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Old 16th March 2007, 07:45 PM   #1
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Default Keeps frying diodes

Hi folks,

I've got a Gallien-Krueger 800RB bass amplifier head in my basement that I've been trying to fix for a friend. It's a bi-amp head with one 100W and one 300W power amp section. My background is NOT as a tech but as a mechanical engineer, so I know some basic electronics theory and have accumulated some real world experience (mostly with tube amps), but I'm no EE and I'm no amp tech. This one's got me stumped! Here's the breakdown:

My friend came to me with this amp and told me it popped a fuse and smelled funny and now it won't power on. So I pull the amp apart and find a couple shorted output transistors and a couple smoked diodes.
I ordered and replaced every piece of silicon on the power amp board, including output transistors, diodes, drivers, EVERYTHING. I also checked the power section, unloaded, and confirmed the power feed voltages were to spec. I put it all back together, happily finding no shorts between rails, at the output, or across any caps.
I power it on (wish I had a variac) and POP go the same two diodes. I pull the power amp board out again and desolder all output transistors and they test good. I verify that all the parts I've replaced are in correct orientation and that there are no shorts on the circuit board. I replace the diodes again and put it back together. POP, again the same diodes are blown! These diodes are d11 and d6 (I believe - it's hard to read on the schematics).
So, now I'm scratching my head - what did I miss?!?! Any ideas?

Here are the schematics:

Power Supply board: http://www.oatsoda.com/800rb-1.gif
Preamp board: http://www.oatsoda.com/800rb-2.jpg
Power Amp board: http://www.oatsoda.com/800rb-3.jpg

Thanks for your help - this one's causing me to loose sleep!

Shane
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Old 16th March 2007, 07:57 PM   #2
Netlist is offline Netlist  Belgium
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Shane,

I think I found D11 on the third schematic.
You have something more readable? You can email the hi-res version (hugorx and hot mail) and I’ll try to post something better.
Or post a schematic with the parts circled in color.
You will find enough clever guys around here to help.

/Hugo
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Old 16th March 2007, 09:01 PM   #3
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It *looks* like D11 and D6 are 1N4002 diodes between the output and rail serving as inductive kick-back protection diodes. They are on the right, top and bottom of pg-3 if I can see them correctly.

1) Their orientation is very critical because if they are backward they will try to short the output to the rail.

2) I would suggest 1N4004 (400 Volts Vs. 1N4002’s 100V) and while the likelihood of them blowing due to excess reverse voltage is slim, if you got a bad lot or 1N4001 (50V) they might self destruct.

3) It looks like D11 should have the cathode (striped end) towards the +85V rail and the anode (non-striped end) towards the speaker output.

4) It looks like D6 should be the opposite of D11 – cathode towards the speaker output and anode towards the -60V rail.

Double check the above and see if it helps. Otherwise, there could be additional problems – open traces, open resistors (that look good, measure weird/okay in circuit, but bad out-of-circuit), shorted caps, AC on the +/- rails, etc.

Maybe try installing 1-amp fuses in-line with the power rails to protect the diodes (and everything else) during troubleshooting.

Good luck
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Old 16th March 2007, 10:25 PM   #4
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Found them...

I actually never new what these diodes did in amplifiers.
Thanks for that.
Anyway, it seems strange that 1N4002 couldn't do the job although I too would replace them with something higher in value.
Would severe oscillation cause them to blow instantly? But why only D11/D6 and not D12/D5.

/Hugo
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Old 17th March 2007, 12:19 AM   #5
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If the inputs to the two power amp sections (1 and 5) are each other's inverse, the problem might lay in the preamp, because a high DC voltage can cause the diodes to go into reverse breakdown, since the amps are DC coupled.

