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Thread: Ampeg G110 Hum / no signal

  1. #1
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    Ampeg G110 Hum / no signal

    Hey Everyone-

    Got a Ampeg g110 on the bench, does not pass signal but it turns on and produces a hum. The hum is controllable via the volume pot, so I checked the circuit before with a signal scope (poor man's Oscilliscope). Signal is lost at IC1, a vintage looking 8-pin circular piece. Voltage at pins 2 & 3 are 26v instead of 13.5v I'm guessing this is the issue because reverb works fine, treble and bass controls change the tone as well.

    HOWEEEEVER the rail voltage is off- Coming from A is listed as "48v" but I'm getting "-45v" on a limiter bulb. That might be a typo, but after R44 dropping resistor the voltage only drops to -36v instead of "24v". R44, R43, and R52 all check out.

    Any guesses here? Thanks in advance.

    Attached a schematic.
    Attached Thumbnails Attached Thumbnails ampeg-g110.pdf  

  2. #2
    Supporting Member The Dude's Avatar
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    You say, "Signal is lost at IC1". Is it lost at pin 3 input or pin 6 output?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    It is very faint at pin 3, put pin 6 is just a hum. Shot IC? Which begs the question where can I find a replacement? It's not the usual black rectangle, but a metallic, 8 legged, raised cylinder. Printed on the top is ca3060 RCA 723, but the '6' is scratched and may be an '8' or an 'S

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    Supporting Member The Dude's Avatar
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    I might try injecting a signal at pin 6 (cap isolated) first to verify a bad IC.

    Edit: Verify first that pin 6 is not shorted to ground. If it is, injecting signal there will prove nothing.
    Last edited by The Dude; 03-03-2017 at 11:53 PM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Thanks Dude, gunna check this on Tuesday and let you know.

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    Supporting Member The Dude's Avatar
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    If the IC is bad, looking at the datasheet and comparing to the schematic, I believe it's a CA3080 and I believe this will work providing lead spacing isn't an issue. It's looks to be a different case style, but the same part.

    CA3080EZ Original New Intel Integrated Circuit | eBay

    If you want to spend a bit more and get the same case style:

    CA3080A Op Transconductance Amps OTA IC CHIP | eBay
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  7. #7
    Senior Member Enzo's Avatar
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    CA3080 was not uncommon. The old metal ones had the same leg count, same pin assignments, just in a circle instead of two rows. A tiny piece of perf board and some bare wires and you can make one up from a DIP.
    Education is what you're left with after you have forgotten what you have learned.

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    For me the usual tell tale sign of a 3080 is the control pin 5, which in this case is used to create the tremolo effect.

    Based on the schematic, there is no negative voltage source in this amp. It is a single sided power supply with all of the op amps biased to 1/2 the supply voltage. So I can't see where a negative voltage reading would come from.
    g1 and The Dude like this.

  9. #9
    Supporting Member Jazz P Bass's Avatar
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    I took notice in Post #1 that you have the amp on a limiter.

    If the amp itself is stable, please remove the limiter.

    As to 52 Bill's post, pin #5 turns on the internal amplifier of the CA3080.
    So you will need at least +0.7 Vdc on the pin for the IC to function.
    What voltage do you measure at pin #5?

    ca3080-a.pdf

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    Thank you everyone, let me answer all your questions in one post-


    Dude: Injecting a signal at pin 6 does indeed produce a signal, so we know that the issue is either the IC or it's control circuit, yes?

    Enzo- good tip! Probably going to replace with same style casing but I will definitely use that hack in the future.

    52 Bill / Jazz P Bass: Voltage at Pin 5 is just a hair under .7 at .670


    Since a few of these voltages are off compared to the schematic, I've included all the measurements of IC 1.

    1: hovers around .0 mv
    2: 23 v
    3: 25.7 v
    4: 0.1 mv
    5: 0.67 mv
    6. 24 V
    7: 37V
    8: 0.02 MV

    With these measurements and with the injected signal success, can we effectively diagnose this IC as defective?

    Thanks as always for your wisdom.

  11. #11
    Supporting Member The Dude's Avatar
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    Some of those voltages are a bit off, but I suspect it's because the IC is bad. I'd say time to replace the IC.
    Last edited by The Dude; 03-07-2017 at 11:31 PM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  12. #12
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    Back in the days when the metal can package was fairly common, we would use cheap wire wrap sockets to convert the circular patterned pc boards over to the DIP packages. The wire wrap sockets had long legs that you could bent enough to fit them into the pc boards. But a lot of the pc boards already have the typical DIP pattern traces and have the circular leads bent to match that.

