Rockmaster preamp outputs nothing but a loud hum

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dicianne
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Rockmaster preamp outputs nothing but a loud hum

Post by dicianne » Sat May 15, 2021 10:33 pm

I turned on my rockmaster preamp today only to hear a very loud hum on all outputs. Even if the volume is completely down, or if even there aren't any instruments plugged in. It sounds like a sawtooth wave signal at around 120hz. Any ideas what could be wrong with it? None of the fuses are blown out.
The only things I've done so far were cleaning out the 2pin molex connector that connects the tube board, and the clean out the tube sockets. Changed the tubes as well. Tried different power outlets. But none of that did anything. It's still making the same hum noise. Some pointers as to how I should I troubleshoot this would be welcomed.
Thanks
Al
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Enzo
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Re: Rockmaster preamp outputs nothing but a loud hum

Post by Enzo » Tue May 18, 2021 8:46 pm

Got a scope? Find out if it is a sawtooth and 120Hz. If so it means power supply riple. Most likely a lack of filtration in the B+ supply Also all those JFETs near all the jacks could inject hum if the RELAYS supply loses filtration.

dicianne
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Re: Rockmaster preamp outputs nothing but a loud hum

Post by dicianne » Tue May 18, 2021 10:44 pm

Enzo wrote:
Tue May 18, 2021 8:46 pm
Got a scope? Find out if it is a sawtooth and 120Hz. If so it means power supply riple. Most likely a lack of filtration in the B+ supply Also all those JFETs near all the jacks could inject hum if the RELAYS supply loses filtration.
I don't have a scope. I honestly don't know which type of wave it is. I know it's not a pure sine wave as there's some harmonic content to it.
The meters and freq analyzers in logic pro read like this though. (see attached)
Would it be a misguided effort to replace all big caps and all relays?
Thanks Enzo
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The Guitar
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Re: Rockmaster preamp outputs nothing but a loud hum

Post by The Guitar » Thu May 20, 2021 10:43 am

I don't think it's the issue, but have you cleaned all the loop jacks in the back? Also take a look at this thread if you haven't already and see if their issue is like yours;
viewtopic.php?t=28942

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Enzo
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Re: Rockmaster preamp outputs nothing but a loud hum

Post by Enzo » Thu May 20, 2021 2:00 pm

I am not sure how the relays could cause oud hum. I referred to the relay power supply, not the relays. That supply not only powers the relays but also the JFET circuits.

If you have a reasonable meter, set it to AC volts and measure the DC supplies. It SHOULD settle to AC zero volts if the DC is smooth. If you get a lot of AC, then the filters are not working.

dicianne
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Re: Rockmaster preamp outputs nothing but a loud hum

Post by dicianne » Fri May 21, 2021 12:12 am

Enzo wrote:
Thu May 20, 2021 2:00 pm
I am not sure how the relays could cause oud hum. I referred to the relay power supply, not the relays. That supply not only powers the relays but also the JFET circuits.

If you have a reasonable meter, set it to AC volts and measure the DC supplies. It SHOULD settle to AC zero volts if the DC is smooth. If you get a lot of AC, then the filters are not working.
I see. Power relay. I have a barebones digital mmeter. Not sure how useful it will be. Where at on the board do I measure this? I have the schematics.
I just realized that even when I lower the input volume on my recording interface, I still get the hum. To recap, the volume is all the way down on both rockmaster (all knobs to zero) and also interface. And it's still registering a loud hum. The recording interface works without issues with other rack gear.

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