Peavey 450 Tour

[lowell]

So this came in with no sound. I discovered a faulty 7815 regulator as there was no + low v supply. I replaced that and now still no sound.

The PNP side of the PA heatsink is getting quite hot. The fuse is not blowing however. The Vc on the PNPs is about -24v and NPNs are +40v. I also realized that the new 7815 I put in is getting quite a bit more hot than the 7915. There is also only +3v on its output and +30v on its input. Something is pulling the + low V supply down. I read in another thread that some opamps/zeners were fried in a poster's 450 when his 7815 went south. I get 1.5kohms across D6 and 4kohms across D7.

So thoughts? I wouldn't be sure where to start with this. Do I need to pull every dang opamp?

Finally, I replaced the 7815 with one that would need an thermal pad between it and the heatsink. However, I looked and it doesn't seem that this heatsink is in any way connected to ground so I'm thinking that it's okay that I used no thermal pad.

[Enzo]

Don't pull them. ANy op amp loading the +15 down to +3 will be a lot warmer than the others.

[lowell]

Okay so how do I do that without electrocuting myself?

[g1]

Don't touch the 120VAC .
I usually use the back side of my index finger, it's more sensitive than the fingerprint area. The op amps are running on low voltages, so no risk of shock from them.

[Enzo]

I use my fingertip, it is tougher, and less likely to get burnt if a chip is flaming hot. Two techs, two techniques.

ANd I discovered another one. I use a yellow china marker, also called a grease pencil, to mark on the chasis or parts. I had one in my hand and just touched it to the top of each IC in something I had that was loading its 15v. All I was doing was keeping track of where I was, but I found the hot ICs melted this yellow crayon leaving a bright color dot on the top of the hot ones.

Come on, Lowell, you've done this long enough to know where the voltage is in an amp.

[lowell]

Ha I KNOW where voltage is, but isn't it current not voltage that is the concern? It's like a car battery 12v vs a 9v battery. One hurts, the other doesn't. Humor me?

[Jazz P Bass]

Ha I KNOW where voltage is, but isn't it current not voltage that is the concern? It's like a car battery 12v vs a 9v battery. One hurts, the other doesn't. Humor me?

Not too sure what you are getting at.
I= V/R.
Whether it's a 12 volt battery or a 9 volt is not the real concern.
It's the R part of the equation that will enable either one to be capable of welding.
The car battery will simply weld longer.
Anyway, have you found any overheating IC's?

[g1]

Ha I KNOW where voltage is, but isn't it current not voltage that is the concern? It's like a car battery 12v vs a 9v battery. One hurts, the other doesn't. Humor me?

Yes, it takes very little voltage or current to kill you. But you must consider the third part of the equation, resistance. Typically, your resistance is probably a couple hundred k ohms. Maybe less if you are sweaty, more if you are dry. Check yourself with your ohm meter.
So to get any kind of current through that resistance, you need high voltage. A car battery does not hurt, you can touch it all you want.
In order to have a problem with lower voltage, you would have to pierce the skin. Then the resistance goes WAY down.

[lowell]

I guess i always thought (thanks hollywood) that even the low 12v of a car battery could kill you, or even just give a bad electrocution. Wrong? Finally, maybe a car battery can deliver more current than a low v audio amp supply. Not sure what the Zout of such a supply is.

[g1]

Yes, a car battery can supply much more current (enough to weld), but it is not a factor. The current in the circuit will depend on the voltage and the resistance. The same current will flow whether the battery can supply 1 amp or 1000 amps.
But the actual current to stop your heart is less than 100 microamps at the heart itself.
It takes around 100 milliamps to stop your heart if you are touching a voltage source, unpierced skin.
With pierced skin, your internal resistance is probably around 500 ohms, so much less voltage is needed to be electrocuted.
One more thing, you can withstand DC better than 60hz AC at the same voltages.

[lowell]

Interesting I thought DC was more lethal due to steady current.

[lowell]

So anyway back to this amp. I found that TR3 is getting VERY hot. It's the triac. Not sure how this works but I think it's related to the protection circuitry. There is no screw installed holding the tab to the pcb. Should there be one? Wondering if this is the problem.

[52 Bill]

TR3 is the output/speaker protector crowbar, is there dc on the output? Is the protection relay turning on?

[lowell]

This schematic doesn't make sense. I think there's a line where there shouldn't be one. In the schematic the output is grounded to the chassis.

[Jazz P Bass]

P18 is indeed at ground.

P19 has the zobel network & the triac between itself & ground.
C36 should block any resistance measurement & Vdc.

The triac out of circuit should measure very high resistance between any & all pins.
The triac does not rquire a heatsink because in a functioning amp it does nothing.
Although in a blown output section it usually gives up the ghost to 'save' the speaker.
In that case it will measure as a short.