    Back to the problem, if you fiddle with the tremolo controls, does the voltage at pin 5 fluctuate?

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    Old Timer J M Fahey's Avatar
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    At mere $5 a pop, I´d just get the proper metal can one and call it a day.

    FWIW my first Op Amp based preamps (early 70`s) were built on eyeletted boards simply because I had been building Fender clones on same kind of boards, was properly equipped, experienced and very comfortable around that construction technology.

    No big deal on transistor amps where you just spread legs a little more than usual (eyelets can only be *that* small and so *that* close and not less) but impossible on DIP ICs.
    But I standardized on 8 pin round metallic 741s, then the standard workhorse.
    Last edited by J M Fahey; 03-10-2017 at 01:57 AM.
    Juan Manuel Fahey

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    OK so the new chip is in and it works.... kind of..... not really.

    Amp produces a sound, but there is signal being passed even with the volume at 0, but it is distorted and about the level of a bedroom practice session. volume control does work turning it up. The tremolo is always on even when the intensity is set to 0. The speed control affects it. No signal on pin 3 or 6 now. Injected signal at 6 produces working amplifier.

    Voltage:

    Pin 1. 1.8mv
    2. 31v
    3. 3.4v
    4. 0mv
    5. 10.8v
    6.1.2v
    7. 40v
    8. 1mv

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    Bump! Anyone have an idea what this issue might be? I lose signal right after the .022 from buffer stage, replaced it with a known good one and still no dice... poked around for a short to ground from around there too but not finding much....

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    When you fix an amp it's good to understand its schematic. And understand how the amp works. In your case you need to understand at least the power supply in this amp:
    - the power amp is supplied with +48V,
    - then you have R44 resistor and D2 diode (this is most probably 27V Zener diode) - they provide voltage for the preamp,
    - +27V supplies IC1 and IC3,
    - R20 and R14 resistors form a "virtual ground" for IC1 and IC3 (they call it "COMMON BIAS LINE" on the schematic) and this virtual ground voltage is 13,5V (half of 27V),
    - on inputs 2 and 3 of the IC1 you should have 13,5V (on the output - pin 6 there should be also 13,5V),
    - pins 1 and 8 are not used in this amp so there is no point listing them.

    As you can easily see, almost all voltages on IC1 are incorrect - starting from +27V power supply. It means that, most probably, there is a problem with D2 Zener diode. Also there is a problem with "virtual ground" divider - R20 and R14 resistors.
    This is how I see it - I assume that the measurements that you made are correct.
    Please fix the problems with DC voltages and then you may start looking for the signal. Check D2 diode and R14, R20 resistors.

    Mark

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    Mark, thanks for taking the time to help me understand. I'm self taught so I have a lot of gaps in my education. I really appreciate this forum because I have no other direct human resources.

    -I see the 48v, I had tracked it across the R44 beforehand and watched it drop only to 40v instead of 27v. So this means somehow the common bias line, which is a "virtual ground circuit" is not functioning properly (or something downstream...) R44 measures healthily out of circuit, so the draw must be somewhere else??

    -Both of the virtual dividers (R14 / R20) and the Zener measure fine out of circuit, but we can assume that the issue is in the power supply. Would you say the next step is to work backwards from the virtual ground circuit and see if I can find something amiss?? Perhaps c15, or some other reference to ground after R44 in the supply?

    If I understand correctly, our goal here is to drop the B+ to 27 for the common bias line and then divide that properly to sueach IC.

  18. #18
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    No. The 27V should be at one side of R44. Once you get that right, the common bias line should be correct (13.5V).
    If D2 is installed with correct polarity and it's connections are good, it must be bad, as you say the 27V line is 40V if I'm understanding correctly. Anode side of D2 should go to ground.
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  19. #19
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    The issue with the Zener diode is really simple. Either it's bad (open) - needs to be replaced, or its connections (e.g. to the ground) is broken - fix connections.
    Another possible problem is that the integrated circuits could have failed because they were getting 40V power supply. I would remove both ICs and fix the Zener diode and virtual ground. Only, when you have 27V and 13,5V you can put back the ICs into the circuit (assuming they are OK).

    Mark

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    Senior Member Enzo's Avatar
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    "zener measures fine out of circuit" Well, just how did you measure it? If it measures like a simple diode, at least it isn;t shorted, but a zener is used in breakdown mode with reverse voltage across it, no way to test that with your hand meter, unless you create additional circuits for it to put voltage across it. A 15v zener might check OK as a diode, but in a circuit, it might not "zene" at all or might zene at 6 volts.
    Education is what you're left with after you have forgotten what you have learned.

