Mark Bass Combo Head II SMPS Woes

[tdlunsfo]

RT2 measures 2.6ohm, D24-D27 have already been checked out of circuit and test good with diode check. C39 and C40 checked good ESR with one leg desoldered. I ordered more parts today. My plan is to do as you suggested and to supply power to the chip to check the pulses going to the mosfets with my scope (it's only 50Mhz but will give it a try). The chips I currently have do not appear to be behaving correctly or my scope is struggling to read the pulses and I'm hoping a new chip will give me that answer. I did see where provisions were made on the board to separate the 80v rails if needed. When I get to the point where I feel okay applying mains power I will likely use my dim bulb assuming I can get enough voltage to startup the chip.

[tdlunsfo]

Here is my waveform on pins 5 and 7 on a brand new chip. Note the volts/div difference. I have a good strong pulse out of the Lo pin but something odd out of pin 7 (HO). This is with 11v from my bench dc supply on pin 1. Pin 4 is being used for the ground for the DC supply and the oscope probes. No FETs installed so I am wondering if the lack of Vb and Vs is causing this. Thoughts?

[x-pro]

If you look closely at the topology of the top and bottom output switches, you will see that the bottom switch has a gate capacitance associated with its source. This is your the ground for the DC supply and the oscope probes. And the upper switch has a source that is not connected to that point. It has its own gate control line. Hence the different oscillograms.

[tdlunsfo]

x-pro "bottom switch has a gate capacitance associated with its source"

Are your referring to C69? If so this also connects to pin 4 of the IR2153D. If I understand correctly, I need to move my ground for the upper fet gate signal?

[x-pro]

No. It's not about C69.

[x-pro]

x-pro "bottom switch has a gate capacitance associated with its source"

I need to move my ground for the upper fet gate signal?

yes,
But only for the oscilloscope probe

[x-pro]

If you don't have an AC line isolation transformer when testing the combo on AC power, turning on the oscilloscope and measuring with it is fraught with failure of the oscilloscope

[tdlunsfo]

Understood on the scope and isolation. I actually learned that the hard way years ago but luckily was on a dim bulb. No AC applied in this case, and just testing waveforms using the DC bench supply. I'll move the other scope probe and see if my results change. Just for clarification if not C69 then which capacitor are you referring to? Are you referencing something internal to the chip?

[x-pro]

A field-effect transistor with an isolated gate has some gate capacitance, inside the chip

[tdlunsfo]

Got it. Here is the new image with the HO scope probe moved to pin 3 of the HO FET. With my limited knowledge this looks okay to me with adequate dead time. Thoughts?

[tdlunsfo]

Well... it lives. I installed the FETS, plugged the amp into my dim bulb and the dim bulb into my variac and it came up. It played audio and stayed alive on the DBT for about an hour, so I removed the DBT and played it through a dummy load for about half an hour at around 10W. It performed flawlessly. I never did find the smoking gun though. I replaced the FETs, the IR21531 and R78. I'm betting the FETs got too hot in this little combo cabinet and shorted taking out R78 and the chip.

[Mikestorm]

As the title suggests, I was begged to try my hand at fixing a Mark Bass and I reluctantly agreed even though I have only worked on a handful of class D, SMPS designs. Apparently nobody else in town would touch it and he was desperate. So... I'm the sucker...lol.
Findings: Blown mains fuse and shorted Mosfets in the power supply (T22/T23). It seems this is pretty common when these amps fail and simple enough to replace but here is the rub... I can't find anything else wrong with it. While in circuit, the Final Mosfets test good as well as the dual diodes in the secondary D24-27. I've checked all around this board and everything checks out. No other shorts. So, I replaced the mosfets as well as the driver chip, fired it up and pop... back to square one.

So, obviously this amp still has another failure somewhere that I am missing. Any thoughts/experiences? My next step was to start removing the finals and dual diode and test them out of circuit just to be sure. I guess I could reorder the parts, disconnect the F1 fuse and the +/-80V supplies and fire up the SMPS on my variac to see what happens (probably what I should had done the first time but we live and learn). I stole the schematic below from another thread so please ignore the X's, or at least the X on D25... for now.

Hi to all

I have received a Mark Bass head amp, and it just wont turn on, no shorts, but completelly dead.

One question:

At Vdc point of the first part of the power supply, in other words between the positive of C39 and ground, how many volts I should see?

thanks in advance
Mike

[x-pro]

What is the mains voltage at the outlet?
Multiply this voltage by 1.4 and you get the desired

[tdlunsfo]

The voltage at Pin 1 will be approx 12v. R78 was blown on mine along with the FETs/chip

[x-pro]

The voltage at Pin 1 will be approx 12v. R78 was blown on mine along with the FETs/chip

Again? This means that the 21531 chip was not getting enough power from C70 and was constantly powered by R78.
R78 is only needed at startup, then power comes from the transformer winding via C67 D34 C69 R80 D21 C70 D40 C52.
Again. If the voltage at CN5 is 110V, then Vdc=110*1.4