Markbass Little Mark II a new post

[Techknowman]

I didn't want to keep taging info onto the bottom of an old thread.
Anyway, I have one of these Markbass Little Mark II.
and on the schematics found on the internet pins 1 and 4 are wrong on the drawings.
I did install a IRS2153D in the place of IC5, replaced T22 and T23.
the main board has jumpers so that you can disconnect the +/- 80v rails from the outputs which I did to start up the unit.
Hooked it up to my variac and powered up slowly.
Unit came up to power, kind of the unit is generating +/- 80V at C45 and 46
no +15 or -15
+18V is low but its there
CN6 pin 1 is 170vdc+, pin 3 is +8v

The SMPS is making a clicking sound at about 10 to 15hz this gets a little faster and the voltages at the testpoints get less stable as the voltage at the variac is moved around at 110-120VAC.
Its not drawing a lot of current, less than 2amps at 110VAC.

I have checked F1 and F2 (on board PCB fuses) they are fine.

I'm at a loss to what is the issue. Any ideas?

[Enzo]

So you disconnected the +/-80v from the power amp. The SMPS looks like a stand alone unit, so did you disconnect all the other secondary voltages from the amp circuits? DO so.


SMPS going tick tick tick usually means to me that it is trying to start but is loaded down and stopping itself. Tick tick tick? Ve haff vays of making you tock.

My initial reaction is something loading down the secondary. Other than the main 80v, you have three other branches, each fused. Lift those fuses. COnnect them one at a time. With F3 only, does the B+ come back? Or at least we stop ticking? WIth F2 only, does +12 come back? And with F1 only, do the +/-15 and the 18 come back?

If none of those work by themselves, then we have a more central problem. But if some of those work, but one set does not, then that one may be the whole problem.

When I have a problematic SMPS, at this point in life I just start right out by checking all the rectifiers for shorts. SO D29,30,31,32,33. ANy shorted? And each of those suppl;ies has several filter caps. Check each for shorted. Even the ones after those regulators.


And just for grins, over on the primary side, pins 1 and 4 are the power to the IC. What voltage is there between them?

WARNING: remember this SMPS rectifies the mains directly, it is extremely dangerous. On the primary side, all those circuits are NOT REFERENCED TO GROUND. THAT EMPTY TRIANGLE SYMBOL IS NOT GROUND. The circuit reference is to the negative side of the rectified mains. SO just carefully measure from pin 1 to 4 directly.


Just in case, the 270k R78 provides power to the IC for startup. But the IC really powers itself after power up. The lower right circuit, zener D21 then through D40 are what keeps it running. If that little powr supply fails, the IC will try to run on just that kicker resistor. Long shot but worth a look.


You say pins 1 and 4 are wrong?

[MarkusBass]

If the fuses are OK, I'd say that either +/-15V rails are shorted, or RG1/RG2 are shorted. Is there a voltage on C64, C65?
I told you that there is a mistake on the schematic (pins 1 <--> 4).

Mark

[Techknowman]

I pulled all fuses up and it still ticks.
I'm afraid the transformer may be bad or has some issue.
The +and - going into the RG1/RG2 do have a small amount of voltage, +/- 3v but not enough to exceed the dropout voltage of RG1/RG2 so nothing comes out.

WARNING: remember this SMPS rectifies the mains directly, it is extremely dangerous. On the primary side, all those circuits are NOT REFERENCED TO GROUND. THAT EMPTY TRIANGLE SYMBOL IS NOT GROUND. The circuit reference is to the negative side of the rectified mains. SO just carefully measure from pin 1 to 4 directly.

Yea im not able to use my scope in there because its ground and the "ground" you mentioned would not get along. I can of course use my meter. I have an isolation transformer somewhere for my scope. 110Vac 1:1 unit for just such an occasion. need to get me a scope thats already isolated.

And just for grins, over on the primary side, pins 1 and 4 are the power to the IC. What voltage is there between them?

get back to you on that

[Techknowman]

9.96VDC

waveform at f3

[MarkusBass]

Transformer failures are very seldom. Have you checked other semiconductors? After the circuit starts, it is supplied from additional winding of the transformer (through C67, D34, R80, D40 - are the all OK?). What voltage is on C69 capacitor and on D21 Zener diode? It is not marked on the schematic but it should be something like 15V. I think that 10V is not enough - maybe the additional power supply circuit is faulty. Can you check it?

Mark

[Techknowman]

I replaced C69 and 70 even though they read good. It was just a hunch that they were breaking down. Wasn't the case.
I'll get v measurments and get back.
I did pull F1-F3 to remove any external load as well as the jumpers on the +/- 80V rails and this had no effect on the power supply at all.
I also took a bench power supply with F1-3 removed and put +24VDC at L5 and regulator output of +15 worked.
I then reversed polarity and put -24V at L7 and and regulator output of -15 worked.
I also put +24VDC at L8 and regulator output of +12 worked.
So its not Downstream!

[Techknowman]

Voltages are with F1-3 reconnected. +/- 80V jumpers out
Measured with fluke vom so voltages are averaged based on sampling rate of meter.

IC5 voltages
PIN 1 (VCC) 9.96Vdc {note. schematic pins 1 and 4 are swapped}
2, .5V
3, .4vdc (measuring here changes pitch of clicking, makes sense since this is part of an RC)
4, 0V {note. schematic pins 1 and 4 are swapped}
5, .28vdc
6, 27.5vdc fluctuating
7, 26vdc fluctuating
8, 35VDC

Also of note when I turn the power switch off. the unit will sit there and click for about 3-4 minutes getting slower and slower as the primary buss fades.
It doesn't discharge or use the current up very quickly.

parts replaced.
IC5
T22 and 23 (shorted all 3 legs)
C69 and 70 on a hunch.
T11 in amp. (shorted gate to source or drain)

[MarkusBass]

I don't know what you mean by "+/-80V rails kind of working". Does it mean they work without a load? In this case I'd say that IRS is working only the MOSFETs do not get strong enough signal. Have you check R76 and R77 gate resistors? Aren't they open?
The power supply for IRS should be more than 10V (and less than 15V). Again the Zener diode is not marked but I'd say that it should be somewhere between 12 and 14 Volts. The diode may be 12 or 13 Volts. So the transformer is not working at full power. This may be transformer failure but first check if the gate resistors are not open. Of course MOSFETs are replaced with correct type?

Mark

[Techknowman]

I have put the jumpers back in and the +/- 80v is getting to the amp output mosfets.
What I mean by "kind of working" is the buss voltage is low as is all the secondary busses.
The gate resistors are good (R76 and 77)
I replaced the mosfers with the exact same part numbers that came out of it from the same mfr.
I have went through all the components on the primary side and all "appears" good.
I'm going to borrow an isolated techtronix digital scope from work so I can probe around on the primary side without blowing it up.

[Enzo]

Are we at the right house?

Did we check the 120/240 selector or jumper is set correctly? In other words, is Vdc about 340vDC or is it only sitting at about 170vDC? You coulod have knocked loose or left off the 120v jumper when you were in there.

[Techknowman]

Both jumpers are set for 110V operation.
The main buss reads 335VDC. they are hard soldered on the bottom of the board so no knocking them off

[Enzo]

Darn, that would have neatly explained... something.

[Techknowman]

I'm taking an isolated digital scope home tonight to get some good measurments on the primary side of the circuit.
My earth grounded crt scope would not get along with the amps "primary dc ground"

[Enzo]

Much rather see you power the unit from an isolation transformer. In fact, do both things.