Markbass Combo head II issue

[blindboybenton]

Hi
I had a markbass combo head II come in with smoking problem, On inspection i could see 2 x 0.1uf 400v block caps with all there guts blowing out, Replaced and replaced a relay that was damaged by the melt. Tested the amp all was good, sounded good

Phone call the following day, saying it was smoking again, Check it out and found another block cap had gone with its guts all blowing out, Replaced again the amp tested fine without issues

Left it on for a while running with signal gen and a dummy load, Again all seemed ok, wasn't til i hooked up scope that i could see the output was just mush on the scope, When i disconnected the signal gen i could see the waveform in the picture

Channel one being used

Also the larger of the two coils is getting hot. I believe too hot can't even touch it

while testing with a speaker, amp is clean sounding no noise or hum
Two test points on the amp with 80+ and 80- both are correct. Ive been in touch with markbass italy was sent to Usa and then sent back to italy who then sent me to a uk firm, Tried to have a chat and maybe some help or a map.. We told to send the unit into them .. So no help. I really prefer working with a schematic. this one has me stumped at present

The unit is currently not on the bench and i will recheck and add more info

If anyone has ideas... Fire them at me

BBB

[mhuss]

Based on the sweep time on your scope (and the relative amplitude), I'd say it was oscillating. A lot of HF going through a cap (particularly if it's an RC or RLC across the output) will overheat it and cause self-destruction.

[erginy]

Hello. I have markbass combo ii. Can you help me?

[Chuck H]

Hello. I have markbass combo ii. Can you help me?

Rather than resurrecting an old thread, it's usually best to start a new thread with your amp model, a schematic if available and a description of the problem you are having.

[blindboybenton]

Better off starting a new post all all the information you have

[erginy]

I am writing to inquire about an issue of my Markbass CMD151P Jeff Berlin amplifier.

I turned the amp on and practised for about 5-10 minutes. The sound was good, bright, full, nice. After that, the sound started to be weak, volume is little bit down, feeling is hard to play and not responsive. So, I stopped to play and wait for about 5 minutes (wait with turn the amp off or wait without turn the amp off), the sound became full and rich again but only for 2-3 minutes. Then again the same issueis appeared.

Even though the offical service in my country (Turkey) couldn't reilize an issue, they accepted to replace my amplifier with a new one. The issuse pursued in the new amp and I decided to change other elemets; my guitar has also replaced other related parts such as socket etc. However, the issue is still exist. I started to belive the issue can be from a specific batch production.

I would appeciate if you help me to solve this problem or give technical advice and check procedure that I can apply as an Electronic Technicihan.

[blindboybenton]

Better off to create a new post.about your issue . This post is old and has been completed.
BBB

[Valvehead]

Hi
I had a markbass combo head II come in with smoking problem, On inspection i could see 2 x 0.1uf 400v block caps with all there guts blowing out, Replaced and replaced a relay that was damaged by the melt. Tested the amp all was good, sounded good

Phone call the following day, saying it was smoking again, Check it out and found another block cap had gone with its guts all blowing out, Replaced again the amp tested fine without issues

Left it on for a while running with signal gen and a dummy load, Again all seemed ok, wasn't til i hooked up scope that i could see the output was just mush on the scope, When i disconnected the signal gen i could see the waveform in the picture

[ATTACH=CONFIG]38567[/ATTACH] Channel one being used
[ATTACH=CONFIG]38568[/ATTACH]
Also the larger of the two coils is getting hot. I believe too hot can't even touch it

while testing with a speaker, amp is clean sounding no noise or hum
Two test points on the amp with 80+ and 80- both are correct. Ive been in touch with markbass italy was sent to Usa and then sent back to italy who then sent me to a uk firm, Tried to have a chat and maybe some help or a map.. We told to send the unit into them .. So no help. I really prefer working with a schematic. this one has me stumped at present

The unit is currently not on the bench and i will recheck and add more info

If anyone has ideas... Fire them at me

BBB

Im having the EXACT same problem with the exact same model-------2 blown caps c87/88 and the 2 , 5w res . next to them ------did you find the cure ??

[blindboybenton]

Hi
I replaced the main board from a uk supplier,
Im now retired from amp repairs and business has been handled over to someone else

BBB