Hi everyone, I have a Peavey Mark VI bass amplifier head with a 402 module on the repair bench. When I first got it it was blowing both F1 and F2, 1 Amp slow blow, after about 5 seconds. I found 15 v zeners cr101 and cr124 both shorted, cr123 1N4003 shorted and Q105 a J174 fet shorted as well. After replacement I can trace the signal from J100 pin 1 through C102, to R102 but then it dies out after it exits R102. Interestingly both R100 and R147, which are 2.7K at 1 watt, get hot. Also, as per help on this forum, I removed CR115 and CR116 in case they were shorted. Also the other transistors test good in circuit. I don't understand the circuit well enough to proceed without help. It shouldn't be that hard but I'm stumped. Thank you.
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R100 and R147 get hot normally, they are dropping resistors from the main power rails down to the 15v zeners. I forget what the main Vrails are, probably 55-60v. So with say 55v dropping to 15v, there is 40v across the 2.7k. That is about 6/10 of a watt, which is plenty to get them warm.
If Cr115 triac is not shorted, you can leave it, but now that it is out, that's OK too. You can measure it where it sits for shortedness.
OK, so you are tracing signal, may I assume it no longer blows fuses, and all voltages are present and more or less what they ought to be? I am assuming so for now.
Often you cannot measure signal right at the input of an op amp. Look up virtual ground for that. If it is present at the output, we can assume it got to the input. And to check the input circuits, pull the IC and probe the empty socket (or pads).
But the circuit offers some clues anyway. JFET Q105 is a power up mute, it prevents thumps from being amplified at turn-on. Note that regardless of how it is drawn, when it conducts it connects pin 5 at the input of U1b to ground, see? JFETs by their nature are ON until turned OFF by a voltage at their gate. Here, +16v is applied to its gate through R107. But before it can turn off the JFET, it has to charge up C105. That just takes a moment, but long enough to get past a thump. If you measure source to drain with an ohm meter, you will get some low figure like maybe 50-200 ohms. When the gate is turned on the resistance goes way higher.
If Q105 shorts, then the input is grounded off. If Q105 is OK, but gets no voltage at its gate, it is still ON and grounds off the signal. Yours had blown up, so it is possible the gate drive is funny. The amp will work fine without it, though may thump at turn on. You can remove it during testing. So is there a positive voltage on the gate leg? Or a different test, power the amp, put your meter on ohms and measure S-D resistance. if it is low, then the part is bad or its gate is not turned on. If it is hard to get at the legs, remember source to drain is the same as ground to pin 5 of the op amp.
But also connected to the pin 5 input is the output of U2, the 87478 IC. That is the DDT chip and is a limiter of peaks. I THINK we are fortunate enough to have sockets under the ICs, and you can pull that IC. The amp will work fine, but no longer have the DDT function. I do that during repair just to eliminate it. Whatever blew up Q105, was of course also connecting to U2 and may have damaged it. And while we are at it, if there is 15v on pin 5, then one of those diodes are shorted, CR102,103.
That ought to get you started.
If Cr115 triac is not shorted, you can leave it, but now that it is out, that's OK too. You can measure it where it sits for shortedness.
OK, so you are tracing signal, may I assume it no longer blows fuses, and all voltages are present and more or less what they ought to be? I am assuming so for now.
Often you cannot measure signal right at the input of an op amp. Look up virtual ground for that. If it is present at the output, we can assume it got to the input. And to check the input circuits, pull the IC and probe the empty socket (or pads).
But the circuit offers some clues anyway. JFET Q105 is a power up mute, it prevents thumps from being amplified at turn-on. Note that regardless of how it is drawn, when it conducts it connects pin 5 at the input of U1b to ground, see? JFETs by their nature are ON until turned OFF by a voltage at their gate. Here, +16v is applied to its gate through R107. But before it can turn off the JFET, it has to charge up C105. That just takes a moment, but long enough to get past a thump. If you measure source to drain with an ohm meter, you will get some low figure like maybe 50-200 ohms. When the gate is turned on the resistance goes way higher.
If Q105 shorts, then the input is grounded off. If Q105 is OK, but gets no voltage at its gate, it is still ON and grounds off the signal. Yours had blown up, so it is possible the gate drive is funny. The amp will work fine without it, though may thump at turn on. You can remove it during testing. So is there a positive voltage on the gate leg? Or a different test, power the amp, put your meter on ohms and measure S-D resistance. if it is low, then the part is bad or its gate is not turned on. If it is hard to get at the legs, remember source to drain is the same as ground to pin 5 of the op amp.
But also connected to the pin 5 input is the output of U2, the 87478 IC. That is the DDT chip and is a limiter of peaks. I THINK we are fortunate enough to have sockets under the ICs, and you can pull that IC. The amp will work fine, but no longer have the DDT function. I do that during repair just to eliminate it. Whatever blew up Q105, was of course also connecting to U2 and may have damaged it. And while we are at it, if there is 15v on pin 5, then one of those diodes are shorted, CR102,103.
