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Fender 1270P Powered Monitor Schematic

Started by paelgin, May 04, 2008, 08:59:24 AM

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paelgin

Here is a schematic emailed to me from Fender for a Fender 1270P powered monitor.  This is not DIY, but it is solid state...Interesting to see what they did.  So, I hope it's OK to post this here. 

I got this because I bid on Ebay (USA) and won a broken one.  (I should get the shipment tomorrow.)   I will be trouble-shooting and seeing if it is salvageable (and usable). 

I found this forum Googling while looking for schematics, and have had fun looking through what is here.  I downloaded the book from Teemu, but haven't had a chance to look yet (thanks for sharing!).

Not sure what I can add to the Forum, but can try to help with questions or trouble-shooting, and may post what happens with my monitor project.

Thanks for everyone who has been helping make and keep this Forum going.

teemuk

Concerning the bi-amplified output section, the circuits are pretty much the same that are presented by the datasheets of the chips in question. TDA7294 uses the class-G -type "high efficiency" circuit with switched supply rails, whereas the TDA2050 is just a basic non-inverting opamp (of course it's one that is capable of providing higher power output). Fender has pretty much followed the application notes and restrained from doing something adventurous or experimental. Nothing wrong with that, their amps seem to be more reliable than Marshall's that use the same chips in different type of configuration. Anyway, if you have troubles understanding the circuit you get pretty far by reading the datasheets.

paelgin

Now, I'm in trouble-shooting and repair.  I found the 3 amp primary fuse blown. 

I replaced the fuse, unplugged the power amp board, then turned it on. 

I checked the AC voltage from the transformer (OK), then the DC voltages referenced to ground.  First the + and - "Lo" sides (plus and minus 17vdc) (OK), then the + and - "Hi" sides (plus and minus 43vdc) (OK).  Then when I checked the + and - 16 vdc for the preamp chips, I got +16 and -3.  Bad... (On the negative side, 40 vdc was being dropped through R52, a 390 ohm 7 watt resistor).

I unsoldered and removed D11 (one of the 16v zehner diodes) then bench tested it.  I didn't like the readings on my diode test from my meter, so I hooked up a bench power 20vdc, and a series 1kohm resister, and the diode zehnered at 3 vdc (bad...).  Got a new one coming (actually, 2; from Mouser).

I inspected the power amp board and found the TDA7294 power-amp chip is cooked (pic attached, hopefully).  Got a new one coming from Mouser.  I unsoldered it and removed it from the board in prep for the new one in a couple days.

I also hedged my bets on the 4560 dual op amps in the preamp circuit.  Since the -16vdc supply was bad, there is a good possibility that there is more bad stuff, so I ordered 2 new ones of them too. 

I don't know about the power-amp board yet though, since I don't want to power up without the -16v supply being properly "regulated".

Just having fun, and hoping it is not going to be too expensive to fix.   I also have not checked the speaker!  Oh, no, I hope it's OK. 

paelgin

Here is an another attempt at attaching the photos I tried to get last post.

One is the bad power-amp chip cooked and partially exploded, and the other is the board I got it from.  Maybe I'll try the big pic too showing the whole project.

paelgin

While waiting for parts, I looked closer at the power-amp board for issues.  I metered the diodes and found nothing unusual, but found a burnt/broken trace on the board. 

The circuit board trace from the +Hi supply to the power-amp chip was burnt open.  I considered options and decided to add a wire in parallel from one component lead to another component lead providing the feed needed.  Here is the pic I posted earlier blown up and the broken trace circled. 

If curious, I can post pics and more detail of my "repair".  I am tempted to also stabilize my added wire with hot glue to avoid future breakage from flopping and flexing.  After I soldered it on, I wished I had shortened it though; it's probably about 2" longer than it could have been.

