I received a Teac AG-790 stereo receiver with burn damage to the PCB underneath resistors 7R41, 7R47, 7R?? (unmarked, next to 7R47) and 7R48. I repaired all of this damage, cleaned up the carbon that had been left behind and soldered to the nearest remaining good portion of the board.
Up to this point, the receiver did what it has done since I got it. It played through the right channel only.
I kept on looking and found a capacitor at location 7C27 (220uF, 16V) that had blown out the rubber plug at the bottom. So I replaced that. Now nothing works. I know the new cap to be good. The unit will power up just as it always has, there are no blown fuses and my repair job seems to be bulletproof (and correct, per the schematic in the service manual). The audio is gone, replaced only by a faint humming on speakers and headphones alike, in the right channel. Turning the volume dial does not change this. There is still nothing from the left channel. The unit is not in protection and nothing is getting hot, burning up or worse.
In addition, the display on the front of the unit will now indicate "E0" (or maybe it's an O...no way to tell). This is not mentioned in the service manual, nor did I find anything in a web search. Operations from the front panel remain possible.
I'm beginning to think that it may be too far gone to fix. Anyone out there with an eleventh hour idea?
Also, is it possible that maybe the protection circuit is now working since you put the cap in, which is why you are now getting the E0 error. This could be caused by yet, a problem that remains in the bad channel..
Assuming this is like most protection output circuits, you may want to test the output of the bad channel before the relay circuit that joins the speakers. .YOu could have a channel that is outputting DC above the safe point for the initial on cycle. Which would lead to say that you still have a problem, ofcourse!
'blown plug' usually means the acid is out and quite possibly eating away the traces near by. Replacing the cap may have finished off a nearly gone trace. Do you have the prints so you can 'ohm it out' and verify the connections are actually there?
The new cap is installed properly. I made careful notes of how the old one had been installed and the schematic is in agreement with my notes.
I do believe the "E0" indication is some kind of error. But what it means is a mystery, and the service manual does not help. I'm going to ask TEAC, although so far they have not been helpful at all. The output of the amplifier does not appear to be blocked--it is amplifying this humming sound. It's not outputting DC to the speaker connections.
Outside of the burned area, no traces on the board were ruined.
If I knew what E0 meant, I suppose it might be very helpful.
I may try pulling the cap (since the original was completely open circuit, I doubt it can hurt) and seeing how the behavior changes.
Is it a double-sided (or worse - multilayer) pcb? If the pcb designer used one or both of the capacitor pins as vias then you could possibly have removed sufficient of the hole plating to lose e.g a power or ground through connection.
No, it appears to be a simple PCB with one layer on one side.
I would not call this receiver the greatest design I have seen. Compared to the Sherwood and Sony receivers that I have been working on, it strikes me as being unnecessarily complex. The main PCB is littered with all sorts of components where the others are much cleaner and simpler.
Well, not really. I was just trying to find a way to better help you with your diagnosis. Twice I had asked what the function of the cap was, and twice you had not replied.
Fair enough. A reasonable assumption, given the amount that's said on here about bulging caps being at the bottom of many faults.
I assumed that as you had the service manual, and were engaged in fault finding to component level, that you would have sufficient understanding of circuit design to be able to just look at the cap on the schematic, and determine its function without a second thought. It seems to me, pretty fundamental to this process, that you know the function of the cap you have replaced, considering that the fault symptoms changed dramatically after replacing it. You seem to have put this down to coincidence, and indeed, it may be, but from many long years of experience, it usually isn't. Either something went wrong with the process of changing it - print damage, component damage, via damage, un-noticed solder splat, etc, we've all been there - or the fact that it was faulty before was masking some other problem, that is now the dominant one. That was the reason that I felt it important to know what the function of that cap is. I don't know how much experience you have of component level faultfinding, but if it's not that much, don't be afraid to admit that. There are plenty on here with a lifetime's experience who will try their best to help, given the necessary information, which ultimately helps others to learn.
OK, no probs.
It depends what they meant by "shotgunning". Once it has been determined which stage a fault lies in, sometimes the best approach is to shotgun all the electrolytics, or sometimes all the transistors. Semiconductors will sometimes 'hide' their faultiness in that they've been weakened by obviously failed nearby devices. This can - and does - result in bounced repairs, sometimes with a cascade failure that you thought you'd successfully repaired, repeated all over again ... :-(
So usually, if a manufacturer recommends "shotgunning", it's for a good reason. Of course, whoever was replying to you might just have been being facetious, and was actually suggesting that you stood it on a post at the end of the yard, and took a 12 gauge to it .... :-))
Well, I'm a bit bored at the moment so having a peek. :-)
The service manual is up on eserviceinfo. Page 13 is relevant.
