Newbie troubleshooting a power amp... suggestions?

[oxide54]

first you need to check PSU output voltages are ok.

also look for obvious burnt out components etc,

then in general terms you need to systematically work through the signal path from input to output tracing the signal to see where the signal is what is expected and where it turns to sh!t, a scope would be best for this, without a scope you could be asking for trouble probing a power amp with a speaker or crystal piece.

[oxide54]

tried editing the previous post but i didn't have permissions ???

edit: get the schematic / cicuit diagram and it will help a lot/

oh and be seriously careful, especially around the PSU region.

your death is your fault and your fault only, if in doubt pay some one.

[The Real MC]

Any novice trying to fix a power amp is risking fire and death. This is one job that should be left to the professionals.

[JohnRoberts]

Common failure mode in power amps is shorted output devices, so measure between C and E with VOM... (with power off obviously_

Repairs can involve replacing other parts in the vicinity that are also damaged.

If you only half fix it, it may break again so repair will be educational but could get costly.

JR

[21doors]

your death is your fault and your fault only, if in doubt pay some one.

Any novice trying to fix a power amp is risking fire and death. This is one job that should be left to the professionals.

...very good. I think that put the fear into me. I definately won't be messing with it while power is on. Not for a while anyway.
I've been learning about this stuff, a little at a time, and I am extremely paranoid. Even unplugged and after discharging caps, I have plastic arms that are taped to probes, with one arm behind the back, fire extinquisher and sprinklers ready, and escape route planned, etc.

Actually I'm learning a lot, and slowly getting more familiar ...and brave. I know from skateboarding that being overly confident leads to pain.

Thanks for the comments.
I think I need to get a variac and a oscope as my next purchase, before I get too ambitious.

[Th3_uN1Qu3]

Originally Posted by 21doors

I think I need to get a variac and a oscope as my next purchase, before I get too ambitious.

The scope can wait, the variac: DEFINITELY. Will save you many expensive transistors. Also remember that the variac doesn't provide isolation from the mains supply, so plug it into a GFCI.

The death risk is way overblown IMO (have been shocked myself more times that i'd like to admit, mostly due to my own stupidity), but accidentally getting your fingers in a live circuit will hurt, and could indeed prove fatal if you have heart issues.

[21doors]

Thanks unique,

I have been shocked once when I was younger, a fender showman cap. Felt like my arm was bitten off at the elbow, haha.

And I did work sales in a shop that sold and repaired amps of all types. The repair guys were getting shocked on a regular basis., ie once a year they'd be scared by it enough to tell everyone about it. Really they were just being lazy, cutting corners, etc: I asked them about safety pointers, and they showed me a lot. They were just super busy, Always in a rush, and safety would slip. So many things break nowadays!

Thanks for GFCI tip, and the variac suggestion. I will definitely get one first. Then Ill try some of the suggestions you guys mentioned.

[The Real MC]

Originally Posted by Th3_uN1Qu3

The scope can wait, the variac: DEFINITELY. Will save you many expensive transistors. Also remember that the variac doesn't provide isolation from the mains supply, so plug it into a GFCI.

GFCI does *NOT* isolate the mains. Even if it is not tripped and conducting current, the mains is NOT isolated.

GFCI will only trip if there is excess current between ground and neutral. It will NOT trip if there is excess current to the load in neutral or hot. Subtle but important difference.

The death risk is way overblown IMO (have been shocked myself more times that i'd like to admit, mostly due to my own stupidity), but accidentally getting your fingers in a live circuit will hurt, and could indeed prove fatal if you have heart issues.

Do not underestimate the risk of high voltage. Even though you drain the caps, there are other places it can zap you. If you do not know the proper precautions, it CAN kill you. If you can't even differentiate the GFCI then you should not be spreading myths like this.

It is stuff like this which is why these kinds of kobs are left to the professionals.

[nosebleedaudio]

I have worked on MANY power amps and do not like to work on them for several GOOD reasons..
Someone that does NOT know electronics GOOD should NOT be trying...
My .02 cents...

[S2udio]

Originally Posted by 21doors

Hello,

I'm kinda new to circuit troubleshooting.

As already mentioned.

Then This is not the piece of kit to practice on !!!

[Th3_uN1Qu3]

Originally Posted by The Real MC

GFCI will only trip if there is excess current between ground and neutral. It will NOT trip if there is excess current to the load in neutral or hot. Subtle but important difference.

Good catch.

The best solution is an isolation transformer, but one of that size is going to be expensive. To this day i do not have one, but instead took another approach: I use an inverter and battery whenever i have to poke around with my scope in a primary side circuit. That amp of yours has a mains transformer, so it is isolated. So unless you put your fingers on the mains side where you shouldn't have any business anyway, that circuit will not bite. It's a low power amp too, so it does not contain hazardous voltages in the secondary. Plug the amp in and turn it on, and check voltage on the big caps with your multimeter. We'll continue from there.

Instead of a variac you can use a lightbulb (the incandescent type not CCFL) in series with the mains. Use a 60 or 100W. Like this: www.antiqueradio.org/dimbulb.htm If there are any shorts downstream, the bulb will light and will limit power so you have enough time to disconnect the amp before it sends any transistors flying. Even though the fuse did not blow in your case, it's still a good idea to have the lightbulb in series to protect from accidental damage due to probes slipping. I've done that more times than i care to admit.

[S2udio]

Then this is what you require..... ?

PKNM-6/1N/B/003-PT Moeller | F&G 1P+N 10kA type B RCBO,6A DIN rail | 341-4424 | Welcome to RS Online

You can get cheaper, this is RS !

Obviously some still dont know the difference between ,OverCurrent and leakage !

6A overcurent L to N
30mA , Trip ,20 ms OR BETTER..... any conductor leaking to earth.

If this dont protect you...then nothing will.

The lightbulb has been used for years as a current limiter,and works well...oh my CRT days.

But to be honest is not essential for everything....

Some SMPSU's dont like 40-60v input limiting....they take up smoking !

Just make sure you still stand back when it gets full whack.......

Suprises still happen !

Once bitten twice shy..........
Or just know what your dealing with

http://www.youtube.com/watch?v=q4uF1...eature=related

Real Power !

[LeeYoo]

Crown D-45
This is only a baby amp. 25watt@8ohms. Only the mains power parts are dangerous.
And ofcourse, a tech ALWAYS uses an isolation transformer.
Needed, because most of the testing gear (scope, soldering station, etc.) is grounded.
And a varactor or lightbulb is almost useless with transistor gear.

Unplug the amp, and wait 'till the main caps are discharged.
An old tech told me once that by using your eyes and your nose, you can find 50% of the problems.
I would start by measuring the output transistors with an ohm meter. Look for a dead short.
If you find a shorted output transistor, don't just replace it.
Expect the driver transistor to be blown as well.
And maybe the 0.22ohm emitter resistors.
Check the soldering. Maybe it all started from output transistor dryjoints.
Before starting up, remove the rail fuses, and replace them with 470ohm/5watt (cement) resistors.
I have a set, sodered to some old blown fuses.
No speakers connected, power up and listen for the protection relay to click in.
If the amp has no relay, measure if the red speaker output terminal is 0volt.
If ok, connect speakers, and run amp with very low volume.
If ok, put the fuses in, pray, and power up.
Leo..