peavey tnt 150 has a buzzing/ humming

[rriddy]

I have a tnt 150 on the bench that has a buzzing or I guess it would be considered hum by some. Original problem was the 7815 regulator had blown along with the cr17 on the preamp board. I've checked all voltages as far as the +/-45v, +/-16v, and made sure there wasn't any ac on the output. I also used my bench speaker to verify no speaker issue. The problem is in the pre amp. If unplug the tether from the preamp to the power I get virtually no noise. Once the preamp is plugged back in I get the noise and there is no output other than the noise. It doesn't change volume at. I;ve also checked all the solder joints and checked all the 4558's. Any help would be greatly appreciated.

[Enzo]

Nothing official about it, but me personally, I use the term hum for electrical noise, and I reserve the term buzz for vibrational issues. But that is just me.

So the preamp hums.

Power first. You do realize there are two sets of +/-15v rails? The power amp has a pair zener derived for the power amp op amps. The 7815/7915 supplies are on the power amp board but are only for the preamp.

So, ON THE PREAMP board are both +/-15v at the same voltage and clean? Make reading on the board right at an IC or two, and also ground your meter to the board, not chassis. An open ground trace for example could do this, but readings to chassis could remain close.

And you measured 15v? But did you scope it? There could be little ripples in it you'd never be able to measure, but would raise a lot of hum. Check this. Those 15v rails are derived from 25v rails. Do either of the 25v rails look lowo, and flip the meter to AC volts and see if either 25v rail has a large ripple. Or scope it.

Cr17 is one of a pair of diodes that protect J111 Q3 from the footswitch jack. You will find Peavey protects most of their jack circuits with such clamping diodes. ANy others shorted?

How did you check all the 4558s? I would go down the rows myself and check pins 1 and 7 of each for DC offset. Not worried about a few millivolts, looking for several volts. That would be bad.

If the powst control turned to zero does not kill this hum, then it would seem the hum was either coming from the output stage U10a or coming in through the chorus.

[rriddy]

peavey tnt 150 has a buzzing/ humming

Enzo,
After checking the voltages again, I guess I totally overlooked the zener string. At any rate I am getting -14.93 and 5.12 on pins 4 & 5 of the u1 on the power amp board. Would that point to the zener being bad. You'll have to excuse me I spend 99% of my time on tube amps, so I'm slowly learning ss as I go. Thanks for the Help.

[rriddy]

peavey tnt 150 has a buzzing/ humming

I was going through it again a few minutes ago and now the + rail is reading 87vdc and the negative is -1.15vdc. Previous to this I found that Q3 was getting hot and that cr8 on the power board was shorted also. Looking at the schematic all my voltages were really close to what is listed on the schematic at the cr6 & cr7 junction and also the cr8 &cr10 one. As listed above my rails were +/-45v. I seemed to lose the voltage at c6.
I also had .2v on pin 1 of u1. All of this but I cant figure out where i was losing the +15v. After finding cr8 the voltage still only went to 6.23v. So I took a little break (football banquet) and came back and went to turn it on to do some more checks and now the rails have jumped to one side. Any suggestions? Thanks in advance

[Enzo]

Look at the schematic, this is a grounded emitter output. SO when the amp swings over one way, it takes the power rails with it. Your power rails are +/-45v, which means about 90v apart. If you measure them with respect to ground you may get something way unbalanced like 87 and 1, but add them up and there is your 90. If you measure from B+ to B-, I bet you get a steady 90v or so all the time.

SInce those zener derviced 15v rails are born from the 45v rails, if one of them falls lower that 15v, then so will that 15v rail. Right? Well, they would, but that is what those 1k resistors to the 25v rails are for, they hold up the 15v rails when the 45v rails are dragging around.

[Zouto]

Hi ,
a friend has a TNT150 with the same symptoms -loud raw hum. Could you please post the schematics, so that I can check the amp?
Thanks a lot
Zouto

[Jazz P Bass]

Peavey TNT 150 Schematic

Here you go.

[Zouto]

RRidy, could you fix it by now, which parts were blown?
Thanks a lot
Zouto

[jojopick]

Fixing 120Hz hum issue for Peavey TNT 150 amplifier

I was going through it again a few minutes ago and now the + rail is reading 87vdc and the negative is -1.15vdc. Previous to this I found that Q3 was getting hot and that cr8 on the power board was shorted also. Looking at the schematic all my voltages were really close to what is listed on the schematic at the cr6 & cr7 junction and also the cr8 &cr10 one. As listed above my rails were +/-45v. I seemed to lose the voltage at c6.
I also had .2v on pin 1 of u1. All of this but I cant figure out where i was losing the +15v. After finding cr8 the voltage still only went to 6.23v. So I took a little break (football banquet) and came back and went to turn it on to do some more checks and now the rails have jumped to one side. Any suggestions? Thanks in advance

I had same problem where hum of 120Hz in speaker was actually intermittant. Bang on it and it would go away for awhile.
I did go ahead and replace power supply caps and the problem went away for few months with nice quiet operation.
But then hum came back and would not go away by banging on it.


I believe the problem could be the mounting brackets for the PC boards. I've also seen this problem on Peavey Tube Fex where ground lugs to circuit boards lost contact. In the TNT150 they are riveted to the aluminum chassis and do not make tight contact which makes them candidate for a poor electrical connection. The power amp has three of these brackets that connect to the printed circuit board ground. The preamp board has one of the lugs that connect to ground on the PC board while the rest of the lugs on the preamp board do not connect to ground. What I did was take both boards out, and solder a wire to the ground that is on the preamp circuit board. Then on the power amp board I soldered two wires... one at each end of the ground trace that runs the length of the board and connects to these mounting brackets. I then drilled a hole in the chassis close to the grounding bracket on the preamp board and soldered these three ground wires to a termial lug bolted to the chassis.

This procedure has put the amp back to clean quiet operation with no hum.
Now, only time will tell if that completely solved the problem. .. but so far ... so good!

[Gtr_tech]

Just to add to this...when chasing down a hum problem, be sure to check the fuseholders on the pwr/output board. I just finished up on one that had this problem. No bad op amps, pwr rails seemed clean with little ripple. But....one of the fuse clips had lost tension. Cleaning off the fuses and retensioning them took care of the hum and improved the sound quality.

[FELIBAR12]

Here you go.

could you upload the file again? This link does not work, thanks!

[Jazz P Bass]

And again.