Mark bass Little Mark 3 repair

[diydidi]

Hi Guys.
I have been trying to repair 3 of these buggers.
They all had the SMPS blow and the amplifier sections. The power amplifier is easy to fix. Mainly just mosfets.
The smps on the other hand is a bit of a bitch.
Firstly I tracked down the controller chip. Its a IR21531D. I couldn't find the correct mosfets (IRFP27N60K), so I put in some STW26NM60N Fets.
The switcher now works and all lights on, no DC offset at amp output. BUT!, the voltage rails for the amp measure ±108V instead of the indicated 80V as in the schematic (see attached).
I soon realised the smps has no feedback from the secondary side, so How will it know to bring down to 80V? It can't regulate it at all.
This means the Fets need to have the proper specs to ensure the 80V output. Maybe they need to have certain on off times? Maybe a gain issue?
Can someone elaborate on this? I can get the original Fets, but will take weeks of shipping and they are damn pricey!!
Is there a way to bring this voltage down? Maybe by changing the timing cap or resistor??

[Enzo]

So it is free running. Most linear supplies are free running too, in their own way.


Note the output of the switching drive at C68. See how ther are three taps on the primary winding on the switching transformer, and three sets of jumper connections to the C68? Which set are shorted together now. I suspect those jumpers adjust the output. If it is not there, try J3.

[MarkusBass]

Is there a way to bring this voltage down? Maybe by changing the timing cap or resistor??

It looks like you need original transistors but if you find it difficult, there are jumpers on the primary winding of the transformer which allow changing the output voltage. Of course, before you try it, make sure you know how to change it (it was described in some other schematic form Markbass).

Mark

[diydidi]

It looks like you need original transistors but if you find it difficult, there are jumpers on the primary winding of the transformer which allow changing the output voltage. Of course, before you try it, make sure you know how to change it (it was described in some other schematic form Markbass).

Mark

Hi. I should have mentioned it, but I have the jumper on the outer winding. This was in an effort to get the voltage lowest. In the original center position, the output voltage was even higher. Something like 118V or so if I remember correctly.

[MarkusBass]

I should have mentioned it, but I have the jumper on the outer winding.

Yes, you should have mentioned this . In this case, can you explain the problem with the original transistors? I can get them for tomorrow and they are not that expensive. What do you mean by high price and long delivery?

Mark

[diydidi]

Markus, a local company here in South Africa, Mantech, require I buy a minimum of 25 Pieces. They are almost 250 South African Rands each. thats about 16USD each.
I know one shouldn't do the direct conversion.
I was just hoping there is a modification I can do to make other Fets work...

[MarkusBass]

Can you check here: IRFP27N60KPBF Vishay Semiconductors | Mouser ? This is Mouser South Africa. Minimum quantity is 1 and the price is twice lower.

Mark

[diydidi]

Can you check here: IRFP27N60KPBF Vishay Semiconductors | Mouser ? This is Mouser South Africa. Minimum quantity is 1 and the price is twice lower.

Mark

Thanx Markus. I just placed an order. Ordered 25 and shipping is free. You're a life saver!!!

[diydidi]

So I got the original Fets today. They work!!
So there you have it folks. One has to use the original Fets to get the secondary voltages exactly right.
If anyone has found alternative Fets that will work here, please share for future reference.

[km6xz]

The timing of the switch transistors is important to match those of the originals. Heating results from too slow transitions from high to low and low to high. Gate capacitance impacts that.

[MarkusBass]

I'm glad that you solved the problem. I wonder which parameters are important in this case. Looking at datasheets I see that both input capacitance and turn-on delay time are different for the MOSFETs.

Mark