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Author Topic: Ampeg SVT450  (Read 1055 times)

aruption

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Ampeg SVT450
« on: February 14, 2019, 07:21:55 AM »
My friend just picked up an Ampeg SVT450 that's definitely got some issues. Plugging either speaker output into any speaker, any notes played on any instrument are super distorted, and do not sustain. Using the line out post, and note played is distorted and sustains infinitely. Only the line out pre sounds normal, but that of course is because it's bypassing pretty much everything in the amp. What should we first take a look at under the hood?

Jazz P Bass

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Re: Ampeg SVT450
« Reply #1 on: February 15, 2019, 11:18:15 AM »
How are you setup test equipment wise?

There are signal voltage references on the schematic that may help narrow down where the issue lies.


aruption

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Re: Ampeg SVT450
« Reply #2 on: February 16, 2019, 10:53:31 PM »
Thanks for the schematic. My testing setup is limited to a DMM. Do you think the issue is likely to be in the preamp or power amp section?

Jazz P Bass

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Re: Ampeg SVT450
« Reply #3 on: February 17, 2019, 12:27:35 AM »
Use the FX jacks to help narrow it down.

The Send to another amp is your preamp.

The Return is pretty much a line into the power amp.
You can use the Send from another amp or any instrument.

aruption

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Re: Ampeg SVT450
« Reply #4 on: February 19, 2019, 11:48:58 AM »
In that case, it's definitely the preamp. Changing the EQ settings or limiter/pad switches don't do anything to make the sound better or worse (although the pad will still reduce the overall volume). Engaging the other EQ section doesn't change anything either.

phatt

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Re: Ampeg SVT450
« Reply #5 on: February 19, 2019, 06:09:10 PM »
Hum? That is not a definitive observation.
Try this, Plug your guitar into the *POWER AMP INPUT*
You should hear clear undistorted signal.
If there is still bad distortion then the power amp has issues.

To test the preamp;
(Assuming you have another amp?)
plug a lead into *EFFECTS SEND* and send that signal to another amp.
You should be able to engage the Graphic EQ,, but master will be bypassed.

If it's clean and all controls work (Except Master volume) then plug into the *PreAMP OUT* and send to another amp.
Now the master should work as well as all preamp controls.
Report results.
Phil.

aruption

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Re: Ampeg SVT450
« Reply #6 on: February 19, 2019, 07:48:35 PM »
Sorry, that was unclear. I was saying that the problem is definitely in the preamp, because I did the tests as you described, and had a clear undistorted signal from the power amp input. The rest was still super distorted. There's no issue with hum, just distortion of any signal being input.

Enzo

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Re: Ampeg SVT450
« Reply #7 on: February 19, 2019, 10:13:22 PM »
SO check the preamp, are both 16v rails clean and more or less the same voltage  ANything in the 15-16v range is OK.

Check all op amps for unwanted DC offseet on teh outputs.  It looks like they use quad op amps, TL074, so the four corners are the outpu tpins.

aruption

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Re: Ampeg SVT450
« Reply #8 on: February 21, 2019, 05:43:30 PM »
Both 16v rails are right in the 15-16v range.

If I am reading the pinouts correct, then on IC1 there's -14v on output 2/pin 7 (0v on the other three outputs), on IC2 there's -14v on output 1/pin1 (0v on the other three outputs), and no voltage on the output pins of the IC3 and IC4.

At the test point near D4, I am reading 14v on one side of D4 and 0v on the other side. Is that correct? Because it looks like the side that is reading 14v is the side that the test point should read 0v, because that is the side that connects to R13.

Enzo

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Re: Ampeg SVT450
« Reply #9 on: February 22, 2019, 12:08:36 AM »
TL074 - the corner pins are the outputs.  That is pins 1,7,8,14.

R13 should be at zero volts DC.

aruption

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Re: Ampeg SVT450
« Reply #10 on: February 23, 2019, 08:19:07 AM »
Bad op amps then?

Enzo

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Re: Ampeg SVT450
« Reply #11 on: February 23, 2019, 05:19:59 PM »
I suspect any with DC on the output pins.

aruption

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Re: Ampeg SVT450
« Reply #12 on: June 27, 2019, 08:09:46 AM »
Finally got around to replacing the op amps after dealing with a divorce lol. Unfortunately this did not fix anything (with the amp). What's the next thing I should take a look at?

phatt

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Re: Ampeg SVT450
« Reply #13 on: June 28, 2019, 09:51:41 AM »
D4 might be shorted,,Maybe check the other Diode points marked *P*,, D5 & D6 and see if they have similar readings.
Those *P* points go to R40 Which looks like the peak detector circuit.
Maybe Q3 might be stuffed.
Phil.

 

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