Earth Amp BT-2000 Bass Traveler Bridge Rectifier Issues

[JerkyPudding]

Hi all,

I have an Earth Amp BT-2000 Bass Traveler 45w on the bench. It has two burnt diodes in the bridge rectifier (a tech before me insulated the circuit board from the chassis with painters tape, it wore through and POP!). The remaining diodes have ICC 170 printed on them, but I'm having no luck finding a cross reference guide. I looked up the Peavey Pacer schematic (which is very close to the BT-2000) and it calls for 1A/200PIV diodes (1N4003). I measured the voltage on the PT's secondary: 54VAC.

Bottom line, is the 1N4003 diode a safe replacement for this? I appreciate the help!

Thanks

[52 Bill]

...(a tech before me insulated the circuit board from the chassis with painters tape, it wore through and POP!)

That was no tech.

The remaining diodes have ICC 170 printed on them, but I'm having no luck finding a cross reference guide.

Could that be IRC 170? I seem to remember 170 diodes as being "Universal" replacement diodes, rated at 1000V - 2.5A. A number of companies sold them Motorola HEP, GE, International Rectifier Co, etc.

While a 200V - 1A diode will probably be just fine, why not use four higher rated diodes if they will fit the board? I think I keep 1N5408 diodes around just for that purpose.

[JerkyPudding]

Yeah, I suppose "tech" was a bit of an overstatement.

I'll go with the higher rating. Better safe than sorry. Thanks for your help, I appreciate it!

[JerkyPudding]

So I've fixed the bridge rectifier, and taken care of the short circuit to ground issue. Now, I have volume in channel 1, but nothing in channel 2, and when channel 2 volume is turned up, it pops very loud at the bottom of the pot, then nothing. The inputs are using each side of a LM1458 opamp. Also, channel 1 is showing some clipping on the scope. Any thoughts?

[Jecarroll7]

How do all your voltages look?

[52 Bill]

...when channel 2 volume is turned up, it pops very loud at the bottom of the pot, then nothing. The inputs are using each side of a LM1458 opamp. Also, channel 1 is showing some clipping on the scope. Any thoughts?

Have you read the voltage on the pins of the 1458? Check to see that you have the correct plus and minus voltages on pins 8 and 4 respectively and then check to see that there is almost no dc voltage on the rest of the pins (ins and outs).

[JerkyPudding]

Ok. Do I measure those voltages to ground? I have no experience with opamps.

Thanks

[52 Bill]

Ok. Do I measure those voltages to ground? I have no experience with opamps.

Thanks

Yes, connect the black lead to ground and carefully touch the red lead to each of the 8 pins. The power supplies go to pins 4 and 8. The outputs are on pins 1 and 7. The rest are inputs.

The pins are numbered in a counterclockwise direction starting with pin 1 at the upper left. Often pin 1 will have a small dot or marking. Sometimes the chip is marked at the top edge with a notch. The pins run 1-2-3-4 and then up the opposite side 5-6-7-8.

If you search online I'm sure that you can find lots of diagrams that will be much easier to understand than my verbage here.

[JerkyPudding]

Yes, connect the black lead to ground and carefully touch the red lead to each of the 8 pins. The power supplies go to pins 4 and 8. The outputs are on pins 1 and 7. The rest are inputs.

The pins are numbered in a counterclockwise direction starting with pin 1 at the upper left. Often pin 1 will have a small dot or marking. Sometimes the chip is marked at the top edge with a notch. The pins run 1-2-3-4 and then up the opposite side 5-6-7-8.

If you search online I'm sure that you can find lots of diagrams that will be much easier to understand than my verbage here.

So I measured the voltages. Inputs read 12v and -12v respectively, output on pin 1 reads 11v when volume pot (for channel in question) is dimed. Every other pin reads very low voltage, negative on inverting inputs. Output on pin 7 increases from .003v to .16v with volume increase (this channel functions).

Seems like 11v hitting the volume pot is causing the loud pop on channel two. Is this a faulty opamp causing this?

[The Dude]

I don't see the schematic, but likely, yes- shorted op amp. The pot wiper is probably on the output pin of the op amp and other side of the pot grounded. When you move the pot wiper towards ground, the voltage is grounded. When you move the wiper away from ground, the voltage goes up. There shouldn't be voltage on the output pin of the op amp.

[52 Bill]

So I measured the voltages. Inputs read 12v and -12v respectively, output on pin 1 reads 11v when volume pot (for channel in question) is dimed. Every other pin reads very low voltage, negative on inverting inputs. Output on pin 7 increases from .003v to .16v with volume increase (this channel functions).

Seems like 11v hitting the volume pot is causing the loud pop on channel two. Is this a faulty opamp causing this?

Yes, bad opamp. Your post says Inputs read 12v and -12. Do you mean on pins 2 and 3, or are you talking about pins 4 and 8? When you post information it helps if you are clear about your readings.

I would replace the chip and see what happens.

[JerkyPudding]

Yes, bad opamp. Your post says Inputs read 12v and -12. Do you mean on pins 2 and 3, or are you talking about pins 4 and 8? When you post information it helps if you are clear about your readings.

I would replace the chip and see what happens.

Yes, pins 4 & 8 (I get ahead of myself and forget details ) I've been researching and learning much about opamps the last couple days, they're not as mystifying as I thought (I got into audio electronics through tube amps, go figure).
I'll get a chip in there, and let you know how it goes.

I appreciate everybody's help on this, thanks!

[Enzo]

negative voltages on the inverting input will result in a positive output

But voltage on an output will usually be fed back to an input pin. So, it seems the IC is shot, so remove it., While the IC position is empty, fire it up and measure to see if that voltage is still present on the place the output pin was soldered. And ion the input pin pads as well. That will either tell us there is some voltage present that ought not to be and is confusing the IC, or it will all read zero like it should and so the IC itself is all that is wrong.

[JerkyPudding]

negative voltages on the inverting input will result in a positive output

But voltage on an output will usually be fed back to an input pin. So, it seems the IC is shot, so remove it., While the IC position is empty, fire it up and measure to see if that voltage is still present on the place the output pin was soldered. And ion the input pin pads as well. That will either tell us there is some voltage present that ought not to be and is confusing the IC, or it will all read zero like it should and so the IC itself is all that is wrong.

So I got the new IC soldered in before taking this advice. It's behaving identical to the old IC. It sounds like some voltage is present and confusing the IC, as you stated. I'll remove it and measure the vacant leads. If it's possible to figure it out with the IC still soldered, I'm all ears.

Thanks for your help.

[52 Bill]

If it's possible to figure it out with the IC still soldered, I'm all ears.

This is where a schematic would help us all to understand what is going on. I couldn't find one online. Do you have one with the amp?

If not perhaps a few photos would help. A lot of Earth products were near direct copies of Peavey amps. If we knew more about the amp, maybe we could find a Peavey schematic that would help in the cause.