By the way, Teemu, thanks for your analysis of the circuit.  I don't work in the industry and got my Associate Degree in Electronics Technology in 1985.  Lots of changes in electronics since then, and I do not know what a 'class-G -type "high efficiency" circuit with switched supply rails' output circuit is.  But it sounds like it would be good to have to get the power rating claimed.

teemuk

I suppose it's pretty much a life-long project to keep in track with this electronics stuff anyway.

Anyway, in short: Classes G and H refer to rail switching topologies, which means that the amplifier is switched to use additional, higher voltage supply rails when the output signal swings past a certain point. Since a higher supply voltage always introduces more losses it is more efficient to run the amplifier, say, from dual 15V rails and switch on, say, dual 40V rails only when the output signal must swing past the 15V limit. Class-G refers to a switching (also called "commutation") mechanism where the switching of the rail is done in On/Off-like manner with a sharp transition from one supply voltage to another, whereas Class-H refers to a topology where the higher voltage rail is modulated to "track" the output signal so that the rail voltage remains only few volts above the output signal. This provides a further increase in efficiency.

The "high efficiency" circuit of the TDA chip seems to be latter type: When signal swing exceeds the lower rail limit a "tracking" higher voltage supply rail is switched in.

The definitions for class-G and H are reversed in some areas. Also, there are many variations of the basic principles and each manufacturer has tended to give these their own individual "trademark" name. Nowadays it is basically quite impossible to know to which one of the methods of operation people refer to. I suppose it doesn't matter that much but it surely can be confusing some times.

ferrante

Hi,

did they send it to you as .jpg?
If you additionally cropped and saved it, chances are that the numbers became unreadable like they are now.

Is it possible to send the original as-is? ;-)

Best regards


Quote from: paelgin on May 04, 2008, 08:59:24 AM
Here is a schematic emailed to me from Fender for a Fender 1270P powered monitor. 
...
Thanks for everyone who has been helping make and keep this Forum going.

paelgin

I did receive the original as a JPG.  It didn't seem very big when I got it, but I was thrilled to get it!

Here is the file as received.  I cannot see any difference in the clarity however.  Hopefully, you can see what you need however. 

Phil

J M Fahey

Dear Paelgin, many many thanks for sharing. It´s great to have, now and then, a "modern", "commercial" example of what's really being done today. I get tired of the endless tube schematics, boringly similar or identical, which can be found everywhere.   Thanks again.

amp_tramp

I just fixed one of these.  Here's what I found:

The TDA7294 output IC was blown.

The AC fuse (3 amp) on the input circuit board was blown.  Have fun changing it...

There were three burned traces on the output board.  Two were on the foil side, and easy to see.  The third was on the parts side, under a big coil (L1)  where it could not be seen.   To check for this, measure from pins 7 and 8 of the TDA7294 to the 8 pin cable that goes to the input board.  Both should connect to one of the pins.  If either one doesn't, you have a burned trace to find.

Hope this helps.

paelgin

Quote from: amp_tramp on May 01, 2010, 03:40:54 AM
I just fixed one of these.  Here's what I found:

The TDA7294 output IC was blown.

The AC fuse (3 amp) on the input circuit board was blown.  Have fun changing it...

There were three burned traces on the output board.  Two were on the foil side, and easy to see.  The third was on the parts side, under a big coil (L1)  where it could not be seen.   To check for this, measure from pins 7 and 8 of the TDA7294 to the 8 pin cable that goes to the input board.  Both should connect to one of the pins.  If either one doesn't, you have a burned trace to find.

Hope this helps.

Hey, thanks for your post.  As I read through my posts, I realized I never followed up: I fixed it, and have been using it for most of two years now.  It's plenty loud.

While working on it, after I put a new TDA chip in it, to troubleshoot a very low output, I tried to measure V on the pins, and touched several together by accident, blowing the fuse and smoking the TDA.  After that, I decided to dry test all connections with no power using the schematic and verifying continuity.  Then, when I needed V tests, I used an alligator clip attached while off to get my VOM connected to the circuit.  I need micro clips on my meter leads to really work live.

