Fender 1270P Powered Monitor Schematic

[Roly]

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.

That sounds right.

Careful and close inspection of your wiring should be all it needs.  Of course you can plug anything into it, another light should produce a glow in each in inverse proportion to their powers.  You can try a low load appliance like a fan and it should glow related to the speed selected, but not much, or a high load appliance such as a jug or toaster when it should light brightly.

*** You don't mention the safety earth which must be carried directly through from the lead to the outlet.


From the results you got;

1. do not run the amp without the limiting lamp until we get the fault sorted;

2. leave the speaker(s) disconnected until advised otherwise.

I got a reading of -19.85

Around -20 volts on the output is a clear sign that something is dead in the power amp, and this itself will cause excessive hum.  There are several things it could be, but the most obvious candidate is the chip amp TDA7294.

I ran my electric guitar in and tried to play it and no sound came out

Which isn't too surprising since the amp has a fault (and even when they don't they don't work normally on reduced supply voltage).

If you look in the bottom-left (@A,B,5,6,7) of the circuit you will find the power supply with Test Points TP1 to TP10; we need a table of the voltages you find at those TP's.

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)

eh?

Looking at the circuit (right-hand middle), @1B the orange is the LF output hot, brown is cold, while @1C the red is HF output hot, and black is cold.  These should go to the High Frequency speaker/horn.  Note TP22 which is the HF output should have a residual voltage lower than 125mVDC, and TP23 is the LF output where you measured about -20 volts and should be less than 100mVDC.

[States02]

Re: the earth.  Yes I ran that to the ground post on the outlet.

We live in an RV so I can't leave projects out.  I'll probably open it up again tomorrow and try some of the tests, will let you know.    Regarding the red and black vs orange and brown, that's how it was hooked up when I opened it yesterday.  I'm assuming the HF is the main speaker connections, and the LF refers to the connections on the unit labeled 'powerline' which is at the very back of the speaker.  This thing unscrews from the speaker itself.   I'll leave both of those sets of wires disconnected during the tests. 

I can see pretty clearly the transistors the other tech replaced. I'll see if I can identify them so we know what's been done.

Regarding the test points.  What am I testing between?  Is this with the amp powered up? I really haven't done anything like this since high school.

Also, I'm wondering if I can really tackle this.  That fine unsoldering and soldering is not something I'm particularly talented at.  I do have a little Peavey KB1 that I aquired that works pretty well as a stage monitor for us.  Perhaps I should just try to sell this cheaply to someone who wants to tackle it.  Thoughts on this are appreciated. 

If I do work on it, I think in my case it will be best to remove the stuff I'm working on from the case so that I can minimize my clutter as I'm testing.  Again, we don't have that much room, and I also have three cajon drums I'm building in the living room at this time.  Thanks again.   Unless I get other advice, I'll probably open this up again tomorrow morning, run some tests and post my results.

[States02]

I can't tell you how excited I am that I was able to get readings.  They don't say what the schematic wants, but we'll see.  Since I'm not sure of myself on the procedure, I'll share what I did and you can laugh or cry.   

First, I disconnected the speakers and plugged the amp into the light limiter (which I'll always do)  :tu:

Second, with the guts pulled away from the box. I connected the black lead on my DMM to the black ground wire that normally runs from the board to the speaker frame.  I then touched the red lead to what I think are the points we're speaking of.  I just went to one side of (say D12 then D14) or the other.  I quickly figured out which side was which.  I hope this is right.  Again like where D12 goes into the board I just touched the red lead of my DMM there.  (I know some of you are cringing right now)

Here are the readings I got.  Oh and also, since some were asking earlier I rechecked the speaker output and it was... on the brown and orange lead -5.75  on the black and red leads, it was .13.

TP1     7.5 VAC
TP2     7.6 VAC
TP3     7.82 vdc
TP4    -7.73 vdc
TP5     19.1 vac
TP6     19.1 vac  (first time out it was 20.8 for TP5 and TP6)
TP7     19.1 vdc
TP8    -19.1 vdc
TP9       14.4 vdc
TP10   -15.6 vdc

There you have it.  I think, according to the schematic, that all of these are pretty low.  Also I noticed on that gray multiwire tape that runs from the power supply to the amp, the far left wire on the amp side was loose.  I pushed that back down into the clip and hope it's connected properly.  I did that all before the testing.  Thanks in advance for the advice.

