Yeah. Once I get the amp working, I will certainly look into more, simple, projects like that. A new preamp would be nice too, but we'll see how that goes. As for the bug, I'm not sure I need another addiction (which is what this kind of thing ends up being for me) considering the speaker one has had me since the beginning starting with my little Altec Lansing PC speakers way back in the day...
Hi Paco,
This bug is a good one to have. 😀
You will learn things that will apply to many other hobbies you may already have. There's nothing better to do once it's nasty outside.
The quality you can build will surprise you I think. I suspect you would really enjoy building things you can listen to.
-Chris
This bug is a good one to have. 😀
You will learn things that will apply to many other hobbies you may already have. There's nothing better to do once it's nasty outside.
The quality you can build will surprise you I think. I suspect you would really enjoy building things you can listen to.
-Chris
Yeah, hobbies make good use of spare time for sure
Hey, I just thought of something that may or may not have been answered already, but when matching the transistors, am I looking for ones that are within certain parameters or just two (or four) that spec close to each other for the new diff pair? I hope that makes sense.
Hey, I just thought of something that may or may not have been answered already, but when matching the transistors, am I looking for ones that are within certain parameters or just two (or four) that spec close to each other for the new diff pair? I hope that makes sense.
Hi Paco,
You only need to have matched pairs for this design.
-Chris
You only need to have matched pairs for this design.
This is the right way to get matched pairs.
-Chris
Hi Paco, chris et all, Believe me Chris, ive spent many, many hours, weeks, months, years...researching everything, Naturally you have to respect what you are doing in light of the hazards within, You'd be a fool not to, My opinion is, if someones taken the step to join something like DIY audio, they have an inherent interest, And a thirst for knowledge, Ok you will aways get the cheap fix exponent looking to save some money!
I understand what you are saying Chris, As a pro you know all too well the pitfalls, and have seen the bodged repairs carried out by over enthusiastic amateur's..But if someones prepared to go the distance as Paco seems with such determination, He's going to learn how to do the best he can!
I started on suicidal latter day amps, that I found dishearteningly depressing, with low tolerance levels, Then got something I cared about, which led me to a voyage of discovery, As with paco, I dug deep and found out everything I could! Its not just about repair, its a utopian path!
I dont mean to be rude, I hate the word respect.....But, you have taken your chosen field, Profession, And are offering help to individuals, sometimes you are high handed, sometimes pedantic, But your a fair and decent guy, I would stress, if I was in your position, the importance of soldering properly to your uncultivated individuals, For me the crux, axis of everything, I let myself down early on with the same....
Or failing that ask a pro!!!!
Dont let us down Paco!😀
I understand what you are saying Chris, As a pro you know all too well the pitfalls, and have seen the bodged repairs carried out by over enthusiastic amateur's..But if someones prepared to go the distance as Paco seems with such determination, He's going to learn how to do the best he can!
I started on suicidal latter day amps, that I found dishearteningly depressing, with low tolerance levels, Then got something I cared about, which led me to a voyage of discovery, As with paco, I dug deep and found out everything I could! Its not just about repair, its a utopian path!
I dont mean to be rude, I hate the word respect.....But, you have taken your chosen field, Profession, And are offering help to individuals, sometimes you are high handed, sometimes pedantic, But your a fair and decent guy, I would stress, if I was in your position, the importance of soldering properly to your uncultivated individuals, For me the crux, axis of everything, I let myself down early on with the same....
Or failing that ask a pro!!!!
Dont let us down Paco!😀
Sorry everyone for not having said anything in a while, but I finally got the new transistors is and.....it made it worse.
The crackling, popping, went from occasional to constant static, so I was like WTF?
I went back, left the new transistors in and resoldered some connections for some of the caps (a friend of my fathers said the the problem was an arcing capacitor) and all the noise went away until I accidentally touched two ends of some speaker wire together that were plugged into the amp.
At that point, I saw a small flash that was most likely the fuse for the right channel and now the amp has developed a ground loop hum on the right channel only, but there's no popping or clicking, any of that....
I haven't double checked the fuse as this was late last night that this happened, but I've got my fingers crossed that it's nothing serious...
The crackling, popping, went from occasional to constant static, so I was like WTF?
I went back, left the new transistors in and resoldered some connections for some of the caps (a friend of my fathers said the the problem was an arcing capacitor) and all the noise went away until I accidentally touched two ends of some speaker wire together that were plugged into the amp.
At that point, I saw a small flash that was most likely the fuse for the right channel and now the amp has developed a ground loop hum on the right channel only, but there's no popping or clicking, any of that....
I haven't double checked the fuse as this was late last night that this happened, but I've got my fingers crossed that it's nothing serious...
