Pyle PTA2 - Schematic, or Just Identify the Part?

[Toolworker]

This Pyle PTA2 did what Pyles do - it smoked a month after the seller's return window closed. Now I know why they call it a PTA.

If you happen to have one, would you feel like peeking inside and identifying the part which used to be where the scorch mark is on the circuit board? It looks like a resistor and measures 10 ohms in its current fried state.

Thanks for any help.

[The Dude]

I don't have a schematic or parts list. I will say, however, that since resistors don't short, the part is likely still good and probably a 10 ohm resistor. It's obviously been hot, so I'd look for shorts on devices the resistor supplies. The output IC would be a good start. Have you asked the company for a schematic? What is wrong with the thing- what are your symptoms?

[Toolworker]

Thanks for your reply. For a couple of days the amp worked fine, except for the burning odor. This morning there was a low AC hum. The resistor goes from the output of the diode bridge just above it, fed from the blue wires which are a 10 Vac 0.15A output from the power transformer.

The little glass bead to the right of the scorch mark goes back to the other side of the bridge. I don't recognize what it is, do you? It looks like a glass bead with copper foil inside. It measures 8.6 ohms, which would heat up the resistor pretty good.

[Helmholtz]

The little glass bead to the right of the scorch mark goes back to the other side of the bridge. I don't recognize what it is, do you? It looks like a glass bead with copper foil inside. It measures 8.6 ohms, which would heat up the resistor pretty good.

That's a diode, probably a Zener diode.

[The Dude]

The "little glass bead" is a diode. I'm guessing without a schematic or a look at the trace side of the board, but it's probably a zener diode on the "output side" of the resistor. If it measures 8.6 ohms, it could be shorted, or something else on that supply could be shorted. You'll have to track it down. The first step would be to unsolder one side of the diode and measure it out of circuit. Also, measure the pads where it soldered after it's removed see that the short is not there.

Edit: Helmholtz snuck in while I was typing (and removing a cat that wandered into my warehouse ).

[Jazz P Bass]

What is up with that resistor in the middle of the board, stuck up in the air with hot snot on it?

BTW: $49.99 Free shipping will solve the problem.
https://www.ebay.com/itm/Pyle-PTA2-2...7&toolid=10049

[Toolworker]

The "little glass bead" is a diode. I'm guessing without a schematic or a look at the trace side of the board, but it's probably a zener diode on the "output side" of the resistor. If it measures 8.6 ohms, it could be shorted, or something else on that supply could be shorted. You'll have to track it down. The first step would be to unsolder one side of the diode and measure it out of circuit. Also, measure the pads where it soldered after it's removed see that the short is not there.

Thanks! I think I understand the problem now. It is exactly a zener diode across the output of the resistor. The black band on the diode was burned or worn off the top side, but visible when it was removed.

The zener diode measures 0.008v both ways on the diode test range of my multimeter, and still 8.6 ohms on resistance. So I guess that's the defective part, and it's probably pulling the full 2 watt output of that transformer winding through the resistor. If the transformer output were greater the resistor might be ashes.

But the amp's working, except maybe with a little hum, because either the 14v is feeding the logic in the front panel, not the audio, or else the 14v has a little ripple in it that the zener diode should be taking out.

So I just need to find out the zener spec, if Pyle will tell me. Or is it guessable?

What is up with that resistor in the middle of the board, stuck up in the air with hot snot on it?

Pyle quality. There's worse in there.

BTW: $49.99 Free shipping will solve the problem.
https://www.ebay.com/itm/Pyle-PTA2-2...7&toolid=10049

Or $47.99 on Amazon, also with free shipping, plus you get the kind of support that gave me a refund and didn't even ask for the unit back. I was just curious to see if I could fix it. But I'm not sure I want another Pyle. I had one of their mixer amps that failed, and now this.

[The Dude]

If you use a magnifying glass, you might be able to read numbers on the diode itself. If you have hum, it may be that there is a bad filter cap on that supply. Lots of AC content on that supply might be the cause of the shorted zener. It's not the job of the diode to filter AC. Its job is to clamp the DC voltage to a given level. It's the job of filter caps to filter.

[Toolworker]

If you use a magnifying glass, you might be able to read numbers on the diode itself.

Thanks! In the part of the black band that's worn off, there's what might have been a couple of numbers, or just as likely part of the black band. Magnifying goggles couldn't make anything out.

If you have hum, it may be that there is a bad filter cap on that supply. Lots of AC content on that supply might be the cause of the shorted zener. It's not the job of the diode to filter AC. Its job is to clamp the DC voltage to a given level. It's the job of filter caps to filter.

There's an electrolytic in that circuit that took a charge with the multimeter. I'll slap a scope on it after the zener is replaced, if I get that far.

[The Dude]

Can you figure out what that power rail connects to? Op amp supply? Output IC? Digital circuitry? Etc. We may be able to determine what ZD should be there by knowing what the supply is for.

[Jazz P Bass]

You must specify the zener voltage & wattage when you replace a zener diode.
I see two TO220 packages. Most likely the + & - 15 voltage regulators, which would explain the large high wattage resistors near them.
So what is a zener diode doing there in the middle of the board with a 10 ohm resistor across it?
I would try to trace out a simple drawing of what & where the mystery diode/ resistor connects to.
Then an informative decision can be made.

[The Dude]

I don't think the zener is across the resistor. If it were and it shorted, the resistor would not get hot as current would be through the shorted diode. I think it's like this:

[Jazz P Bass]

Good point!!
I still would like to see a drawing.
If there is a capacitor involved then it may be the turn on voltage for the TDA output ic startup circuit.

[dmeek]

Please post photos of the whole circuit board, component side and foil side.

[Toolworker]

Called Pyle. They don't provide diagrams or parts.

You must specify the zener voltage & wattage when you replace a zener diode.
I see two TO220 packages. Most likely the + & - 15 voltage regulators, which would explain the large high wattage resistors near them.
So what is a zener diode doing there in the middle of the board with a 10 ohm resistor across it?

The power "transfomer" has two outputs, the main one, 14 VAC @ 1.5A, which goes to the two large caps at the top, the bridge rectifier, and the IC on the large heat sink, and 10 VAC 0.15A, which is feeding this circuit.

Please post photos of the whole circuit board, component side and foil side.

I'll do that later today.

I don't think the zener is across the resistor. If it were and it shorted, the resistor would not get hot as current would be through the shorted diode. I think it's like this:

Yes, that's right. That's what I was trying to say when I said the zener is across the output of the resistor.