Actually, I checked the ac voltage coming in and that's around 76 volts, I guess due to the light limiter.

[States02]

Update....  I went ahead and began checking some of the other test points, since the thing is open, I have those numbers too, however, part way through the testing I noticed that the limiter bulb dimmed significantly.  I actually couldn't see the 100 watt bulb glowing, I put a 75 clear in there and could see some glow but it was pretty dim.  I went ahead and finished with the idea of posting this.  I went back to look at a few things on the circuit as I was reading some earlier posts and found a flaw in the board.  It seems to be the link from c27 to earth?  Not sure I'm reading it right.  Here's a pic.  Does this put me out of the game?  Can I drop a bit of solder on to repair? 

[DrGonz78]

Well that explains why there is not a change in TP7 & 8 in relation to the voltages of TP5 & 6. You see on the schematic where C27 is connected to Hi is where pin 7 is on the TDA7294. Basically that is where that +43v goes from TP7. Pin 7 & 8 get the -/+43vDC for the chip. *EDIT* I also need to say that pin 13 & 15 on the chip use a -/+17vDC voltage to power the output section of the chip too. Looking at the data sheets on the chip it talks about how the chip is designed to use 40vDC and 20vDC power supplies. Almost thought for a minute I had it all wrong but it looks right to me again.*EDIT*

So yes you need to repair that trace to continue onward with the repair. However, this is a double sided board right? Is there a ground wire that is supposed to be plugged into one side of the trace? I see where there is a soldered component on one side of the broken trace and would assume that is one leg of C27. What is supposed to be plugged into the hole on the other side of the broken trace? **Double Edit** That is probably after you disconnected the B+ rail where that hole is missing. Tough to understand pictures and the board layout. **Double Edit**

Fixing traces is not too hard and yes some solder coupled together with a bit of copper wire will work wonders. Once you have the trace fixed make sure to test the trace with the amp off. Test with ohm meter to see that it is reading very low resistance 1ohm or less. Do not test anything voltage wise until you fix this trace. 40 watt bulbs might be the way to go as a way to be even more careful and lower test voltages even lower. Right now you have reduced the voltage down a great deal using the 75 watt bulb and that is better than 100 watt bulb. Good job so far getting some data and jumping in head first! 

Edit: I read some things wrong and am fixing what I wrote to make more sense... What I first wrote was correct and I started to change it. Then I cleared my thoughts about pin 13 & 15 vDC voltages and just got confused for minute there... These IC's are crazy!

[DrGonz78]

Oh yeah I liked looking at page 10 on this link for the TDA7294. It shows how this chip is used in a more practical basic situation. I think reading the Fender schematic is a bit confusing  :duh. I like how you can reference functions of each pin on this linked pdf file better. It shows the chip in use in a very simple circuit compared to the Fender circuit. None of the circuits are really simple to be honest.

http://aes.sdsu.edu/documents/TDA7294.pdf

[Roly]

I'm wondering if I can really tackle this.

Sweat it not - you seem to be doing just fine so far.

I'm assuming the HF is the main speaker connections, and the LF refers to the connections on the unit labeled 'powerline' which is at the very back of the speaker.  This thing unscrews from the speaker itself.   I'll leave both of those sets of wires disconnected during the tests. 

I'm at a bit of a loss here.  The HF or High Frequency output should go to a treble horn or similar, the LF or Low Frequency output should go to a normal speaker.  Yes, both pairs need to be disconnected.

Regarding the test points.  What am I testing between?  Is this with the amp powered up?

Between the Test Point and a chassis ground common.  Yes, powered up.

Tick, tick, and tick (or if you're American, Check, check, and check).

The voltage table looks good; you are running with about half the normal supply voltage (due to the limiting lamp).

on the brown and orange lead -5.75  on the black and red leads, it was .13

This seems to confirm the expected, that the treble/HF amp is okay, and the main/LF amp isn't.

Here's a pic.  Does this put me out of the game?  Can I drop a bit of solder on to repair? 

