peavey tnt 130

[pontiacpete]

I'm working on one of these --- it powers up but all that's there is a hummmm.
I tried swapping out the filter caps in the power supply, also have on hand 4558s and a TL074 and tried changing them with no luck. Resoldered both boards. So what am I left with? The 87476? Are there any subs that can be used for this IC? If there's an open component or a shorted transistor would it power on at all? I'm not running it through the speaker, I'm using a known good speaker. '
Does anyone have any suggestions?

pete

[Lee_ranaldo]

It could be the power transistors in the output stage

check them if they have shorts in it with a DMM . You should replace all of them
Q6,Q7,Q10,Q11 if they're shorted

give us some voltage readings so we can guide u in this process

good luck!

[52 Bill]

If you plug into the power amp in jack what happens to the hum?

If it goes away, the problem is in the preamp, if not the problem is in the power amp or power supply.

[Lee_ranaldo]

if it's a loud HUMM it comes from shorted power transistors, but if it's that kind of hum that sounds like when u're not plugging ur guitar and turn the volume up. check for bad solder joints in the pre-amp.

The best way to diagnose this is to make a signal tracer. This can be done with any amplifier (including multimedia speakers) and a test probe with a capacitor in series to avoid DC voltage to flow throught the amplifier.

I usually do that and follow the schematic to trace the signal. you can use a signal generator (any kind) that gives ua constant signal at line levels or even your guitar or an ipod playing a test tone as a last resort.

[pontiacpete]

I resoldered the pre amp board and yes it does hum when plugged into.

If it is the power transistors are these available or would a mj15024 and mj15025 be good subs?
I will post some voltages tomorrow. thanks for the help.

pete

[Jazz P Bass]

Peavey Parts

I would order the transistors diect from Peavey.
They will be current matched.
You really should consider replacing the driver transistors.
Copied from there site:
To order replacement parts, please call us at 877-732-8391 (toll free in the US) or 601-483-5365. The Parts Department extension is 1386.

[Lee_ranaldo]

You can contact peavey there They are very helpful
they might have some replacements
so drop them a call just in case.

What u can do is to check with a DMM in "continuity test" mode
yea that one that buzzez everytime u touch the leads.

If any of the junctions is "shorted" the amp will hum like a maniac, also if a driver transistor is shorted it will hum too.

If the amp has a Loop effects (i see it has one) try bypassing the pre-amp by connecting some signal into the power amp in.

If the hum continues. it's clearly the power amp.
if u connect the pre-amp out to another power amp
or effects loop "RETURN" input u could test the pre-amp alone

good luck!

[Enzo]

Before we start just replacing lots of stuff, determine what is wrong.

Isolate the problem.

Look at the power amp board. On the far end from the fuses is the four wire cable to the preamp. Pull it off. Now the power amp is running by itself. Still hum? If so, the power amap has the issues. If the hum goes away, then either the preamp is faulty or the +/-16v supplies on the power amp are faulty.

If it is the preamp, then re-connect it, and measure the +/-15v rails coming through that four-wire cable. Are they both up to voltage and free of ripple?

And if the problem is in the power amp:

Measure for DC voltage across the speaker wires. if there is substantial DC on the speaker leads, then we need to repair the output stages of the amp.

If there is no DC or under half a volt anyway, then set your meter to AC volts an measure the +42v supply and the -42v power supply for AC. There ought to be very little AC on them. If you find something like 20VAC on one of them, then you have lost a filter cap.

And check the zener regulated supplies, the +/-15v for the power amp.

Look in the center of the power amp board. There is a crude looking coil of wire next to the speaker wire connections. On either side of it there should be a 700 ohm 5 watt resistor. They are convenient test points. On the end of them nearest the speaker connector, you should find +15v and -15v DC and almost no AC. On the other end of each you should find the +42VDC and the -42VDC, and very little AC on each.

If one of the 42v supplies has a lot of AC on it, then either the filter cap for it is bad, or it has cracked free of its solder. The two big blue cans next to the fuses are the filter caps. Look underneath to see if the solder is 100%.


Note that there are two separate pairs of 15v supply. The power amp has its own separate from the one for hte preamp. They are independent, so checking one tells you nothing about the other.

[pontiacpete]

That's all for helping. Did a couple of things you guys suggested.

Still hums after disconnecting the preamp.

No AC on speaker terminals, there is 28vdc across spk term.

@the 700ohm Rs , the one on left is 14vdc & -1vdc on the other. No AC

Tried the continuity test lee suggests , touching either side of R38,44,46 and 47, no buzz. Touching CR13 on + end there is a snapping noise on the other no snap, CR12 both ends snap.

Which are the 'driver transistors" Q5 &9, or 4 & 8 or all of them?

Is there anything else I that I might have missed?

[52 Bill]

Unhook the speaker from the amp before you damage it.

The hum you are hearing is the 28vDC across the speaker terminals.

As Lee suggested earlier, check the power transistors, the drivers, etc.

[pontiacpete]

that's probably why it was blown when it came to me.

will do. thanks.

[Lee_ranaldo]

That's all for helping. Did a couple of things you guys suggested.

Still hums after disconnecting the preamp.

No AC on speaker terminals, there is 28vdc across spk term.

@the 700ohm Rs , the one on left is 14vdc & -1vdc on the other. No AC

Tried the continuity test lee suggests , touching either side of R38,44,46 and 47, no buzz. Touching CR13 on + end there is a snapping noise on the other no snap, CR12 both ends snap.

Which are the 'driver transistors" Q5 &9, or 4 & 8 or all of them?

Is there anything else I that I might have missed?

IF u have DC on the speaker it's a clear sign that one of more of those power transistors are SHORTED. you should see a voltage around 0 volts DC in the speaker output. If u keep pluging your speaker with DC voltage for a long period of time it will get a bit hot and could damage the coil of the speaker by overheating.

Those 4 power transistors are mounted in the heatsink.
You should have +/- 42 volts or less in the colector of those transistors. and around 0 volts in the emiters.


I will post u a pic later when i have some time, of what u should check using the original schem u provided. Contact peavey to get those power transistors if they have them on stock or u can contact mouser or any other supplier.

[Enzo]

I disagree, I doubt there is a shorted output, though I could be wrong.

The clue is the missing -15v at the 700 ohm resistor. Either the resistor is open, or the zener it serves is shorted. Very simple power supply to sort out. WHen one 15v rail is missing, then the op amp in the power amp will not have even rails, so the whole deal there will be skewed over to one side, and that will push the output over one way as well.

I am assuming the -42v is present at the other end of the resistor. If -42v is missing, then of course the amp will slam the output over to the +V.

SOlve the missing -15VDC, and I bet the whole thing falls into place.

Feel that 700 ohm resistor. If it is cold, then it is probably open. If it is blazing hot, then the zener or cap is shorted.

[pontiacpete]

Thanks Lee, apparently there are voltages on all terminals, the bases', emitters and collectors, of each power transistor, from 27vdc to 37vdc. Seems like they are just fused together on the insides.
So these are bad and should be replaced I presume.

What about the driver transistors? Not very clear which ones they are. ???
Q5 & 9 or Q 4 & 8?

[Enzo]

Don't assume they are all shorted because the voltages are the same on the legs, If you think that, pull the part and check it with an ohm meter.

Please read my above post - I think we simul-posted.