Yes, but those were resistance readings between transistor pins and rails. Did you do the tests I mentioned in #32?
When you talk of resistance readings I'm not sure what Fredlf is actually measuring and what kind of meter he uses (or did I miss that?). I use a Fluke 8060A in diode check mode and the number it reads is the voltage drop of the diode. The readings in post 28 make no sense to me but the measurements in post 29 pretty much sealed it for me. The large base current and low emitter current combined with the 82 volt output makes me think Q4 has an open E-B junction and a leaky B-C 'junction'. I put that in quotes as I don't believe that transistor does anything any more.
If my 555 ever croaks I'll have a pretty good handle on it.
G²
If my 555 ever croaks I'll have a pretty good handle on it.
G²
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Yes, I agree, especially since the transistor is one of the ones I had problems with in my Adcom, and one of the ones I suggested to check in the first place. I'd still kind of like to see it verified with some diode check readings of the transistor itself, though. I thought, back at the beginning of this, he had mentioned something about diode check with his meter.
Also, if Q4 is bad, I think he'd be well served by swapping out the other Q4, both Q7's, both C3's, and both C5's. (With good Matsushita, Rubycon, or Nichicon 105 degree caps for the C3's and C5's). At that point, the amp should be good for a few years longer... although it might also need some filter caps in the not-too-distant future.
Cheers,
Paul
I'll re-do those tests as you suggested in post #32, but I ordered new Q4 and Q7 already (the exact transistor from MCM!) and I will add the clip-on heat sinks G2 recommended.
So I had already ordered new C3 and C5, on the general principal of replacing old electrolytics, but I'm confused by the fact that my parts list (from the service manual) lists C3 as 100v, but what is mounted is 10v. It's hard to find a 100v cap that would in the space (5x11mm is what's in there).
Edit: now that I look at the PCB in the manual, I see that my PCB is a slightly different rev. than what's shown. In the manual, C3 is clearly a larger 100v cap.
Aren't those filter caps going to cost a fortune? What would you use?
Needless to say, I won't do any of this part swapping until I get things running again!
cheers,
Fred
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Wrong cap, and, IMO, insufficient advice on cap quality.
C3 is 47uF 100V.
I would recommend something like this: Digi-Key - P5596-ND (Manufacturer - ECA-2AHG470)
It's about .4" diameter and about .5" high. IMO, and in my experience, Matsushita (Panasonic) makes generally the best long life at high temp capacitors out there.
If you want to order from Mouser instead, the Nichicon 105 degrees are pretty good too, as are the Rubycons available elsewhere, but I still think the Matsushistas are the best.
Cheers,
Paul
C3 is 47uF 100V.
I would recommend something like this: Digi-Key - P5596-ND (Manufacturer - ECA-2AHG470)
It's about .4" diameter and about .5" high. IMO, and in my experience, Matsushita (Panasonic) makes generally the best long life at high temp capacitors out there.
If you want to order from Mouser instead, the Nichicon 105 degrees are pretty good too, as are the Rubycons available elsewhere, but I still think the Matsushistas are the best.
Cheers,
Paul
"Aren't those filter caps going to cost a fortune? What would you use?"
"Wrong cap, and, IMO, insufficient advice on cap quality."
What, Adcom filter caps aren't good enough?
As regards C3, any good cap that fits will be OK. 22µF will work, 100µF will work, 6.3V will work (as long as the amplifier is working).
I always replace these when the amp blows up, or if older than 7 years.
"Wrong cap, and, IMO, insufficient advice on cap quality."
What, Adcom filter caps aren't good enough?
As regards C3, any good cap that fits will be OK. 22µF will work, 100µF will work, 6.3V will work (as long as the amplifier is working).
I always replace these when the amp blows up, or if older than 7 years.
I re-measured Q4 and Q7, this time with my Fluke 2707A set to diode test (*ahem*). Looks to me like Q7 has a B-E short:
Q7 (1210):
Bad Channel:
B+/C- O/L
B-/C+ .580
B+/E- .000
B-/E+ .000
E+/C- O/L
E-/C+ .580
Good Channel:
B+/C- O/L
B-/C+ .607
B+/E- .530
B-/E+ O/L
E+/C- O/L
E-/C+ .625
Q4 checks out okay, but I'll replace it anyway.
Parts are on order so we'll see in a few days (fingers crossed).
Assuming this is the fault, should I replace the nearby resistors?