Ok. I'm officially giving up. I went and plugged in the amp today to see if it would still pass a signal at all and the bias transistor blew up, like let out the magic smoke and everything. So now I'm completely stuck and am going to end up taking it somewhere because the last time I managed to make an amp smoke, it was a $200 dollar fix...f*** me
I'm surprised that no one here has warned you to always power up a solid state amp that you're working on with some sort of current limiting before applying full power. If it won't come up with the current limiting in place then something is wrong.
Some use a Variac, or a light bulb in series with the main line voltage. I for years used current limiting resistors in the +/- supply lines.
Simple, safe way to go.
This is probably the first thing to learn after how to use your meter and soldering iron.
I built a Citation 12 from scratch as a young kid, even formed the chassis out of sheet aluminum.
Pete B.
Some use a Variac, or a light bulb in series with the main line voltage. I for years used current limiting resistors in the +/- supply lines.
Simple, safe way to go.
This is probably the first thing to learn after how to use your meter and soldering iron.
I built a Citation 12 from scratch as a young kid, even formed the chassis out of sheet aluminum.
Pete B.
You might want to consider repairing it yourself now that you know.
Do you have a decent meter?
Do you know how to test transistors at least with an ohm meter?
As a minimum, I would replace all the small electrolytic caps. They are past their useful life.
Check all the solder joints for cold or aged joints. Any in hot spots will tend to be stressed and perhaps crack and oxidize. Reflow them - better desolder them and apply fresh.
Give all the connectors and screw terminals a good coat of Deoxit.
I would not be surprised if the above would have fixed your poping problem.
Then debug the blown channel, does one at least work?
Do you have the repair manual?
Are you comfortable setting the output stage bias current and checking for DC offset at the output?
Pete B.
Do you have a decent meter?
Do you know how to test transistors at least with an ohm meter?
As a minimum, I would replace all the small electrolytic caps. They are past their useful life.
Check all the solder joints for cold or aged joints. Any in hot spots will tend to be stressed and perhaps crack and oxidize. Reflow them - better desolder them and apply fresh.
Give all the connectors and screw terminals a good coat of Deoxit.
I would not be surprised if the above would have fixed your poping problem.
Then debug the blown channel, does one at least work?
Do you have the repair manual?
Are you comfortable setting the output stage bias current and checking for DC offset at the output?
Pete B.
Technical manual, note that someone wrote 2N3055 as a replacement for the outputs, this is not good advice:
http://manuals.harman.com/hk/Service Manual/Citation twelve sm.pdf
Pete B.
http://manuals.harman.com/hk/Service Manual/Citation twelve sm.pdf
Pete B.
Do you think there was any one thing that caused that transistor to fry? Obviously it would need to be replaced and I want to assume that since there's only one on the right channel, I wouldn't need to worry about matching any like I had to do for the diff pair...
Also, do you think it would be worth it to reinstall the old diff pair or just leave them as is and keep the original part on the left channel too?
The left channel still works, which is nice. It would be easy enough to go back and resolder everything for the right channel. Probably would take me about 1/2 hour or so to do all the connections.
Where could I buy caps for the amp? I don't really know of a place to buy decent electrolytics, nor do I know of a brand to look for...
And no, I do not have the repair manual...
Also, do you think it would be worth it to reinstall the old diff pair or just leave them as is and keep the original part on the left channel too?
The left channel still works, which is nice. It would be easy enough to go back and resolder everything for the right channel. Probably would take me about 1/2 hour or so to do all the connections.
Where could I buy caps for the amp? I don't really know of a place to buy decent electrolytics, nor do I know of a brand to look for...
And no, I do not have the repair manual...
And the outputs are fine as far as I can tell. It was the input that Chris (anatech) suggested to me to replace, which I did. The original pair was replaced with 2N5087, actually as Harman did with later versions of the amp...
paco161 said:
The bias transistor you mean Q707 or Q708? The only way for this to go up in smoke is for there to be a short somewhere, or more likely for the driver transistors to have failed.
The 2N5087s are fine, but make sure you got them soldered in correctly. If so, leave them for now, but be sure that they have not failed.
No point in resoldering at this point change the caps first if you plan to. Digikey is the obvious choice for small quantities. Panasonic 47uF 50V is part #P13464-ND at Digikey, this is a 105 deg long life part, and costs $.14. You can up the power supply bypass to something like 220uF ( P10326-ND at $.80) and they will probably fit just fine with the size reductions, but of course check first. Select Panasonic, 5000hrs at 105 deg, radial type, and working voltage, then select the uF to find each one.
I've actually never worked on a real Citation 12, as I built it from the repair manual. Are the main PSU caps screw terminal type? This is a good place to put your current limiting resistors. These can be 100 ohm half watt but they will go up in smoke if something is wrong, otherwise use 5 or 10 W.
