Fender 1270P Powered Monitor Schematic

[J M Fahey]

Agree.
Now it looks like it can be plugged the normal way.
Do it, then feed it some nice MP3 you like and let it work 1/2 hour, checking now and then the heatsink temperature.
If reasonable raise power to what you feel is about 1/2 full and give it another 1/2 hour workout.
If fine, give it 5 minutes full power (anymore and family/neighbours will kill you) and you can pronounce it healthy.
Heatsinks may seem way too hot, but if you can still touch them with your bare hand for a couple minutes, it's acceptable.
Congratulations.

[WannabeGeek]

Thanks J M,

I ran the monitor for several hours today and it passed all the tests.  I'll be using it for a gig on New Year's Eve.  If it survives that, it will become a permanent addition to my PA system.  I appreciate the time you took to help me out. The world needs more people like you.

[Roly]

Congratulations.   :dbtu:

[States02]

Not sure if anyone will see this as it's an old thread.  I also have a 1270P that is not working.  I don't do my own work, but took it to a guitar shop tech who's reputable. I don't have a list of what he replaced, but he did several replacements as he went through it.  The original problem was that as soon as you switched it on there was a loud hum, I mean really loud.  The 3 amp fuse was blown,  I replaced that, and that's when I was able to switch it on and get the hum.  So the guy says that it's now in the power supply and he doesn't have what he needs to test it.  I've considered sending it to Fender to see what they can do with it.  The thing is, I don't have much in it, but it's a perfect sized unit for what we do, I think if it works it will be a perfect addition to our stage setup. 
Is this worth pursuing? Might anyone from this thread have some insight I can give my tech guy?  I can post a list of replacements if it will help.  I appreciate any advice I can get on this. 

[WannabeGeek]

When you switch it on, does it work normally aside from the very loud hum, or are you just getting a hum and nothing else?

[DrGonz78]

Well first thing first if you are getting a loud hum on the speaker then turn off the unit and stop turning it on. The hum could be DC voltage from a shortage component in the power amp that is now humming on the speaker. Last thing you want is to burn up your speaker right? Disconnect the speaker and test with a voltmeter to see if there is DC on the output.

[J M Fahey]

Well, the schematic was posted at the beginning of the thread.
Any Fender authorized service Tech should be able to repair it with that info; parts are not esoteric or unavailable.
A non Fender authorized but experienced and properly equipped one should also work.
The Fender factory itself won't directly deal with it.

[Roly]

Hi @States02.  Welcome!

as soon as you switched it on there was a loud hum, I mean really loud.  The 3 amp fuse was blown,  I replaced that, and that's when I was able to switch it on and get the hum.

Fuses blow for a reason, that's why Limiting Lamps.  Make one now - you're gunna need it.

Well first thing first if you are getting a loud hum on the speaker then turn off the unit and stop turning it on.

Amen bro.  It will only cause more, and more serious, damage.

If you have a multimeter and a soldering iron and some idea of how to use them, you can certainly at least do some diagnosis of the problem.

First thing, you must make up a Limiting Lamp and only power the amp via that until we know it's healthy again.

Second thing; disconnect the speaker, power up via the LL and measure the DC voltage coming out of the amp output.

Just that will tell us a bit.

I would not jump to the conclusion that there is "something wrong" with the power supplies, there may be, but we come to that next.

First I'll jump to the conclusion that a transistor has failed in the output stage, a very typical fault in s.s. amps (almost as common as replacing output valves ;-) ).

[States02]

I've made a point of not turning it on since I got it back from the tech.  He was using a limiting lamp.  I'll see about making one. He said that the last time he had it powered up through the limiting lamp, there was a small hum.  He also told me that he has replaced all output transistors on the power amp.  He further said everything he replaced is attached to the power amp heat sink.  I have a solder iron, very limited high school electronics (in 1971) and a volt ohm meter.  The tech suggested that he needs an oscilloscope to test it further?  It's possible that I could wait until May when I'm back in Colorado, where my step-son may have access to the O scope.  I'll look up the limiter and see if I can build that so I can power it up and tell you all more. I'm a bit excited about attempting this myself, however, I'm a little skeptical of my abilities.  (the forum gives me confidence.)  Thanks for your input.  (is there a way to turn on notifications so my email tells me when someone posts?)

[States02]

Oops, got my email notifications set up.

[States02]

Sorry for the additional post.  I looked up the light bulb limiter.   I'll build that today.  Looks too easy.  Looks like a 100 watt bulb is recommended. I have a 75 on hand, but can grab a 100.

[Roly]

The tech suggested that he needs an oscilloscope to test it further?

Sorry?  The "Tech" said "he needs an oscilloscope"?

What sort of "Tech" is this?   :loco


One thing at a time.  This "Tech" says he needs a 'scope.  Here we have yet to conduct the most basic DC tests, and once we do I'll bet we have a reasonable idea of what is wrong, which will logically lead to how to fix it, and (while I love my oscilloscope to bits) it can normally be done with just a DMM, soldering iron, and some commonsense.


75W will do fine.

Disconnect the speakers!

Using the limiting lamp; bright/dims?

What are the supply voltages?

What is the voltage on the output of each amp, bass and treble?

[States02]

Just about done building the light limiter.  Is there a test for it?  Does the light come on only when there's a load plugged into the outlet?

[States02]

Here's my load limiter light.  Hope it works okay.  I'm going to go back to the page for that, but basically I ran the common to that side of the switch, ran the hot to the switch, from the other side of the switch to the light and from the other side of the light to the hot on the outlet.  Just wondering if there's a test before I plug it in.  Thanks so far.  (you can see my tip jar in the background of the photo..... empty as usual.

[States02]

Okay, I plugged the amp into the limiter light unit and turned it on.  Then I turned on the load limit switch.

The 75w bulb illuminated but not too bright, maybe halfway light.  I got a hum in the speaker but not too loud.  I ran my electric guitar in and tried to play it and no sound came out at all as far as the guitar.  The hum remained.  I did this all pretty quickly and got it shut off again.  That's where it stands at this point.  I guess I'll take it apart and put a meter on the speaker outs instead of the speaker and see what that says first.

When I do the next batch of trouble shooting, should I just unplug the speaker without the meter so I have it for other trouble shooting?

I disconnected the speaker.  The orange wire was on positive and the brown on negative so I tested that way.  With positive lead of multimeter on orange I got a reading of -19.85 or so. After I switched off the unit it slowly dropped the voltage back to zero.  I also tested the red and black leads to the powerline unit attached to the back of the speaker (which seems to be loose from the speaker) and they showed .11  That's the extent of what I'll do until I hear back.