iampoor1 said:
Output power devices are the most common failure in power amps.
Funny you mention that, the voltage drops are not consistent on some of the parts. I also started testing the other "good" channel, and I noticed that after the amp start heating up the DDT protection circuitry would active at seeing random.
DDT basically compares output to input and triggers whenever output is not correct, so it will trigger from clipping, or current limiting, or even too much distortion (rarely).
Sometimes under full load, sometimes under 1/16th of a full load, and sometimes not at all for a few minutes at full load. I am certainly suspect of all of the output transistors now. Unfortunatly, I dont think replacing them all is economical.
Power transistors don't fail as softly as small signal drivers. Once you get it powered up without smoking you can look at the emitter degeneration resistors voltage drop to see if any of the power transistors are not sharing current equally (a sign of problems).
Are there any tests to check for blown opamps, other than physical damage, shorted rails, or just not acting right in circuit?
op amps will generally work or not...in that amp probably a cheap op amp in a socket, so if suspect just change it but more common faults are power transistors and associated drive circuitry.
Maybe I should build a test jig for opamps...Testing them in circuit seems practical in most gear, with the exception of power amps of course! :'(
Thanks for those tips, I will check bias resistors in the future, I checked the emitter resistors (also do as a matter of precaution) and then all read .33 ohms, or thereabouts.
the emitter degeneration resistors are pretty robust designed to tolerate massive overload (for short time) so unlikely to be bad.
So are you responsible for the nifty little addon module with 4 transistors per side? 8)
I like how the circuit uses the standoffs!
No I did not do the packaging... by that time I was no longer doing engineering as my primary job duty but product management (feature set definition, marketing, etc). I was responsible for all Peavey power amps (and more).
You can take the boy out of engineering, but you can't take the engineer out of the boy, so when that amp series got refreshed I made an engineering call that the new improved PV2000 needed more output devices than the former PV 1.3k. The PV amp series was an exercise in making low cost amps and IMO they cut one corner too many. As product manager I had responsibility for pricing and profit margin so made the relatively unusual decision to add cost (2 more power devices per channel IIRC). I was more than willing to take slightly less profit, to sell a more robust power amp. Happiness is not having to get all those service calls... but stuff still happens apparently. Just be glad you have the 2000 and not the old 1.3k hand grenade (had thermal runaway issues).
Yes I do, unfortunalty, its only a 4amp variac...so its a bit small for this amp. I have a big bank of load resistors too.
If the amp is running with no load a 4A variac is more than adequate to bring it up for testing. An old school trick is to wire a 100W light bulb in series with the amp line cord to serve as a fault current limiter, during troubleshooting.
Yes it does, I checked that along with all of the diodes.
I spoke with the owner of the amp today....it got sprayed by sprinklers during full power at a DJ gig. I guess that explains the corrosion on one of the boards.
At this point, I think thats my only option. Time to remove all the transistors......for the third time. :'(
The PV2000 is one of the last old class AB heavy iron dinosaurs... so a pretty simple design. Bring it up to voltage and see what is not behaving properly.
Of course be careful probing around inside a power amp under power, as a slipped probe could cause another failure.
JR