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    Hey G1- thanks for the input. The diode in question is a IN4148, polarity correct. It measured good out of circuit with a diode check function on the multi meter. Replaced with a new one just to be sure since that is the consensus on here, but same results.

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    Mark! great advice about the IC's. I will do this presently. The Zener is a second brand new one in case of the first being defective, connection to ground at anode is 0.0.

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    Thanks for pointing this out Enzo! I will do some research on reverse voltage, I've only heard a little bit about it. I replaced the new zener with another new zener just to be sure as I've got abtu 100 IN4818s lying around.

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    The diode in question (D2) is supposed to be a 27V zener, not a 1N4148.
    Certified Dotard

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    Senior Member Enzo's Avatar
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    This^^^

    1N4148 is not a zener, and will do nothing to regulate voltage down.

    1N4148 is a very common part, but it is only a plain old signal diode.
    Education is what you're left with after you have forgotten what you have learned.

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    Wow. I'd like to take this time to apologize to my friends, my family, and my lovely wife for the pain I have caused them by being an idiot.

    So I realize now how little I understood Zeners. I put two 15v zeners in series to get it ballpark enough to test- Power supply sitting at a 30v until I can get a 27v Zener in the mail. Beginning to understand why they are instrumental in supply voltage.

    Amp sounds pretty good, now the only remaining issues are...

    A: The volume pot at 0 still produces a good amount of sound. Gets very loud very quickly, but sounds amazing at breakup levels. Check around the volume pot, yes?

    B: Reverb does not work. Pot produces hiss when turned up, you can hear the springs when tapping on the can.

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    Quote Originally Posted by Mr_bibbles View Post
    Wow. I'd like to take this time to apologize to my friends, my family, and my lovely wife for the pain I have caused them by being an idiot.

    So I realize now how little I understood Zeners. I put two 15v zeners in series to get it ballpark enough to test- Power supply sitting at a 30v until I can get a 27v Zener in the mail. Beginning to understand why they are instrumental in supply voltage.

    Amp sounds pretty good, now the only remaining issues are...

    A: The volume pot at 0 still produces a good amount of sound. Gets very loud very quickly, but sounds amazing at breakup levels. Check around the volume pot, yes?

    B: Reverb does not work. Pot produces hiss when turned up, you can hear the springs when tapping on the can.
    Any chance reversing reverb cables works?

    nosaj

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    reversing the cables does not work, Gunna start with the volume issues.

    Replaced the pot with a 250 just to eliminate that from the suspects. removed and check the variable resistor, measures at 1.3m out of circuit. It looks like the volume control here is not just your everyday voltage divider.... maybe a feedback controller for the IC's?? I'm really at a loss for most SS devices...

    Again the symptoms are a huge jump in volume from 0-1, not in the way a linear pot behaves but in a "bedroom quiet signal at 0 ---> LOUUUD ASS SIGNAL at 1"

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    Quote Originally Posted by Mr_bibbles View Post
    measures at 1.3m out of circuit. It looks like the volume control here is not just your everyday voltage divider....
    The schematic show a gain control that is a 1M5 pot and a volume control that is a 200K pot. Which one are you talking about?

    Have you tested the inside tank wiring? If the tank makes noise through the speaker when you rattle the tank, then you know that the reverb tank output is okay and the amp's recovery circuit is working. If the tank's input transducer is open circuit or the driver chip is bad, then the reverb will work the way you describe.

    Read the resistance across the tank's input jack.

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    The "gain control" mini-variable resistor measures 1.3m out of circuit, but while measuring the the voltage on either side of it it stays constant 16v across the entire range of the resistor. Am I correct in thinking that this controls functions of the reverb as well?

  31. #31
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    Quote Originally Posted by Mr_bibbles View Post
    The "gain control" mini-variable resistor measures 1.3m out of circuit, but while measuring the the voltage on either side of it it stays constant 16v across the entire range of the resistor. Am I correct in thinking that this controls functions of the reverb as well?
    The reverb drive signal is sent from the emitter of Q2, so yes the gain control will affect the drive signal going to the tank.

    The voltage on either side of the gain control will not change that much as it is being fed voltage from the output of the first chip as well as the bias supply line. The volume control seems to be wired as part of the feedback loop for the first IC, so the gain control fine tunes the maximum resistance of the volume pot which in turn sets the maximum gain of the stage.

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