampeg.com/support/files/Schematics/V%20Series/V-4B%20(1971)/V4B%20Schematics.pdf

Not clear on exactly what your asking here so I'll give you vac and vdc for every tube you mentioned. V5 pin#2(19vac/8vdc), pin#7(15vac/8vdc), V201 pin#1(15vac/10vdc), pin#12(19vac/10vdc), V1&V2 pin#4 & 5(18vac/11vdc), pin#9(22vac/11vdc), V3 pu #4 & 5(17vac/9vdc), pin#9(20vac/9vdv), V4 pi #4 & 5(15vac/8vdv) pin#9(19vac/8vdc).

[g1]

V5 pin#2(19vac/8vdc), pin#7(15vac/8vdc), V201 pin#1(15vac/10vdc), pin#12(19vac/10vdc), V1&V2 pin#4 & 5(18vac/11vdc), pin#9(22vac/11vdc), V3 pu #4 & 5(17vac/9vdc), pin#9(20vac/9vdv), V4 pi #4 & 5(15vac/8vdv) pin#9(19vac/8vdc).

Possibly the hum balance pot is bad so measuring to ground is giving poor results. Put one probe on each of the pins mentioned and measure AC volts. For example, V5, one probe on pin2, other probe on pin7.

[Enzo]

To expand on g1's post.

The tube heaters must have current THROUGH the heater. So we measure for AC voltages ACROSS th heater, as in end to end. SO we don't want pin 2 to ground and pin 7 to ground, we want pin 2 to pin 7.