I have a bass head peavey mark VI XP with an internal compressor in the pre (not the DDT limiter). I got it second hand and the previous owner told me that something was wrong in the pre.
The problem is that when playing it produce some distortion and some peak get over driven.
Actually as a superficial solution I found that pulling away the IC C3080 of the compressor section, solve the problem: the amp sound as it should. but as consequence the compressor is unavailable.
Is there someone who had a similar problem?
At the same time I have a second minor problem: in the pre there is a chorus effect. Suddenly stopped to work the switch to activate it, I changed the two FET transistors connected to it and now it works only via foot switch: I mean I can activate/deactivate the effect only via foot switch, if I disconnect the foot-switch the effect is always on independently by the position of the switch on the amp.
The problem is that when playing it produce some distortion and some peak get over driven.
Actually as a superficial solution I found that pulling away the IC C3080 of the compressor section, solve the problem: the amp sound as it should. but as consequence the compressor is unavailable.
Is there someone who had a similar problem?
At the same time I have a second minor problem: in the pre there is a chorus effect. Suddenly stopped to work the switch to activate it, I changed the two FET transistors connected to it and now it works only via foot switch: I mean I can activate/deactivate the effect only via foot switch, if I disconnect the foot-switch the effect is always on independently by the position of the switch on the amp.
Oh, this is old. They haven't been making 3080 since the nineties. The last ones were from Harris and labeled 13080, but the originals were CA3080 from RCA (defunct). I think Peavey used them because they were the fastest amp IC available back then. Not pin compatible with anything modern.
Weird distortions are often caused in old equipment by dried up electrolytic capacitors. The rubber oxidizes, cracks, the water leaks out and the capacitance and ESR go way off. Any low power, or frequency effects, I just change out all the electrolytic caps in old gear before wasting too much time debugging what else is wrong. Other typical cause of symptoms is oxidized connector pins, particularly on pin and socket connections that are tin plated brass, or copper. Removing and replacing these often can cure a problem.
Change your caps one or two at a time between tests, amateur solderers often cause problems with their work. If you test after each one, and it got a new symptom, you know just where your problem would be, the last part you put in. Also, mark the board for plus as you pull the old ones out, if you get them backwards they blow up. I use a sharpie marker.
Basics- disconnect the power plug when working on it, discharge the big e-caps with a resistor before working on it. Measure any metal at less than 10 v to ground before touching it. Don't use two hands, measure with a alligator clip lead from the negative probe to ground. Wear no jewelry on hands or neck. Wear safety glasses when desoldering especially, solder can splash in the eye.
While the CA3080 is out, check, the jfet associated with it, probably j174 is my guess. Should be one diode drop (0.6 v) from the base lead to the two ends, 9999 or ----- if you measure backwards.
Don't lose your 3080, if you have to get another you'll probably have to order it from Peavey parts department. I bought some from Jameco six years ago, but they were old stock.
Weird distortions are often caused in old equipment by dried up electrolytic capacitors. The rubber oxidizes, cracks, the water leaks out and the capacitance and ESR go way off. Any low power, or frequency effects, I just change out all the electrolytic caps in old gear before wasting too much time debugging what else is wrong. Other typical cause of symptoms is oxidized connector pins, particularly on pin and socket connections that are tin plated brass, or copper. Removing and replacing these often can cure a problem.
Change your caps one or two at a time between tests, amateur solderers often cause problems with their work. If you test after each one, and it got a new symptom, you know just where your problem would be, the last part you put in. Also, mark the board for plus as you pull the old ones out, if you get them backwards they blow up. I use a sharpie marker.
Basics- disconnect the power plug when working on it, discharge the big e-caps with a resistor before working on it. Measure any metal at less than 10 v to ground before touching it. Don't use two hands, measure with a alligator clip lead from the negative probe to ground. Wear no jewelry on hands or neck. Wear safety glasses when desoldering especially, solder can splash in the eye.
