old MP2, new problems

[ang3lus]

Hey all,

after fixing my MP1 i got a MP2 which has the "it takes me 10 mins to output sound" problem which i understand is somewhat normal.

other than that i have 2 very strange issues, one of them is having a missing resistor on the board (R398), which looks factory, is it important to have?

2nd one, on the tube patches, i dont get any sound if the Drive or OD are less than 68, and if i increase the master to something more than 25 i get terrible hum, granted the unit appears to be stock, only has the 1.41 eeprom but other than that the capacitors look stock.

engaging the effects loop on Loop A (and also playing with the mix up and down) in any patch results in more hum, even without anything plugged into the input.

wah/compresor/tremolo and all other effects work fine.

[Harley Hexxe]

Hey ang3lus,

   I would start with a fresh pair of tubes first of all and see what happens after that. Most of what you're describing sounds like tubes that are almost dead, and since the MP-2 has no solid state voices, you won't get any audio with out good tubes.

Harley 8)

[ang3lus]

Tubes are new mesa 12ax7, forgot to mention

[Harley Hexxe]

That sound like the tubes aren't getting enough voltage, hopefully it's something minor like cold solder joints or something similar.
Maybe MJMP might have a better idea what's going on with it.

Harley 8)

[rnolan]

Hey ang3lus, EPROM v 1.41 is the latest available for the MP2 so your all good there.  Main upgrade was faster midi program switching, you can set it to fast or slow (however slow is close to useless).  The initial MP2 release was very slow (irritatingly) but they issued a new EPROM very quickly to fix it. 

R398 is in the tube area (under V1), on the PCB adjacent to where the tube board attaches to the PCB.  I checked the only internal pic I could find and it's supposed to be there (from what I could see)  MJMP can hopefully advise.  The next row (under V2) has the sometimes problematic (noise at high gains) R913 (replace R913 with 240Kohms 1/2w resistor) see https://adadepot.com/index.php?topic=1066.0 I've attached the MP2 loading chart in case you don't have it. Also attached is the instructions for replacing R611 to eliminate noise gate chatter.  So check that while you have the lid off.

Re no sound with drive or OD less than 68, is the noise gate on? if it is on possibly the gate threshold is set too high (just a guess).  Otherwise there is something wrong.

Master > 25 with lots of hum is not good  , normal master settings are > 60. You may need to do the noise mod and replace the old stock caps, unfortunately the older electrolytic caps go off over time.

Re loop A issues, how is loop B?  does it work ok? It may be loop A jacks need a clean??

[ang3lus]

thanks for the tips, to answer your questions.

no sound with drive or OD, noise gate is off.

loop b works fine, loop A doesn't seem to need a clean-up, rotating the mix values gives a louder or lower hum

the hum itself sounds like heater hum, but if it increases and decreases with volume, i assume it might come from somewhere else ?, so i guess the capacitors are dead in the water, they seem original, they say LEAK on them i think ? not really sure.

I will  do the noise mod, just trying to figure out what's wrong with the "no sound on od less than 68" first

im used to building and repair amps, but i took this one to an electrical engineer who fixes audio equipment and he told me he can't repair the thing. so trying my luck

[rnolan]

Hey ang3lus, it's good you have some repair experience as the noise mod needs some skill to get the big caps out without doing damage. 

@MJMP what are your thoughts ??  This is very much your territory.

It's possibly a dud tube (as Harley suggested).  Mesa used to check the tubes they rebadge pretty thoroughly.  They are most likely rebadged JJs but they source them from other places as well.  Then since Gibson bought them out, who knows...  Anyway worth checking the tubes if you can. 

Since loop B is good (and loop A isn't) it seems, as you say, upstream from there, possibly around the chorus where the signal gets turned into pseudo stereo.  The stereo loop is very late in the signal flow chain.  BTW there's a good signal flow diagram at the front of the MP2 user manual. 

Hopefully MJMP will chime in soon and help get it sorted.

[ang3lus]

well...

checked HV at the PCB and it showed around 220v, so i changed the resistor, now getting 187v, no more hum at all, patches all work.

amazing

thanks guys

will do the full noise mod after as this unit is almost 40 years old with original capacitors

also, no hum on fx loop anymore

just a weird problem, display goes on the fritz after a while and showing weird stuff, the unit passes sound but i can only scroll thru one patch before it freezes, controls wont work but still passes sound, maybe a battery change ?

