Another amp fault (sorry!)

[johndenim]

My trusty Peavey 2600 has developed a fault.

 

I pushed it too hard the other night and whilst the red clip led's were lit, the sound was not distorted.

 

I was using it to drive 2 xt118 bass cabs.

 

Now, one channel does not work at all, whilst the other is fine.

I have opened her up and there is no physical sign of damage, and the two fuses are OK.

 

When the amp is powered up the red clip leds flash momentarily, but it always did that with the circuit protection on. (DDT)

I have tried the amp with this circuit off and also back on but no change.

 

Any ideas where to look?

I have a multimeter but not an oscilloscope.

Nor do I have the knowledge to use one!

 

Any help please chaps?

[Chris L]

Hey

 

This is likely to be overcurrent through the transistors - they use fairly cheap ones in the PV range. You'll probably need a new power stage which would cost about £70 or so just for the parts.

 

It may have not sounded distorted, but that doesn't mean it wasnt damaging the amplifier/speaker. If you have the DDT on, it acts as a very heavy compressor, so if you drive the input so it's on constantly then you increase the RMS (average) of the power output. This heats it up more and can lead to the transistors getting too hot and going into "thermal runaway" (where they heat up, which lowers their resistance, causing more current to flow, so they heat up more etc... until the semiconductor is damaged).

There are other threads on here that explain about limiters (DDT) and what that does to RMS level, particularly with bass frequencies.

 

That's just my take on what might have happened - having looked at how the DDT behaves on a scope, it's best to only flick the clip/DDT lights a little bit when in use: It's not really designed to run constantly.

 

HTH

Chris

[KevinE]

they use fairly cheap ones in the PV range

 

Toshiba 2SC5200/ 2SA1943 usually in these so standard transistors used across the industry!

 

There is quite alot of protection in the PV2600, and any slight fault is enough to mute the channel in question if it detects something amiss at power up. Most major faults will make the amp go into protect which will show on the LEDs.

 

I assume youve checked the obvious such as swapping the inputs and outputs over to eliminate leads and speakers.

 

If it's still misbehaving then pop the board out and do cold checks on the major semis in case anything's popped. As well as the components you can see on the top of the board, there is a raft of other surface mount on the underside.

[wmcalin]

Hi Jon

 

Peavey are based in Corby Northants.... might be worth rturning back to the factory....

also I'm sure these have a 5 year warranty, take a note of the serial number and give the factory a call...

 

Regards

 

Will

[johndenim]

Thanks a lot guys, I'm really a sparks by trade and know nowt about electronics!

That helps a lot Chris, cheers.

I only hope it won't cost £75 just for the parts, I could get a ta 2400 for about £250.

 

Kev, how do I do a basic test on semi's?

I sort of know their role but.......

 

I think the warranty has run out, it's the mk1, but might be worth a quick call.

[Chris L]

The transistors are £5 each from peavey - you can probably find them cheaper on farnell, not looked.

There's lots of protection on the schematic for the thing, but given the number I've had in pieces with power stage blow-outs, it doesn't seem to do anything useful - the time constants in the systems are too slow to catch thermal runaway. Only thing I've ever seen any trip out for is S/C faults.

The DDT is done by a limiter-type VCO (Peak and RMS based) with a very fast time constant on one chip. Which is why it's so lethal.

 

The parts you'll need for a whole power stage are:

5x SA1943, 5x SC5200 (power transistors)

1x MJE15033, 1x MJE15032 (power transistors)

1x 2SA1837, 1x 2SC4793 (more power transistors)

1x 8550, 1x 8050 (signal transistors)

The large white biasing resistors will all have gone (x10) (possibly .33ohm 5W)

Probably the biasing resistors for the other 6 transistors (don't know the value as I only have a PV1500 schematic in front of me)

And the diodes attatched to the heatsink usually go.

 

Once you've done that, it should work again :(

 

If you're not confident getting down and dirty with a soldering iron, send it to peavey, they have a nice friendly man who helped me fix the first couple I did.

 

HTH

C

[Mixermend]

There is a PV 2600 power amp and a GPS 2600 made by Peavey - and they are very different designs! The GPS 2600 uses speaker protection relays, whilst the PV 2600 uses a triac for DC output protection.

 

If the front panel LED's seem to indicate that all is well - you are very unlikely to have damaged power transistors - as this constitutes catastrophic failure - and there would be other clues evident.

 

After checking that your speaker is indeed fine (continuity check) - you could test the triac (assuming it is a PV 2600), by simply doing a continuity test across the speaker terminals (or Speakon connector) - if the triac has failed short circuit, you will see this on the faulty channel (compare with the good channel). The amplifier should NOT be powered up for this test. No need to even remove the lid!

 

It is fair to say that spurious triac short circuit failure is rare, although it will often fail this way if the amplifier goes DC.

 

Peavey in Corby are fantastic - and will help you in any way they can - so I would advise contacting them.

[KevinE]

Aye John D well with due respect if you're not ok with checking bipolars then I wouldnt take it much past the stage of checking the obvious things which I guess you've already done. They can be little tinkers to bottom out and occasionally the fault can spread into the LV rails.

 

The PV2600 is a little more complicated than a PV1500 as it used class G technology (mosfet rail commutated) and these have to be working correctly as well as the bipolars.

 

It should not go into thermal runaway for anything as it is fully thermally protected with tempistors that ramp the fans up from tickover to flat out over a 3-step period, then unload the outputs if the stage continues to overheat.

 

And as John G has said, any major power stage fault will result in a fault condition (ie the power lights not coming on). Does the softstart relay go 'click' after power-up? This is linked to the self check.

 

If you connect a signal (no speakers connected) and turn the volume up, do the sig lights flash? Anything else shows? Do the DDT lights flash in sympathy with the sig lights on the faulty channel? Are the fans running flat out, or just idling/run then stop?

[Chris L]

It shouldn't go into thermal runaway, but they seem to be able to. I don't think the heat sensors are mounted close enough to the transistors so they don't catch some faults in time. I've had a couple of mine go (never run them with the clip lights more than flicking) and repaired a few others that have. Also, only one of these showed any fault lights, but the power stage was definately dead. (replacing it fixed it!)

 

It could, as other people have said, be another problem entirely, but that's the most common problem I've seen on them.

[johndenim]

Will check everything mentioned tomorrow guys and let you know.

Thanks!

[Rob_Beech]

I'm only down the road if you need something in the meantime. Might have a Crown badge on it though I'm sure you can put some tape over that :P

[johndenim]

I do have a spare, it's an AX 3500 and actually pretty good.

I don't know why, I just prefer the Peavey????

 

Trust if I need to hire in I will always come to you Rob, thank you sir.

Oh, and the Crown badge certainly beats anything I own!!! :P