Do the circuit boards plug in?
I would remove the main fuse for the working channel to protect it as you debug this channel.
You should probably test all the transistors and order replacements while you are ordering caps.
Pete B.
I should add that the bias transistor is in a sense protected by the base emitter drops of the driver and output transistors. At most it should have 3 Vbe drops plus a small drop across the low value resistors, say 3 to 4 Volts at most. Not likely for it to go up in smoke with so little voltage across it. You can remove the bias transistor and jump the C to E connection which will simply operate the amp in class B with no idle current for test purposes. Something else will likely go up in smoke, so put in the current limiting resistors first.
Check the .27 ohm resistors and the 10 ohm 1W resistor with your meter. Do any resistors show signs of heat stress? These should also be replaced and the wattage upped if there are signs of them being highly stressed - this would be as indicated by a working channel not one that has been stressed due to component failure.
Pete B.
Check the .27 ohm resistors and the 10 ohm 1W resistor with your meter. Do any resistors show signs of heat stress? These should also be replaced and the wattage upped if there are signs of them being highly stressed - this would be as indicated by a working channel not one that has been stressed due to component failure.
Pete B.
Ok, so was it just a fluke that a fuse blew up and something fried to begin with?
I only assume that is was the bias transistor because that's where I thought I saw smoke come from, which was Q707 by the way...
The main PSU caps are not the screw terminal type and yes, the entire PCB plugs into connections on the chassis, which is really convenient...
I only assume that is was the bias transistor because that's where I thought I saw smoke come from, which was Q707 by the way...
The main PSU caps are not the screw terminal type and yes, the entire PCB plugs into connections on the chassis, which is really convenient...
Hi Paco,
Sorry to hear what happened. Sometimes lessons cost something and since you have gone this far, may as well continue to try.
Can you do something first? Check over the PCB and outputs / drivers and list exactly what has been blown. Measure those emitter resistors that Pete mentioned. Also measure the other resistors in that area of the circuit.
Do you have a variable AC transformer to use? If not, start with light bulbs in series with the AC supply to the amplifier. Start with low wattage bulbs to purposely under power the amplifier. You can then safely test the currents in different sections of the amplifier without further damage.
Hi Pete,
-Chris
Sorry to hear what happened. Sometimes lessons cost something and since you have gone this far, may as well continue to try.
Can you do something first? Check over the PCB and outputs / drivers and list exactly what has been blown. Measure those emitter resistors that Pete mentioned. Also measure the other resistors in that area of the circuit.
Do you have a variable AC transformer to use? If not, start with light bulbs in series with the AC supply to the amplifier. Start with low wattage bulbs to purposely under power the amplifier. You can then safely test the currents in different sections of the amplifier without further damage.
Hi Pete,
Paco was supposed to have had help from a ham friend on this. Your average ham knows better than to just plug something in and turn it on.
-Chris
Anatech,
I will eventually get to that. Do you want me to measure that transistor too while I'm at it? I can A/B it with the one on the other channel to get an idea of where it should be, maybe...
Also, the key words are supposed to. Unfortunately my friend was stuck in Philly for about three weeks and just got back this past week, but is still sorting stuff out from his trip, so I've never been able to actually get to him...
I will eventually get to that. Do you want me to measure that transistor too while I'm at it? I can A/B it with the one on the other channel to get an idea of where it should be, maybe...
Also, the key words are supposed to. Unfortunately my friend was stuck in Philly for about three weeks and just got back this past week, but is still sorting stuff out from his trip, so I've never been able to actually get to him...
All the resistors are ok. I checked transistors Q703 and Q707 and both still test ok, though I didn't pull the equivalent parts out of the left channel and check them yet.
I suspect that maybe it was cap C709 that bought it, but I'm not sure....those two transistors still pass voltages and all the resistors check out ok, so I'm down to caps...
Also, something else I just noticed. If you look in the tech manual on either link that has been provided (they're both the same) in the "Revised Section" that shows where HK used the two 2N5087s, if you look at the resistor that runs between the emitter of Q702 and cap C703, it's 12K ohms, not 8.2K ohms as it was with the original diff pair. Could it have been this lack of resistance that caused something to fry?
I suspect that maybe it was cap C709 that bought it, but I'm not sure....those two transistors still pass voltages and all the resistors check out ok, so I'm down to caps...
Also, something else I just noticed. If you look in the tech manual on either link that has been provided (they're both the same) in the "Revised Section" that shows where HK used the two 2N5087s, if you look at the resistor that runs between the emitter of Q702 and cap C703, it's 12K ohms, not 8.2K ohms as it was with the original diff pair. Could it have been this lack of resistance that caused something to fry?
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