Samick Power Amp Dead

[Ed_Chambley]

Hey guys, new stuff for me so a couple of questions.  Have a monitor amp blowing fuses.  Visual inspection turned up nothing.  Found the package rectifier bad and causing a short to ground which certainly is one of the issues.

So while I await the correct part I thought I would check with you guys to see if there may be something that could be causing this issue as I don't want to toast the new part, but I did order 2 of them just in case. 

The problem began during a break at an outside venue whereas someone placed a mic too close to the monitors.  I heard feedback begin, but being an outdoor event the soundman was away and by the time I got to the board to pull the volume the amp quit.  We replaced the amp to finish the day out and now I have a repair that I am not all that familiar with.

Other than replacing the diode package and checking the output transistors (they are not shorted) is there anything else I should check before powering up the amp on a current limiter?  Surprisingly the amp seems to be extremely well made.

I have attached the schematic.

[sluckey]

Pull the two fuses for the output stage (actually 4 fuses since it's two channels). Fire it up and check the positive and negative power supply rails.

[Ed_Chambley]

Pull the two fuses for the output stage (actually 4 fuses since it's two channels). Fire it up and check the positive and negative power supply rails.

Thanks Steve.  Waiting on Mouser, but I get where you are going.  This will insure the power section is ok prior to bringing in the output components.  Am I thinking right?

Also, do you know how feedback does this damage?  Just curious mainly.

[shooter]

Am I thinking right?

You are right, were those fuses good?
you can ohm the 4 R's 130 - 134  .5ohm 5 watt on each channel, sometimes they blow before the fuses.

[PRR]

> how feedback does this damage?

It really shouldn't. Any modern transistor amp should take "any" input without anything dying.

Looking at that plan, I see a too-fancy input stage, but NO comprehensive output transistor protection. Q118 senses Q115 current/voltage, but there is no such sensing for Q117, and when Q118 triggers it starts a long chain through U2 to the output relay, which really is not fast enough to protect transistors. (And can't protect against internal faults.)

I suspect you have more damage to find.

Just four output devices (two pairs) for 300 Watts per channel? That's way under-supplied. Robust power amps don't ask more than 75W per pair. At today's transistor prices, that doesn't up the cost much, and you recover some in better heat-spreading.

IMHO, if it doesn't come back to life in an hour and $10, set it aside and get a More Robust amp for live sound.

[Ed_Chambley]

> how feedback does this damage?

It really shouldn't. Any modern transistor amp should take "any" input without anything dying.

Looking at that plan, I see a too-fancy input stage, but NO comprehensive output transistor protection. Q118 senses Q115 current/voltage, but there is no such sensing for Q117, and when Q118 triggers it starts a long chain through U2 to the output relay, which really is not fast enough to protect transistors. (And can't protect against internal faults.)

I suspect you have more damage to find.

Just four output devices (two pairs) for 300 Watts per channel? That's way under-supplied. Robust power amps don't ask more than 75W per pair. At today's transistor prices, that doesn't up the cost much, and you recover some in better heat-spreading.

IMHO, if it doesn't come back to life in an hour and $10, set it aside and get a More Robust amp for live sound.

Thanks for the input.  Glad you shared your knowledge because all I know is appearance and not what makes good power amp components in the transistor world.  I just looked at the massive Power Transformer and main caps and these components dwarf what is in my crown amps.

I did not find any bad fuses, but just wanted to see if I could fix it really. The good thing (I think) is used power amps have gotten cheap.  This was not my PA rig, but I did loan one of my amps to finish the show.

I am currently looking for a used replacement and finding that it is not as simple as once was as Crown and QSC have a lot of different models.  Some seem too cheap.

[Ed_Chambley]

Am I thinking right?

You are right, were those fuses good?
you can ohm the 4 R's 130 - 134  .5ohm 5 watt on each channel, sometimes they blow before the fuses.

Fuses are good, but something had to kill the diodes.  I will know soon as the parts will come today.  The package diode is bolted to a heat sink with thermal paste.

See how much I know.  I thought it was a high quality amp because it is huge and heavy for 600 watts.  It is about 50% taller and heavier than my old Crown 800's.


PRR says it is not a good design and I have already learned quite a bit about them and have only been messing with it for a short time.  The coolest thing is now I have an excuse to shop for gear!


UPDATE:
Thanks for the input guys.  Just the package diode was the only problem.  That is great as I did not spend much time fixing it.  Did as suggested and nothing went boom.

Tested the amp with my Ampeg preamp and it is a nice sounding amp.  Great sounding bass guitar.  I can now return it to the owners as they were concerned having to get a new one.  Don't know how long it will last, but good now.