Acoustic 450

[doctorvalve]

I have an acoustic 450 head, it sounds fizzy at all volume levels, it needs the quiesent bias current set up properly, anybody know the correct settings/voltages/test points to do this? i have the schematic for this amp.

regards, doctorvalve

[gbono]

Here is a picture of the 450 output board - I don't see any trim pots for bias/symmetry like the 470 has. Are you sure the distortion feature is turned off? Opps maybe the 470 only has distortion?

[Enzo]

The 450 should have a power amp in jack. Plug a signal in there. SOund bad now? And plug your signal into the regular input out front, run a cord from the preamp out jack to some other amp for a listen. NO speaker on the 450 for now. How does that preamp signal sound through the other amp? Those tests should isolate the problem to the preamp or popwer amp.

The 170051 board had no adjustment, but the 170045 version did, both bias and offset. Is this amap one with aqdjustments? If so, get out the scope and set for crossover notch just disappearing.

If the preamp sounds bad, my fist suspects are all those little small value electrolytic caps. I see 1uf, 4.7uf, and 10 uf scattered about. Oooh, there's a 50uf too. And I wouldn;t bet my life on the couple in the power amp.

[brianbb]

Hello, I'm new to the forum.

I'm having a similar issue with a 450 I just acquired. The output is very weak, even at full volume. Using Enzo's suggestion above, I have isolated the problem to be the preamp. I have checked all the electrolytic caps and voltages that I could find. I even sprayed the rear jacks with deoxit in the event one was shorting out the preamp. Please help.

Thanks, Brian

[gbono]

Hebergement d'image et upload image - Casimages

Check out these pictures with the associated voltages. You may also need a scope and signal generator to see where you loose signal integrity.

[brianbb]

gbono,

Thanks. I have used these diagrams and found most of the voltages check good. The ones that didn't I attributed to my lack of knowledge in exactly what to look for. The DC voltages looked okay but the AC voltages read very high and came down to an acceptable reading. I'm not sure how to check these.

[gbono]

Did you look very closely at solder joints? The AC voltages are best read with a scope.

[Enzo]

How did you "check" all the electrolytics? I worry not about the larger filter caps, but I worry about the smaller ones in the signal path. They may not read shsorted and may have a value close to spec, but that doesn't mean they don't leak or haven;t dried out.

A scope, AC voltmeter, signal tracer each could be used to trace a signal through the signal path to find where it weakens.

[brianbb]

I looked at all the solder joints and touched up a few. I have a scope that a friend gave me but don't know how to use it.

[brianbb]

I checked the electrolytics with a multimeter on cap setting. Most gave a reading close to stated value. A few caps read open until I removed them and then they checked good. They still may be dried out as you say. I have a scope and multimeter but not familiar how to trace the signal path with any degree of certainty.

[Gtr_tech]

Just replace *all* of the electrolytic caps in that thing. They are 40 years old now and won't do the job as they should. Your meter only tells you a portion of the story. Value is one thing, but dielectric absorbtion, ESR, and leakage are other parameters of a lytic that need to be in spec. get 'em outta there and install fresh parts.

[brianbb]

Okay I'll start there first; replace all the electrolytics. Thanks Gtr_tech.

[brianbb]

Good news. I think I found the problem. In the process of replacing all the electrolytics I found one completely missing. The one adjacent to the small coil near the middle tone control pot has been cut out. Please tell me the value and orientation of this cap.
Thanks, Brian

http://nsm01.casimages.com/img/2009/...8903345701.jpg

[brianbb]

I found something else clipped from the board, a resistor. Looking at the image from the link above there are two caps, an .047 and .022, between the bass and distortion pots. Between those two caps are two resistors and one is missing. Please give me the value of those two. I don't trust my own ability to trace it from a schematic.

Who would cut out a resistor and cap from an amp an put it back together again? Oh well, thanks for all your help.

Brian

[Enzo]

I don't get it. Is this a photo of someone else's board? I don't see a clipped out cap next to the coil. And I have to say I don't know which control is which in the picture. The color codes are visible in the photo, so if this is a photo of a good board, there is your value.

Just a guess, but look on the underside of the board to see if anything else has been done there, but mainly, find out what your missing parts connect to. If your mytery cap has one end on some control or other recognizable part, we can find it in the schematic.

ANy chance the missing parts are C111 and R121? Those are a little brightness network across the right-hand distortion control in the schematic. Maybe someone removed it to tame the distortion.