Mark Bass Little Mark III Blowing Fuse

[x-pro]

It is not necessary to unsolder transistors T22 T23 to check for short circuits. It can be checked directly on the board in the multimeter mode on the probe. Moreover, most parts can be checked in this way.

[misterc57]

In circuit with no power applied.

The outer pins (pins 1 and 3) of T22 T23 are about 90 ohms to each other.

Would this imply a short between pins 1 and 3?

[misterc57]

A lot going on in this power supply that I am unfamiliar with. Not your typical AC to PT to DC.

I read that T22 T23 are a switch but I do not understand the function.

I am only seeing about 10% DC from the incoming AC. Something is sucking the DC. T22 T23 seem like a possibility but I wonder about other components on the primary side of the PT.

[x-pro]

What's the yellow crocodile doing? What's being fed to it?

[misterc57]

What's the yellow crocodile doing? What's being fed to it?

That is my mains ground.

The yellow crock is connected to the Neg probe of my DMM

[misterc57]

My mains connection

[x-pro]

In circuit.

The outer pins (pins 1 and 3) of T22 T23 are about 90 ohms

Would this imply a short?

I need to measure between 2-3 of each transistor.

[misterc57]

I need to measure between 2-3 of each transistor.

318K in one direction, probes reversed 10M

[x-pro]

BR1 is OK

[x-pro]

"Example at about 25 VAC coming into the bridge I only see around + and - 2 VDC at the bridge outputs."

Apply this 25V to pins 3-4 of the bridge. Do you measure this voltage on these pins?

[misterc57]

"Example at about 25 VAC coming into the bridge I only see around + and - 2 VDC at the bridge outputs."

Apply this 25V to pins 3-4 of the bridge. Do you measure this voltage on these pins?

I can follow the VAC from where it enters the board to the two center pins of the bridge. There is no loss in VAC.

[misterc57]

BR1 is OK

Are you saying it is okay or asking?

I see about 10% DCV of my VAC inputs to the bridge.

Example. 25 VAC at the two center pins of BR1 and only seeing about + 2 VDC and - 2 VDC on the outer pins of BR1

[x-pro]

This is very strange when BR1, C39, C40, T22, T23 are in good working order.
There are no other elements that cause high current consumption, so that after the bridge it becomes 2V.

[x-pro]

Measure the transitions of the bridge diodes in the multimeter "diode" mode, and announce the voltage drops across their transitions, please.
There should be about 600-700mV on each bridge diode.
Do not supply power to the device!

[misterc57]

Measure the transitions of the bridge diodes in the multimeter "diode" mode, and announce the voltage drops across their transitions, please.
There should be about 600-700mV on each bridge diode.
Do not supply power to the device!

+ to AC pin 3, 0.541 VDC, reverse OL
+ to AC pin 4, 0.541 VDC, reverse OL

- to AC pin 3, 0.533 VDC, reverse OL
- to AC pin 4, 0.533 VDC, reverse OL

+ to -, 0.956 VDC, reverse OL


Readings are not always instant on the OL side and need a few seconds to settle.