Mark Bass - Little Mark Tube 800 - Will not turn on

[LS_Tech]

We have a Mark Bass Little Mark Tube 800 unit that will not power on. No shorted supply or output transistor. We changed the switch mode chip (IC5) which we think is the same as the Little Mark II, a IR21531D. We only read about 8.6V from the chip's Vcc (pin 1) and the COM (Pin 4). The data shows the there is a under voltage detect in the chip will trip when Vcc < 10V. So, the 8.6V on the Vcc pin seems wrong. We tried replacing C52 and D40. We are using the Little Mark II service manual which look to be the same circuit. If anyone has any ideas or the correct service manual or schematic please let us know.

Jim

[Patolin]

We have a Mark Bass Little Mark Tube 800 unit that will not power on. No shorted supply or output transistor. We changed the switch mode chip (IC5) which we think is the same as the Little Mark II, a IR21531D. We only read about 8.6V from the chip's Vcc (pin 1) and the COM (Pin 4). The data shows the there is a under voltage detect in the chip will trip when Vcc < 10V. So, the 8.6V on the Vcc pin seems wrong. We tried replacing C52 and D40. We are using the Little Mark II service manual which look to be the same circuit. If anyone has any ideas or the correct service manual or schematic please let us know.

Jim

Hi. You can solve this problem?. I'm working in the same amp.

[akser]

replace C68 1uF 250V it might be open also check for cracked pins on rectifier diodes attached to heatsink

[mastersplinta]

Hello there,

I see this tread has gone a bit cold recently but i thought I'd see if anyone can help. 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.

[Scraptru]

Yellow blob is on the secondary side