Trying not to walk away from Mark Bass Little Mark Tube 800 repair

[nsubulysses]

Ok, I have not gone far into this so far but I would like to ask for advice since I am not familiar with this type of amp at all. Let the learning curve begin.

Here's what's up.

No output at speaker jack. There is output at effects send and also at tuner out. There is a signal at effects return when effects send and return are patched with a cable but not without a cable. Even with effects looped patched together there is still no output at speaker jack. Have not gone farther than this so far.

A schematic would be a good start. Anyone have one?

Edit: I just took the amp off my bench to work on something else and when I set it down there was a spark near the effects send/return PCB. Plugging it back in now it appears to work the same though. could have just been dust or something. who knows.

[nosaj]

No but I worked on another variant of the mark amp It's Master Vol had a connector to the board that would vibrate out so I hot glued it, after that I discovered the speaker jack is cheap and internally broken so I cut it off and put a new switchcraft on . Crazy difficult smps amp with super simple fix, I just over analyzed it myself.

nosaj

[nsubulysses]

I am getting the AC signal at some connections of the master volume pot. I wonder if I have an SMPS problem. There seems to be a slight buzz coming from the power supply area and when I turn off the unit it winds down with a whizzing noise. This thing works up until the effects send/return and master volume which I would assume are both right before the output section.

Schematic ??

[km6xz]

I don't have a schematic for this model but if there is a problem with the SMPS, all outputs will be affected. If the low voltage rails are OK, find out if the power output stage is getting its higher voltage, around 80volts for the other models. It is certainly possible that it is in the supply for the output section, after the supply since the supply monitors the output of the supply, look after the supply. I don't where in the circuit that are using the tube, is it in the preamp or a later stage after the inserts? If the later,make sure the B+ for the tube, and the heater voltage are present at the plate pins of the tube. It has muting circuits which complicate matters without a schematic.
If the tube is before the insert send jack you know the plate voltage is good since the sends have signal.

[nsubulysses]

Stan thanks for chiming in. The tube is in the preamp. I will check around and see what other info I can come up with.

A SMPS SMD amp with no schematic is not my specialty by any means. I would be happy to increase my knowledge and complete this repair but if I have no schematic or service info I will just pass on it after I waste a few hours routing around in the amp and looking around online for schematics. Then on the next one I will know not to waste a few hours.

However I would rather fix this and future ones if possible, so thanks!

[Jazz P Bass]

Try contacting them: MARKBASS | Bass Amplification

See if they will fix there own crap.

This is nuts.



I would hope that all of that goop is not silicone.

[J M Fahey]

Saturday afternoon a customer picked 2 x 12speakers for his Bass rig.

He declined buying one of my old style (basically unchanged for the last 20/30 years) all discrete/analogic MosFet/conventional supply B300 amps (think an 80's SS Ampeg or Peavey style) to get a brick sized (literally) 250W TC Electronics amp

all digital/SMPS weighing a mere 3 pounds or so.

At least he was bright enough to choose a regular size separate 2 x 12" cabinet instead of the shoebox sized combo: single 12" or wall shaking 2 x 8"

Told him: "pray it never ever fails, or you'll have to ship it back to Denmark for repair, nobody else does, not even in USA"
I guess it also has one of the ubiquitous ICE Power modules, only "repaired" (I guess replaced) at the original Factory.

On his own told me if that happened, he's straight come buy one of mine ... after a few minutes he added: "I'll come back next month and straight buy one of yours now, as a backup"
"Be my guest"

[Drewline]

It is unfortunately. Makes it obvious to me it was meant to be replaced rather than serviced. Former company I worked for was warranty for Mark Bass & it was mostly exchange.

