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Thread: Gretsch Fury Reverb issue

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    Gretsch Fury Reverb issue

    i just finished replacing all of the filter caps, quite few coupling caps in my buudy's Gretsch Fury and the amp it's sounding great, but for the life of me, i can't figure out why the reverb isn't working.
    i've repaired reverb circuits on 100's of fenders, and it was always either the tank (broken leads to the tiny xformers in it, or an open one), the RCA cables, the reverb drive transformer or reverb drive tube, or the recovery circuit or the tube.
    on this one, when i turn the reverb level up, the (unreverberated) signal level drops in level thru the speakers quite a bit. when i shake the amp, i get the spring crash thru the return, so it's definitely in the send/drive circuit. it uses a 6973 (RCA) to drive the tank and i tested it on my triplett 3444 and it tested at new spec, i also checked every resistor, cap, and even subbed a fender drive tranny in it, and also ohmed out the reverb tank's input and output xformers (all good), but still the same results. i'm getting absolutely no signal on the scope on the drive signal to the tank. does anyone have idea of what i'm missing here?

    here's the schematic of the circuit:


    my voltage readings on the 6973's (V4-A) tube socket:
    pin 3: getting plenty of signal with it's level controlled from the amps input volume pot
    pin 7 : .035V (IMO, this is the problem!)
    pin 8: 314V
    pin 9: 384.5V


    any help would be greatly appreciated as i'd like to get this one off of my bench.

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    Last edited by johnk_10; 03-14-2012 at 05:21 AM.

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    i'm stumped and since i don't have another 6973 on hand, i'll have to wait to try another one in it until a couple of them that i ordered arrive on thursday.

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    crickets chirping..............................

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    fixed it today.

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    Hi there, I have the exact same issue on my Fury piggyback. Can you tell me how you fixed it, I am totally stumped.
    Thank you - Dc

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    Lifetime Member Enzo's Avatar
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    No reverb is such a broad complaint, it could be a ton of things.

    Unplug the cables from the reverb unit, then measure resistance across each jack. Is either end open?

    Now you have a drive circuit, and a return circuit. Play a guitar or other ssignal through the amp. Plug some speaker into the Reverb footswitch jack. DO you hear the guitar through that? That tests the drive.

    You have the two cables for the pan, leaving then unconnected still, one should have a shielded wire, the other may have just one wire. With volume up and reverb control up, touch the tip of the one with the shielded wire. DO you hear a loud hum? That tests the return.

    These three tests will tell you where the problem lies, the pan, the drive, or the return.

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    Firstly, thank you for the response!

    Neither of the RCA cables is open, tested that already.

    I do get a hum when I touch the top of the shielded line.

    I have tried three pans, all of wyI believe to be functional. I can hear the spring when I crash the pan.

    I have not tried the speaker trick you mentioned, but based on your suggestions, Iím led to think it may be the drive circuit.

    I do think there may have been some work done previously on the amp. The footswitch jack is completely different than all the others (itís a square black plastic box as opposed to the standard switccraft style with the PCB board material sandwich). Also, the RCA lead and the signal lead were both soldered to the tip. The wiring diagram I have (looks to be the same as yours) shows signal to ring and RCA to tip. I resoldered them accordingly, but still have the same symptoms. I get signal, no reverb effect, and I get a volume/tone loss as I turn the pot up.

    Thanks so much. - Dc

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    In your situation, do you recall what the fix was?

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    Quote Originally Posted by dc74 View Post
    I do think there may have been some work done previously on the amp. The footswitch jack is completely different than all the others (itís a square black plastic box as opposed to the standard switccraft style with the PCB board material sandwich). Also, the RCA lead and the signal lead were both soldered to the tip. The wiring diagram I have (looks to be the same as yours) shows signal to ring and RCA to tip. I resoldered them accordingly, but still have the same symptoms. I get signal, no reverb effect, and I get a volume/tone loss as I turn the pot up.
    The jack may have been replaced, but that shouldn't matter.

    You have read the schematic wrong, both the output from the drive transformer and the hot of the reverb tank drive cable must be connected to the same tip connection of the foot switch jack. When the foot switch is used, the signal going to the tank is grounded, turning off the reverb.

    The reverb control is like a balance control between the reverb signal and the straight signal. As you turn up the reverb the straight signal is reduced, so what you are experiencing is normal.

    Have you checked the driver tube? Do you have a voltmeter? Do you have the skills to read high voltages in the amp? If you do, read the voltages on the driver tube.

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    Thanks, Iíll switch the lines back to the tip when I replace the jack.

