Mark Bass Little Mark Tube 800 malfunction

I had a similar fault on a LM3 which turned out to be a faulty pre amp board; the one that attaches to the front panel with all of the controls soldered to it.

 

I would expect that this is something a qualified service tech can sort out easily.

 

Some of the outputs can be used to test and narrow down which section of the amp is faulty. These are the effects send, the D.I., and the tuner out ports on the back.

If you are comfortable, you can open the top and check the tube, whether it glows or not. Additionally, you read the tube's spec sheet and measure the voltages on it. That's easier if the tube leads are exposed.

If your Markbass is like mine, there's too much goop to really replace anything but minor stuff at the component level. It's basically board swapping. Getting boards is tough for this brand, which means an authorized service center. Which in turn means a good chunk of one's paycheck in these parts.

Too bad, it helps creates a throw-away, pollution-oriented society. Manufacturing semiconductors, whether active or passive, is a high pollution endeavor.

So in the end, all you are doing is narrowing down what needs to be replace so that you can evaluate the tech's price better. Around here, some are willing to rip off the unaware.

 

While I agree with Andy, if the amp has an effects loop you could try linking the send and return with a signal cable to see if it helps.

 

As the others say, check for signal from XLR out, tuner out and effect send.

The below part is only if you have some basic understanding about electronics:

I have not worked on that exact model, but the LMII and LMIII have a fair amount of internal cables.

After leaving the amp unplugged (from the mains) a couple of hours, it is OK to remove the top plate and one by one detach and re-attach the cables from the connectiont points on the boards. A small (!) amount of deoxit in the connector sleeves can be applied as well. Reinstall the lid and try the amp again.