Q7 (1210):
Bad Channel:
B+/C- O/L
B-/C+ .580
B+/E- .000
B-/E+ .000
E+/C- O/L
E-/C+ .580
Good Channel:
B+/C- O/L
B-/C+ .607
B+/E- .530
B-/E+ O/L
E+/C- O/L
E-/C+ .625
Q4 checks out okay, but I'll replace it anyway.
Parts are on order so we'll see in a few days (fingers crossed).
Assuming this is the fault, should I replace the nearby resistors?
Sorry, I missed the part that we were talking about the filter caps as well. That's a great source you posted for the filter caps. I was talking just about C3.
For C3, are you saying that both the parts list, and the schematic, are incorrect about this being a 100V part? If a 10V part is in the board now, then that's probably what I'd replace it with, if the solder was original (i.e. I could tell that the part was original).
Regardless, there's no reason to start changing values and such to fit the part. Mouser and DigiKey both carry a WIDE variety of caps, and correct replacements are easily available with no minimum order.
For C3, are you saying that both the parts list, and the schematic, are incorrect about this being a 100V part? If a 10V part is in the board now, then that's probably what I'd replace it with, if the solder was original (i.e. I could tell that the part was original).
Regardless, there's no reason to start changing values and such to fit the part. Mouser and DigiKey both carry a WIDE variety of caps, and correct replacements are easily available with no minimum order.
If they're not burned, they're probably fine. You can do a quick check of their resistance once you have the transistors out (or replaced), but I think you'll find that that one transistor is the main problem.
The reason I recommended replacing both the 2SA1210's and the 2SC2912's, is that, a week after I first fixed mine by replacing a bad 1210, a 2912 went out in the other channel. You can see that these transistors run a bit hot. I think that when one goes, the rest are generally not too far behind.
Given these numbers I agree your first failure was probably Q7. Q4's job is to be a current source at 4mA with an average of 80 volts on it - .32 watts. With Q7 shorted, Q4 would have 160 volts on it at 4mA doubling the power to .64 watts. Without a heatsink it would fail. If you had installed only Q4 you would have still had +82 at the output but it would take a few minutes to overheat and ruin Q4 (without a heatsink) While you have Q7 out of the board I would re-check Q8. it might be better to temporarily remove it from the board _or_ lift 1 end of R7 and R7 so you get valid readings.
G²
Yes, both the parts list and the schematic have C3 as 100v. However, the PC board graphic in the manual is slightly different than the PC board in my amp. The diagram in the manual shows a larger cap for C3. On my board Q7 is rotated 90 degrees from how it is shown on the diagram, and there are few other little differences. (see pix)
When I get the board outs I'll look at the p/n. I suspect there was a rev. of the board and some parts changed slightly. I'll replace C3 with a 10v, identical part.
You want to know how _I_ would approach a sick power amp? Having done this stuff for a lot of years, the things most likely to fail are the ones that run hot. Power devices are most likely to get hot. I first check for E-C shorts on each leg (pos and neg) of the amp. Multiple transistors in parallel are no problem as the shorted one reads just the transistor but the good ones read the shorted one plus both emitter resistors so you can easily identify the bad one. I've never seen more than one device in a leg (3,4, or 5 in parallel) fail. but almost _always_ one device in the other leg. Remove the bad transistors and turn it on with no load and run test tones. If you see normal signal levels (use a scope and run it all the way to clipping. It should be symmetrical) just plug in the new power transistors and set the bias and ship it. The way _I_ set the bias on the old amps was to run 30 KHz at a volt or 2 P-P and look for crossover notches on the scope. Increase the bias until the notches are gone as that is the junk you actually hear. If the current gets excessive and the notches _aren't_ gone, you have more checking to do before you're done. For the tough dogs that old curve tracer can give a wealth of info. The dogs are REALLY RARE in my experience. "Dogs' almost always have multiple failures which makes them interesting to solve. Statistically, capacitors are the most common failure. Sometimes acid leaking from the cap(s) will damage the traces on the board, sometimes so fine you need a microscope to see them but the Fluke meter in resistance 'beep' mode can find bad traces as fast as you can move the probes. Resistors blow because of heat often induced by shorted semicinductors. Semiconductors blow from shorted speaker wires and ventilation problems from dust. Back the the heat again.
Testing transistors in circuit is done (by me) using the Fluke 8060A in diode test mode but you need to read the schematic and sort out when to check the device_out_ of the circuit to get valid answers. The E-B and B-C reading will be similar but never exactly the same. Same means bad. E-C leaks shouldn't be there (though you_might_ see some), E-C shorts are bad. Having a known good transistor to compare against is helpful and you found that comparing channels is very good.
