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Old 10th February 2010, 05:20 PM   #1
_vi is offline _vi
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Default noob with a perplexing problem.

Hello I have a perplexing problem and am hoping someone may be able to shed some light on the matter. First of all I am an electronic and computer engineer mainly specialising in digital design, occasionally dabling into analogue when I need a power supply for something. Analogue was never my strong point and BJTs always seemed a bit like black magic!

Any hows here goes...

I have a peavey pvx-125, schematic here: http://www.geminidj.com/disc._prod_m...25/PVX-125.pdf

When the amp powers up it goes strait into protect. Now as I am aware this is either due to a problem with the DC bias or a problem with one of the output transistors supplying the rail voltage strait to the speakers.

So my first step was to test all the output transistors. They are 'SC3281' http://datasheet.octopart.com/2SC328...eet-101323.pdf

Now as far as I am aware to check an npn transistor with a multimeter on diode mode you would check base(b) to (=>) Collector (C) then b => emitter (e). Both should read around 0.5 to 0.8 v.

I have checked the output BJTs they are (on the schematic) Q112, Q111, Q113 and Q212, Q211, Q213. I get the same result on each and every BJT. whn measuring B=>C I see 0.509v. When measuring B=>E I get 0.136v.

Are these values too far out? Is every single output transistor toasted here? They all have almost the EXACT same value.

I have also checked Q107 and Q207, I believe these are the DC stabilising transistors. They are 2sc3421:http://www.toshiba.com/taec/componen...c//66/7767.pdf

When measured in the same way (B=>C then B=>E) they show a good healthy 0.752v each. I see from the schematic that Q107 and Q207 are in a darlington pair configuration with Q106 and Q206 respectivly. There companion transistor read B=>C 0.8, B=>E 0.8v (the same for both).

In addition to this, I see that there is a potential difference of around 0.6v on each speaker output channel. (2 channels).

At my disposal I have a trusty old crt oscilloscope, a very nice function generator, multimeters and some power supplies.

So I am a little stumped. What could be going wrong? Bear in my mind I that this is the first time I have tried to repair an amplifier of this complexity. I have done plenty of work with small battery powered D class amps, but this is a whole other league! So please any suggestions no matter how obvious they may seem.

Thank you
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Old 10th February 2010, 06:12 PM   #2
h_a is offline h_a  Europe
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Quote:
Are these values too far out?
Are you measuring with the parts still installed? Sounds a lot like it. Because then you get identical readings for the outputs because they are paralleled.

You can't reliably measure the parts still installed, you have to take them out.

Before that you should measure how much DC you actually have on the output and measure also other working points.

I'm a little short on time right now to have a closer look, sorry.

Have fun, Hannes
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Old 10th February 2010, 06:16 PM   #3
Mooly is offline Mooly  United Kingdom
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Just had a very quick look at the circuit... as h_a hints at, those 270 ohms paralled across B-E will stop you reading them correctly.
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Old 10th February 2010, 06:23 PM   #4
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Measure and report the DC voltage on R122 and R222... this is the true output of the amp... what does it read ?
Measure on both ends of each resistor... as they could be open circuit.
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Old 10th February 2010, 07:38 PM   #5
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Thank you for your suggestions.

R122 has a PD of 204mV.
R222 has a PD of 123mV.

This is with speakers disconnected, the outputs switched of and nothing on the input.

I apologise for the time it takes for my responses to get through but my posts have to be moderated as I am a new member.
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Old 11th February 2010, 07:12 AM   #6
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I just noticed you measure 0.6V DC on the outputs; seems to me that you only need to readjust the offset. 0.6V DC does not indicate that something is blown.

On the other hand I'm stumped by your last post; if it's really true that you read these values across the 0.1R resistors, bias current is 2A and 1.2A respectively. In that case the outputs are close to blowing up as they don't stand 80W idle power dissipation for a long time - do they become hot?

Have fun, Hannes
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Old 11th February 2010, 07:20 AM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by _vi View Post
Thank you for your suggestions.

R122 has a PD of 204mV.
R222 has a PD of 123mV.

This is with speakers disconnected, the outputs switched of and nothing on the input.

I apologise for the time it takes for my responses to get through but my posts have to be moderated as I am a new member.
I wasn't very clear on what I meant perhaps. I see it can be taken in two ways
I meant from ground, and then measure to each end of each resistor. Is that what you have done. If one were OC you would have many volts difference end to end.

Perhaps you could also measure across each as well and see what the voltage is (and calculate the current). As Hannes asks... "are they hot" ?

If the output of the amp is at essentially zero then I can't see why the protection has tripped.
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Old 11th February 2010, 09:35 AM   #8
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Apologies I should have been more explicit about what exactly I measured. I have found 4 test points, one at either end of those resistors. Those resistors are massive by the way 1/4"*1". Anyhow I have measured the voltages with respect to what I believe to be the ground rail. That is where the black wire from the transformer attaches to the PCB point labelled GND. As I power up and power down the amp the readings start at these values and slowly count down as I measure them.

R122 = 13.8mV
R222 = 82.9mV

I see these are vastly different from what I measured before and I can only assume as I was trying to press the probes against the resistor leads I didn't take very accurate readings. The voltage is exactly the same at either end of the resistor and measuring across the resistor shows no voltage difference. Measuring the resistance R122 and R222 shows an exact resistance of 1.3ohm.

The resistors are cool to the touch.

So if my ohms law is correct the current across those resistors should be

R122 = 10.6mA
R222 = 63.7mA

Does that seem somewhat...unbalanced?

When I quoted the value of 0.6v on the output it would seem I was WILDLY out. I have re-measured this morning. When the output is switched off there is a potential difference of:

left = -2.5mV
right = -1.6mV

When the output is switched on:

left = 109.1mV
right = 11.2mV

Sorry for the madly incorrect information . That is the potential difference between the - and + output terminals for the speakers.

I am having difficulty figuring whether it is the left or right channel that corresponds to the output stage which has a higher voltage than the other.

When I say there is no input I mean there is nothing connected to the amplifier input. The speaker channels 1 and 2 both have on/off switches. They are both set to off.
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Old 11th February 2010, 11:56 AM   #9
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Those readings seem fine.
Just to confirm, read also R124 and R224 on ohms (0.15 ohm) as these are in series with the output to the speakers. Sure to be OK.

It's not easy following a circuit on a PC screen
First thoughts... check the protection circuit... there may well be nothing wrong with the amp as such. These stand out as "typical failure" looking items due to the way they are used. Worth a quick check... if that shows nothing you start at the protection LED and work back, finding why it is being turned on.
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Old 11th February 2010, 01:19 PM   #10
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Quote:
The voltage is exactly the same at either end of the resistor and measuring across the resistor shows no voltage difference.
Then you don't have any bias current in the output transistors (current=no voltage/0.1=no current), in other words the output transistors are off.

What bothers me more is that completely contradicts your first post where you say that you find a DC offset of 0.6V. Your above readings say DC offset is 13.8 and 82.9mV, respectively. That's generally too small to trip the protection.

If unsure, measure with one lead attached to the enclosure (if metallic). Enclosure must be grounded.

Have fun, Hannes
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