That ought to get you started.
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Thank you very much, your explanation made the jfet circuit understandable. It's interesting how it is designed. I removed it for simplicity of further testing along with U2, the "limiter" chip. Should have mentioned all the voltages are correct. Also, I'm probing the op amps with a scope probe which should be high enough impedance not to load the circuit, at least I thought. Signal at Pin 5 and 6 of U1 is ok. Signal at pin 1 (output) is not good, lower amplitude and riding up on about .6v dc for some reason. Have rechecked diodes and transistors with none bad found so far. I'll keep chipping away at it. Thanks again.
The op amp input thing is not about loading really. If the IC is in a socket, power down and remove it, then feed the input signal into the jack and see what ends up on pin 5. The amp need not be powered for this, since it is just a couple resistors and caps. Then put a known good IC in the socket, power up and try it again.
Just because the transistors and diodes test OK does not mean there is not an open resistor or cracked trace somewhere. If by all the voltages are correct, you mean the supplies, we also need to know it is distributed. So are there 16v levels on pins 4 and 8 of that IC? And you have 0.6v on the output pin, but is there any DC offset on an input pin, regardless of whether signal is good there?
And have you substituted another 4558 in place of U1? If not, why not?
By voltages OK, are both main rails at the same voltage to ground, other than polarity?
What signal, if any, makes it to the speaker terminals?
Just because the transistors and diodes test OK does not mean there is not an open resistor or cracked trace somewhere. If by all the voltages are correct, you mean the supplies, we also need to know it is distributed. So are there 16v levels on pins 4 and 8 of that IC? And you have 0.6v on the output pin, but is there any DC offset on an input pin, regardless of whether signal is good there?
And have you substituted another 4558 in place of U1? If not, why not?
By voltages OK, are both main rails at the same voltage to ground, other than polarity?
What signal, if any, makes it to the speaker terminals?
OK, after removing U1 from it's socket and injecting a signal at the input I have good signal to pin 5. When powered, the signal output at pin 1 of U1 is bad. Got caught only checking both 16v supplies but not to the chips themselves however proper voltage ultimately verified on all 3 chips. No offset at input. Should have mentioned I did swap in good 4558's for testing with no change. Also, measured 55.3 and -55.4 voltages on the rails with negligible ripple. No signal passes to the speaker output. My schematic shows a CR106 but I don't have that part on my board and I don't understand how the circuit distributes both halves of the waveform without it. I'm starting to suspect a bad CR112, a dual diode.
CR106,109,112 are the bias string for this amp. If one is open, then the bases of Q100 and Q112 get pulled towards their respective power rails, turning them on hard, which in turn turns on the outputs hard, and when opposing outputs are both on, that blows fuses.
I have no reason to suspect your dual diode.
CR106 is right by Q100, there are three or four little diodes in a row in front of it. Are you saying there is an empty place where it ought to be? No wire jumper?
The bias is not adjustable, so if at some point this amp was running hot, one simple fix is to short across that diode. That cools the bias by one diode drop.
One test you could make: Use your diode test, power off, check from the base of Q100 to pin 1 of U1. You should get something less than two diode drops. (the parallel resistor to CR109 will make it read lower drop) By doing that we verify the circuit is there whether we see the parts or not. Likewise, from the pin1 to the base of Q112, we should read the dual drop of CR112. It is more valuable to test it this way than just going right to each part, because while we are testing the part, we are also testing the connections and board traces.
I have no reason to suspect your dual diode.
CR106 is right by Q100, there are three or four little diodes in a row in front of it. Are you saying there is an empty place where it ought to be? No wire jumper?
The bias is not adjustable, so if at some point this amp was running hot, one simple fix is to short across that diode. That cools the bias by one diode drop.
One test you could make: Use your diode test, power off, check from the base of Q100 to pin 1 of U1. You should get something less than two diode drops. (the parallel resistor to CR109 will make it read lower drop) By doing that we verify the circuit is there whether we see the parts or not. Likewise, from the pin1 to the base of Q112, we should read the dual drop of CR112. It is more valuable to test it this way than just going right to each part, because while we are testing the part, we are also testing the connections and board traces.
I was mistaken about CR106. I have noticed several discrepancies between the parts layout drawing and the parts actually on the board. Peavey has been pretty good in my limited experience but other manufacturers I have dealt with more are notorious for mistakes and omissions on docs. It goes with the territory. I measured .58 from Q100 base to U1 pin 1 and 1.1 from Q112 base to U1 pin 1 which both seem good. Seems likely one or both of these transistors are bad although seem ok in circuit.
repair in progress
I've got some exciting progress to report... daylight at last. Q100 was bad, conducting in both directions and was replaced. I have some sound at output although distorted. U1 pin 3 signal is abnormal and U1 pin 1 appears as a square wave. Also, I have to tattle on myself because I had the wrong connector attached to the speaker output previously so probably had something at output and didn't know it. Now I'm focusing on the U3 chain which I think provides feedback to the preamp board.
