Ampeg B-410 Blown Fuse Diagnostics Help

[misterc57]

Hey Folks. I have a B-410 head that I am hoping to bring back to life.

Initial observation is no power. There are two power supply fuses in series. A 6A and a 4A. The 6A is blown. I do not understand why it is set up this way. There is also a convenience outlet in the mix.

The 6A fuse is internally soldered and the 4A fuse is in a chassis fuse holder. Why are there two fuses and can I get away with using one fuse for testing purposes by jumping the blown internal fuse? What value fuse is needed if using one fuse?

There is also a white gizmo in this fuse network that looks like it could be a circuit breaker but I am not sure exactly what it is or how to confirm it is not failing. I believe in the schematic it is depicted as the component between the power /polarity switches and the primary side of the PT.

Schematic is attached.

With P2 disconnected I did jump the 6A fuse just to see if there was AC in the secondary of the PT, and there was AC.

Thank you, MC

ampeg_b115_b410_sch.pdf

[misterc57]

Some pics

[The Dude]

The internal fuse is an "idiot" fuse in case someone installs a fuse value in the user fuse holder that is larger than recommended. The "white gizmo" is likely a voltage selector for 120V or 240V operation (can't see the other side in pics). If the internal 6A fuse blew, it's likely someone over-fused the external holder. The unit should be fused at 4A. You can temporarily jumper the internal fuse while you work on the unit, but I'd restore it to factory fuses when the repair is complete. There must be a short somewhere if it took out the 6A fuse. You'll need to track it down.

[g1]

The white block seems to be what is called the 'interlock switch' in the schematic. Not sure what it is interlocking.

[misterc57]

More pics of the interlock switch and power board.

So the amp is drawing high current. I am trying to find the issue(s).

Was looking to lift one leg of R305 and R306 (5W, 680 ohms) to isolate the power supply, but I cannot locate those resistors, or D301 and D302. I will keep looking but there are no components marked over 300 on the board.

[misterc57]

I am also noticing that P2-1 and P2-10 show +/- 16V after R305 and R306, yet the upper section of the schematic show the same points at +/- 56.5V

So I think R305, R306, D301, D302 do not exist.

And, it looks like one of the large caps, C303 or C304 was replaced with a 50V rated cap which is not a high enough V for the 56.5 V shown on the schematic output board.

[misterc57]

I am also noticing that P2-1 and P2-10 show +/- 16V after R305 and R306, yet the upper section of the schematic show the same points at +/- 56.5V

So I think R305, R306, D301, D302 do not exist.

And, it looks like one of the large caps, C303 or C304 was replaced with a 50V rated cap which is not a high enough V for the 56.5 V shown on the schematic output board.

I found these components that I thought were missing on the other end of the chassis where it supplies V to the preanp boards.

[g1]

The interlock switch looks like it may be set up to disconnect power if the heatsink is not mounted to the chassis.

[misterc57]

The interlock switch looks like it may be set up to disconnect power if the heatsink is not mounted to the chassis.

That is interesting

[misterc57]

Trying to find the excessive current issue I struggle again with diagnostics on SS circuits.

I touched every leg of every transistor, diode and cap to test to ground and found nothing shorted. I tried to test transistors and diodes in circuit but can not tell if there is a failure.

I removed the connections (which are screwed on) to the new large filter cap and it tested fine.

I disconnected P1 which supplies power to the preamp boards. Still have excess current.

I hate to remove all components for testing. How does one proceed?

Thank you

[Helmholtz]

I am also noticing that P2-1 and P2-10 show +/- 16V after R305 and R306, yet the upper section of the schematic show the same points at +/- 56.5V

So I think R305, R306, D301, D302 do not exist.

The opamps in the preamp use +/-16V, the power amp is supplied from +/- 56.5V.
The power amp supply is not explicitly shown on the schematic but is probably taken from before R305/306.

[Helmholtz]

As always, remove load (speaker) and check for DCV at output.

[The Dude]

The most common failure of a SS amp is shorted output devices, so I'd start by checking those. Test the actual parts and look for shorts between C-B-E. A test to ground is somewhat meaningless.

[misterc57]

As always, remove load (speaker) and check for DCV at output.

In this case I cannot get to that point as the amp is pulling way to much current. I am dialing up on a variac and it starts to draw too much current with only a small amount of AC applied. Should I use a bulb limiter here and then test for DC on the output?

[misterc57]

The most common failure of a SS amp is shorted output devices, so I'd start by checking those. Test the actual parts and look for shorts between C-B-E. A test to ground is somewhat meaningless.

Will do. When checking these should I reverse leads and check in both direction?