    I replaced the driver tube, and it should be good. I will check the voltages, from the tube as well as the drive transformer to see if thatís where it lies.

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    I get just about 4 volts dc off of the tube socket pins that the input leads (? red & blue) for the driver transformer connect to, and there is no voltage on the black leads (which connect to the lead that runs to the footswitch jack).

    So I am assuming with my rudimentary knowledge that the transformer is dead. I cannot identify the type of transformer it is however, other than the schematic I have indicates "24:1", the transformer itself has no real manufacturer markings. Just reads: T - 109 (model?) 524 67 07 (date code?

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    If the tank is the same input impedance as the standard Fender type, I'd just put the standard Fender transformer there.

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    Lifetime Member Enzo's Avatar
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    Why are you assuming the transformer bad because there is no voltage on it? I see two 500 ohm resistors in series from the B+ supply. Are either one open? Is there B+ at both ends of each?

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    Or are you reading the voltage across the transformer primary, from red to blue?

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    Yes, there is voltage across both that reads at 388v, which is high (according to the schematic, should be 340v feeding to pin 9 & the transformer correct?)...I suppose that means they are bad. One does not look very good, the ceramic is cracked, hadnít noticed it because it was hidden under a wire. There is also a wire that has melted onto them that feeds from the 450v caps.

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    What is the voltage at pin9 of the 6973? If you have 388VDC there, that is close enough. It does not mean the transformer is bad.
    What is the voltage at pin 7?

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    Across means one end to the other. If you have 388v ACROSS the primary, that would suggest 388 on one end and zero on the other. If you have about 388v to ground from each end, that is very different.

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    I think he is talking about the 500 ohm resistors now.

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    Quote Originally Posted by 52 Bill View Post
    I think he is talking about the 500 ohm resistors now.
    Yes, there is actually 402v across those resistors (there is no voltage at first, but then it comes through, maybe as the amp warms up)

    There is no voltage across the primary leads to the reverb driver (pin 2 red, pin 2 blue) and no voltage on pin 7 of the 6973.

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    Quote Originally Posted by dc74 View Post
    Yes, there is actually 402v across those resistors (there is no voltage at first, but then it comes through, maybe as the amp warms up)

    There is no voltage across the primary leads to the reverb driver (pin 2 red, pin 2 blue) and no voltage on pin 7 of the 6973.
    The readings you report are not consistent. Do this
    1) Set meter to DCV
    2) Clip black lead to ground and leave it there
    3) Report the reading with the red lead on the +ve end of C2
    4) Report the reading with the red lead on the +ve end of C5A
    5) Report the reading with the read lead on pin of 9 of V4
    6) Report the reading with the read lead on pin of 8 of V4
    7) Report the reading with the read lead on pin of 7 of V4

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    3) Report the reading with the red lead on the +ve end of C2 - 406 vdc
    4) Report the reading with the red lead on the +ve end of C5A -1.8 vdc
    5) Report the reading with the read lead on pin of 9 of V4 - 1.8 vdc
    6) Report the reading with the read lead on pin of 8 of V4 - 1.75 vdc
    7) Report the reading with the read lead on pin of 7 of V4 - 0.1 mV

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    Quote Originally Posted by dc74 View Post
    3) Report the reading with the red lead on the +ve end of C2 - 406 vdc
    4) Report the reading with the red lead on the +ve end of C5A -1.8 vdc
    5) Report the reading with the read lead on pin of 9 of V4 - 1.8 vdc
    6) Report the reading with the read lead on pin of 8 of V4 - 1.75 vdc
    7) Report the reading with the read lead on pin of 7 of V4 - 0.1 mV
    Seems the 500 Ohm resistor between C2 and C5A is defective (high resistance).
    Otherwise it would be dissipating around 330W (!).

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    Last edited by Helmholtz; 02-08-2019 at 04:22 PM.
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    I am thinking that is the case after running through the suggested checks...much thanks for that!

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    Replaced the 500 ohm Resistors and a few melty looking wires and now read:
    Click image for larger version. 

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    402v before resistors
    359v after resistors

    359v on Pin 2
    14v on Pin 7
    350v on Pin 9

    No voltage on the green and black lines from the transformer...now I'm feeling more confident that's dead too.

    Oddly I can crash the pan, and when I tested the front end of the resistors the "pop" that happened when I contacted the meter lead had reverb on it...

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    Quote Originally Posted by dc74 View Post
    Replaced the 500 ohm Resistors and a few melty looking wires and now read:
    Click image for larger version. 

