I bought a QSC RMX 2450 off a guy on ebay for £100, with one channel not working (channel 1). After some detective work, I've managed to find the culprit - one blown 2SC5200 NPN power transistor.
I've pulled the little bastard out with the intention of replacing it. However, the thought struck me - should I just replace this one, or replace all four? If I'm going to replace all four, should I replace the PNP complementary output transistors too?
Then I got to thinking, what about the drivers? What about the transistors on channel 2? I mean, where do you draw the line?
What are your thoughts? 😱
I've pulled the little bastard out with the intention of replacing it. However, the thought struck me - should I just replace this one, or replace all four? If I'm going to replace all four, should I replace the PNP complementary output transistors too?
Then I got to thinking, what about the drivers? What about the transistors on channel 2? I mean, where do you draw the line?
What are your thoughts? 😱
Just one. They produce them with quite tight parameters for that model of semiconductor. But it should work with the transistor out of the circuit. If it doesn't it may be easier to replace eight. If it still doesn't work then both drivers. If it still doesn't work then look at the input.
Remember when an OT blows, high voltage comes out of the base line and goes where?? Are all the drivers and capacitors and stuff rated for that voltage? I'm trying to repair a PV1.3K and I've got over a pound of blown up stuff, over 70 components. If you luck out great, but if not it is an interesting learning experience. Parts cost now $50 for 20 OT's and drivers and new electrolytic capacitors (rail capacitors were not blown but 14 year old). the other 50 parts were about $.20 a dozen, diodes, resistors, capacitors, SCR's. Just because an output transistor acts like two diodes on your DVM, does not mean that it is going to hold off full rail voltage. Best test I've found so far for OT's on no money, short b-e together and put 18 VDC off car battery charger through 47k to C. If it leaks current it is bad.
While you are paying freight charges, get some big load resistors for a power test, (not speakers) as resistors are not dC sensitive. Also some 100W light bulb sockets for a low power tester (in series with the AC line).
While you are paying freight charges, get some big load resistors for a power test, (not speakers) as resistors are not dC sensitive. Also some 100W light bulb sockets for a low power tester (in series with the AC line).
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Peavey builds cheap amps. Crown & QSC figured out long ago repairs are part of the cost of ownership. When they come off lease they will change all the outputs and power supply caps and sell them as good as new for what was the original dealer cost. They have built amps where when one part fails so do others. Pro's avoided them and between warranty costs and end of life sale they were a bad idea.
Peavey has a different market. Their customers want the cheapest watts per dollar. That is what they sell.
Peavey has a different market. Their customers want the cheapest watts per dollar. That is what they sell.
A lot depends on the reason the transistor blew in the first place.
Very few fail for no reason.
It could be the amp was driven too hard into too low an ohm speaker.
However when a transistor blows it can often take out the drivers too so simply replacing the output transistor results in another meltdown.
The advice of just testing with the a single output pair is a good idea along with using a light bulb mains limiter to reduce current.
One trick I learned early o nwas to completely remove the output transistors and feed the driver output back into the ltp.
This created a feedback loop so I could test the driver stages live. I would check for right DC offset along with right bias voltage.
High voltage comes out the base? Only if the transistor shorts collector to base. Much more common for them to short collector to emitter.
The drivers are expected to take the full rail to rail voltage, because under normal operation, the driving signal can run close to rail to rail.
Of course it is good practice to check the drivers whenever an output fails.
With all due respect, to say a blanket statement like Peavey builds cheap amps is hogwash. Yes, they make amps that cost less than their Crest line or less than Crown or QSC. But over the last 25 years, I have found them to be reliable performers. My shop has been an authorized repair shop for both Peavey and QSC, and frankly, I saw a larger percentage of the QSC amps coming in for service. And while there are times an amp blows up pretty hard, as in Indiana's case, most times they need a couple transistors and back out the door. Failed QSCs tended to need outputs, drivers, commutators, and those little surface mount controller cards.
The drivers are expected to take the full rail to rail voltage, because under normal operation, the driving signal can run close to rail to rail.
Of course it is good practice to check the drivers whenever an output fails.
With all due respect, to say a blanket statement like Peavey builds cheap amps is hogwash. Yes, they make amps that cost less than their Crest line or less than Crown or QSC. But over the last 25 years, I have found them to be reliable performers. My shop has been an authorized repair shop for both Peavey and QSC, and frankly, I saw a larger percentage of the QSC amps coming in for service. And while there are times an amp blows up pretty hard, as in Indiana's case, most times they need a couple transistors and back out the door. Failed QSCs tended to need outputs, drivers, commutators, and those little surface mount controller cards.
Well that explains why only one set of drivers etc was blown on my Dynakit ST120 amp in three OutputTransistor failure events. I was there at the last two blowups and shut it down. Of course if the previous owner tries to operate the amp with a shorted OT BE junction (one dead channel), it overheats the drivers even if they are rated for the rail voltage.
I think from looking at the design the Peavey PV# line was a low cost alternative to the CS#00 line which was more expensive. The CS#00 line also have more resale value on Craigslist than the PV# models, which is now I ended up with the PV1.3K. I'm not paying $250 for a working 15 year old CS800 rev b, or a QSC either. Like I say I've got $115 in this so far, and have learned a lot of things not to do and a few basic facts. The internet and contributors to diyaudio in particular are wonderful teachers.
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Actually Mr. Peavey likes to tell the story about how he used to build the circuits straight out of the RCA manual until he hired the guy who designed them. He then found out that there were much better circuits.
The QSC failure rate at 1 year and 5 years are much lower than the Peavey numbers. So your experience may not be typical. (I suspect you are only seeing a part of the QSC line and not the units that really are used in large quantity.)
As to Peavey being cheap, look at the designs! For example look at their mixer circuits. There is a lot of DC coupling between stages. Even through volume control pots. That is not to improve the sound, it is to avoid spend money on small parts. It doesn't matter if there is some small DC on the pot, the noise is not an issue with their customers and even though it will cause early failure that is not a concern, because once anything is out of warranty they don't want to see it again.
Cheap is fundamental to Peavey's marketing approach. It is what their customers want and expect.
I want adequate quality made in Nafta countries or Europe. Design improvements I can modify in, perhaps, as I've done the ST120. I also want complete disclosure of schematics and repair parts. I have to buy import caps and resistors and transistors, but at least I can get them from from the second world, not the third world.
And I love my new/used speakers, the best that are in stock in this market from anywhere.
You will see from my signature, the far east didn't invent consumer electronics.
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Most of the time the base-emitter charge-suckout resistor provides this connection for you. I've run lots of Phase Linears without outputs (or even rail fuses) to make sure everything else is fixed first back in the day.
The only PV product I've ever really liked was the old boat anchor CS800's. The alternatives were either a)not powerful enough, b)ridiculously priced, or c)blew up like the 4th of July running hard on 4 ohms. QSC was kind of a game changer when they matured - it was the only reduced-parts-count design that was worth a darn. And you could run them hotter than the blazes of Hell and Damnation and not instantly lose $100 worth of output transistors.
Imho... I believe it depends on type of damage
sometimes the other transistors lose their characteristics if they are too stressed
then you need to change all them.
sometimes you just need to change transistor blown.
sometimes the other transistors lose their characteristics if they are too stressed
then you need to change all them.
sometimes you just need to change transistor blown.
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