Help with Acoustic 470

[drewl]

Blows mains fuse.....power supplies are okay
All the NPN output transistors are okay.
Tested all the pre drivers on power board
along with diodes, power resistors, caps..
bringing up voltage on variac slowly the fuse blows about 1/4 way up...
the positive side power transistors feel warm, neg side okay....
I need to isolate the power board and send voltage to pre amp to make sure it's okay.

[Enzo]

How did you determine the power supplies were OK if it blows the fuse at 30v on the mains?

WHen an amp blows fuses, the first thing I look for is shorted outputs, if they are not short, then I look for shorted main rectifiers.

How about a main filter that is real leaky over half its voltage?

REmember this is a single supply power amp, so there is a speaker cap. MAke sure it is not shorted, and of course work without a load.

I'd connect an ammeter in place of the fuse. WHy waste fuses. You can see the current ramp up before you even hit that magic 30v - or whatever - and back off instantly.

SImple enough to isolate things, pull the wire off the power amp card that supplies the +92VDC. If it still blows fuses, you have a main PS problem. The LV rails are derived from the +92 in the PA.

My approach would be to disconnect the +92 from the rest of the unit and see if the PS alone draws current. If so, does the rectifier alone draw current - ie no filter cap. If so does the PT alone draw current.

[drewl]

Well, that's what I did...pulled the red 92 volt wire off the output board and the supply came up fine.
While pondering I remembered I had dc voltage on the output....but with a cap onthe output?
It shouldn't be should it.....I checked the cap and it wasn't shorted....maybe it's flaky....

[Enzo]

Your meter uses a volt or less to test things. That won't tell us if something leaks terribly at 45v. Disconnect that cap and see if the output bus comes up to half B+. There should never be DC on the output jack itself unless the cap leaks. There is about 34k from the - end of that output cap to ground, or should be.

The +92 also goes over to the 170046 board. My guts tells me the problem is on teh power amp, but you should for sure check that out as well. Or disconnect the +92 from that board to see.

[drewl]

Well the cap is bad, not shorted but starts to pull alot of current over 30v.
The amp still has the problem though. I disconnected the lv outs to the preamps.....still no good.
I'm gonna start pulling all the transistors working backwards.
I feel like just bringing up the power supply until it starts blowing the bad component(S)......

[Enzo]

Have you just done junction drops on all the semis? That might point out a faulty one without pulling it first.

[drewl]

I checked all the transistors and diodes and caps, most resistors, all the power resistors.
I pulled all the power transistors and drivers and checked them.
Someone was in here before, so it might be easier to just rebuild the output section than check every component to see if something in here is wrong.
I'm taking the weekend off from this one....
For every one or two a month I have trouble with or am stupid with, a bunch more go out the door!

[acousticcontrolcorporation]

www.acousticcontrolcorporation.com

[Bruce / Mission Amps]

www.acousticcontrolcorporation.com

If you are going to take the time to type... why don't you participate instead of leaving your website URL?
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.

[kupervaser]

I was wondering, have you got any luck to fix that Acoustic 470 head?

Thanx
Jeff

[gbono]

What's the issue with your 470?

[kupervaser]

I don't have one yet. I can get one which is not working. But I was just reading the forum and I was wondering if you have fixed the issue.

[jrfrond]

When all semi's check good (again, this is with a low-voltage meter) and I am absolutely positive, I will check the bias circuit next.

Again, as Enzo pointed out, you don't know what is going to happen under load with semi's, even if they check good. If the problem isn't obvious, sometimes the more efficient way is to shotgun the semi's, that way you are positive they are good. Over the course of 30+ years, that has worked for me countless times. In a low parts-count circuit like that, it is highly feasible and in the long run, cheaper. That is, if you are running a service center. It becomes time $$$ vs. parts $$$.