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Messages - Enzo

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1
Amplifier Discussion / Re: Power supply/ power amp question
« on: January 17, 2021, 09:45:40 PM »
Transformers produce AC voltages, and those voltages are called RMS voltages.  SInce AC voltage changes all the time, we use an average.

36v AC, moves from zero up to a peak of About 51v, then back down through zero and out to -51v and back.   The average of that zero to 51v is 36v.  When you rectify and filter that, the caps charge up to the peak of 51vDC.   This is how it works.  You take the RMS AC voltage of the transformer, and multiply by 1.414 to get the peak voltage, which will be the rectified and filtered result.We might subtract a volt or two for losses in the rectifiers or some such, but that basic formula is close enough.

2
The Newcomer's Forum / Re: Mike700 transistor and mje800 transistor
« on: December 17, 2020, 11:48:26 PM »
Um... how did you test the op amps with a battery?

I don't know how you could get 77 volts on an IC, the highest voltage in the amp is 46v.


Ther op amps run on +15v and -15v, the 15v supplies are not AC.

Note the schematic voltages have polarity signs, and that means they are DC.  AC has no polarity

3
The Newcomer's Forum / Re: Mike700 transistor and mje800 transistor
« on: December 16, 2020, 09:13:25 PM »
He said it was the Studio Pro

4
The Newcomer's Forum / Re: Mike700 transistor and mje800 transistor
« on: December 14, 2020, 09:04:18 PM »
There are some test voltages on the schematic, start with those.

5
Amplifier Discussion / Re: Acoustic G120-112
« on: December 02, 2020, 09:27:22 PM »
What?   Just what large cap were you expecting?  This is not a single rail amp like older Acoustics.   Look at your schematic.  RUns on plus 42 to minus 42 volts.  Not plus 90 and ground.  The output sets at zero.  No need for a fat cap.  The fact it is quasi doesn't matter.

Your schematic shows no output jacks.   But if I were to guess, I'd say the extension jack puts the extra speaker in series with the main, rather than parallel.  SO the sleeve can be grounded, and the tip cutout goes to the sleeve of the main jack.   Thus the main jack needs to be isolated.

Why would they do that?  If they made it parallel jacks, then any moron could plug a couple 4 ohm cabs in there and load the poor amp to 2 ohms and blow it up.  Now a couple 4 ohm cabs makes an 8 ohm load.

We do find schematic errors now and then, but I think it is wise to resist the urge to immediately label something we don't understand as a mistake.

6
Amplifier Discussion / Re: baldwin c2 solid state amp rebuild question?
« on: November 25, 2020, 09:05:18 PM »
I rebuilt many many many jukebox amps over the years.  Mostly Seeburg tube amp and Rockola solid state.  But Rowe or AMI or whatever, we did it all.   I don't recall ever seeing cap kits.  I don't doubt they exist, but I just maintained a stock of parts and had no need.  I did a ton of CRT monitor repairs for the arcade trade, and they did offer cap kits for those.

That said, I am not aware of any cap kits for amplifiers, and especially for a relatively unusual one like that.  But such parts are easily available.

7
Amplifier Discussion / Re: peavey musician series 400
« on: November 20, 2020, 10:47:38 PM »
I am not where I can post at the moment, but google "Peavey semiconductor cross reference"  or "peavey semiconductor guide".   Many places will have it for download.

8
Rail is just a informal term for power supply.  For example, your amp has a +27v and a -27v "rail", also has plus and minus 15v and also 13v rails.   There is no physical rail thing, it just means a point in the circuit that multiple things connect to.  SO in your schematic, every place you see a arrow pointing up with +27 next to it is connected to every other place with an up arrow and +27.   And that imaginary common point is the +27 rail.

SO as g1 suggested, check ALL of those supply voltages to see if any are missing.

9
And did we unplug the footswitch?

Is the FS the real Fender FS for this model?  Or is it some stomp switch you had from something else?

Is there ANY sound in the speaker?  BAckground hum maybe?

Turn up the reverb and bang on the amp to crash the springs.  Does that noise come out?

10
Amplifier Discussion / Re: peavey musician series 400
« on: November 18, 2020, 09:02:26 PM »
Oh, don't sweat any that are off a bit.  We are concerned they are shorted or not.   I'd wager your testing is not done at full current, but rather with a meter, and that is like tuning a race car engine at idle.   You are blowing fuses, and gain variation will not cause that.

If you are blowing fuses, suspects will be:  output transistors shorted.  Bias string open somewhere (perhaps at the sensing diode connector).  Shorted rectifier.


11
Amplifier Discussion / Re: Amp help for a(n old) newbie
« on: October 27, 2020, 09:35:05 PM »
The Hot Rod series do have some things that happen to a lot of them, BUT, there is a difference between "When they fail, THis is what often happens" and "This usually fails in an amp."

WHEN an amp comes in with channel switching problems, I know to look at the two hot power resistors.  But that does NOT mean that all the aamps do that.  Most of them do not have a problem.

"Leaky" caps don't usually look different.  The leak refers to electrical operation, not some goo oozing out.  That can happen too, but it rare.

My point being I generally do not like pre-emptive wholesale parts replacement.  We would be more likely to cause problems than to prevent them.

12
The Newcomer's Forum / Re: Soldering Station
« on: October 16, 2020, 10:37:26 PM »
Visit shops or repair techs in your area and ask what they use, and if they would buy another one when that one dies.  What do they like and dislike about it.

I have used Weller irons all my electronic life.  I like the WTCP series, but they make a variety.  I also use Pace equipment.  More costly but I like it, been using my Pace desolder station for decades now.

But there are other good brands like Hakko that I have never used, but many like them a lot.  So you can find good equipment that is not what is in my shop.

13
Amplifier Discussion / Re: Peavey VSS-20 amplifier with chorus noise problem
« on: September 17, 2020, 12:50:41 AM »
U3 and all those caps are the chorus clock circuit, they are not in the audio path.

U2, U7 Q6, Q7 are the chorus path.  MAybe Q5 too but I doubt it.  Turn chorus on, you get hiss.  Power off, remove U2, power on.  His still there?  You won't get a chorus signal with U2 gone but we are looking for the noise now.  Still noise?  Remove U7, same question?

Make SURE not to install ICs backwards.

14
Amplifier Discussion / Re: Help with Crate CR112 Tonestack
« on: September 09, 2020, 01:38:53 PM »
Hamburger claw meat just isn't that good to start with...

15
Amplifier Discussion / Re: Help with Crate CR112 Tonestack
« on: September 03, 2020, 12:07:51 PM »
It is fun to try to tweak an amp for tone, but remember, no matter what you do to a hamburger, you will never make it taste like a lobster.

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