Markbass Little Mark II

[MarkusBass]

It is written just a few posts below - it's IR21531D. I replaced it in several amps and they work until now.

Mark

[verano]

Supply won't start, no burned components, the fused resistor measures fine. disconnected the amp stage ( -80 +80v ) The filter in the VAC makes a high whining sound, replaced the Elco of 47uf on the IR2153D ic. Checked the main transistors that pulls the transformator Also the resistors in front of them. Checked the zenerdiodes. Somebody told me to replace also the 10uf and the 100uf elco's in de switching powersupply ( C69 and C70 ) .

[MarkusBass]

Are you sure that IR2153D is the same as IR21531D: http://www.irf.com/product-info/data...ta/ir21531.pdf ?
Check whether IC has correct voltages to start. For example what is power supply voltage?

Mark

[verano]

I don't think the IC is broken, because the coil in de Vac is whining... That means that current is being pulled, i think... I ques something is pulling down the +and- 80 volt rails..

[verano]

Found it ! The capacitor that is in line with the transfo was bad... 1uF 250v did the trick !

[maikjawa]

hello. Tell me denomination variable resistors in equalizer (preamp) and denominations optocouplers, thank you .. I'm sorry for the English Ukrainian

[mastersplinta]

Hello there,

I put this post on a different thread but not sure if anyone saw it so thought I'd try this one too. I too am repairing a Little mark tube 800 and have found a few faulty parts. Both the mosfets in the SMPS section short and also one of the RF2001 rectifiers in the 80v section short. I have replaced all parts and also taken out the transformer to check that too. Does anyone know what sort of resistances I should expect to see accross the windings? I have never had to fault find on an SMPS transformer and all I am finding is very low resistance (appears as shorts) accross the windings. I assume this is correct and the resistance is so small it is loo low for my meter to measure. Also does anyone have a pin out diagram of the transformer. The only schematic I have is the Little Mark II diagram which seems slightly different to the 800 circuit around the secondary section. One last thing! Slightly embarrasing, I removed the transformer assuming it would be easy to tell the primary from the secondary side. The only thing which may indicate it's polarity is a yellow blob on the corner. Does anyone know which side this indicates. In most pictures on the net I have found it seems to indicate the secondary side but wanted to check if somebody knows for sure.

Thanks,

Gav

[nev23]

I've got an orange dot on the secondary of this little mark II.
All the windings will read short circuit on a multimeter.
To test a SMPSU transformer, you need to build a circuit to do it, a 12V power supply and an oscilloscope. The circuit is an oscillator of about 50kHz that makes narrow pulses, feeding a high voltage transistor. You connect a winding of the transformer between 12V and the collector of the transistor, with a capacitor of about 1nF in parallel. The collector also goes to the scope. You should see the pulses ringing on a good transformer.
All the windings will behave similarly. Be careful if you use this to test a LOPT; the EHT lead will have several hundred volts on it.

[hitch]

the sound and portability of these amps is desired by many of the musicians that I know of. and having a switching type power supply obviously helps reduce the weight. however, the design issue that angers me is that the power supply is on the same board as the amp. so, now that my power supply is dead, the whole board is dead. so rather than replacing only the power supply, I now have to pay for a whole new board that does 60% of the work in the box. this is an annoying and expensive decision on the part of Markbass, and could have been avoided with little or no increase in product size or weight.

[MarkusBass]

What about fixing the amp? This is actually quite a simple task (in most cases).

Mark

[Enzo]

Agree. Replacing whole boards is not how we repair amps.

[hitch]

What about fixing the amp? This is actually quite a simple task (in most cases).

Mark

Thank you Mark

I am taking the amp to a specialist recommended by the local Markbass distributor. we will see what he finds out.

my concern is that the power and the amplifier parts handle very different issues. ie, power manipulation and sound manipulation. so the failure modes might be very different. I know that the most common thing to die in my computers are the switching power supplies, and that other items that are lower power and that are protected by the power supply tend to not fail nearly as often. so, when I see the power supply in the amp failing, I come to a similar conclusion that it would be much easier to fix if the two functions were on separate non-integrated parts.

I will make sure to update as to what is determined to have failed. the amp is treasured for its portability and sound.

[Enzo]

Actually, the power supply provides the current for your speakers, the amplifier circuit merely controls it. The power supply and amp are very much in the same business. The power supply forms the power and the amplifier does the manipulating.


When I repair a circuit like a power supply or an amplifier, I tend not to care what else is on the board.

[MarkusBass]

This is a topic for a completely different discussion. I fix amps and that's true that the bigger the boards, the easier they can be fixed. But in case of MarkBass I would say that doesn't matter. I think that they put two modules on one board in order to decrease the cost of making the amp. Also with separate boards you would be tempted to replace the one that failed (instead of fixing it) and this would increase the cost of the fix.
BTW, I also play bass guitar and I even own MarkBass amp (F1) but I don't think that the amps are desired by musicians due to their sound. Rather due to their portability. If you want a good sound buy Ampeg SVT-CL. If you want to have good sound and portability you may consider Epifani UL902 or one of Demeter amps (there is one in 1U rack enclosure with 800W power amp - this is good sound).
MarkBass is famous of VLE and VPF filters that most probably no bass player uses. If you use the filters, you would get hiss and poor sound - that's why I never used them.

Mark

[g1]

The point being missed here is that it is more and more common for the smps to be a non-repairable part. To the point where some new documentation does not include schematics for the smps sections.
I don't think replacing a whole smps is all that uncommon any more, but putting it on the same board as the rest of the amp just pushes the whole unit closer to being a disposable item (I guess the modern "spin" they put on that would be calling it recyclable).
There are apple laptops that do not have a replaceable battery. Out of warranty you toss it. I hate seeing amps going in that direction.