Roland Blues Cube SMPS issue

[saint gillis]

I'm trying to fix the PSU of a Roland Blues Cube combo. I've retro engineered the primary side of it, there seem not to be any feedback loop in the SMPS :



On the secondary, there are some little 1.6A fuses, then diodes, inductors, and filtering capacitors.
First time I had my hand on it, one of the fuses, and one the diodes was blown on the secondary, I've changed them and I've also changed some 100µ filtering capacitors on the secondary. The mosfets also were blown.

Then I've changed U1, Q1 and Q2. The amp worked fine for about 30min, then boom again.. U1, D3, Q1, Q2, Q3 and R7 dead...

I thought it was because I've used AOTF20N60 for Q1/Q2, as the AOTF20N60s have a weak power dissipation factor... Soooo, I've replaced them with IRF740 as at the origin, replaced U1, D3, Q3 and R7, the amp worked fine... for about 30min again.... BOOOOM, U1, D3, Q1/Q2 dead...

I'd like to avoid to replace the mosfets and the driver ad libitum... anyone experienced with this kind of circuit ?

 

[Khron]

First time I had my hand on it, one of the fuses, and one the diodes was blown on the secondary

Tracking down the possible reason for this might be interesting..? Which power rail are those on?

Are that fuse & diode still in one piece, after the repeated primary-side failures?

What about the current-sense circuitry? As in, the stuff between R11 and Q3.

 

[saint gillis]

Tracking down the possible reason for this might be interesting..? Which power rail are those on?

A -12V audio rail

Are that fuse & diode still in one piece, after the repeated primary-side failures?

Yes, they're all ok, even for other power rails

What about the current-sense circuitry? As in, the stuff between R11 and Q3.

Only Q3 seems to suffer on each fail..

 

[Khron]

Especially on first-start-after-repair, you'd want to hook SMPS's to the mains via a series tungsten-bulb limiter

That's an odd-but-clever-but odd charge-pump (D1-D2-C3) they used, for the chip to power itself after start-up, though. No series current-limiting resistor (in circuits powered by an auxiliary winding on the transformer, you'd see a 10-47 ohm resistor before or after the rectifying diode). Wonder if everything is still ok there...

 

[NoisyIndividual]

I'm trying to fix the PSU of a Roland Blues Cube combo. I've retro engineered the primary side of it, there seem not to be any feedback loop in the SMPS :

View attachment 139555

On the secondary, there are some little 1.6A fuses, then diodes, inductors, and filtering capacitors.
First time I had my hand on it, one of the fuses, and one the diodes was blown on the secondary, I've changed them and I've also changed some 100µ filtering capacitors on the secondary. The mosfets also were blown.

Then I've changed U1, Q1 and Q2. The amp worked fine for about 30min, then boom again.. U1, D3, Q1, Q2, Q3 and R7 dead...

I thought it was because I've used AOTF20N60 for Q1/Q2, as the AOTF20N60s have a weak power dissipation factor... Soooo, I've replaced them with IRF740 as at the origin, replaced U1, D3, Q3 and R7, the amp worked fine... for about 30min again.... BOOOOM, U1, D3, Q1/Q2 dead...

I'd like to avoid to replace the mosfets and the driver ad libitum... anyone experienced with this kind of circuit ?

It is important to ensure any parts you replace, particularly the Mosfets and the controller IC, are from genuine sources.
Otherwise they may be fake.

Genuine does not include Ali Express or Ebay.

 

[saint gillis]

Yes I always use geniune good brands parts