Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: Ed_Chambley on May 28, 2015, 12:04:21 pm
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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.
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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.
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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.
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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.
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> 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.
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> 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.
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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! :laugh:
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.