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Thread: Marshall VS65R signal path help?

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    Marshall VS65R signal path help?

    Hi, I have a VS65R with very low and very distorted output. Same at the headphone socket and line out socket. Plugging directly into FX return gives me nothing at all. So, looking at the diagram, If Im reading it right it looks like theres an extra op amp thrown in for good measure? IC7. If this was faulty it could make a no go situation when plugging into FX return. Am right? ok, so I checked the LT and HT lines both for voltage and AC and they seem fine. I changed IC7. Still the same. SO, I figured I would be methodical and started injecting a tone here and there. If I inject at pin 2 of CN1 which goes stright to the power amp I get a good smooth healthy tone so Im calling the power amp ok with that. I put it on pin 7 of IC7 and got a good output. Moving backwards I tried R59 which im guessing is the input to IC7a (I think its 'a', its a bad drawing) One side of that resistor is stupidly loud and the other is a good normal output. This Im also calling ok. So Im thinking (and this is where I may be wrong and in need of advice) that pin 1 of IC7b shoud also give me a good output as it goes to R59 via VR10. Surely if VR10 is turned fully down its should basically be a short between the pin 1 and R59? therefore a signal on pin 1 should give the same output as a signal on R59? It doesnt of course, which is why Im asking! Should I be squirting a signal in somewhere else?


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    Last edited by allante666; 10-13-2017 at 12:35 PM.

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    Supporting Member Jazz P Bass's Avatar
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    Quote: "Surely if VR10 is turned fully down its should basically be a short between the pin 1 and R59?"
    Only you can answer that one.
    Measure the resistance between the pin 1 and R59 when V10 is fully down.

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    I did, and its like, negligible, an ohm or less. What Im saying is surely the signal should pass through there as though it was a direct path?

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    Old Timer J M Fahey's Avatar
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    I see something abnormal here, but not what you think.

    You must NOT inject signal successfully either on pin 7 or pin 1 because those are Op Amp *outputs* , which among other things means very low impedance points, practically ground for externally injected signals.

    So no sound injecting on pin 1 is whatīs to be expected and hearing it "normally" when apparently injected on pin 7 is the wrong bit.
    Are you injecting straight on IC leg (on the component side?) or on the copper side, either on IC solder pad or somewhere else on the tgrack "since itīs all the same"?
    SWhich it "shouldW", except you could have a broken track or cracked solder there.
    Check continuity in that area and look at it with a loupe and under good light.
    Also check voltage at all IC pins, but at the IC itself.
    Juan Manuel Fahey

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    Hi Juan. So, for future reference, are you asaying I should inject signals on the inputs of the opamps? Like pin 3? Anyhow, I was injecting on th eleg of the IC and also on either side of C10, all of which gave me a good output. The negative of C10 goes to pin 2 of CN1, isnt that the main input to the power amp? And are you saying that injecting the signal on pin 7 should give me no output as it will more or less ground my signal generator or because it will damage something?? What should happen if I put a signal on pin 7? Should I get no output but get an output from there if I put it on C10 -VE? Could it be IC7 missing its ground connection? (Im back home now so going from the diagram)
    Last edited by allante666; 10-13-2017 at 07:07 PM.

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    Old Timer J M Fahey's Avatar
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    You will break nothing, no real *power*n involved there, but connecting your generator to an Op Amp output (pin 7 in this case) is the same as connecting it to a , say, 100uF capacitor with the other end grounded ...
    So if it did not get swamped to ground even wnen directly connected to pin 7 at the IC body itself, then probably "thereīs nobody home" meaning the IC is either dead open or all other pins are floating .
    In any case, such IC is not working.
    Juan Manuel Fahey

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    Thanks for that, Understood. Ill check again on monday, I was just thinking though, I definitely checked the supply to the chip, on the chip and I had a good + and - 15v on 4 and 8 but, I measured this in respect to chassis ground. If the chip isnrt working, is it likely that the chip itself has no ground? that would show correct voltages to chassis but not to the chip yes? Does it get its main ground through pin 5?

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    Old Timer J M Fahey's Avatar
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    "Hidden in plain sight"

    Thereīs 2 ways it acts like an *audio* ground, in a way similar the a large capacitor with the other end grounded, thatīs why I used one as an example.

    |) It gets "audio ground" or "ground reference" through the power rails, in this case the +/-15V rails.
    If you try to inject, say, 1V RMS from your generator into +/-15V rails your audio will disappear
    Why?

    A voltage supply will keep the voltage itīs offering steady no matter what ... meaning if you inject 1VRMS you wonīt have *modulated* 15V, meaning 15V sweeping up and down from roughly 13.5V to 16.5V (signal peaks) but plain steady 15V, no matter what ... or the power supply isnīt doing its job, meaning it will absorb/kill what you are injecting.

    Even the cheapest cheesiest Op Amp rail supply, made out of a voltage dropping resistor from main rails, a Zener to ground and a 10uF to 100uF capacitor to ground will do that.
    Even if itīs "not ground" because itīs 15V away ... for DC that is, but not for AC since you can NOT sweep up/down ... which is the essential of an Audio signal.

    Since Real World stuff does not behave exactly like in a textbook, you may have a ***tiny*** Audio signal there if supply is fine and maybe way more if you have a problem, such as open/dry/high ESR caps.

    Thatīs why Enzo many times suggests "scope the supply rails" to check certain problems, such as motorboating/instability/oscillation .... you should have nothing there (besides DC of course) or *a little* residual ripple.

    Sorry, a Customer is waiting, just stopped a little for a cup of tea (and my morning diabetes and blood pressure pills ) , later will explain the other "invisible in plain sight" ground.

    As usual, I get carried away

    EDIT: just found a related example: suppose you are in a balloon and at the end of the trip to, say, New York, you want to tie it down so wind does not carry it away, not even move up/down because passengers suffer nausea.
    Whatīs the simple answer?
    Anchor it to ground with a strong wire cable.

    Ok, what if for any (imaginary) reason you are not allowed to drop your anchor on proper ground? (imagine some stupid City ordinance).

    Ok, no problem, you tie it to Empire State building *roof* .

    Is it real "ground"?
    Nope, itīs 200 or 300 feet up.
    If you fall to ground from there you will definitely be smashed to pulp.
    Will it *behave* as ground, meaning a balloon tied there will NOT swing up/down in the breeze?
    You bet it will
    It will keep *steady* 200/300 ft (whatever height it has) no matter what.
    Juan Manuel Fahey

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    Thanks for taking the time to explain so far, its much appreciated. I do get the fact that 'ground' isnt necessarily zero volts and can be anywhere as its only ground 'relative' to the signal as it were. In much the same way the empire state building is only 'ground' relative to your balloon. The ground labelled 'signal ground' comes from the junction of ZD1 and ZD2 so is zero volts relative to the other ends of those 15 volt zeners and not 'necessarily' the same as the chassis although it probably is in this case. Am I correct in my statement though that if pin 5 was in fact floating, the + and - voltage on 4 and 8 would still read correct to the chassis but the chip itself would have, as you said, nobody at home. And as it wasnt getting an actual supply as in + and - and a ground relative to those, my injected signal would be free to make its way to the power amp without the chip swallowing it up?

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    quick update, I called in for literallly 5 minutes today with a borrowed scope and did a couple of quick checks. Scoped the 15 volt rails and they are as smooth as a smooth thing. I checked voltage again of both rails on the chip legs itself using pin 5 as the ground and got + and - 15v. Checked continuity between pin 5 and junction of ZD1 and ZD2 adn all is well there.R68 is fine as is C10, R59 and C57. Thats all I had time to do but if I understand Juan's description correctly and that injecting a signal on to pin 7 should just swamp the signal due to its low impedance then am I correct in thinking the fault must be in the IC7c part rather than the IC7b part? Im assuming IC7a is simply the power supply part of the chip, which incidentally is the third IC I have tried in it so Im pretty much ruling out a faulty chip (I fitted a socket the second time). Im not sure where else to check or how else to isolate the fault any further?

    Edit: just rememebred something else, when I injected aforementioned signal into pin 7 and got a good output from the power amp, I had the other end connected to chassis. Should I have used the 'signal ground' (i.e. pin 5 for instance) instead?
    Last edited by allante666; 10-15-2017 at 06:16 PM.

  11. #11
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    Quote Originally Posted by allante666 View Post
    Im assuming IC7a is simply the power supply part of the chip, which incidentally is the third IC I have tried in it so Im pretty much ruling out a faulty chip (I fitted a socket the second time).
    IC7 is a quad op-amp. It seems parts A and D are not in use. IC1 below it is an example of all 4 sections in use.

    Do you have good signal coming out of the Send when using the guitar input? Can you run the send out into the FX return of another amp?
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    I thnk IC7 is just a dual op amp not a quad, its an 072. However, no, I have nothing coming out of the send apart from the same very low, very distorted sound that I get out of this amp.

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    Well I pretty much give up on this one. If i inject a signal into the input, I get the tone (albeit low) from the speaker (a guitar will also make it through the amp but very low and distorted). I can follow the signal though the amp and pick t up in all the right places buut its low (apart from on pin 1 of IC1 in the reverb channel where its much louder so I deduced from that the signal must be making it from the wiper of VR10 ok) (shorting the reverb jacks out simply made the amp louder but still distorted). Power amp is all ok as I swapped it out with a spare one to try it. Im beginning to wonder if im looking in the wrong place? The signal is actually making it through the amp but just not cleanly. Best way I can describe it is when you turn an amp off sometimes while still playing, it fades away to a faint farty sound in th ecouple of seconds before it stops? Thats what it sounds like all the time. I tried swapping the tube for a good one but no different. The tube heaters are lit and the HT fuse is ok (checked with the tube removed) I cant pick up the signal on pin 6 of IC7 but I can on pin 7 so as Enzo pointed out in another thread it must be present or it wouldnt be at pin 7. Any ideas before I throw it at the wall??

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    So you have signal at VR10 wiper. How about each side of R59?
    Pin7 of IC7 ?
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    I cant actually pick up any signal on either side of R59 but it must be there as I get it on pin 7 ok (the tank isnt connected so it cant be sneaking in that way either). I get signal all the way through the amp but its just very low and very very distorted and broken up.

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    Supporting Member Jazz P Bass's Avatar
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    You have to find where the problem Starts.

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    Yes, and thats where I need help with the signal path. Its all very well putting a tone in from a signal generator but it comes out the other end still as a clean tone. very low volume but clean. Its when I put a guitar type signal in that it comes out totally distorted and broken up.
    Im guessing I would be better off inputting a guitar type signal and trying to pick it up at different parts of the signal path to see where it starts to get broken and distorted but im not sure where are the best places to look for it? Can you suggest particular points on the schematic to look?

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    I don't think its a problem with test tone vs guitar signal, rather just a problem of signal levels.
    If you don't know what to expect anywhere, you may get signal and think it's ok, but it could be 10 times less than required.
    Use your meter on AC range. Set it up so you have 100mV going into the return jack. Post the AC voltages you get at IC7 pin1, pin7, then further into the power amp like T4 base, T4 collector, and at output jack.
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    Youre right it is indeed a level thing, its just difficult to hear any distortion on the tone but easy on the guitar signal. And yes it probably is 10 times less than it should be. OK, My signal generator is only a cheap and cheerful thing its only got two settings, AF or RF! I can however measure what it puts out in mV so we have a comparison. So measuring the AC voltages at the IC you would use the signal ground (pin 5) not chassis yes?

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    Yes. Use signal ground as the reference for all your readings. Sometimes the connection to chassis gets lost or degraded, especially when things are partially or fully disassembled. Signal ground will still give a good reference.
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    ok thanks ill do that and get back to you. I think I did check signal ground and chassis ground and there was about 15 ohms between them.

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    Forgot to ask g1, When I take these measurements, what settings should I have on the amp? Full up, ful down, halfway etc?

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    For the test I suggested in post #18, you're plugging in to the return jack, so no controls should affect it, except fx mix, which should be set to full.
    For now, this will just test the power amp section.
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    You did say that, sorry. was thinking you had said the input jack.

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    ok, Ive taken the measurements suggested by g1. My generator signal is 120mV and I input it into the fx return. With my meter set to AC The readings I got on IC7 pin 1 was 1.48vac and on pin 7 was 880mVac. Where do I go from here? Do bear in mind that I did swap out the main power amp board for a working one so I ruled all of that out as it had the same results. However, IC7 and associated bits arent on that board they are on the preamp board. In other words everything the other side of CON1 seems to be ok?

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    So when you are set up like that, what do you measure at the speaker out jack?

    Maybe the power amp board is ok, but you still don't know if the proper signal is getting through to it via connectors.
    If you don't hear a good signal when plugging into the return jack, then the problem (or at least one problem) is after that jack.
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    At the speaker output all Im getting is 4mV and thats with or without the generator running.

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    Ok, same conditions that gave you 1.5VAC at IC7 in post #25.
    What do you measure on either side of R1? How about at T4 base?
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    With the same setup as in post ##25 I measure 1.78vac on all three of the places you suggested.

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    How about (same conditions) Q4 collector? T9 base? T9 emitter?
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    ok so Im not getting any AC reading on either T9 base or T9 emitter (well, 2 mV if that counts). I cant see a Q4 so just in case you have a different diagram Ill try post the other two pages of the ones I have.


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    Sorry, I got my Q's and T's mixed up.
    So you had 1.78Vac at T4 base, I wanted to know T4 collector. Since you said zero at T9 base, it looks like there will be nothing at T4 collector, but check it anyway.
    While you're at it, check that R7 (at T4 emitter) and it's connections are good.
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    No, nothing at T4 collector. R7 measures 22R as it should (measured in circuit) and have checked connections and continuity to other components etc.

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    Well, at least the problem area now seems to be isolated.
    How about DC voltages (no signal applied) at E,B, and C of T4.
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    E= -40vdc
    B= -39vdc
    C= -1vdc

    Measured in respect to signal ground, no signal applied

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