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.
There are some test voltages on the schematic, start with those.
What in the world is a 'Crate Gaza'?
Sorry there were a lot of auto corrects I didn't notice. Crate gfx 1200h head
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.
I cannot find a schematic for this model amp.
Do you have one?
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.
He said it was the Studio Pro
Thank you, Enzo.
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?
Now actually I see it's neither ac or dc labeled. I think the diodes indicate dc right?
Peavey schematic indicates Volts dc.
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.
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?
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
Ok. With my negative lead on chassis ground I take positive lead and check and all the voltage readings say positive. I think my prospective from working on auto and truck dc is the problem because your dc comes from a battery there and not a rectifier bridge. How would I see a negative on one side? Or dose this mean I have some bad diodes? And we're should I put my negative lead?
Leave the Black Lead attached to chassis when taking voltage measurements.
The negative voltage can be read at R 63. (Red Lead)
The rectifier side should read -46Vdc and the other side has a zener diode (CR30) attached which will drop the voltage to -15Vdc.
Ok thanks Jazz! The voltage is reading properly correct polarity where it should be and the 77volts I claimed on out put of the op-amp were actually my. It%u2019s actually reading 205milly volts at pin 7. I%u2019m about to finish replacing the diodes and the two transistors I was talking about and then I will update on what happens after that. (Je800 and je700 with the four diodes around them)
Ok hum on speaker is not so loud but is still present. The speaker pulls in when kicked on. I'm not getting all the heat at the transistors or at r58 and r57.
Ok I have 46v+ between r49 and r47 but 37v+ between r47 and c25. It should be 23v positive. Dose this indicate c25 being a bad? Or were would I want to go with that? Thanks for all the help so far I'm having fun learning.
Ok this was fun looking at schematic and figuring out. You all probably all know but don't replace a mje800 with a mje700. When I followed the schematic it explains why I didn't get the 24volts.
Omg so finally got my transistors from China and soldered the correct pair in. Now I have signal coming out but it is not very loud and gets quieter longer it's on. I noticed one capacitor getting just a little warm. Thanks for all the in put. I am very happy to have gotten this far with everyone helping. What should I check now that I have signal coming out? Thanks to everyone in advance.