Good.  No.  Not really.  What has happened here is that something (the TDA7294?) has shorted and the track has blown just like a fuse.  Trim back the loose bit of track and bridge the gap with a short length of wire, being careful that it doesn't contact anything else (since it's one of the main power supply outputs).  The little hole it runs to is called a "plated-through" hole which connets to the other side of the board.

Other may have different views, but personally at this point I'd take a punt (bet) and try replacing the TDA7294.  With the fused link replaced and a new chip amp in place I'd measure the output voltage and if it comes up very close to zero that would be looking very good.  If not then we'll need to get into the surrounding transistors, Q1 - Q8, but at this stage I think the chip amp itself is a pretty good bet.


{I hope you are not living in your RV in snow up to your nose.  I'll be thinking of you next week when it hits 40ºC here for the next week or so (Ballarat, Victoria, Australia).}

[States02]

Okay, thanks everyone.  I'll look up some info on what wire to use to bridge the trace.  I'm also going to check and make sure, but I'm pretty sure the tech that had it replaced that tda7294.  I think I'll talk to him some about what he did and tell him what I'm doing.  At least I'm having some fun working on this. 

@Roly, We're living in about the middle of Texas right now, USA.  I have a brother who lives in Warrandyte, Vic.  It was pretty cold the last couple of days but not like in the north. 

@DrGonz, I'll look over that sheet.  What I do is just pretend I know what I'm doing and so far so good.

The only weird thing on the numbers was at TP18 where it's supposed to be a +14.6 and I had a -.7  I might recheck this after fixing the trace. 

So I guess I just need a real fine wire for that trace repair (like the size of the connects on the other components?  I forget where I need to use heat sinks.   

Thanks all, this is quite fun.

[States02]

Roly, Regarding the speaker.  I've attached a photo.  Again, sharing my ignorance.  I would assume that the hf (red and black) leads would go to the clips that go to the main speaker cone (further down toward the front) and the lf (orange and brown) go to the connectors on the very back of the speaker (where it says powerline) Of course, they are remaining disconnected until we feel it's okay to connect. 

[J M Fahey]

Well, you had been hiding a very important piece of information from us.
I sort of imagined something of the sort when you mentioned "Powerline" which is a Motorola Trademark used in their Piezo drivers.

Rearranging the data:

1) this is a coaxial speaker ,made by Eminence.
2) the LF part is the main speaker, made out of stamped steel, a magnet, voice coil and cone.
3) The HF part is a Piezo driver, made of plastic, a piezo disk and a small (hidden) cone, and it bolts in the cone shape perforated speaker polepiece, which is the beginning of the HF horn (which you don't show or even mention).
I'd love to see a front picture of that mystery speaker.

And, in case it is not clear, LF refers to the actual big speaker; HF refers to the small driver and horn.

[States02]

Here's a picture of the front.  Hope this is what you meant.

[J M Fahey]

Yes, that's it.
Not very clear, it *looks* like it might have a small horn under the dustcap.
Anyway the point is that I hope now HF and LF connections are sorted out, they were confused.
What DC voltage do you measure across each terminal pair?
I mean without s[peakers connected of course.
And please re-confirm wire colours coming from each amplifier.
Thanks.

[States02]

Thanks that helps.  Here are the voltages I got with speakers disconnected. 

brown and orange lead -5.75 
black and red leads,    .13

[J M Fahey]

Ok, .13V red black is  is fine , so TDA2050 , HF amp  is fine.

-5.75V on the LF out is bad.

Problem is that LF amp is quad rail, low and high.

To separate areas, first pull TDA7294,measure and post all pin voltages,measured at the PCB pads.

[Roly]

Warrandyte's a nice spot; used to go skinny dipping in the Yarra there at Pound Bend when I was a great deal younger (and it was much more rural).  I'm about 100km on the other, West, side of Melbourne now in a lovely little town called Creswick in the middle of the old gold rush area.

What I do is just pretend I know what I'm doing and so far so good.

:lmao:

That's worked for me most of my life.   

As JM says, that's not really what I expected, something more like this...

...was what I was visualising, but now we fully understand your confusion.

I'll look up some info on what wire to use to bridge the trace.

Just about any hookup wire will do, from a dead computer power supply for example.

To separate areas, first pull TDA7294,measure and post all pin voltages,measured at the PCB pads.

Good thinking Batman.   :dbtu: