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Ampeg VT-120 Restoration / Repair - hum on channel C

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  • g1
    replied
    Jfets are 'on' til you turn them off. So a low resistance reading from source to drain unpowered is normal.
    I think John just made a typo.

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  • q9522678
    replied
    JFETS are used as switches in this application and are either on or off. If you use your DMM on Diode test, when powered off they should read open circuit Source to Drain. Don't let them confuse you!
    John - I have the board out again at the moment and though I would test the JFETs (J176s) in the circuit just out of curiosity. I have my DMM set to diode test and am placing probes Source to drain, but none of the JFETs on the board are reading open circuit. Most read 0.05v.. I’m guessing doing something wrong.. does this test work in circuit?

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  • q9522678
    replied
    No problems. Appreciate the suggestion! And I learned a bit about JFETs in the process!

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  • Enzo
    replied
    50 or 60, equivalent in terms of what we are doing.

    OK lifting those caps clears my JFETs. So much for that. Thanks for checking them.

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  • q9522678
    replied
    Just lifted C28 and C29 and hum remained ..

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  • q9522678
    replied
    Enzo,

    My little project today is to look at the JFETs and work out whether theyíre the cause.

    Being in Australia I think itís 50 cycle hum. Looking at my Petersen strobe app whilst holding close to the speaker appears to be perfect 50 cycle ..

    Click image for larger version

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  • Enzo
    replied
    I worry more about JFETs in the gate area. The Source/drain path is a low resistance until turned off by the gate. But the gate can get leaky to the S-D path. And that can inject noise into the system. Easy to determine, and in my view worth a check.


    And did we ever determine if your hum was 60Hz or 120Hz? In my JFET scenario, the control signals run off supply F which in fact is half wave, so ripple there would be 60Hz.

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  • q9522678
    replied
    Hi John,

    I think I’ve ruled out it being any kind of tube problem as I’ve tried multiple new tubes through everyposition including preamp and power section. So I think tubes are out.

    But like you, I do suspect it is a grounding issue. The hum reminds of previous grounding issues. I just stumbled on this which is essentially a blog of a repair of one of these by an amp repairer from around 9 years ago. It’s an interesting read https://www.google.com.au/amp/s/atom...c-overhaul/amp
    Funnily enough I already completed a lot of the work that he has - seems to be some common issues caused by underated components and resistors burning out.

    However he has a long treatise on hum. Below is a direct quote from the page about the hum and how he dealt with it:

    The biggest deal: the grounding scheme. I read on some forums (yeah, I know, ugh) that the amp had a ground loop, which was what caused the excessive hum. So I traced out the entire ground circuit with a sharpie on the solder side of the PCB. This was miserable and took forever – only to demonstrate conclusively that there is NO ground loop. I messed with rerouting a few things to more local points, but that only made things worse.

    Eventually, I just implemented a version of the Valve Wizard multi-star scheme (which I used in the Atomium Model A, a very quiet amp), with jumper wires and cutting traces. V1 and the entire front panel are grounded to the bottom of C9. This is area A. Area A connects by one jumper wire to Area B, which all the grounds in V2 go to. Area B connects by a jumper wire to Area C, which has SOME of the reverb circuit grounds and V4, and connects by a jumper to Area D, which has the other reverb grounds. The connection point between C and D then runs back to the main star point, which is the center tap of the PT B+ winding (green wire spade connector, J31). I cut the jumper wires between A, B, C, and D, and cut them all off from the main star. I ran separate wires from A and B back to the ground of C8. I connected C and D at their jumper point and ran a wire from there to C8’s ground (C8 supplies V2-4). I used 18-gauge solid-core copper wire


    I still find it difficult to believe they hummed this bad from the factory but I suppose anything is possible. I do have time on my hands and could implement a similar grounding scheme to see if that would improve things.

    I appreciate the advice about eating the elephant one bite at a time. It can certainly initially seem overwhelming with so much stuff to consider and look at. On the up side, I’m in no rush and have some time up my sleeve, so will just be patient with it.
    Last edited by q9522678; 03-28-2020, 10:04 PM.

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  • q9522678
    replied
    Hi Enzo,

    Thanks for the detailed reply. With continuity - I was primarily look for shorts to ground and was looking physically for solder bridges or something connecting to something elsewhere shouldn’t be...

    Understand that the spec of a resistor won’t cause the hum. But I’ve had some luck, mostly with carbon comps, with weeding out noisy resistors that were also out of spec. But acknolwedge this hum is unlikely to be caused by a resistor. I suppose I was just curious to see if everything was in spec and functioning as it should be.

    When I say I traced out the circuit. This was literally using a sharpie to trace the circuit board - using both the schematic and layout. This is more to easily physically identify the parts on the board than anything. The board is hard to access, with components on the underside and no easy means to get to them without have to pull a whole bunch of connectors. So scribbling on the bottom of board helps me locate components easily. By the end of this I’ll probably have all the components on the bottom of the board labelled!