[rnolan]

Well that's great news  :whoohoo!: so R913 was the issue (as Harley eluded to).  So it seems R398 isn't required, go figure.  IIRC the voltage needs to be no more than 190v.  When I checked all my MP-2s years ago the were all around 187v. 
So you can replace the display.  Generally it's because it fades over the years and becomes hard to read though.  I've had the screen go weird when I ran it off generator power (which MP-2s' don't like).  The fix was boot it, and then I started using a computer UPS when I played at that house (in the bush).  Others use a power filter, MP-2's are quite sensitive to power fluctuations, much more than MP-1's.
So MJMP generally has some displays for sale but you can also get them elsewhere.  It uses the same display as the Alesis quadverb.  There's a bunch of post about it, they also come in a variety of background colours.  It's an easy swap, just be careful you don't dislodge or damage the ribbon cable when attaching the display connector. 

The battery will need changing, particularly if it's the original.  It's mounted vertically, again quite few posts to help with the options.  The battery just maintains the RAM,  which is where all your patches are stored.  The factory presets are in the EPROM, which you load into RAM (same as the MP-1).

[ang3lus]

I dont think its the display, I'm more suspicious of the power (electricity is really bad where i work), i guess a power filter is in order, i'm already using a step up transformer (unit is 110 and our voltage is 220) so that might be anothoer case.

this is one of the best units ive ever used... i really love it, afraid bad things will happen to it :D

[Harley Hexxe]

I think those displays are still on ebay.

Once you get the MP-2 recapped, it's pretty dependable. There may also be a few resistors that need to be replaced too because resistors drift over time, but MJMP would know specifically which ones they would be. Aside from needing fresh tubes periodically, I've found the MP-2 to be the most robust preamp for road use. I could still gig with mine if I wanted to and it would do a show no problem. I just don't want to. All that rack gear is a pain in the back to haul around and set up, especially if you can't crank it up and let it rip. The only place I can do that any more is at outdoor festivals, but they have to pay me for that. 

The only thing you might want to do with it is try some different tubes in it. Most of us here like the reissue Mullards in them,(long plate), but YMMV.

[rnolan]

I dont think its the display, I'm more suspicious of the power (electricity is really bad where i work), i guess a power filter is in order, i'm already using a step up transformer (unit is 110 and our voltage is 220) so that might be anothoer case.

this is one of the best units ive ever used... i really love it, afraid bad things will happen to it :D

This is why I now have 4 MP-2s  ;) , Best guitar pre-amp ever IMHO.  They don't like flaky power and that can cause the issues you are having.  I went with a battery isolated computer Uninterruptible Power Supply (UPS), cheaper than fermins etc. and work better.  Ideally you can get one with the right power output for your unit.  So the ADA units came with either 117v US, 110v Japan or 240v for the normal world transformers.  Is yours a US or Japanese model?? it matters... 
Tube wise, see lots of posts, the Mullard Long Plate reissues work really well in MP-2's, very 3d tube (and sound).  The messa SPAX7s sound ok, great feedback etc. but I found they break up in not such a good way on the bottom E (82Hz)   

[ang3lus]

i have the 117v US model, it has a 117v sticker on the power transformer, i will order some long plates and try, thank you all.

i might get the furman, because i suffer from ground loops and noises because the electricity is flakey where i work.

[rnolan]

Hey ang3lus, so that's a US model.  Lots of others here like the furman, convenient if you need to rack mount it.  Check out the various computer UPSs' as an alternative.  The one I bought is completely battery isolated, but it's about the size of a small tower computer and the battery is heavy.  They are also quite a bit cheaper than a furman and do more than just filter the power and may work better for your flakey power (as the battery supplies good clean filtered power and the power input trickle charges the battery).  From what you describe I suspect either (furman or UPS) will fix your problem when the display goes "fritz" that sounds exactly like the issues I had running off a generator. 

Hopefully you can source the Mullard Long Plates, they are Russian tubes and may not be easy to get?

[ang3lus]

there's an importer here for mullard tubes, so thats no problem, they stock it regularly