While the CA3080 is out, check, the jfet associated with it, probably j174 is my guess. Should be one diode drop (0.6 v) from the base lead to the two ends, 9999 or ----- if you measure backwards.
Don't lose your 3080, if you have to get another you'll probably have to order it from Peavey parts department. I bought some from Jameco six years ago, but they were old stock.
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I already re-cap it completely, 'cause it had a leaking capacitor in the power amp section. so just to be on the safe side I replaced all the caps (electrolitics). the sockets are all new too.
I have some 3080 chip (CA3080) but if I swap the old one with one of these, the problem remains.
I have some 3080 chip (CA3080) but if I swap the old one with one of these, the problem remains.
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RCA3080 is similar to a uA741. A TL071 is ideal as a replacement if you do not connect pins 1 & 5 as they affect the DC offset.
In my experience, the 3080s work or they don't. Probably an offset issue or as Indianajo states, dried out and leaky electrolytics around that stage.
Check for 0volts at pin 6 of each one and if you use TL071, uA741 etc, do not connect pins 1 & 5.
In my experience, the 3080s work or they don't. Probably an offset issue or as Indianajo states, dried out and leaky electrolytics around that stage.
Check for 0volts at pin 6 of each one and if you use TL071, uA741 etc, do not connect pins 1 & 5.
I tested with an 741 but the result is a super distorted output even with the volume of the amp turned down to zero and the compressor not engaged. (of course pins 1 & 5 not connected)
with the CA3080 I have, it dist (even when comp not active) only in the peaks or it start to produce more distortion when I increase the compression ratio.
with the CA3080 I have, it dist (even when comp not active) only in the peaks or it start to produce more distortion when I increase the compression ratio.
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Surely my analysis was wrong from the beginning:
now I tested leaving the CA3080 on place and taking away two of the OP amp (4558) in the comp. circuit, and the result is the same as I should take away the CA3080: it sounds clean as all works right.
So the problem is another, considering that the OP amp are new and the elecrolitics are new.
now I tested leaving the CA3080 on place and taking away two of the OP amp (4558) in the comp. circuit, and the result is the same as I should take away the CA3080: it sounds clean as all works right.
So the problem is another, considering that the OP amp are new and the elecrolitics are new.
The '3080 is NOT any sort of '741 opamp. Nor spectacularly "fast".
The '3080 is designed to be used as a Variable Gain Amplifier. Work it open-loop. Current into the Iabc pin raises the gain. This is an obviously-necessary part of a Limiter.
A batch of '3080 were baked 2 years ago at Rochester Electronics. The retail source is Small Bear Electronics. Many-many-many other sources turn out to be fakes; Steve knows where his '3080s came from, he and his large customers batch-test them.
Schematics and voltages would help a LOT.
The '3080 is designed to be used as a Variable Gain Amplifier. Work it open-loop. Current into the Iabc pin raises the gain. This is an obviously-necessary part of a Limiter.
A batch of '3080 were baked 2 years ago at Rochester Electronics. The retail source is Small Bear Electronics. Many-many-many other sources turn out to be fakes; Steve knows where his '3080s came from, he and his large customers batch-test them.
Schematics and voltages would help a LOT.
1988 Peavey Mark VI Series 402 Bass Amp Schematics
This is for a Mark VI. It has a '3080 (page 4, U2, Peavey-special # 87478) as a power-amp overload controller, NOT in preamp as OP says.
This preamp page does not(?) show any '3080 or related circuit. It does have VCA in the Chorus, but the NE572 telephone compander not a '3080 in this plan. This does however show the foot-switching which was also remarked.
This is for a Mark VI. It has a '3080 (page 4, U2, Peavey-special # 87478) as a power-amp overload controller, NOT in preamp as OP says.
This preamp page does not(?) show any '3080 or related circuit. It does have VCA in the Chorus, but the NE572 telephone compander not a '3080 in this plan. This does however show the foot-switching which was also remarked.
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