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Author Topic: Mike700 transistor and mje800 transistor  (Read 3443 times)

Green leaf

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Mike700 transistor and mje800 transistor
« on: December 14, 2020, 02:54:29 PM »
When I was a kid I made the stupid mistake of plugging my crate Gaza head out to the guitar input of my peavey studio pro and the crate stopped working. An amp tech fixed it and said the transistors had blown. Well weeks later the peavey stoped working with this loud buzz hum sound. Using it properly of course. Since then I’m 37 years old now and have learned a bit since then. So I have pulled my peavey out of the attic and am ready to repair it myself if possible. The hum is constant, so I shut it off and pulled speaker wires and measurements show 5volts dc to speaker. So I have pulled chassis and gave it a look and see the mje0800 and mje700 and according to the data sheet I believe it said the output should be 5volts dc but I am not sure how to test them. I do know that when I hit them with temp Gun they were very hot, and the sand block resistors got hot. I am changing the transistors and the four diodes that pass through the heatsink. How can I check the transistors if possible? And thanks for adding me on the network I am very happy to be a part of it. I would like to add some pictures to show the work area but am not sure how to add attachments.

Enzo

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Re: Mike700 transistor and mje800 transistor
« Reply #1 on: December 14, 2020, 09:04:18 PM »
There are some test voltages on the schematic, start with those.

Jazz P Bass

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Re: Mike700 transistor and mje800 transistor
« Reply #2 on: December 15, 2020, 12:52:31 AM »
What in the world is a 'Crate Gaza'?

Green leaf

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Re: Mike700 transistor and mje800 transistor
« Reply #3 on: December 15, 2020, 02:17:30 PM »
Sorry there were a lot of auto corrects I didn’t notice. Crate gfx 1200h head

Green leaf

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Re: Mike700 transistor and mje800 transistor
« Reply #4 on: December 16, 2020, 03:53:11 PM »
Also looking at the circuit I see the three op-amps I’m pretty sure payed the price of the signal the crate was shoving right through them with no mercy like the limits were a big deal right? Just a joke but I am just now seeing the ignorance of my Youth.

Jazz P Bass

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Re: Mike700 transistor and mje800 transistor
« Reply #5 on: December 16, 2020, 07:27:01 PM »
I cannot find a schematic for this model amp.
Do you have one?

Green leaf

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Re: Mike700 transistor and mje800 transistor
« Reply #6 on: December 16, 2020, 08:04:55 PM »
No but I believe peavey would send me one if I ask. It’s got 3 4558p op-amps in it which I find pretty cool.

Enzo

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Re: Mike700 transistor and mje800 transistor
« Reply #7 on: December 16, 2020, 09:13:25 PM »
He said it was the Studio Pro

Jazz P Bass

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Re: Mike700 transistor and mje800 transistor
« Reply #8 on: December 17, 2020, 12:40:24 AM »
Thank you, Enzo.

Green leaf

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Re: Mike700 transistor and mje800 transistor
« Reply #9 on: December 17, 2020, 09:09:44 AM »
That was awesome Enzo. I am used to dealing with dc voltage and schematics so forgive  my ignorance, and I am still learning. When it shows the 15v + and - for the op-amps I think that is ac. Is this the case seeing that is how it is labeled?
« Last Edit: December 17, 2020, 10:27:20 AM by Green leaf »

Green leaf

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Re: Mike700 transistor and mje800 transistor
« Reply #10 on: December 17, 2020, 10:29:18 AM »
Now actually I see it’s neither ac or dc labeled. I think the diodes indicate dc right?

Jazz P Bass

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Re: Mike700 transistor and mje800 transistor
« Reply #11 on: December 17, 2020, 11:34:21 AM »
Peavey schematic indicates Volts dc.

Green leaf

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Re: Mike700 transistor and mje800 transistor
« Reply #12 on: December 17, 2020, 04:46:02 PM »
Wow! Schematic proves helpful. Power supply is good, but for some reason the 15 votlts on the op-amp u1 are good but pin 7 out put is 77volts. I’m a little puzzled. Any ideas? I’m still checking.

Green leaf

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Re: Mike700 transistor and mje800 transistor
« Reply #13 on: December 17, 2020, 05:30:03 PM »
Ok I pulled all 3 op-amps and bread board tested each one separately with a 9v battery and guitar signal. Both Chanel’s all 3 they weren’t passing signal they just push full battery volts through. I checked on my o-scope to verify no signal was passing. At this point I am thinking it’s going to need a new set of op-amps. I’m going to replace with lm833. Anything you guys think I’m over looking?

Enzo

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Re: Mike700 transistor and mje800 transistor
« Reply #14 on: December 17, 2020, 11:48:26 PM »
Um... how did you test the op amps with a battery?

I don't know how you could get 77 volts on an IC, the highest voltage in the amp is 46v.


Ther op amps run on +15v and -15v, the 15v supplies are not AC.

Note the schematic voltages have polarity signs, and that means they are DC.  AC has no polarity