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Advanced Member
Username: terryc

Post Number: 397
Registered: 11-2004
Posted on Sunday, December 16, 2007 - 8:12 am:   Edit Post

Sorry but it isn't an Alembic problem but this is the bestplace for advice.
My son's Peavey TNT 115 has developed a hum, it doesn't get worse when the gains are turned up or any of the tone controls..just a continuous hum.
The amp works fine, I checked the condition of the mains plug and they were okay BUT before I remove the amp and spend time with my multi meter I think it could be earth problem with the transformer or the grounding somewhere along the innerds of the amp
DFung or anyone...any definite reasons why this
has happened. Suggestions welcomed
Advanced Member
Username: dfung60

Post Number: 287
Registered: 5-2002
Posted on Monday, December 17, 2007 - 12:51 pm:   Edit Post

Since the hum level doesn't change with the gain or tone controls, the culprit is in the amp's internal sound path. This probably won't be trivial to debug.

The problem might be a bad ground connection or failed component in the amp. Since the amp seems to work OK other than the hum, it's possible that the main path through the amp is OK and a side path has a problem. If you have the current TNT 115 it looks like there are a number of things to check out - it has a built-in chorus and an electronic crossover - so see if the hum level follows the gain controls on those functions if you can.

Also, there are a lot of patch jacks on this amp - the electronic crossover outs as well as a preamp out and power amp in. I'd be really suspicious of the preamp out and power amp in. Those jacks are set up with a switch which is thrown when you insert a plug (e.g., when you use the preamp out jack, it not only feeds the output, but disconnects the preamp from the power amp internally). If those rarely used jacks have corrosion, the switching function often introduces some noise. So, I'd try inserting and removing a plug in these jacks (power off, please!) to see if you can make sure they are clean.

In most newer amps the preamp components are ICs (sometimes even in the power amp) and the wiring is all in a PC board, so a failure in a chip or the board will be hard to correct.

Write back if that doesn't work,

David Fung
Advanced Member
Username: terryc

Post Number: 399
Registered: 11-2004
Posted on Monday, December 17, 2007 - 3:10 pm:   Edit Post

David..many thanks for reply. It was my old amp which I donated to is from 1993 so it is getting old.
As you say it is a PC board with IC's although the power transistors are mounted onto a heat sink at the rear of the chassis
I will do all the operations you describe and get back to you.
Again thanks for diagnosis and hopefully I will be able to fix this without major cost.

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