    Thanks for the info on the JFETs -again because of the absolute pain of access the component side of the board, trying to test things in circuit where I can. But understand this can’t always be achieved.
    Last edited by q9522678; 03-28-2020, 11:00 PM.

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  • Jon Snell
    replied
    Q7,8 & 9 act like a switch, either on or off and increase the gain of that particular stage.
    Eg; Q9 effectively places a 0.1uF capacitor across the cathode resistor increasing the gain above the CR point so above about 6kHZ there will be a 6dB increase in level. Q8 does the same at a lower frequency.
    JFETS are used as switches in this application and are either on or off. If you use your DMM on Diode test, when powered off they should read open circuit Source to Drain. Don't let them confuse you!
    Ampeg have had a lot of trouble with JFETS failing in the effects loop from dodgy pedals that are not earthed properly killing them, not their fault.
    When removing C27, the signal path is interrupted before that point, if the hum stops, the hum is being picked up before that point, if it doesn't, it is being picked up after that point.
    Be logical, keep a note.
    This is not about testing components, it is about isolating and fixing the cause.
    The cycling beep from the pots is a red herring, I think.
    Check your ground points go to the ground and not just floating.

    Years ago, when I had the odd Television, and colour first entered the UK, I quickly learnt to divide the fault up and that there was only ever one fault plus my saying was, "It's only a telly". If it had a video fault, decide what sort of video fault and go down that path. If it had an audio fault, likewise.
    So many good engineers got lost by trying to look at the whole scenario and that circle on the schematic that contains the fault got much bigger for them.

    So, it's only an amplifier and the hum is to be isolated at the one point and fixed.
    I still reckon it is a bad ground or a faulty valve. Did you replace the ECC83 with another one, to check it?
    Have fun.

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  • Enzo
    replied
    Don't use continuity tests, use resistance. Using continuity, yo8u can't tell if continuity means 1/10 of an ohm or 110 ohms.

    R50 opens the current path through the tube, so it is essentially the same as pulling the tube.

    Hum is not going to be caused by an off spec resistor. In other words if a 470k resistor in that circuit is really 560k or even 330k, it may affect the gain a little or maybe the tone, but would not cause hum. Kinda like if your guitar gives you electrical shocks, it won't be because the strings are untuned.

    Carefully traced out from V2? Why not look on the schematic, all those parts are right there showing their relationships.

    JFETs. They can get leaky. You wanna see if one is involved? Either remove them one at a time and see, or lift C28 or C29 to disconnect them from the circuits.

    A leaky JFET can allow DC from the control line into the signal path. The two caps block it, but it can still admit ripple.

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  • q9522678
    replied
    Just an update on progress or lack thereof.

    Lifted R48 and hum remained.

    Then lifted R50 (off pin 3 of V2) and hum stopped. I reconnected R50 and hum came back. So thought I’d narrowed it down to something off pin 3 V2.

    Then carefully traced out the path off V2 to identify the components: R50, R51, R52, R53, R54 and R55, C28, C29 and FETs Q8 and Q9. All resistors and caps tested ok. No shorts or open lines and resistors with spec. Not an expert at testing FETs - is there any way to test in circuit? If it’s not the FETs, then stumped as all components off pin 3 of V2 seem ok.

    As an aside - when I was doing some continuity testing to ground I discovered the middle pin of channel C “High” control had intermittent ground continuity ie the test would beep rhythmically (rather than a solid tone). This terminal also connects to the one of the outer pins of the channel C and B gain controls and these show the same behaviour and on the continuity test (not the ground pin side of gain control. Unsure if this is normal or could be related. I note the “high” pot is connected to pin 3 of V2 via the “pull bright” switch on that pot. Probably just incidental.

    Enough for today. Will try some more tomorrow

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  • q9522678
    replied
    Hi John,

    I donít think I have the means to test for ripple. But point F is showing a pretty stable 10.6vdc. To be sure I tried another 1000uf cap in there from a different manufacturer - no change to the hum.

    Will try your other suggestions shortly working back from V2 through V1.

    Cheers

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  • Jon Snell
    replied
    Shot in the dark;
    If point F on the power supply has lumpy DC, the 1000uF capacitor (C10) is not smoothing properly, that will cause odd 50HZ ripple.

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  • Jon Snell
    replied
    No worries.
    Just it is difficult to figure out what it may be. Work backwards.
    The best way is to start by removing V1.
    Lift R48 and check the hum has gone. If it hasn't the hum is within the circuitry of V2 the first lower gain portion, pins 1, 2 & 3.
    If it has, move back to the second portion of V2 pins 6, 7 & 8 and lift R38.
    Continue backwards towards the 1st stage of V1.
    Hope that helps.

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