Mark Bass Little Mark III Blowing Fuse

[misterc57]

My friend dropped off his MB head for me to check out. The amp is 12 years old and it has seen tons of gigs. I replaced the blown 8A mains fuse and brought it up on a current limiter.

I am using a 60W bulb and the bulb starts to light up as I dial up the mains AC and at full AC (about 120 VAC) is glowing like a standard bulb. Seeing around 0.75 A current draw on the variac meter.

There is no DC on the output.

If I bypass the bulb limiter I am reaching 1 A current at about half way point (60 VAC) on the mains AC dialed up on the variac. I stop there. I do not think I should be exceeding 1 A.

Anyone know what the normal mains current should be reading for this amp?

I would like to try to isolate the excessive current draw but have doubts I will be able to easily isolate things such as decouple the power supply from the rest of the circuit.

For current and future reference purposes I am showing some pics and a service manual I found online.

Thank you, MC

parsek_mark-bass_little-mark-iii_sm.pdf

[x-pro]

• broach the subject
  https://music-electronics-forum.com/...d-ii-smps-woes

[Jon Snell]

I would disconnect CN3 & CN4.
Check the power supply is cool and the voltages are correct, then, allow the electrolytics time to discharge and check the bias voltage will adjust across R19 & R20 junctions with 1R0 2W resistors across both CN3 & CN4 to allow for current measurement and act as a fuse limiter.
We are looking for 20mA ish current with no load or signal.
Check for leaky MosFets; T11 - T16.

[misterc57]

• broach the subject
  https://music-electronics-forum.com/...d-ii-smps-woes

All that is out of my league...

[misterc57]

I would disconnect CN3 & CN4.
Check the power supply is cool and the voltages are correct, then, allow the electrolytics time to discharge and check the bias voltage will adjust across R19 & R20 junctions with 1R0 2W resistors across both CN3 & CN4 to allow for current measurement and act as a fuse limiter.
We are looking for 20mA ish current with no load or signal.
Check for leaky MosFets; T11 - T16.

Disconnect CN3 and CN4 (removed jumpers).

Using a bulb limiter

CN4 pins. -1.7 VDC on one pin, nothing at other pin
CN3 pins. 3 VDC on both pins

[misterc57]

D26 pin 1 and D27 pin 3 seeing 0.25 VAC

[misterc57]

I do not see on the schematic where the mains is connected to the power supply.

[Jon Snell]

Page 6 of the manual is where the mains enters. (see attached)
From your description, I would say you have a faulty power supply.

[misterc57]

Only seeing 8.5 VDC at T22 (IRFP27N60K) pin 2.

Difficult to measure most sectinos of the PS with the board in place. I have removed the board. I am suspicious of the bridge rectifier, I have very low DC + and -.

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

[x-pro]

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

Are T22 and T23 soldered out?

[misterc57]

Are T22 and T23 soldered out?

No. I was wondering if they could be pulling down the VDC

[x-pro]

C39, C40 check for short circuit

[x-pro]

BR1 is OK

[x-pro]

this translation is so contradictory...
Are there transistors T22 T23 on the board? If so, they are probably short-circuited.

[misterc57]

In circuit C39 and C40 show no shorts and tested good as caps.

T22 T23 are still attached. Nothing has been pulled yet. I guess removing T22 T23 is the next test.

In circuit T22 T23 did not show shorts.