So, first and foremost, get a test generator and scope and know how to use them along with a good meter and a very good soldering iron and know how to use them too. Personally I use Tektronix scopes and consider the rest toys. Give me an old 465 over most anything else for the price. If you're rich, the TDS3054 will set you back around $10K. It's what I've been using at work for 10 years. A Weller temperature controlled soldering iron (magnetic thermostat) is the minimum I would use if forced but I much prefer my Metcal. Good tools will not make you good but will allow/assist you to to be good.
I eventually got out of consumer repairs and went into broadcast maintenance as the pay is better, the test gear is better, the equipment is better, the hours more consistent and you see the same machines and people every day AND you get to 'get to the bottom' of the occaisional dog. With good maintenance records it gets pretty easy to keep hundreds of machines (dub house) in good repair. You have all seen my work on DVDs and network TV.
G²
Geez, G2, I was just wondering what your method for testing trannies was, and you have exceeded expectations with a short course in electronic repair with some career advice thrown in! Many thanks.
My very first paying job was assembling PC boards for a guy down the street who ran a medical instruments company out of his garage. I have been fascinated with electronics since, but my career(s) led me far afield.
Now I just want to be able to competently repair my own aging gear and maybe help out some friends. I'm saddened that consumer gear has become so disposable and that it's ever harder to find qualified techs who will do consumer repairs.
Back on topic: parts are on the way so I should be able to hit the bench at the end of the week. Updates to follow...
My very first paying job was assembling PC boards for a guy down the street who ran a medical instruments company out of his garage. I have been fascinated with electronics since, but my career(s) led me far afield.
Now I just want to be able to competently repair my own aging gear and maybe help out some friends. I'm saddened that consumer gear has become so disposable and that it's ever harder to find qualified techs who will do consumer repairs.
Back on topic: parts are on the way so I should be able to hit the bench at the end of the week. Updates to follow...
Some questions about replacement transistors. Several of the trannies in the spec call for specific hfe grades (GR or BL). Is the GFA-555 one of those circuit designs that will tolerate less precise gain? In other words, can I sub in trannies of lower grades?
I ask because I'm having a hard time finding manufacturers who will list the grade, or if they do it's out of stock and 20x the price. (e.g. 2sc2240 - MCM Electronics Search )
I ask because I'm having a hard time finding manufacturers who will list the grade, or if they do it's out of stock and 20x the price. (e.g. 2sc2240 - MCM Electronics Search )
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I went hunting for a suitable replacement for the 2SC2440 and I have what looks to be a good candidate in the Zetex ZTX1055A for $1.93 in 1's and $1.45 in 10's at Digikey. The search was for NPN 100MHz, 120V and 200 hFE minimum. I think you'll find this OK. I would use them in my Adcom (or Phase Linear) if I needed to replace. For the drivers I'd use the aforementioned MJE15030 and MJE15031 and for outputs MJ15024 and MJ15025.
G²
Okay, some progress tonight (a week of in-laws is no way to progress on projects, 'nuff said). I pulled Q4 and Q7. I will re-test them. I lifted R7 and R8 (I assume you meant R8) and tested Q8 with the following readings:
B+/C- O/L
B-/C+ .626
B+/E- O/L
B-/E+ .641
E+/C- 1.968
E-/C+ O/L
Would that indicate a leaky E-C junction?
Possibly. If Q7 is off the board AND one end of R7 AND R8 are lifted, Q8 is out of circuit. If you have another 2SA970 (Q8) I would compare new vs used but once I have a device out, its just as easy to install the new one (I'm assuming you have one). Your symptoms point to a Q7 failure causing a Q4 failure and I would be surprised if Q8 is bad.
I'm curious enough to offer to check your questionable transistors on the old Heathkit curve tracer and would be happy to post photos of yours vs known good devices.
G²
Yep, Q7 was off the board and one end of R7 and R8 was lifted when I did those tests. I don't have another 2SA970 because I'm having a hard time figuring out an appropriate replacement. Any suggestions would be most welcome.
I'd be thrilled to send you the pulled transistors since I'm very curious myself.
I generally try to replace any component I've pulled on general principal, new is better than old, and heat (via solder/de-solder) is the enemy. Of course I test components before installing.
I'd be thrilled to send you the pulled transistors since I'm very curious myself.
I generally try to replace any component I've pulled on general principal, new is better than old, and heat (via solder/de-solder) is the enemy. Of course I test components before installing.
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