mikeydee77 7/3/2018 4:27 AM
Mesa Boogie Simul Class 2:90 1/2 Drive LED stays on at start up, is that normal?
I have just been repairing an MB Simul Class 2:90 with a channel switching flicker problem - random switching and interplay between LEDs. I think that I have fixed it by re-capping the switching decoupling caps (physically leaky 10000uF x 2) and cleaning up some dry joints. The switching seems stable and positive now, except for one thing. On start up the 1/2 drive LED stays on for about 15 seconds before going off. This only occurs after everything has had time to discharge.

I have never seen this amp before so wondering if this is normal or whether there is work still to do. I couldn't see anything in the user manual saying this was normal either.

Amp is running on 240v and schems below.

g1 7/3/2018 1:24 PM
15 seconds seems a long time. Is that the light P2 in the schem.?
Does the 3.2V supply at C41 come up right away or is that where the delay is?
How about the caps across LDR2 and LDR1 led sections?
mikeydee77 7/3/2018 4:18 PM
Hi g1,
thanks for taking the time to respond.

With the amp having been off for a couple of hours, and then turned on... The 3.2 supply at C41 came up reasonably quickly, may be 1-2secs, whilst the LED continued to light for several seconds more. This measurement point is the exact point where the earlier fault would have had this voltage jumping all over the place (measured wrt chassis earth) - one minute stable at 3.2, then climbing up to 20+v over 10secs, and then apparently discharging and falling back. That problem seemed to be connected with the earth point that the -ve side of C41 connects to which seems to track back to the centre tap of the 6.3v supply.

Most of the switching components have been swapped including the LDR and their 100uF caps and the two biggies 10000uF caps during my hunt.

Was it coincidence but the LED did go off when I probed the connector that carries the power transformer windings so makes me thing that the earth connection is not 100%.

mikeydee77 7/5/2018 1:23 AM
BAH! The fault that I thought was fixed has returned. So I think I can answer my own question and say probably not normal.

I suspect this board has a broken earth connection inn the switching circuit. Maybe a cracked plate through connection or similar hard to find fault.
g1 7/5/2018 12:03 PM
Yes it does sound like a grounding issue.
Maybe try measuring for voltage between heater CT and ground end of C41 ? If any voltage appears, can you run a jumper?
mikeydee77 7/8/2018 2:07 PM
Hey g1, sorry been unable to respond to your suggestion until now.

I was sure that you were on to something with your suggestion but I could see no voltage between the two points heater CT and C41. But in the course of making those checks I did discover that there is a relarionship with the bias supply. That is the voltage on the -ve side of the LDR seems to be pulled -ve by the bias supply. If I lift one of the bias resistors (825 or 3K3) then the light is immediately extinguished, makes me think that there is a little short there somewhere. Something small that disappears once the amp is warmed up. I say warmed up as it usually occurs during the 15-30s start up but occasionally when the amp is running for 20mins or so. So, making the fault repeatable is tricky. If I discharge everything it will sometimes repeat the fault but often it needs to be on its own for two hours sulking.

Bear in mind that the conditions are that the HT is not applied and that the valves are all removed. But what I can see is that if I power off the amp. Then discharge the channel switching supply, both sides of the 6R8 resistor, then the voltage at the - side of the LDR goes slightly negative maybe -1 of a volt. and then if I discharge the bias supply, then the LDR- rises up to zero volts.

I have had a couple of Mesa boogies drive me nuts before and the fault turned out to be a partial short between a resistor body and the track and I suspect that this could be the sort of fun and games with this one.

Any thanks for your help, unfortunately I am going to have to leave this one for 3 weeks or so due to some work commitments but I will be back on it then.

Cheers, Mike
nevetslab 7/9/2018 8:51 PM
Unless you find the issue with yours, I'm curious to see what my Stereo Simul Class 2-90 does in the same regard. Thus far, I haven't gone digging into mine, though it still has the unnerving LF oscillation problem that seems to common to Mesa products. I'd love to find what it is in their amps that causes that 'big truck rumbling down the back alley' sound.
g1 7/9/2018 9:24 PM
As far as the bias voltage affecting the switching, if the schematic is correct for your unit, there should be no connection whatsoever.
In that case, your theory about a resistor body shorting to a track makes good sense.
You might even want to check if the bias voltage is fluctuating anywhere when the light is 'stuck'.
mikeydee77 7/10/2018 3:25 AM
It's certainly a strange one... i was hoping it was a tube with a partial short but since I removed them and put them in a box ....

My big lesson is to remove the pcb from the chassis. There is so little room to manoeuvre or trace the tracks. But there are so many wires to desolder of course I avoid it.

I doubt you have it in the US but the 240v model has a dropper resistor for the 110v fan. There is a bit of a burn there so that could be involved.

Thanks for all help and comments good to have the support. I am away for a bit but will resume in a couple of weeks.

mikeydee77 7/30/2018 4:57 AM
Ah got it!

By applying an external bias supply of -50v I found that the 1/2 drive jack had a surprising -5v potential on it. It should be zero volts until heater supply present.

Following the track I found that the bias supply snakes its way across the pcb but the last 1/4 of the board it runs parallel with the 1/2 Dr connection. I couldn't see any stray connection but there was some slight powdery residue. I am concluding the latter but having cut the unused section of the bias supply out I am confident that I have stopped it in its tracks.

Amp sounds ok no truck rumbling that I noticed.