Phil

WannabeGeek

#11
I just acquired a non-functional Fender 1270p Powered Monitor.  The TDA7294 was also fried; in fact pin 7 (+vs signal) was completely blown off.  The fuse was also blown.  I removed the chip and replaced the fuse.  I already have a replacement TDA7294.  There are no other physical signs of damage on the boards.  I decided to test the voltages on the traces to the TDA7294 first.  The + - vs power (across pins 13 and 15) measures 33v.  The voltage across the traces where pins 7 & 8 would be (+ - vs signal) is 72v.  I think that's within the max ratings for the chip, but I'm a little afraid to solder it in, as it was burned out at that spot.  I'm not an electrical engineer, just a hobbyist, so I'm not quite sure where to look for a possible voltage surge.  There's an 8 pin power supply connector between the power supply board and the amplifier chip board.  I'm getting some high readings there too, so I'm assuming the power supply board was at fault or some object just shorted out the chip and everything else is ok. Does anybody have a suggestion as to which parts are most likely to be causing this sort of problem?  I would appreciate it if somebody could tell me where I should logically start looking.

Update:  One of the 4700uf caps was bad, so I replaced it.  Now I have + and - 43v where pins 7 & 8 would be and + and - 17v where pins 13 & 15 would be.  That is exactly what the schematic shows and according to the TDA7294 datasheet the chip can handle a maximum of + - 50v.  The only thing that puzzles me is why the original IC was destroyed.  Does anybody have any idea?  I still haven't replaced the amplifier chip because I only have one of them.  I guess I'll wait a while for a reply before I risk it.

J M Fahey

Well, stuff fails , that's why there's servicing ;)
Clean everything (soot/carbonizing/solder drops) replace the chip, and test the amp.
2 details:
1) build and use a lamp bulb current limiter (search this Forum) to protect your chip if there's some other problem you didn't notice.
2) start with the amp disconnected from the speaker.
You want to first check all volteges are fine, specially no DC voltage on the speaker out.
3) post what you find before re-connecting the speaker.

WannabeGeek

#13
Thank you J M Fahey for the advice.  I built the series lamp bulb limiter, cleaned up the boards, soldered in the new TDA7294, disconnected the speaker and flipped the switch.  The bulb lit up for about a half a second and then went very dim.  I hooked up the speaker wires to my oscilloscope and only got a few millivolts AC.  I decided it would be safe to hook up the speaker.  As a test, I connected a microphone to the XLR jack and turned everything back on.  The bulb flickered again and went very dim.  I got a nice, loud, clean signal with the mike and no hum.  :dbtu:  Awesome!  The only thing I haven't tried yet is running the monitor without the series lamp bulb limiter.  I'm assuming the surge when switching it on is normal.  Is there any reason you can think of why I shouldn't put it back together and try it out?

Roly

Thanks for the detailed writeup @paelgin, always helpful to those who are just starting out.

@paelgin> I tried to measure V on the pins, and touched several together by accident, blowing the fuse and smoking the TDA.

Never mind, we've all had days like that.  One time I (accidentally) tried to measure the resistance of the power mains right in front of a client, and reduced my multimeter to a smoking ruin.    :-[

@J M Fahey> Well, stuff fails , that's why there's servicing

Ever had one of those days when nothing seems to go wrong?   Just standing around waiting for somebody to blow something up?  :lmao:

{Hope you had another nice BBQ or two over during The Madness - I gorged as usual, ham, chicken, seafood, choc-dipped strawberries, followed by being a beached landwhale snoring on the couch all afternoon.}

@WannabeGeek - that looks like a result to me, however if you really want to be cautious you can try another limiting lamp or two of higher wattage, but if you have the lamp only flashing as the power supply cap charge, then goes dim, the amp half rail is very close to ground and there is no significant DC across the speaker, and above all it sounds okay (at low level) with a test signal, then I would consider that well out of the woods and call it a wrap.   :dbtu:

If you say theory and practice don't agree you haven't applied enough theory.