I've got some exciting progress to report... daylight at last. Q100 was bad, conducting in both directions and was replaced. I have some sound at output although distorted. U1 pin 3 signal is abnormal and U1 pin 1 appears as a square wave. Also, I have to tattle on myself because I had the wrong connector attached to the speaker output previously so probably had something at output and didn't know it. Now I'm focusing on the U3 chain which I think provides feedback to the preamp board.
Feedback? Are you sure that IC path doesn't just drive the clip LED? I am not near my schematic of the 402.
You are right, that connector is dedicated to the switch and clip led for the compression... that's it. Still can't see any principled reason for a bad waveform at U1 pin 3 except that it ties indirectly to chassis ground which would open up numerous possibilities such as maybe Q106 or Q107 to start?
U1-12 drives your output, U1-3 is feedback from the output. If U1 doesn't see good signal from the output it will try to "correct" that by increasing the signal. I'd wager that is why it is driven to square waves. You still have no signal to the speaker?
With huge square waves at U1-1 we might expect huge square waves at the speaker. So why are they not there? Are you working with a load? If so, what does the output look like with no load? And power down and measure resistance across the speaker/load. Do you measure the resistance of the load or is it shorted. If shorted, that triac CR115 is probably short. Happens when outputs blow.
What does the distortion of the output look like? heavy clip? One side clip? Half waveformn missing? other?
You have square waves at U1-1, are they more or less rail to rail (15v rails, so 30v p-p)? Or are they much smaller? You replaced Q100? So is that square wave at its base? And is it also at its emitter (the output base)? and check the neg side too.
With huge square waves at U1-1 we might expect huge square waves at the speaker. So why are they not there? Are you working with a load? If so, what does the output look like with no load? And power down and measure resistance across the speaker/load. Do you measure the resistance of the load or is it shorted. If shorted, that triac CR115 is probably short. Happens when outputs blow.
What does the distortion of the output look like? heavy clip? One side clip? Half waveformn missing? other?
You have square waves at U1-1, are they more or less rail to rail (15v rails, so 30v p-p)? Or are they much smaller? You replaced Q100? So is that square wave at its base? And is it also at its emitter (the output base)? and check the neg side too.
Issue with che C8030 in the compressor circuit.
Have someone had a general issue caused by the compressor?
I mean: playing with this head, I used to get some distortion, or really some extreme peak.. I didn't found the cause but I solved removing che C8030 from the compressor circuit. Now it sounds perfectly clean, but, of course, the compressor is no longer usable.
Have someone had a general issue caused by the compressor?
I mean: playing with this head, I used to get some distortion, or really some extreme peak.. I didn't found the cause but I solved removing che C8030 from the compressor circuit. Now it sounds perfectly clean, but, of course, the compressor is no longer usable.
Scope the amp with the chip in place, see if your distortion coincides with clipped peaks.
The Peavey compressor is just a peak limiter. Pretty much it only affects the sound when you would be clipping. Instead of the raspy clip, it just keeps the level even.
The Peavey compressor is just a peak limiter. Pretty much it only affects the sound when you would be clipping. Instead of the raspy clip, it just keeps the level even.
You should replace the '3080. The problem is there are many-many more fake '3080s than genuine working '3080.
In the US, Small Bear Electronics has made special effort to source good '3080, even working with Rochester Electronics to run a wafer of brand-new ones. He will ship to Germany but the postage and customs and delay may be brutal.
In the US, Small Bear Electronics has made special effort to source good '3080, even working with Rochester Electronics to run a wafer of brand-new ones. He will ship to Germany but the postage and customs and delay may be brutal.
I will check with the oscilloscope even if I can say that it already sounds as you say. Anyway my problem is that it sound bad in both the cases the compressor is turned on or off.
U2 is the compressor chip, but it is not required for the amp to work. SO if turning the thing off makes no difference, remove it from the board. Now see if the amp acts funny on peaks. But be careful to keep it below clipping for this. We could be chasing some other problem.
According to the scheme I have here it is the U109.
I don't had time yet to test with scope.. but I will let you know asap.
for the moment thanks for you help
I am looking on the power amp board.
More specifically, I am looking at Peavey Mk VI Bass with 402 amp. I don't see any 100 series ICs on the whole thing. What drawing are you working from?
More specifically, I am looking at Peavey Mk VI Bass with 402 amp. I don't see any 100 series ICs on the whole thing. What drawing are you working from?
it's Peavey Mk VI Bass XP on the preamp board sorry I didn't specify before. And what I mean with compressor is not the "DDT" is the compressor in the preamp section.
.... and probably I'm going out of topic 'cause this thread is about the power amp/power supply module 402
.... and probably I'm going out of topic 'cause this thread is about the power amp/power supply module 402
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