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Author Topic: earth g2000/fender dual showman vibrato troubleshoot  (Read 4743 times)

ilyaa

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earth g2000/fender dual showman vibrato troubleshoot
« on: October 15, 2014, 04:16:40 PM »
amps fine but vibratos not working....

thought it was the LDR/optocoupler so i replaced that but no dice

so far the only issue i can see is that both cathodes of are at 0 V, instead of 2 and 13DVC.

but ive never really troubleshot this circuit before - good place to start?

its not actually a dual showman, but it seems to be pretty much exactly this schematic:

http://www.thevintagesound.com/ffg/schem/twin_reverb_sf_100_schem.jpg

edit: ive got -44V at the grids of the vibrato tube - is that right?

thanks!!
« Last Edit: October 15, 2014, 09:07:29 PM by ilyaa »

ilyaa

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Re: earth g2000/fender dual showman vibrato troubleshoot
« Reply #1 on: October 16, 2014, 02:39:00 AM »
hm alright i made some progress on my own

the person who gave me the amp claimed the vibrato had been working fine and there had never been a switch. upon closer inspection, the switch that turns the vibrato 'on' was OPEN, so the vibrato was definitely OFF. once i closed it, i started to get a tiny vibrato effect, but not quite. THEN i realized that the grids of the vibrato tube - the LFO + neon driver - were directly connected. ?????. i got that back to stock and its working but not that 'intense' -

i guess i need to go through it and see whats up....the bypass caps on both cathodes were 35 uF - someones been in here and fudged around, i think!! im also starting to doubt that the vibrato ever worked.......was my buddy fibbing?!

g1

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Re: earth g2000/fender dual showman vibrato troubleshoot
« Reply #2 on: October 16, 2014, 12:11:39 PM »
  Is there a switch or a footswitch jack?  Some require the footswitch to work.
So maybe your friend had a way of turning it on.
Otherwise, weak tubes and caps (especially the cathode caps) can make the trem weak.
 Maybe he had some work done later that mucked up the grid wiring and forgot?
Anyway, try to find the real schematic.

g1

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Re: earth g2000/fender dual showman vibrato troubleshoot
« Reply #3 on: October 16, 2014, 12:18:53 PM »
Here you go, the grids are in fact paralleled.

J M Fahey

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Re: earth g2000/fender dual showman vibrato troubleshoot
« Reply #4 on: October 18, 2014, 01:41:11 AM »
If I were Leo Fender I would be very pissed at Earth , they only copied 99.98%  of a Twin reverb  :o

What's wrong with that 0.02%  ????? 

That shows lack of respect !!!! :trouble

Roly

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Re: earth g2000/fender dual showman vibrato troubleshoot
« Reply #5 on: October 18, 2014, 05:26:41 PM »
Quote from: ilyaa
ive got -44V at the grids of the vibrato tube - is that right?

In some of his amps Leo used the OP stage bias supply to control the trem Low Frequency Oscillator.

One of the problems that can arise with oscillators generally, and in particular if the gain is marginal so that you get a reasonably undistorted sinewave, is sluggish starting.

With a LFO that has marginal gain it is possible for a delay out to seconds before the oscillation is stable.  This is because starting initially depends on circuit noise to start the oscillatory build-up.  This delay is musically unacceptable, you want the effect to start the moment you hit the stomp switch, not some variable time later.  Generally speaking a truly noise-free oscillator won't.  Try it in the noise-free environment of LTSpice.



Note that part of the Phase Shift Network, R4, is connected to the emitter rather than ground.  This puts a small voltage on that point of the PSN where the switch is also connected, so when the switch opens this point will suddenly jump to a volt or two, injecting a voltage step into the PSN/Base circuit and kicking the oscillator into life (trace above).  {The V4/D1 work-around clamps the oscillator PSN to ground to turn it off, and when V4 goes positive D1 is reverse-biased and the oscillator un-clamped, but it's the emitter voltage that actually provides the initial kick.}

Leo's circuit does something similar, only in reverse because he used the negative bias supply that was handy, so that when the switch is open you get the -44V you measured and the valve is well cut off and stopped.  When the switch closes a step of +44V is injected into the oscillator grid and it starts very promptly.  Most builders arrange their circuits so that the trem runs if the footswitch is disconnected (reasoning that you can always turn the Depth down to zero).

Quote from: ilyaa
the person who gave me the amp claimed the vibrato had been working fine and there had never been a switch.
...
im also starting to doubt that the vibrato ever worked.......was my buddy fibbing?!

Quite possibly.  One of the things that you encounter as a tech is that the vast majority of people are lousy eye-witnesses and may swear blind that something happened that is technically impossible, as here.

I'm a tech, not a psychologist, but there are a number of reasons why this might happen, mis-observation mainly, but the most obvious being that the owner did something stupid they don't want to own up to (in which case you are diplomatic and simply accept what they say).

You also sometimes encounter what I call "forcing" - the reasoning seems to be that if they give you an incomplete or false report that it will somehow "force" you to fix what some other tech may have told them was difficult or expensive, that you somehow won't notice e.g. the output transformer is a smoking ruin and will find a cheap way around that if you aren't told about the clouds of smoke.  This is the "negotiable reality" so beloved of politicians and lawyers, but we techs have to deal with the cold hard reality that you can't negotiate a compromise with a burned out OPT (or whatever).

Once in a while you will encounter somebody who is so fixated that they have it right and the techs they have talked to have it wrong/are crooks, that it can turn nasty.  There are a few clients that it's better not to have and you can only retreat gracefully and suggest that they take their gear to another tech.

I have encountered clients who have given me a forensic report of what happened that exactly matches the evidence, and even made a good guess as to what might be wrong, but they are rare and about as common as the ones that are total idiots who tell you the amp blew up when it wasn't even plugged in (or similar impossibility).

Quote from: J M Fahey
What's wrong with that 0.02%  ????? 

The 10 meg across the neon.  I have always thought that this was an error and should go to ground as a keep-alive for the neon to stop it going out altogether, e.g. my HV-LFO and;


If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: earth g2000/fender dual showman vibrato troubleshoot
« Reply #6 on: October 23, 2014, 10:51:32 AM »
thanks again for the help, roly!

just to update you guys - the vibrato worked fine...once i realized that the switch - which in this amp is an RCA jack - was wired to be off. there MUST have been a shorting jack in it or something, because once i shorted it internally, the tremolo worked. oh yeah, and once i put in the original opto-coupler, as well. the one i got from tube- depot didnt work!

the tube was also an ecc81 instead of an 83 and the vibrato definitely sounded better with an 83 in there.

so its all good, and got a little recapping in the process!