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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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
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.
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.
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?
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:
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.
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. :)
Congratulations. :dbtu:
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.
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?
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.
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.
Hi
@States02. Welcome!
Quote from: States02as 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 (http://www.ssguitar.com/index.php?topic=2093.0). Make one now - you're gunna need it.
Quote from: DrGonz78Well 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 ;-) ).
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?)
Oops, got my email notifications set up.
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.
Quote from: States02The 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?
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?
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.
(http://file:///C:/Users/home/Downloads/20140106_105728.jpg)
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.
Quote from: States02
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.
Quote from: States02I 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.
Quote from: States02I 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.
Quote from: States02I 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.
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.
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.
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? (http://file:///C:/Users/home/Downloads/20140108_132303.jpg)
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!
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
Quote from: States02I'm wondering if I can really tackle this.
Sweat it not - you seem to be doing just fine so far.
Quote from: States02I'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.
Quote from: States02Regarding 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).
Quote from: States02on 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.
Quote from: States02Here'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).}
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.
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.
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.
Here's a picture of the front. Hope this is what you meant.
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.
Thanks that helps. Here are the voltages I got with speakers disconnected.
brown and orange lead -5.75
black and red leads, .13
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.
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.
Quote from: States02What 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...
(http://www.jbhifionline.com.au/rkt/MEDIUM//47/94/479471.jpg)
...was what I was visualising, but now we fully understand your confusion.
Quote from: States02I'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.
Quote from: J M FaheyTo separate areas, first pull TDA7294,measure and post all pin voltages,measured at the PCB pads.
Good thinking Batman. :dbtu:
Thanks all, I haven't opened up again yet. I realize I have some perfect wire for that repair. Now a local guy wants to get it from me to fool with it (or maybe make it into a non-powered stage monitor) I might let him buy it. I'll let you know if I go back in. I'm a little afraid of taking out that TDA7294. I guess I'll need a more pointy tip for my soldering iron.
One other question. I have a little peavey kb1 that I'm going to use as a powered monitor is there a merit in using this (now semi-famous) speaker, in the peavey? Both are 4 ohm.
I'll let you know what happens after I hear from this guy.
Be very careful when pulling that TDA7294 (or any other multi pin chipamp).
Melting solder and wiggling the leg will very probasbly tear the pad off.
You need a hot clean solder tip, a *good* solder pump to suck 90% of solder, then finish with solder wick.
You must be able to pull chipamp legs *easily*.
You can buy a new chipamp fpr a few dollars, you cannot buy a new PCB, so some prefer to just cut all lega, throw the "body" in the junkbin and pull leg by leg, with longnose pliers and melting solder one by obe.
The message is: pamper the PCB.