HM-Acapulco-Gold-2 Hum Noise

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smileyNK
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Joined: 19 Jun 2020, 08:05

Post by smileyNK »

So I slammed an Acapulco Gold into an HM-2 EQ. I've built this a few times before from the same layout and had no trouble.

There is a constant low, loud hum. The timbre changes as I twist EQ knobs but never changes pitch.

I removed the input and output wires to probe around and the Gold section seems to be working fine, but as soon as I touch the output to any part of the EQ stage the problem comes back. Touching the input and output wires to separate opamps, they seem to be okay until I get to the right side of the topmost TL072. When I touch input to pin 5 and output to pin 7 there is nothing but the hum. This is driving me insane. Thanks for any help you can provide!

Pin voltages top to bottom:

1: 4
2: 4
3: 0
4: 0.7
5: 3.5
6: 4
7: 4
8: 9.75

1: 4.5
2: 4.5
3: 4
4: 0
5: 3.9
6: 4.5
7: 4.5
8: 9.75

1: 5
2: 5
3: 3.9
4: 0
5: 4.15
6: 4.2
7: 4.7
8: 9.75


1: 1.3
2: 0
3: 0.03
4: 0
5: 5.25
6: 9.75
7: 5.07
8: 1.29

1: 1.27
2: 0
3: 0
4: 0
5: 4.15
6: 9.75
7: 5.05
8: 1.25

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theblackman
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Post by theblackman »

probably a short in/before the last two op amps.

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smileyNK
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Joined: 19 Jun 2020, 08:05

Post by smileyNK »

Alright I was wrong. The first Opamp stage of the HM2 section seems fine, but then the hum. It's everywhere after that and after cleaning, knifing, triple ten time checking, there are no shorts or layout problems that I've found.

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ivor1980
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Joined: 29 Jan 2018, 05:08

Post by ivor1980 »

:beatdeadhorse: Sorry can't help with debugging. But I'm very much curious how such a beast sounds?!? You mentioned you have built some before.

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smileyNK
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Joined: 19 Jun 2020, 08:05

Post by smileyNK »

It's awesome! My main instrument is a Jazzmaster style Bass VI and this earned what will probably be a permanent spot on my board. The bass is insane. It's the only distortion I've ever played that gives a significant boost to the lows. It's making every other fuzz/dist pedal I have sound kind of thin by comparison. I absolutely love it.

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