mesa boogie mark IV B eq switching on and off issue

[Arbutt]

Hello I had thought i was finished with this idiot of an amp, what the hell were they thinking when they designed it !!
It came in for terrible hum. Changed the tubes/ two o/c bias feed 220k and 2 x 10,000uf which had leaked.. replaced both bias caps as a matter of course
anyway all was good, Sounded good
On final test i notice that the EQ was not switching off, with the LED on the front Just dimming when you flicked the switch on the back ( On - Off - Assigned to R2)
mk4b.pdf
I foot s/w in the back and turned too FTSW still the LED is remaining on or dimmed, still EQ can be adjusted when it should be off.
At this point i do remember when i tested the amp at the start, i saw this , but didn't pay much attention

I have checked the voltage rails all present 5v/23v/12v
All functions are working apart from the EQ switching
Checked the switch on back - OK
Selected the external jacks and tested All working apart from EQ switching

I'M a bit baffled TBH - not hard to do
I feel like handing back, as it is, But i one last chance
any help is going to be well received

thanks in advance

[mikeydee77]

MB MKIV Swithcing measurements.pdf

Hi I have seen something like this before. I Will attach some notes that I made about a year ago... but now make little sense at all to me.

The amp I have has some sort of strange residue on the board making it unstable in many ways but the switch eq problem was just a blown transistor problem.

I can provide comparison measurements in about 1wks time as currently away.

[Arbutt]

Thanks. I did think of that about the board I had issues with the bias voltage bleeding away til I did a little point to point.. Thanks for the help. I'll wait for the notes
Arbutt

[mikeydee77]

Yep that sounds similar story then. I had very lop sided bias that was inconsistent often be OK for 20mins and then drift off... I did find by examining the PCB under dark conditions I could see a little lightning storm appearing on the PCB as little shorts appeared.

[Arbutt]

This one had the 2 x 10,000uf cap leak their s1hit all over the area and cause one or the other 220k to loose upto - 20v and slowly increase it over 20 mins. So I did a point to point.. All that is good now. Just this eq switching issues.

[mikeydee77]

Hi Arbutt,

I just got back to base after some travelling... One other thought that I did have is with the LDRs. They are stacked up in blocks of 3 or 5 in cases. Check that only the ones that should be touching legs are... I have a feeling that can be an issue also. Mike

[Arbutt]

Thanks Mike I think finally today. I have sorted the issues. Been a nightmare. Shit loads of problems. I have modded the eq for switchable. Waiting for 24 hrs before I check it for noises
Thanks. I will recheck the ldrs
Arbutt