[MarkusBass]

You are guys exaggerating - the amp is difficult to be fixed without a schematic but it does not compare to TC Electronic "all-digital" amps. Being frankly you haven't performed even the most basic tests (advised by Stan) .
The very first question is: does the problem is caused by the power supply failure, or the power amp failure (or something else). And you can easily answer this question simply by measuring +/-15V on opamps in the preamp, then by measuring voltages on the tube, and finally by measuring voltages in the power amp. After such measurements you will know whether the power supply is working correctly, or not.
Further, I would investigated the problems reported in the first post:

There is a signal at effects return when effects send and return are patched with a cable but not without a cable.

Is it possible? Isn't the Return jack shorted with the Send jack?

when I set it down there was a spark near the effects send/return PCB. Plugging it back in now it appears to work the same though. could have just been dust or something. who knows.

Do you really believe that a dust on low voltage circuit can cause a spark? I would take a look at the output board. Maybe the problem is as simple as Speakon output incorrectly soldered to the PC board - check it.
The silicone glue is not a problem when it is applied to big parts (first you can see that the parts haven't failed and if it is required, you can remove it with exacto nife). The problem is with small SMD components covered with silicone. You have to be very careful when trying to remove the glue.
But, as I said, first check the voltages and the output board. Only then you can proceed with further steps.
You can also answer some basic questions like:
- is the fan working,
- are the LEDs on,
- do you hear a tick of the output relay when you switch on the amp?

Mark

[J M Fahey]

Dear MarkusBass, *we* never saw a schematic so it's prudent to assume the worst unless proven otherwise.

You say they are easy?
Cool, personally I hold you as a qualified Bass Amp specialist, so don't hold back and feel free to fully help nsubulysses

[nsubulysses]

I will be able to check this again on Friday and will get back with some info

Thank you

[MarkusBass]

Dear MarkusBass, *we* never saw a schematic so it's prudent to assume the worst unless proven otherwise.

I don't agree. You shouldn't make any assumptions if basic measurements are not performed (e.g. does the power amp has power supply?). And in this case no measurements were performed. We only know that there is (sometimes) a signal in the erea of the effects loop and there was some mysterious spark when the amp was switched off. Based on this I wouldn't make any assumptions. Also looking at the photo you can clearly see that the power supply is the same as in other Markbass amps (and the schematic is floating around) and the power supply is not manufactured in SMD technology but on the contrary it is trough-hole technology. Only 10-15 parts are SMD. It's the power amp and the preamp that are manufactured in SMD technology (as you can see on the photo).

You say they are easy?

I haven't said that. I said only that it is not that difficult as it seems at first glance (which is not the same as to say that the amps are easy to fix). What can easily be done is verification whether the power supply is working. This can be done in about 1 minute with basic equipment (a screwdriver and a multimeter).

Cool, personally I hold you as a qualified Bass Amp specialist, so don't hold back and feel free to fully help nsubulysses

That's what I'm doing. I'm not a qualified Bass Amp specialist but I never say "no" when Markbass amp has to be fixed. And all Markbass amps that I got were fixed.
The OP should start with basic measurements and basic information (fan, LEDs, output relay). Without this information I wouldn't even think about fixing the amp.
It seems that even Markbass professional services in most cases replace the modules that failed (but first they determine which module has failed). But if the service is in Italy and you are e.g. in South America it is sometimes better/cheaper to try to fix the amp on your own - without sending the amp around the world. At least that's what I do.

Mark

[Alex R]

The last time I tried Markbass were happy to supply me (in the UK) with a new all-in-one board for one of their amps, I forget which, for about £150. That was about four years ago mind you.

In this game you have to know when to hold em, know when to fold em. I'm not all that good at giving up myself, I tend to think, ok, learning process, but then many unchargeable hours can slip by under that excuse! There is always the danger of making things worse and not being able to find a solution, that's definitely to be avoided.

[MarkusBass]

I'm not sure about the location of the OP but Chiraq it may be Chicago. If this is correct, the price will be different (much higher).

[g1]

I'm not a qualified Bass Amp specialist

Sorry, but I think you will lose this argument here.