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Thread: Fender custom shop 68 Twin reissue Nasty HF Oscillation

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    Fender custom shop 68 Twin reissue Nasty HF Oscillation

    Hello everyone and thanks for having a look here,

    As the title says, I have a 68 Twin reissue Custom Shop on my bench, actually the 2nd one in two weeks with a similar problem of HF oscillation. This circuit is essentially the same as the 65 Twin reissue, except that they have paralleled the two input sections so the "custom" channel also has access to reverb and tremolo. Oscillation occurs first at around four on the volume control and appears as a heavy squiggle on a sine wave on the oscilloscope. Past this, there comes a point where the amp simply explodes into full oscillation and becomes unusable. The owner of the amp first noticed the problem as a sort of background noise to certain notes but actually didn't ever the play the amp very loud and may not have noticed the oscillation beforehand.

    The amps have grid stoppers on nearly every tube grid, preamp and output. I believe they must have noticed an issue in the factory and made some adjustments (This amp also has a reworking of the traditional tremolo section as in Europe LDRs are outlawed. It would have been nice if this was the source of some trouble, but as I removed the tube and the power to the LDR replacement circuit it isn't the case.)

    The source of the trouble appears to be at the tube used for mixing the reverb return and dry signals. Grounding the grid here removes most, but not all, of the oscillation. Also, when I removed the phase inverter and output tubes, oscillation still occurs at the anode of this tube. Leads from the PCB to the socket aren't particularly long, and there is a grid stopper of 10k on the grid of the mix as well as the reverb return! I have tried to reposition these wires (it worked on the first amp) but here there is no difference. As well, a different tube makes no difference. I also tried to snubber the grid with a capacitor from grid to ground, but this only helps with such large values that the amp would end up sounding completely flat.

    Obviously there is a gremlin in this amp, but I can't seem to find it! If anyone has some ideas, I would be most appreciative...
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    What is the approximate oscillation frequency?

    What changes if you short R73 (10 Ohm). Or if you wire a 0.1µ cap in parallel with R73?

    What changes if you disconnect the reverb in/send cable?

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by BiBi View Post
    This amp also has a reworking of the traditional tremolo section as in Europe LDRs are outlawed.
    LDR's outlawed in Europe? First I've heard of this. Anyone have additional info on this statement?

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    Grid resistors in each triode are associated with the incorporation of the two modules that replace the neon/LDR. This happens in all since they started installing it eleven years ago.
    Have you checked the filter capacitors? In some units they degrade quickly and create strange problems.

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    Quote Originally Posted by Pedro Vecino View Post
    Grid resistors in each triode are associated with the incorporation of the two modules that replace the neon/LDR. This happens in all since they started installing it eleven years ago.
    Have you checked the filter capacitors? In some units they degrade quickly and create strange problems.
    IC caps...if one fails it can easily lead to feedback.

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    Supporting Member The Dude's Avatar
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    Agree with the above. I've had a few with this exact symptom. All were found to have at least one bad filter cap.

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    Senior Member TimmyP1955's Avatar
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    While you are in there: I'll bet that the power tube grid stoppers are on the board instead of the sockets. This works OK on the RI amps even with JJs, but not on the Tone Master, at least with JJs. So I'd short the ones on the board and add them to the sockets. Worst case scenario is that all should be well if anyone puts JJs in it.

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    its a feature, not a bug, as all twins have that oscillation.
    As far its the resistor or capacitor of the feedback loop.

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    I'm not sure how to approximate the oscillation frequency. It appears on the top of the sine waveform at around 4 on the volume control then gets larger until it explodes into all out oscillation.

    Shorting R73 and bypassing both have no effect.

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    I've now replaced the filter capacitors but that also has no effect. (I'm not exactly sure what the reasoning behind it is either)
    I have also separated the common cathode connection between the reverb return and the mix, also to no effect.

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    This amp is built in the traditional Fender style in that case: grid resistors go from pin 8 to pin 5, g2 resistors from 6 to 4

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    no this is not a feature. It goes into full oscillation after 5 on the volume control. I know what you are talking about as all Fender designs are prone to oscillate, but this is really exceptional and unusable.

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    Lastly, I have also tried tightening all bolts and connections to ground. No luck here either.

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    Also, when I removed the phase inverter and output tubes, oscillation still occurs at the anode of this tube.
    This means that the oscillation is produced before the power amp.

    One of the usual suspects is the reverb driver V3. Does the oscillation stop when you pull it?

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    Bent Member Chuck H's Avatar
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    Helmholtz asked earlier what happens when you unplug the reverb send to the tank. I'm just repeating the request so it doesn't get missed. You could also unplug the reverb drive tube.

    EDIT: Ah... Helmholtz was reposting at the same time.

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    thanks for the replies...

    yes the oscillation certainly occurs before the power amp.
    Pulling the reverb driver ECC81 doesn't help neither does pulling the opposite ECC83 from the other channel.

    included are some pics of the oscillations at different frequencies before it goes completely haywire.
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    Gut pics?

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    Bent Member Chuck H's Avatar
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    If this model has the big capacitor on the board near the preamp, did you replace that one too? Or just the caps in the doghouse?