This (with the exception of the resistors) is all in output protection past the final stage.
7C26 is in reverse series with 7C27 (i.e. non-polarised configuration) and I guess may be short, open or leaky? Change it? What's across them looks to be a summed fraction of the output of both L&R amplifiers, and also a POW signal to the system micro.
The burning of the resistors first mentioned is indicative of an overload prior to the final output transistors (7V19 to 7V22). Don't know what that is all about, unless the amp power rails themselves have been up to some silly voltage they shouldn't have been.
Seems enough discretes there to enjoy (!) a good troubleshoot, with similar and isolate-able sections.
I am sorry about that. I must have missed seeing it.
You could say that. But I'm still *very* much in the learning phase. I started with studying basic home appliances and household electricity long ago. I started to study the workings of transistors, semiconductors and other components after that. (This particular process continues today.) Then, after much trial, I decided that I was going to learn how to solder properly. And I finally did. My plan is to know how to repair things even if nobody cares to. I don't plan to do this sort of thing professionally. I'm getting there...I've built simple electrical devices to solve problems or improve things in some way, modified circuits, mended broken up circuit boards a few times now and done "other stuff".
I haven't really looked into what that cap is doing...yet. I've been very busy with many other things, including the repair of cars, computer consulting work and "other jobs". I just haven't had the time to sit down with the schematic and look at what's going on.
No, I don't think it's a coincidence. I'm familiar with the concept of more than one part being bad, and more faults surfacing as damaged circuitry is repaired.
Well, I'm pretty sure I didn't do any of those things. When I first noticed the behavior change, I looked over my work very carefully. I wanted to be sure that I had not induced this problem by my own doing.
It's probably on the lower end of intermediate, or just out of the "total beginner" range. Of the things I've tried, though, I've had a
100% success rate...including the diagnosis of my linear tracking turntable's front panel control failure.
As it is, I've got enough stereo receivers, some of which I have repaired. Maybe I have too many...at last count I'd repaired at least four units in some way or another. One was a case of the previous owner experiencing "operator error" so maybe it does not really count.
This is a direct quote. Note the style as well, which has been left intact.
"HI William.We only have what's in the manual too.Since you already have the service manual,it shouldn't be too difficult to troubleshot.Power supply rails o.k.?,D.C. offset? Other than "shotgunning"parts,I don't know what else to recommend."
The power supply rails seem to be OK...they're all doing something (don't recall the exact measurements). I wasn't sure where exactly they wanted me looking for DC offset...I have some idea what it is, and that's about it.
Maybe I'm reading more into it than I should be, but I take what was written to mean "oh, we really don't repair those when one comes in under the terms of the warranty". And maybe they don't?
I asked if there was any documentation for the error codes, or if anyone there might know what they meant. I felt, perhaps incorrectly, that knowing what an error code meant would help. (I do realize that error codes are not always definitive and that they could send me barking up the wrong tree.) I also asked if my manual was complete...if maybe there was a piece I didn't have.
I never heard back.
I don't mind "shotgunning" parts of a circuit, but I have reservations about doing the whole darn unit. I'd have to think whether it is worth that to me...and I'm not convinced that it is.
Being as it was through e-mail, I can't reliably know the intention of what was written. I like to see the best in people, so I assume it probably was serious.
Compared to the other receivers I have worked on, I don't think this one is as good of a design. I may not be qualified to say that, but then again, I did fix the others.
I had this feeling that I should change that cap, if for no other reason than its immediate neighbor was bad. I'd even go so far as to say that said feeling just had something about it, telling me that I shouldn't ignore it.
No, I'm not superstitious or anything like that. But I know when I have a gut feeling, and I *do*.
So I'm going to do that and see what happens.
I don't know either. The only inkling I have is that someone played it very, very hard. (The previous owner didn't want to talk about what happened, but I don't think they were inside it. So it's another one of these feelings.) One channel still played without issue when I got it.
As best I can tell, the final transistors are good.
The only thing that's really driving me batty about this is the fact that it's a pain to do any testing, since I have to put the board back in. I really don't want to rearrange all the internal parts for testing purposes, because I am afraid I will lose some of them.
