Acoustic 300 schematic needed and ???

[tim]

Hi all,
I got one of these in to work on...

Does anyone have a readable schematic for an Acoustic 300 amp? Also, 3 out of the eight power transistors are toast so will be replacing all of them. The devices that are in there now are labeled RCA CVZ 422....I can't find out anything about them on line but the good ones test as NPN so I'm assuming that any decent NPN power transistor should work OK (MJ15022 or the like?)

Any hints on setting idle bias would be helpful...not too familiar with these amps

[Enzo]

The schematic calle for outputs type 48-15 (480015) which are 2N3055. You aer tight, most any good NPN power transistor will work. MJ15022, 15024, 15003 would all be reasonable and most of us stock them. Or should.

I'm not so sure there is a bias adjustment on this, the one tweak pot is for centering the output voltage at half rail - before the output cap that is.

I have the drawing, but it is a Xerox from one on dark colored paper, so a scan is hopeless, but they used the sdame circuit for a lot of amps. Send me an email with Acoustic 300 Schematic as the subject line ro:

tmenzo at msn dot com

And I will get you something that will work.

[tim]

Thanks Enzo,
I found several schematics on line for the 320 and 360, etc but they were illegible....from what I could see, they looked close but it was just too hard to make that determination/assumption.
I have plenty of 2N3055's here so I'll pop some of those in there.
I have worked on a few Acoustics in my life but this is the first 300.
BTW, what is that little momentary contact switch for that is on every Acoustic S/S amp I've ever worked on? It's on the driver board.
I'll drop an email to you probably early next week if I can't get this tank going.
Thanks again,
Tim
Electronic Sound Technologies

[Enzo]

The 370 and the 833 use the same power amp, though the 833 omits that button. My 370 is clear.

MAke sure they are the modern 3055s. The original 2N3055 was a 60v part, and this amp runs on about 90 volts. You need the 100v versions for sure.

Does the amp work with the bad parts removed? It will function with only one output device on each grouping. REmember there are only 6 output transistors, two of them big ones are wired as drivers. SO two drivers adn two outputs and it will amplify.

I point this out so you can verify the rest of the thing works before throwing the rest of the parts in it.

I don't know about the button. It is part of a circuit that shunts the P/A input with a JFET. There are a couple samples of output current and output signal voltage that come into it. Looks to me like if things grow too large, it turns on the JFET. I'd say it is a peak limiter. The gate of the JFET has an e-cap, obviously for timing, so if a peak trips the thing, it might take a second to recover and let go.

The button appears to defeat this circuit. My guess is when adjusting for center voltage, you hold the button to defeat the anticlip limiter so you can adjust for symmetrical clipping without it tripping.

But that is just a guess looking at it.

[tim]

That's good info Enzo, especially on the pushbutton switch...I always wondered but never took the time to find out, either by asking or looking at a schematic...I'm sure you know how it is...money is time. You see things, wonder what they're for but when the amp (or whatever) is up and working, the last thing you want to do is analyze the working part of the circuit...you want to get the thing back in it's cabinet and send it on it's way.
Yup, my 3055's are new...just got some in a couple of weeks ago.
I didn't check the amp with the good parts as I pulled all the transistors out as several on both sides were shorted (the drivers were fine). Being that he outputs were in parallel, it was hard to tell which ones were bad without yanking them all...because there were toasted parts on both sides, I'm just going to replace all the outputs at once...I don't want to mix and match brands and types of transistors....on some of the higher power amps, I don't even like mixing date codes (that may be over doing it but I hate return problems)

[Enzo]

Oh good lord. I just realized there are no ballast resistors!!! They are asking for trouble expecting those transistors to share current well. You better replace the whole set of them, yes. You don't want one from the new batch trying to parallel with an old one.. REally, there should be a low resistance ballast in the emitter legs of each. That one common ballast doesn't do it.

You don't want to mix brands, but mixing lot numbers of the same type is OK, at least when the resistors I mentioned are there. MAking the parts share is the whole point of the resistors.

DO you know this trick? In a normal amp where there is a row of outputs, and each has a ballast resistor for its emitter, they are wired in parallel except for the emiitters, which are paralleled outside those resistors. When an EC short happens, it can be hard to isolate the bad one. Any one shorted makes the whole row measure shorted.

Measure resistance right at each transistor directly E to C. Now going down the row, usually one will measure a lot lower than the rest. That is the bad one. If the transistor is shorted, it measures close to zero ohms. If you are on ine of the others, your meter is actually measuring the resistance from collector, over through the bad part, and back through the bad part resistor, and then up through the resistor where you are. If those resistors are half an ohm, it adds 1 ohm to the reading compared to the shorted one. So if the row measures EC 1.2, 1.2, 1.2, .2, 1.2 ohms, then we know the fourth one is bad.

That rarely lets me down, and if it does? Well I am no worse off, and it only took a minute.

Unfortunately, doesn't work for BC shorts, and in my experience, a BC short on any one transistor in the row will destroy them all.

[tim]

Yeah, I noticed that too and that's one of the reasons I was going to just replace them all. I was wondering though if I was to install, say, a .33 ohm 5 watt resistor on each emitter if that would help...I know it's not designed with that but would it throw it off if I were to do that? I know it would limit current through each device a bit (probably a good thing) and probably therefore not quite as much power out (miniscule difference) but what would happen if ballast resistors were installed in a circuit where they weren't before? It wouldn't be that hard to do on this amp as the emitters and bases are all wired in parallel with bus wire. What are your thought on this?
As always, thanks for your help.
Tim
Electronic Sound Technologies

[Enzo]

Wouldn't hurt it a bit.

Ther already is a common ballast, but that does nothing for current charing.

[acousticcontrolcorporation]

www.acousticcontrolcorporation.com

www.acousticcontrolcorporation.com

[Bruce / Mission Amps]

Starting paying for the free advertising

www.acousticcontrolcorporation.com

Doug, or who ever, you are really becoming annoying with these worthless entries...........
If you are going to take the time to type anything at all ... why don't you participate instead of leaving your website URL as a spamming commercial?

If you have any, how about just sharing some info like the rest of us.
Otherwise your just another annoying free commercial here and that is not in the true spirit of AMPAGE.
There are other places for that.

Really, the worthless, self promoting commercials with no content on every Acoustic question must go... add some text with insight to be respected or just leave.

[acousticcontrolcorporation]

You are getting annoying!!!!

He is asking where to get schematics. That is where to get them. What is your beef with me?

[Enzo]

His beef is that your post doesn;t say "I have that schematic and will send it to you." Your response was the same one no matter what the question was - a link to a web site that offers only email addresses.

[gbono]

Hey Doug,

Where are my inductors?

[Dave Curtis, dB AudioTech]

*You*, Sir, are annoying.

He is asking where to get schematics. That is where to get them. What is your beef with me?

You replying to a post from 2007 (and a great many others by the looks) with an ADVERTISEMENT to a web page with only email addys and no content (that you last updated in 2006). And it's not just his beef, Mr. acousticcontrolcorporation; he's kind of a spokesman around here. Get with the program or leave. Please. If you have nothing to offer but your website, then we got it, OK? Deliver Goods or Lurk & Learn, Buck - O . Keep yer spam in yer lunchbox.