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I fired this baby up, a loud hum, no signal pass.After 3 minutes, pop, the fuse went. Tried another fuse, stupid idea, hum, fuse blew again. One more time for good measures, now dead. At this point I am thinking the outputs may be shorted? They were tested months ago and they read fine. After this fuse issue I switched out the large can 7400 capacitor, no change...where does one go from here? Any help would be grand...Thanks.
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Output transistors are gone. Test them for shorts and emitter resistors for opens. Make sure the rail voltages are ok and op-amp rails. Best to use a variac when powerwing up to run at low voltage to check all the parameters to see if everythings comes up ok voltage wise.KB -
acoustic 470 output devices...help
Ok. So i checked online for a source for the output devices and i can't seem to find a reasonable, somewhat cheap parts source. The outputs are two 2n3055's,three 2n3055h's and one 480037. I had read on the forum that the 2n3055's could be subbed with better choice, the MJ15003's. Does anyone have any leads on any of the output's? Thanks all.Originally posted by Amp Kat View PostComment
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Don't take my word for it, as I'm not looking at a schem or anything, but my guess is they're all 2n3055's. OT's will be in pairs. The NPN & PNP compliments if its a complimentery design. Or if it's quasi-complimentery, which it sounds like your's is, two of the same. So what I think you probably have is 3 pairs of 2N3055. 480037 sound like an Acoustic in house #. Don't know what the h suffix is in 2N3055h, maybe package type or something.Originally posted by wardevans View Post
So it looks like some of them have been replaced in the past. I've learned from the good people on this board, and from experience that when one goes, it's a good idea to replace them all, and the drivers as well.Comment
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480037 is a house number for 2N3055. If you replace with 2N3055 make sure to look at the data sheet. The original 3055 was a 60v part. This amp has a 92v power rail, well over that rating. Later 3055s had a 100v rating, but that leaves a mere 8v headroom.
That is why I recommend MJ15003, the 2N3055 is marginal for this application. In 1974 it was a happening transistor, but now we have better stuff. Allied sells those for $3.67 and has hundreds in stock.
Obviously your amp has been repaired at some point, that is why the mix and match transistors.
So check for shorted power transistors, also check for a shorted main rectifier bridge. If one side of that is shorted, it will dump AC into the B+ and draw a ton of current as well.Education is what you're left with after you have forgotten what you have learned.Comment
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updates...readings
Two of the 3055's are shorted. The rectifier reads fine all the way around. I threw a meter across the 7800 large can cap and i can't get a reading. Someone mentioned i should get at least 15k? I have the old cap and tested that as well, that only read near 8k.Am I correct in thinking these caps are bad?Comment
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Threw a meter?
What exactly did you measure? They are huge caps, not resistors, so when youi first put an ohm meter to them, they will usually show a rising resistance until they show open. Your meter is charging it.
If the cap has voltage in it, that will read as either an open or a "negative resistance" if the leads are reversed.
Put a volt meter to it and see if there is a couple volts of charge stored.
That is one test for a cap. APply some voltage to it to charge it, then remove the wires. Now read the stored voltage. The slower it drops off, the less leaky the cap.
The cap must be disconnected from the rest of the circuit for tests, otherwise the readings will be confused by the rest of the stuff.Education is what you're left with after you have forgotten what you have learned.Comment
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Threw a meter? My old terminology..
Enzo and everyone else, thanks for all your help! Sorry for the obscure terminology...i didn't throw anything. haha... I PUT a meter across the caps. I pulled the caps before measuring them. They are not charging. So I probably answered my own question at this point. Also, I need to track down a new large cap and install that then install those new output transistors. I will try hooking a variable power supply and measure the rails and see what comes up. I have done some research for the 7800 cap and haven't found much that will fit the existing space, it seems like all the newer caps of this type are too tall!!! The existing one is slightly less than 4". So if anyone has a resource for this same size/type cap please let me know.Thanks all!!!