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    so here are the guts...

    obviously this is after some changes I made like separating common cathodes, redressing the wiring from the power transformer, etc.
    nothing has hurt so far but nothing has helped either
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    all the filter caps have been replaced.

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    neither does pulling the opposite ECC83 from the other channel.
    Which tube is this in the schematic?

    At which circuit point were the scope pics taken? What was the time base setting (horizontal deflection coefficient)?

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    Bent Member Chuck H's Avatar
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    Helm is considering the same thing I am. Divide and conquer. Trace the signal path to find the first place in the circuit where the oscillation shows itself. Oh, and the scope settings.

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    Quote Originally Posted by BiBi View Post
    so here are the guts...

    obviously this is after some changes I made like separating common cathodes, redressing the wiring from the power transformer, etc.
    nothing has hurt so far but nothing has helped either
    If you actually replaced C34 you obviously used the same Illinois cap crap as before (sorry Illinois people, love your area and tradition).

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    Quote Originally Posted by Helmholtz View Post
    If you actually replaced C34 you obviously used the same Illinois cap crap as before (sorry Illinois people, love your area and tradition).
    It's alright. I don't think they're made there

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    I don't think they're made there
    Doesn't matter, headquarters are responsible for product quality from all production sites. I know that you can get identical quality out of anywhere if you care, invest and check. Quality management is the key - and installing qualified/experienced quality managers with direct reporting. Also quality improves if the factory is charged with after sales costs (closed feedback loop).

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Doesn't matter, headquarters are responsible for product quality from all production sites. I know that you can get identical quality out of anywhere if you care, invest and check. Quality management is the key - and installing qualified/experienced quality managers with direct reporting. Also quality improves if the factory is charged with after sales costs (closed feedback loop).
    Absolutely. I think the humor I intended must have been missed. And yes, I've had enough experience with Illinois caps that I avoid them. Maybe they make some really good ones, but those aren't going into any guitar amplifiers I'm aware of.

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    I think the humor I intended must have been missed.
    I think I got it. But it's such a shame that a US (Illinois) based company with a long tradition got that bad a reputation.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    I think I got it. But it's such a shame that a US (Illinois) based company with a long tradition got that bad a reputation.
    Hopefully that reputation is just with us guitar amp guys. By that I mean that it's the amp manufacturers that choose what capacitor to pay for. If a long standing, reputable company gives you the lowest bid that doesn't automatically mean your getting their the product that earned them that reputation. I can go to the local deli and buy a grilled Ruben and it'll be delicious. Or, at the same deli I can save some money and just get the bologna sandwich.

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Thanks for the heads up on the Illinois caps-- I had one 22/500 from them still for some reason, but I didn't realize they had such a bad reputation.
    I replaced it with another and it didn't help however.

    By the other channel ECC83, I meant I can pull V1 while using the "vibrato" channel or V2 while using the "normal" channel and there is no change in the oscillation.

    Scope setting was at ,1ms for the photos as I started at around 40Hz on the generator. The scope was connected directly to the dummy load (directly from the amp output jack is the same)
    Like I wrote earlier, I am quite certain the problem is situated around the grid of V4b. The previous amp's problem was corrected here while and I can remove the following tubes and still see
    oscillation on V4b's anode. Grounding V4a's grid and separating the cathodes here have not shown any improvement.

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    OMG Illinois Capacitor is located only a few miles away from me I always thought it was an asian brand that was just called Illinois Capacitor but actually had nothing to do with here.

    what a drag

    Oh well at least I can still enjoy searching ebay for vintage old stancor transformers which actually seem awesome and were made here as well. Bring em back home

    I have seen Illinois caps fail in fender hot rod series that are under 10 years old. Funny in contrast to also occasionally see those old 1960s caps that come in valco, silvertone, sears brand, Danelectro, Supro etc that say "guaranteed for 1 year" and they still work 50-60 years later. I forgot what the brand is called. I'm sure you have seen them. They are red

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    Oh jesus if you try to look up the caps you find this -- https://www.premierguitar.com/articl...to-silvertones

    Never trust this amp man he talks about replacing the 60 year old "guaranteed for one year" capacitors then shows a picture of good modern caps, and it's a picturs of IC capacitors.

    I think the brand of the "old reliable" ones are Planet ?

    sorry for

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    Like I wrote earlier, I am quite certain the problem is situated around the grid of V4b. The previous amp's problem was corrected here while and I can remove the following tubes and still see oscillation on V4b's anode. Grounding V4a's grid and separating the cathodes here have not shown any improvement.
    What changes if you lift one end of C13? Or lift one end of R30 (1M)?

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    Scope setting was at ,1ms
    This makes the HF oscillation frequency something like 60kHz (I count ca. 6 positive HF peaks per division, one full division corresponds to f=1/0.1ms=10kHz). Referring to pictures 2,3,4. Pic1 must be a different setting.

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    Ok, I see where you are going with this now...

    First removed the connection to C13, still looks the same.
    then I removed R30, still the same.
    Then, I bridged R32 and C17 (the 3M3 and 10pF combo) and it is worse! Goes directly into hysterics even earlier.

    Thanks for sticking with me on this one.

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