70s Ampeg V4B Blows Fuses

[Enzo]

At this point, just to clarify where we are, am I correct that we have basically established that the amp does not blow fuses until the tubes are installed?

Also, crappy filter caps can drop bias voltage severely, it generally results in low bias, not completely absent bias. meaning even after replacing the filter caps, you may have an open rectifier diode or a resistor, or just a broken connection.

[ca7922303]

I should have not posted the title Blows Fuses. I have not had it blow a fuse yet. Guy I got it from told me it blows fuses and in my hurry to get advice on this amp, I prematurely posted it that way. But no, it does not blow fuses, tubes in or out.

[The Dude]

Let me rephrase. One end of C16 shows 2.0 the other end reads OL. Unable to get any other reading except OL at 200 or 600VAC.

If you are getting voltage on one side of the cap that is out of meter range (regardless of what it is) and only 2V on the other side, the cap is bad. Replace it.

Edit- and for clarification: Assuming we are talking about this cap:

[ca7922303]

I've got board#210803-1 and it seems identical to the schematic I'm looking at on the ampeg website dated 12/71.

[g1]

I should have not posted the title Blows Fuses. I have not had it blow a fuse yet. Guy I got it from told me it blows fuses and in my hurry to get advice on this amp, I prematurely posted it that way. But no, it does not blow fuses, tubes in or out.

If you do not have the proper DC bias at pin5 of the power tubes, you should not run it with them installed. If it does not blow the fuse it will red-plate the power tubes and damage them.
So double check that. Without the power tubes installed, you should have around minus 60VDC at pin 5 of all power tubes. Maybe it is there and you just need to dig in more with the meter probes.or maybe you weren't on pin5 when you checked before.

[ca7922303]

Underside of board #210803-1 less c15, c16, c23. R28 is burnt and OL on all ranges, will replace. Anything in photos look wrong?

[ca7922303]

Rest of board

[vintagekiki]

Rest of board

Replace damaged R28 look in schematic for value

[vintagekiki]

Underside of board #210803-1 less c15, c16, c23. R28 is burnt and OL on all ranges, will replace. Anything in photos look wrong?

Missing C16 (look in schematic for value). Therefore no bias voltage.
Missing R52, R53 (look in schematic for value). Slowly compare PCB layout with the schematic, see what is different, eliminate what is wrong, and then ask questions.
When you remove all what is wrong, remove the power tubes, measure whether all DC voltages on the rectifier are within limits, and then mount the power tubes.

https://www.ampegv4.com/images/schematics/V4B.jpg

[g1]

Missing R52, R53 (look in schematic for value).

I think these are sometimes mounted elsewhere, off the board. Maybe on the filter cap cans.

[vintagekiki]

I think these are sometimes mounted elsewhere, off the board. Maybe on the filter cap cans.

I also saw this mode of mounting, but in that case there should be a wired communication between the caps and the PCB (maybe I don't see it)

[g1]

there should be a wired communication between the caps and the PCB (maybe I don't see it)

Some wires connect to trace side of circuit board.

[vintagekiki]

Some wires connect to trace side of circuit board.

[g1]

I have no crystal ball, but I do have a V4 (non-B) chassis open here.

[vintagekiki]

https://www.doktorsewage.com/operating-theatre-ampeg-v4b/

https://www.doktorsewage.com/operating-theatre-ampeg-v-4b-part-2/