Originally posted by Enzo View PostComment
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The 470 was designed with the 2N3773 as the OP devices. The 2N3055 may work but you are betting that the BVceo of the transistor is guardbanded above 60V.
These parts are available from Digikey.Comment
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drag....all that and still dead.
I installed the 6 new output transistors,rechecked one of the filter caps I had and it was fine.I re-installed the filter cap...got it all set up...tried to fire up and nothing. I checked all my connections and I am getting no power to the main power switch. I am getting power to the fuse and to the conversion switch, but none to the main power switch. getting no voltage to the rectifier either...Is my transformer toast? Im thinking so. Any help would be great.Thanks all.Comment
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Ward, you can spend a lot of money in tossing in replacement parts for one of these if you don't track down what's really wrong in an orderly way.Originally posted by wardevans View Post
Is AC power getting to the transformer primary?
You say "I am getting power to the fuse and to the conversion switch, but none to the main power switch." Is the main power switch before the transformer? If so, you're not getting power to the transformer in the first place.
Do this:
(a)UNPLUG THE AMP FOR THE FOLLOWING TESTS
(b) set your meter to resistance, and clip one lead to one of the transformer primary leads. Now test for continuity to one of the AC power cord blades, with the power switch set on. You should find continuity to one of the AC power cord blades.
(c) Not test the other transformer primary lead to the other blade.
If you got continuity of both primary leads to the same blade, you have an open in the wiring leading to the transformer. This is what your description seems to say more than "bad transformer". Switches, fuse holders, and wires go bad too.
If you got continuity of each transformer lead to a different blade, you may have an open primary.
If you got continuity of both primary leads to both blades then the primary is not open at least.
Based on the preceding tests, if you think the transformer primary MAY be open, unsolder all but one lead and test for DC continuity on the winding itself. Infinite ohms is an open primary, and your tranny is dead. Do this the smart way and from the schematics be sure you're correctly looking at where primary winding should be in case you have a multiwinding primary.
(d) If all of the primary circuit checks out, Unsolder all except on wire of each secondary. Resolder the primary leads if you disconnected them in a-c. Tape any loose leads off so they cannot be accidentally shorted. Set your meter to measure AC power line voltage and clip it onto the primary leads. Check to see that you have not accidentally shorted something, leave the transformer and the meter on the bench, and plug in the AC power line cord. Operate the power switch. You should be able to turn the wall-socket power on and off to the transformer with the power switch. If not, you still have a problem in the AC wiring to the tranformer. If it pops a fuse each time , disconnect the power cord, unsolder one primary lead and re-clip your meter to the tab where the primary lead WAS. Now re-set it and the meter on the bench and reconnect AC power cord, then flip power switch. Fuses blow? If yes, you still have an AC power wiring issue. If not, you may have a dead PT.
If this shows a possible dead PT, rig up a (shorted-turns detector ) and test the transformer.
If the transformer passes, you have issues in your rectifiers, filters, secondary wiring or amplifier.Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.Comment
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Found the issue with no power
So, first of all, RG you are right in saying this could be costly without properly diagnosing all possibilities of fault. I jumped the gun with installing all those OP's without properly testing each variable. live and learn and spend too!!!
Turns out i found the issue with no power late last night when in my tired state of tracking the issue of "no power", i scanned the schematic last time to find there is a 7amp fuse in line with one of the transformer leads. This little bugger was wrapped in cloth and I really had no idea it was there. It was blown.In my quest to solve this amp's sickness I jumped the fuse, flipped the switch and for a short two seconds all of my honest efforts payed off, then system shutdown. The 5a power fuse popped again...back to square one. So maybe my anticipation and lack of patience and lack of proper diagnosis led me to this, but it is also a learning process. I wish to have this up and running soon, I am obviously not checking something or overlooking the obvious. At any rate, I'll keep plugging away at it until it is the 470 it should be.Comment
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