OK, that's fair enough. At least we now have an idea of your capabilities. When Teac asked you to check for DC offset, they were referring to a standing DC component at the power amplifier midpoint. This is at the junction of 7R43 / 45 for one channel, and 7R44 / 46 for the other channel. The voltage with respect to chassis measured at this midpoint, should be zero. Any positive or negative standing voltage at this point, is termed a "DC offset". If large enough, it could be damaging to a loudspeaker, so for this reason, there is circuitry to sense any offset, and if there is one, block a relay from operating. This relay is normally in series with the speaker feeds. As you are hearing some hum through one channel, this would suggest that the relay is closing (can you hear it close after a short delay from switching on ?) and if it is, that might suggest that there was no DC offset . But, if the protection circuit was not working correctly, it might just allow the relay to close with an offset present. As 7C27 is in inverse series with 7C26, I would be inclined to replace them as a pair. Your first move though, would be to check if an offset is present on either channel.
Yes, the relay closes. I have replaced 7C26 just now. The old cap tested OK per the cap checker built into my multimeter, but the new caps were cheap and I don't want it causing trouble. I've just found a
47uF 80V cap that is also bad. I don't have that one in my stash of parts, so I may have to order one. It appears that the heat which burned the circuit board also dehydrated this cap. Removed from the circuit, it tests open.
There is soft hum from the working right channel. I have yet to hear anything from the left channel. When it was playing (through the right channel only), it was possible to just barely hear distorted music through the left.
I did not notice any DC voltage at the speaker connections, if that's an acceptable place to take a measurement. I tested that before I ever hooked any speakers to the unit.
Yes, as long as the relay is closing, you can measure offset at the speaker terminals, with one slight reservation. In the case that you can hear nothing at all from the speakers, but you hear the relay close, you need to make sure that having closed, the relay is actually staying closed. Sometimes, you can be fooled, as on some designs, the protection relay will close momentarily, and immediately reopen if there is a fault.
What's the circuit designation of the new cap that you've found to be bad ?
I note you say that you are using the cap checking facility on your multimeter. What parameter does this measure ? Capacitance maybe ? If so, and you are aiming to do a moderate amount of repair work, you would do well to consider investing in an ESR meter, such as the "Blue" version of Bob Parker's original design, now marketed as an easy to build kit, by Anatek Corporation in the U.S. Although capacitance readings can be useful, to get such a reading necessitates removing the cap from the board. Apart from any other reason, this can be a tedious procedure if there are a lot of caps to check. However, a much more valid reason for owning one, is that the ESR of an electrolytic capacitor is, these days, one of its most important parameters. A lot of modern circuitry - particularly in switch mode power supplies - relies absolutely on the ESRs of the caps being within tolerance, and a basic capacitance meter cannot tell you this. I have seen electrolytics which show no signs at all of heat distress or bung popping, and which read almost spot on for value, but whose ESR is out of the window.
If I was recommending test equipment on a usefulness basis, then an ESR meter would be top of my 'must have' list.
I am pretty sensitive to the noises of relays and such, and if one does something out of the ordinary, I usually catch it in a sort of "and what do you think you're doing?" moment. (My hearing is also good enough to catch a lot of high frequency noises that some circuits make, and that has clued me in to a few problems on other repairs.)
This one really does seem to stay closed. It produces pretty noticeable clicks and clacks.
I am not near the receiver now and will have to look.
While the meter itself seems to be of decent quality and pretty accurate, the directions that came with it are poor and do not discuss how to do anything except voltage and resistance checks. Even those are very poorly expressed by way of poorly written or translated instructions.
The display reading in the "cap test" mode is expressed in nano or microfarads, although the test seems to be limited to low voltage and capacitance levels. So it would appear to be useless outside of small, low voltage capacitors.
For those that seem to exceed the ceiling on the meter, I've done a simple resistance test on the caps. A neighbor suggested this method, and stated that a good cap would slowly charge up and that the resistance reading would gradually increase while it did so. This seems to be true. Every "bad" cap that I've found using this method has either been open circuit or shown a high resistance reading that did not change much/at all from the initial reading.
At some point I do want to invest in a proper ESR meter, and the Bob Parker meter is the one I planned to get. If and when finances allow...
Every suspicious cap has been removed from the circuit for testing. This most recent one had a heat-shrunken plastic jacket.