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Please help with the preamp on the fritz (Art Powerplant)

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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby Hugh Robjohns » Tue Nov 26, 2019 11:19 am

DC-Choppah wrote:This is new. I did something to it this time.

I'd suggest checking those connectors you opened and cleaned. I suspect you've either aligned them incorrectly, or have bent pins, or maybe the contact cleaner has pooled and is effectively shorting some pins together. It sounds to me like some contacts are either open or short...

As for the original fault... if both channels are affected the same way, that would point towards a power supply problem.

H
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby cyrano.mac » Tue Nov 26, 2019 4:15 pm

Of course, check PSU first. If there's 3V on a 5V rail, fi, look no further.

So what are the voltages?
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby N i g e l » Tue Nov 26, 2019 7:46 pm

When I listened to the recording it reminded me of two faults ive seen before.

[1] signal passing thru duff opamp/transistor or one without adequate power and then being amplified by the next stage

[2] output power-on-delay-thump-remover circuit duff mangling the signal > above a certain level.

in both these cases, low level signals were "ok" but louder stuff was distorted

The cause of these faults were components gone bad, so would neeed a tech to sort it out. It might be straight forward to do but time consuming tracing the signal through the circuit.

It would be helpfull if you could post a recording of the amp with just a pure sine wave ramping up in level, or even just a single guitar string pluck and let it fade away
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby Wonks » Tue Nov 26, 2019 7:53 pm

N i g e l wrote:It would be helpfull if you could post a recording of the amp with just a pure sine wave ramping up in level, or even just a single guitar string pluck and let it fade away

Except that currently one channel works perfectly, whilst the other makes no sound at all. :D
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby James Perrett » Tue Nov 26, 2019 10:53 pm


Shows the connector on the left of the PCB misaligned - it should go one pin closer to the front panel.

Though it looks right on


but what is all that liquid?
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby DC-Choppah » Wed Nov 27, 2019 5:07 am

The liquid was De-Ox overspray.

This evaporated by next day and the problems remained.

I have taken your all’s advice and gave it to an audio repair shop that mostly restores old vintage stereo equipment and also pro audio. They will be following this thread too and I asked them to check out the features you guys described. I appreciate your advise.

This thing really sounds great and is pretty rare. I can’ find any others around. It does a sweet clean tone, drives pro-level line signals into stereo studio rack gear, and has a sweet marshal stack dirty channel that is outrageously good. I hope it can come back to life.
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby DC-Choppah » Wed Nov 27, 2019 5:07 am

Currently, clean channel us broken up like the audio recording, but when the gain is increased the sound gets softer. At 25% up it is silent.

Green clean channel light never lights up.

Dirty channel is extra broken up, but red light comes on and it is loud and all controls work.

The tech is starting out with it like this and it is not an intermittent problem. It stays broke.
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby DC-Choppah » Wed Nov 27, 2019 3:10 pm

James Perrett wrote:

Shows the connector on the left of the PCB misaligned - it should go one pin closer to the front panel.

Though it looks right on


but what is all that liquid?

Great catch man!

Ok the first picture with the misaligned connector was how it was when I first opened it.

After cleaning contacts and reassembling I took the second picture.

I wonder if it is supposed to go in the misaligned way?
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby Hugh Robjohns » Wed Nov 27, 2019 3:20 pm

DC-Choppah wrote:Ok the first picture with the misaligned connector was how it was when I first opened it. ... I wonder if it is supposed to go in the misaligned way?

It's rather unlikely, and since the unit was faulty before you opened it, I would suggest that could well be a significant part in the problems you've been having.

Without an accurate schematic that identifies the wiring through the connector it's impossible to say, but it would seem likely that the misplaced connector has resulted in something being damaged through the resulting circuit mis-connections.

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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby DC-Choppah » Thu Dec 19, 2019 2:48 am

Tech reports today that the issue is cracks in the circuit board near the back of the unit near where the jacks are. He also says that the some of the rear jacks lack the nuts that stabilize the connectors to the circuit board.

These guys who fix vintage audio are gold!

If it is OK with the mods, I would like to give a good word out to: https://www.just-audio.com/
for doing this repair for me!
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby Wonks » Thu Dec 19, 2019 9:36 am

Great news!

And it's always fine to recommend places that give good service. It's just self-promotion that's frowned upon.
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby DC-Choppah » Sun Dec 22, 2019 6:53 pm

The preamp is back in my rack.

Tech fixed the green light problem. That was burned out probably from the connector issue you guys found.

He repaired solder joints on the cracked PC board and redid the hardware for the rear jacks.

Looks like the lack of those jack nuts put force on the PC board that cracked it.

The preamp sounds great.

No noise, no hum, very clean and powerful clean channel, and the dirty channel is the most rocking thing around.

It has pro line level output ('To Pwer Amp') that drive stereo studio effects in the rack, and instrument level signals for the FX loop that drives the send FX foot pedals.

I know they have digitally modelling amps that do the Marshal Stack thing, but they are not as inspiring and sound processed and smoothed out. This thing I guess is 'analog modelling'. A lost Art perhaps.
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby Wonks » Sun Dec 22, 2019 7:41 pm

Great you've got it all working again. :thumbup:
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Re: Please help with the preamp on the fritz (Art Powerplant)

Postby Martin Walker » Mon Dec 23, 2019 1:25 pm

Hi DC-Choppah!

Great news indeed, and your glowing praise of the ART Powerplant inspired me to look it up:

https://artproaudio.com/product/power-p ... ar-preamp/

It's certainly a versatile piece of gear (great shame it's discontinued), but this forum post has got some great data as to why it sounds so good, complete with graphs:

http://jvmforum.com/phpBB3/viewtopic.php?t=8907


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