marshall DBS 7400 issues

[belgamrood]

hi there

i have a DBS 7400 rig, the head and two DBS 2X10 cabs

lately i have some issues with it

after 10 minutes of playing it shuts down? the power indicator is still burning but theres no sound
if i turn the volume up, at some point ( different every time) theres a loud "bop" and then the sound comes back
after some time, 10-15min, same thing all over again

i checked if it gets too hot, thats not the case

is this a known issue with this amps? i did a bit of research but couldnt find anything

had it checked by local amp tech, he claimed to have fixed it
he chainged the master vol pot and the preamp tube, and cleaned it
when i got it fixed there, i played it for about half an hour and no problems.
next day rehearsal with the band, problems start

any help at all would be very welcome

thanx

[icastle]

Without seeing the problem, it sounds like a dry joint somewhere around the volume control to me.

As the amp heats up the joint expands and a contact is broken - that'll give you the 'pop' and sudden volume loss.
When it cools down, turning the volume control allows the dodgy contact to 'touch' again and the circuit is restored.

Needs someone to go through all the joints around the volume control and track down wihich one is bad - although IME, it's often far easier (and quicker) to just de and resolder all the joints within an inch or two of the area affected.

[belgamrood]

[quote name='icastle' timestamp='1338406079' post='1674170']
Without seeing the problem, it sounds like a dry joint somewhere around the volume control to me.

As the amp heats up the joint expands and a contact is broken - [b]that'll give you the 'pop' and sudden volume loss.[/b]
When it cools down, turning the volume control allows the dodgy contact to 'touch' again and the circuit is restored.

Needs someone to go through all the joints around the volume control and track down wihich one is bad - although IME, it's often far easier (and quicker) to just de and resolder all the joints within an inch or two of the area affected.
[/quote]

the pup actually comes as i turn it the volume after breaking down... while playing theres suddenly just volume loss, without pop...

so this would only show when it just brakes down?
would the installation of an extra fan help for future use?

[fuso]

Hi,

I owned a DBS400 for 10 years, and it was mostly reliable.
It did develop a problem where the sound would cut in and out - tried leads, speaker connections, etc.
Solution: Input jacks are in the same signal path as the effects send/receive - if you have no effects plugged in then loop the effects send/return jacks with a cord. These jacks can go high resistance and cut your input signal.
Never had a problem after doing this.

Mike

[belgamrood]

[quote name='fuso' timestamp='1338452291' post='1674656']
Hi,

I owned a DBS400 for 10 years, and it was mostly reliable.
It did develop a problem where the sound would cut in and out - tried leads, speaker connections, etc.
Solution: Input jacks are in the same signal path as the effects send/receive - if you have no effects plugged in then loop the effects send/return jacks with a cord. These jacks can go high resistance and cut your input signal.
Never had a problem after doing this.

Mike
[/quote]

i will try that next rehearsal. where did you find out about the sollution?
do you remember if the master volume dial was kinda loose? i mean real easy to turn, almost spin by itself, not sturdy at all like the other knobs...

[belgamrood]

btw, should this be speaker in instrument chord?

Edited May 31, 2012 by belgamrood

[BOD2]

[quote name='belgamrood' timestamp='1338464495' post='1674923']
btw, should this be speaker in instrument chord?
[/quote]

Instrument cord

[belgamrood]

ok

thankx for the replies!! will check it out first thing next rehearsal, hope it works

[fuso]

Don't remember where I read/found out about the fix.
Master volume was probably easier to turn than the other pots, but not sloppy like.

[slobluesine]

[quote name='fuso' timestamp='1338452291' post='1674656']
Hi,

I owned a DBS400 for 10 years, and it was mostly reliable.
It did develop a problem where the sound would cut in and out - tried leads, speaker connections, etc.
Solution: Input jacks are in the same signal path as the effects send/receive - if you have no effects plugged in then loop the effects send/return jacks with a cord. These jacks can go high resistance and cut your input signal.
Never had a problem after doing this.

Mike
[/quote]

same here, plug a jack into the send/return, Marshall servicing told me about the problem, they even sent me some input jacks FOC. have to say the amps are notoriously problematic, i've know others with endless problems.

[belgamrood]

tried the effect loop fix... still shuts down the same way as before

[Sibass11]

Did you ever get this fixed? I have had the same head since new, but now it is doing exactly the same thing. Its been back to Marshall 4 times but is still not fixed, I'm about to drop it from a great height. Thing is I love the head and don't know what i can replace it with that will sound as good!

[MarkBassChat]

I hope that you are joking about dropping the amp from from a great height. If the amp works correctly (sometimes), then this is just a minor problem, maybe just a dry joint, as mentioned. Or maybe a problem with the volume pot. I wonder how it is possible that Marshall couldn't fix it. Maybe they just couldn't reproduce the problem in their lab.

Mark

[Sibass11]

Sorry, I only just realised that you replied MarkBassChat, No I didn't drop it in the end!! My local music shop Promenade music in Morecambe sent it to a PA repair company in Blackpool I think! They rebuilt one of the boards and its been sweet since, I don't know why Marshall couldnt fix it, maybe they just don't have the facility to put things on bench test for long periods of time?