Help with Montarbo W440A

[diydidi]

Hi All.
I have 3 of these for repair. Just had a quick look around the circuitry. The KSA709 (Q5&Q14) are getting quite hot.
Each of these are dissipating around 700mW. VCE measures around 78V, and there is about 7-9mA flowing through it.
Looking at the datasheet for these transistors, Pd max is 800mW @ 25 degrees celsius.
Is this normal?
I have linked part of the schematic.

[Enzo]

perhaps you could tell us what the problem is with the amps we are trying to repair. That might put it all in a context.

[diydidi]

Enzo. The outputs are noisy.
If I hit these transistors with freeze spray, it goes away. These are little TO-92 packages and are getting to hot to touch.

[Enzo]

If you have three of them, two channels each, that makes six identical circuits. If they are all measuring the same, my inclination wold be it probably works. We may find out different though.

Have you tried setting the idle bias down a little cooler?


And your theory may be correct too. Try substituting a beefier transistor in place or Q5 and find out.

[J M Fahey]

Agree.
MJE340/350 (don't remember which is the PNP) will work fine and dissipate way more, even without added heatsinks (which you may also add, a thumbnail size piece of 1mm aluminum is better than nothing)

And those were designed in Northern Italy, where it snows in Winter.
Not exactly South Africa.

What worked in Milan will be marginal somewhere else.

Not kidding, Fiat 600 cars were famous for overheating the back mounted engine here in Argentina, while they worked well in Milan.

[diydidi]

Thanx JM!!
You have just affirmed my thinking. Only thing is, the original transistors have their base pins in the middle.
Oh well, will see if I can make them fit.

[diydidi]

https://www.google.com/url?sa=t&rct=...zlrr2mD1TPclxl

[J M Fahey]

Are you Riaan?

[diydidi]

Yes it is I master!!

[J M Fahey]

I remember the Riaan Diederick name from somewhere else.
Is it from Pasteg or Basterd?
Or some other Forum?

Congratulations on your article.

Yes, Montarbo makes high quality stuff but I was always baffled by their poor biasing systems.

Having the sensing transistor fully uncoupled from actual heatsink means they have to underbias them for safety .... or else.

You did the proper thing.

And Hexfet Mosfet crossover distortion is even uglier than normal, because they are literally meant for *switching* so threshold is both very high (between 3.5V and 4V) and abrupt.

Ugh!!!

That said, once you go beyond that point, they work fine.