So, power up the power amp with shorted inputs. If this doesn't fail, check the preamp outputs etc. etc.
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Old 17th March 2007, 05:03 PM   #6
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the diodes are there to protect the output transistors from inductive kickback from the speakers. an inductive load (like speakers) can generate reverse voltage on the output, causing the transistors to be reverse biased, which can damage the transistors. the diodes shunt any reverse voltage directly to the power supply rails. as dcpreamp said, the higher voltage devices like 1N4004 (i keep a stock of 1N4007, 1000V diodes, since they replace everything in the 1N400x series) will probably cure your problem if you used 4001's. oscillation would most likely kill the output transistors before they would do anything to the diodes. there's really only 2 things that are bad for diodes, forward biasing them (putting them in backwards) or too much reverse voltage (possible if you replace them with 1N4001).

dc on the input won't forward bias the diodes, because they are attached to the rails, and the rails are as far as you can go, like having concrete walls on the sides of a highway. if you crash, the walls are as far as you can go.

btw, one critical difference between troubleshooting tube amps vs S/S amps..... tube amps MUST have a load on them when you power them up. S/S amps should have no load on them until you're sure there's no dc offset on the output.
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Old 20th March 2007, 01:31 AM   #7
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Right, so I went ahead and installed some 1N4004's, but before hand I checked the polarity and THOUGHT I found that they were in backwards! I did this, of course, by going by the schematic, finding the proper colored wires that would designate the rails and so on, and I triple checked. So, blindly I installed the diodes in the reverse polarity from which I had them.
Well, I turn it on and, vigorously, the diodes explode! I hope it was just the diodes, because it was at this point that I stood up and walked away, waiting for the frustration to boil off before I came back to the project.
I think what I need to do now, is to go back and trace out the power section, to make SURE I know which rails are which voltage, power up the amp with power wires disconnected, and MEASURE them to make sure, and then consult the schematic for proper diode orientation. I will also install a 1N4004 or greater 1N400x replacement. I'll probably need to retest all the silicon, right, to make sure I didn't fry anything else? Do you think a simple in-circuit short test would suffice (after removing the welded diodes, of course), or do I need to tear it all apart AGAIN?

Thanks everyone for your input so far. I'm going to get to the bottom of this thing -- I can't let it beat me!
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Old 20th March 2007, 01:31 AM   #8
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unclejed613: Yes, thank you, I have read this elsewhere and thus far have been testing this solid state amp with no load.

Another thing I just thought about: the basement I'm working in has pretty bad wiring and the circuits are nearly always maxed out (been on the landlord about this for months), could a sag created on the line when this monster is switched on cause some uncommonly high reverse voltage on these diodes at all? I fear my EE ignorance is showing through here, but could it be a cause?

Thanks all!
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Old 21st March 2007, 04:58 PM   #9
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The nice thing about those protection diodes is that they’re not necessary for bench testing the amp. What I mean it, they can be removed (just cut them out) and the amp can be powered up and tested. I would be pro-active and measure all other components after removing the diodes just to be sure nothing else went away. Then, once they’re out, everything measures good, test the amp. If all tests good without them, measure the diode locations to determine the correct polarity. Power down, install them, and test again. All should be good.
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Old 27th March 2007, 02:57 AM   #10
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Ok, so it turns out it wasn't the diodes that fried this time around - and I double-checked the orientation of the diodes, they are correctly installed now. That explains why those diodes kept popping, but it doesn't explain why Q14 smoked this time!

Q14 is an MPS-A56 PNP transistor. Given my track record with this project, the first thing I checked was that it was installed correctly, and it is. I did replace this device, as I did the rest of the silicon, just to be sure (maybe this was a bad idea?). So far, this is the only component that I've found that's failed.

Of note is the fact that the actual circuit differs from the schematics I posted previously in at least one spot that I've found while poking around Q14: the capacitor, C12 (.01 uF), as shown on the schematic is actually a resistor (250 ohm) in application. This is shown as a cap coming off the base of Q14. I did not replace this part, so this is factory configuration, AFAIK. I don't know if this is relevant, but thought I'd point it out.

I, of course, haven't the foggiest idea why this transistor, Q14, would have smoked. I'm starting to feel a little in over my head in diagnosing the problem here

Any ideas? Thanks all!
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