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Messages - DrGonz78

#1
Q11 & Q12 caught my eye too but understanding each transistors role in a circuit is difficult. I don't know what Q11 & Q12 actually do but it never stops me from guessing. Are they current limiting the circuit in case of overload? Not sure if they are pre drivers for the outputs or what. I'm curious to know how bad of a guesser I am.  :o
#2
The 30 vac refers to the AC voltage but after rectification to DC voltage it will be higher. 30vac * 1.414 = 42.42vdc.
#3
There are amps out there like Mesa boogie simulclass where they have a pair of each EL34's & 6l6's. But these are pairs of tubes and the amp is designed to allow this arrangement. No amp that I know of uses one of each different tube as a pair. It just doesn't work in a balanced way. People spend all day trying to create a matched pair of tubes for their amp circuits. EL34 tubes require a power transformer that can handle the current demand which is more than a 6L6 demands on a circuit.
#4
Just a quick reply but perhaps study up on crossover distortion in a solid state amp. The symptoms you describe are similar. Play soft and it sounds fine. A hard attack signal will reveal the crossover distortion. It might be biased cold but that might not be what is going on. Just something to consider.
#5
Quote from: mr.death on May 22, 2024, 04:11:09 PM
Quote from: DrGonz78 on May 19, 2024, 12:19:47 AMWhat method did you use to make the conclusion that the transformer is shorted?
I watched a dlab vid. He shows how to check the transformer. I did his test and it failed. Also when I turned the amp on I could hear the transformer frying, bubling sound. Its toast.

Okay just had to ask or we might have let someone trash a power transformer. I like D-Lab a lot and he is a solid provider of good services. I never heard a bad transformer make bubbling noises. I mean I have heard a buzzing sound from old worn out transformers. But most bad transformers and we're talking fuses blowing when turned on.
#6
I mean from the description of what caused a failure of the amp we have little to go on. What is the status of the main transistors? We have no idea what traces you bridged. With so little information I personally can't say go buy a transformer. With more information then I might have more of an answer.
#7
What method did you use to make the conclusion that the transformer is shorted?
#8
List voltage reading for Q8 & Q9 at  all pin's connection points. Measure all power supplies. What are your -/+40v rails? -/+12v rails? Can you scope a signal through the unit and figure out where the signal goes bezerk?

Edit: If you scope the output is the fuzz actually crossover distortion?
#9
Tubes and Hybrids / Re: Mesa Dual Rectifier
May 14, 2024, 06:26:53 PM
If the rectifier's are not in circuit then there would be no B+ vdc. So I can't immediately imagine the caps are the issue from that standpoint. Of course posting a schematic would be a good thing. It seems the transformer may have other windings -/+15v rails and your heater's winding. Try disconnecting all secondary connections when in doubt. Light bulb shining with secondary taps disconnected from circuit is not good. Right now from what you have written we need to clarify what you are describing and a schematic. We need voltage readings if the light bulb is not shining bright.
#10
I wouldn't use arctic silver for anything other than CPU's on a computer build. It says it has trace amounts of silver in it.
#11
My advice with Peavey designs that failed is to check every single little thing that resembles a diode. Those transistors count hugely but don't forget the 1N4148 diodes on the bias string for example. I am pretty compulsive with such a repair but I had issue with a peavey just like that. Thought I had checked everything but I missed one.
#12
I am still curious, like Enzo was, about what make and model amp we are looking at. Perhaps there is a schematic to look at to figure out what could have burnt up that resistor in the first place.
#13
What are the symptoms with the amp? A roadmap helps repair an amp with a major fault but sometimes we get by without the schematic. Only thing I could find Century related was a 200ii preamp schematic.
#14
I am not saying the 1458 is healthy by any means. I would remove that 1458 opamp and then measure for voltages at the pads for pins 4 & 8. If the opamp was to blame for missing voltages then with it removed you would still see voltage present at those points in the circuit. If the voltages are missing then we must figure out why.
#15
Quote from: pinkjimiphoton on August 31, 2021, 01:02:18 PM
since i can't seem to get a reading for any voltages at pins 4 and 8, and no output after the 1458, should i assume the opamp is toast? a shorted opamp would make it so i get no voltage readings anywhere, wouldn't it?

Actually I would think just the opposite and if the opamp had shorted internally then it would lead to unexpected voltages present on many legs of the IC. I would think there is actually an open condition that exists in your circumstance. Meaning the missing bi-polar power supply that feeds pins 4 and 8 points to a power supply issue.

P.S. Why not start a fresh thread for this repair? It will make the repair easier to follow along as well.