Hi everybody
:tu:
I got to thinking I need something to work on.
A friend has lots of stuff but not really any cool guitar amps
I remember seeing a Realistic MPA amp that looked cool I guess it is a mono PA amp
I just wondered if this would be a cool thing to try to get it working if so I can go get it
xP
(Maybe there are some unobtanium components idk)
I would find out what the model is first and then see if you can find a schematic for that model, I know ElectroTanya has the REALISTIC MPA-40 schematic so there might be more out there.
There will always be a use for an amplifier on your workbench eventually, I have a tube preamp pedal of the Mesa Boogie Dual Rectifier that might sound great through a PA amp as long as you have a nice speaker cab to play through.
At worst there will be many great parts to salvage from it and most likely a fantastic chassis to build a proper guitar amp in, just make a new faceplate and it can look professional.
Yes sir thanks in fact I was gonna post
"I've been around here long enough to know that the absolute first order of business is to come up with a schematic " :tu:
Seems I recall he had at least two different MPA models I'll have to take a look one had a silver face and one was black face.
xP
Just in your favour;
* being Commercial amps, their schematics are often found in many repositories, service pages, Sams Photofacts and maybe even a Realistic dedicated fan page.
Or Elektrotanya as mentioned above.
* those amps were *robust*: a Hotel, Hospital, Supermarket, Auction site, Bus station losing its PA is a far worse event for many more people than a Guitar player missing a rehearsal so they were made long lasting.
Meaning: it´s very possible that amp *electronics* work fine, but you meed to clean pots, switches, connectors, the "mechanicals"
Which does not really require Schematics.
Yes sir thanks :tu:
And...two rusty dusty boat anchors later :loco
IMG_20251011_171737204.jpg
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The cover screws on the MPA-100 were missing not sure what's up with that
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And on the MPA-200 the left volume? knob spins 360 without stopping :grr
I hoped the plastic knob was slipping on the pot shaft but the metal shaft spins 360 :'(
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Before I got there I was gonna mention kinda like you warned...Realistic and Peavey seem to have a "bad" reputation of being unearthed from a pile of crap and still working :lmao:
I'm kinda pumped that one of them is stereo that's kinda cool I guess :dbtu:
Well decided to :loco try em on the ol ugly DBT
First up was the MPA-100 and it was dead as a doorknob :grr
But then I remembered....oh yeah :loco
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Next up was the MPA-200
And I got a bit of a surprise :lmao:
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xP
I wish I had a tuner or something to hook to the RCA inputs but I only have one and I didn't want to unhook it from its current set I have rigged up.
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So I noticed the upper input jacks might take a guitar cord so that's what I did and hooked up two small speakers
It worked but at max wasn't very loud. Then I tried the left side the one with the broken wiper stop(?) and got sound but nothing linear or controlled.
So I opened it up to have a look at the volume pots and here is the back of one
IMG_20251012_184515155.jpg
May be time to Google "A 50k " idk or lmk if you recognize what it is or can recommend a suitable replacement
I figured it would be crazy loud maybe a guitar signal isnt much idk. :grr
Thanks :dbtu:
Pic of the inside
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Kinda confused about these speaker terminals maybe they take a crimped on U-shaped connector
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xP
Two beasts, door anchors in a good way, lots of iron and heatsinks
Connect one or two 8 ohm speakers at the proper speaker outputs and be certain they can handle that much power.
Are those RCA/Guitr Jack inputs on the power amp?
If so, you need to build the proper cables.
It is always good to have spare 1/4" plugs, RCA plugs and a few yards of spare shielded Audio cable just in case, it NEVER is wasted, so next time order 10 of each tp spread post cost or add them to your next "something else" order.
You will most probably need 1 V (or more) to drive those Power Amp inputs, so a direct guitar will barely make any sound at all, same with a Phone pr MP3 player headphone out which gives around 100mV RMS BUT a table radio/Boombox/Tape Recorder etc. *Speaker* out can probably drive that amp loud
Don´t you have some old Transistor Radio (even if Mom´s AM one) lying around?.
Or get one at Goodwill/Salvation Army for $1.
A guitar cable tack soldered across speaker terminals (without removing speaker) will do.
For DECADES I had a permanently ON Lafayette AM/FM receiver on a bench shelf, with an added volume pot and a 1/4" jack as a ready made "known" Audio source.
Since at least 1975 or so!!!!!!!!!!!
Doubles as a background Music source when doing boring repetitive work such as soldering transformer pigtails, filling bobbins with EI laminations, drilling PCBs, etc.
Just tune a good Rock/Jazz/Classic station.
Undocumented Bonus: FM sounds way better/smoother than ANY MP3 and most CDs.
Probably FM coding/transmitting/decoding smooths out rough edges.
Yes sir thanks :dbtu:
I think I lucked out getting the black one xP
Black one MPA-200
-"normal" output :tu:
-Stereo output so I can possibly use it to listen to ball games etc (loudly :lmao: )
-to my noob eyes looks evenly divided two channels each the same
Kinda like working on drum brakes...don't take both sides apart at the same time :lmao:
Silver one MPA-100
-weird multi voltage speaker output :grr :'(
-Output transformer ?! :grr xP
So I'm hella glad to have the MPA-200
I need to order a volume pot just replacing that should take me about six months :loco
The silver one would make a good one to try a guitar on, it is missing the cap on the main fuse but you should be able to borrow the one from the black one to test it with, it should just unscrew.
Try the guitar into a microphone input and see what you get, the speaker out is transformer driven so you can use 4, 8, 16 ohm speakers by just using the right tap, just ignore the 25v and the 75v outputs.
I would turn it on and measure the voltage on the speaker terminals before attaching a speaker, you should always do that when you first start a solid state amp because if there is DC on the speaker output you can destroy the speaker in seconds.
There should be no DC on that amp since it has a transformer but it is better to be safe every time, if you have a preamp pedal that might help drive the black one to full power output as well.
Have fun
Cheers
Mick
Yes sir thanks nice to hear from you :dbtu:
One BIG TO DO on my list is to set up some test speakers UP OUT OF THE WAY that I can CONVENIENTLY :lmao: utilize for scenarios as these :grr