Markbass Little Mark II

[J M Fahey]

Hi Mark.
I live in Argentina and I use EI42 because that's what's abundant here.
To be more precise, I have access to a factory which makes them, for me, on a custom order basis.
They just make the powder, and press and sinter it in whatever die they have available, my choice.
Their largest regular one is the one I mentioned.
The cost is nil : I get 200 E42-21-15 for U$100.
When I go to pick them up personally, I will have a long talk with them about reverb tank magnets.
They can make them for me, although the 5000 unit minimum order is still a little steep for me.
Maybe I'll have to provide the die.

[MarkusBass]

Please note that there are different core materials possible and not all of them can work with 125kHz switching frequency. I suggest checking ferroxcube catalogues.
Another problem is that E42 has square bobbin whereas ETD34 has round bobbin. It makes winding the transformer (with ETD34) much easier. And with rounded bobbins you get better parameters of the transformer (lower leaking inductance because windings can be better coupled). I would test first whether E42 can be used in such a SMPS .

Mark

[J M Fahey]

Yea, I know.
These guys provide datasheets, but I always *measure* everyhing.
Their material is good, though.
I know people who uses it; I even built many 12V to +/-42 or +/- 52 converters for car amplifiers and Street use PA systems with those EI42 cores; now I'm going the other way, rectifying 220 VAC to 308VDC and turning that into +/-70V.
I don't *need* regulation , in fact, remember I'm replacing a heavy 50Hz iron transformer with which I am very happy, except for its weight.
Winding those square bobbins (I do not have a supplier for them yet, the tests will be with homemade *cardboard* ones) will be much easier than winding the much larger, many many more turns square conventional ones; besides I will use no more than 30 or 40 a Month, at best.
The problem that IC solves is that I needed a half bridge driver, not an easy thing to do because both halves of the circuit rest 150V apart.
In fact I do not know how *they* do it.
My 12V to +/- rails converters were push-pull , driven by oscillating CD4027's, believe it or not.
The same solution (not yet discarded) would force me to use at least 800V or better yet 1200V Mosfets, not an easy thing to get.
Link to these fascinating guys:
Ncleos E
Pagina nueva 3
Even more fascinating is that they custom build for *me*, incredible !!.
They were former Engineers at the local Philips plant, which was dismantled in the late 70's, they pooled their savings and bought most of the machines, at scrap iron prices.
They already had the know how.

[MarkusBass]

OK, so they most probably are able to wind the transformer for you. I have also such a company - they would be happy with an order of 100 pieces but I usually order 1-2 pieces .

Mark

[Techknowman]

Have you noticed that pins 1 and 4 (Vcc and COM) of IC5 are reversed on the schematic (if this is IR21531)?

I noticed on the one i'm working on. I was hoping it was an error in the schematic but it is not.
I verifyed continuity to ground at pin 1.
So the IR21531 is not a drop in replacement as far as I can tell. I have looked at just about every self oscilating half bridge drive they sell and nothing that has pins 1 and 4 swapped.
I'm stuck trying to find one of these. Any one have any luck?

[Steve Conner]

Maybe they bent the leads upside down as well as sanding off the markings?

I can't think of any other explanation, as all the other pins, and the circuit, match with the IR2151/2153/21531, and I don't know of any other popular chip that does the same thing.

[Techknowman]

Maybe they bent the leads upside down as well as sanding off the markings?

No the old chip has the manufacturing date stamp embossed on the bottom and the pin 1 detent dent / mark is still visable from the top

On the bottom the only print visable that is silk screened on the part is 3007-S next to the manufacturing date stamp.

[MarkusBass]

I noticed on the one i'm working on. I was hoping it was an error in the schematic but it is not.
I verifyed continuity to ground at pin 1.

I wonder what exactly you are comparing? Schematic with datasheet, schematic with real amp, or datasheet with real amp?
Is the continuity to ground at pin #1 measured with the IC soldered? Isn't the IC shorted? In my opinion there is a mistake on the schematic, this is IR21531 and the IC in your amp is shorted (or a capacitor or some other part that is connected in parallel). Can you actually see the connection, or you just measued it with a diode tester? Can you check the same with the IC desoldered? I don't think that they bent the leads upside down - this would be crazy.

EDIT: I see that you "desoldered" it :-). I still think that some other part may be shorted and the IC could be OK.

Mark

[Techknowman]

I was comparing schematic to readings. I put a socket where the chip used to be and now it looks like things are correct. 1 to VCC, 4 to ground
have replaced the two mosfet drivers and have to order an IRF240 for 1 shorted channel driver / output

[MarkusBass]

I was comparing schematic to readings. I put a socket where the chip used to be and now it looks like things are correct. 1 to VCC, 4 to ground
have replaced the two mosfet drivers and have to order an IRF240 for 1 shorted channel driver / output

I was comparing IR21531 datasheet to real amp and I'm almost 100% sure that everything was correct. And on the schematic there is an error (or it is done on purpose to confuse people who try to fix the amp).
Now I'm also slightly confused. If you are saying that "1 to VCC, 4 to ground" (measured in the amp), so this is exactly as in IR21531 datasheet. Why do you think that this is not IR21531 (assuming that you understand that there may be a mistake on the schematic)? I doubt that there may exist e chip with exactly the same functionality and with two pins reversed (especially power supply pins).

Mark

[Techknowman]

Ya for some reason it was messing with my head! I checked and check and it "seamed" to match the schematic. but i'm now confident the schematic is wrong!
I haven't powered up yet using the IR21531 yet. I have a bad mosfet output on the unit I've ordered and waiting on. one IRFP240 with a gate to source short.

[MarkusBass]

What about the gate resistors and Zener diodes, are they OK? Do you change just one transistor, or all three? The new one may have different Vgs(off).
BTW, do you know about Chip Quik that allows you to desolder ICs like you had in the amp without destroing it: Chip Quik: Easy removal of surface mounted devices ?

Mark

[bpucekov]

One question for owners markbass litle mark tube amplifier.
Is it normal that this amp is very warm. I mean, heat sink of the output transistor is heated to 80 degrees Celsius.
The amp I got broken, I repaired it, the amplifier now operates normally. I was just wondering is it normal to be so very warm.
Thanks in advance
Branko

[Pbassred]

I have a 800 tube. It does not get warm. Then again I don't run it hard. There is a lot of air going through it and the case isn't phisically connected to the heatsink so I would not expect the case to get hot.

How hot?

[verano]

Anyone got an idea which driver ic is used ? IC 5... ?