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ID:	52483Click image for larger version. 

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ID:	52484

    402v before resistors
    359v after resistors

    359v on Pin 2
    14v on Pin 7
    350v on Pin 9

    No voltage on the green and black lines from the transformer...now I'm feeling more confident that's dead too.

    Oddly I can crash the pan, and when I tested the front end of the resistors the "pop" that happened when I contacted the meter lead had reverb on it...
    Well, your new measurements show 9V across the rev. transformer primary. This could be OK and means continuity.
    What is pin 2?
    Voltage on pin 7 looks rather high. Please check 330 Ohm cathode resistor.

    Can you identify green and black transformer wires in the schematic? I assume it's the secondary which shouldn't have any DC.

    Please bend away all wires from the hot power resistors shown in the pictures.

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    Last edited by Helmholtz; 02-12-2019 at 09:17 PM.
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    Quote Originally Posted by Helmholtz View Post
    Well, your new measurements show 9V across the rev. transformer primary. This could be OK and means continuity.
    What is pin 2?
    Voltage on pin 7 looks rather high. Please check 330 Ohm cathode resistor.

    Can you identify green and black transformer wires in the schematic? I assume it's the secondary which shouldn't have any DC.

    Please bend away all wires from the hot power resistors shown in the pictures.
    The Left side picture was the original, I removed those resistors, and replaced them. I also replaced the wires which had melted to them. There is space now between those front leads and the new resistors (you can see in the right side picture).

    Pin 9 is the Red Lead from the reverb Driver, Pin 2 is the Blue lead.

    Pin 7 runs back to a buss bar with the black and green leads from the reverb driver and a series of caps.
    Click image for larger version. 

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ID:	52485

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    Pin 9 is the Red Lead from the reverb Driver, Pin 2 is the Blue lead.

    Pin 7 runs back to a buss bar with the black and green leads from the reverb driver and a series of caps.

    Sorry, not clear. Tube pin 2 is not connected internally, seems to be used as staging post only. I don't feel able to identify the wiring from the picture, looks like a mess to me.
    But it seems that red and blue wires are rev. transformer primary while black and green wires are secondary (just as in Fenders). Given this, your voltage readings make sense - apart from the suspiciously high pin 7 voltage.

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    The voltage drops across the 500 ohms resistors and the 330 ohm cathode resistor are consistent with a cathode current of about 42mA. OTOH the tube will be running at max dissipation. None of that will stop it working though. If the cathode bypass cap 25uF were bad it would lower the gain a little and reduce the reverb but not kill it.

    Try this: Plug a speaker into the reverb footswitch jack and disconnect the tank input. You should get a decent volume out of the speaker. If that is OK tack a 25uf cap across the existing cathode bypass cap. If it's louder the existing cap was bad. If nothing or very quiet then the problem is earlier.

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    There is 14vdc flowing off Pin 7 (of the 6973) into the cathode bypass cap, 13.58vdc flowing out of the cap and into the 330ohm resistor...then no voltage on the other side of the resistor...and and therefore no voltage after that to the secondary lead of the driver transformer...

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    Quote Originally Posted by dc74 View Post
    There is 14vdc flowing off Pin 7 (of the 6973) into the cathode bypass cap, 13.58vdc flowing out of the cap and into the 330ohm resistor...then no voltage on the other side of the resistor...and and therefore no voltage after that to the secondary lead of the driver transformer...
    Those are normal ( we already covered this IIRC). Try the tests in post #28.

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    Quote Originally Posted by dc74 View Post
    There is 14vdc flowing off Pin 7 (of the 6973) into the cathode bypass cap, 13.58vdc flowing out of the cap and into the 330ohm resistor...then no voltage on the other side of the resistor...and and therefore no voltage after that to the secondary lead of the driver transformer...
    You have this wrong, cathode cap and the cathode resistor are in parallel, so nothing flows into one from the other. The cathode cap and resistor connects the cathode of the tube to ground, so there will not be a voltage reading on the ground side unless there is a broken connection. The 14 vdc is probably correct in this circuit.

    Did you try a speaker connected to the output of the driver transformer as was earlier suggested? Have you read the resistance of the two sides of the transformer?

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    I did try the speaker trick, and did have good loud signal...but no reverb effect

    If I am testing it correctly, red lead to blue lead & black lead to green lead:
    .250k Ohms on the primary
    0 Ohms on the secondary

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    Quote Originally Posted by dc74 View Post
    I did try the speaker trick, and did have good loud signal...but no reverb effect

    If I am testing it correctly, red lead to blue lead & black lead to green lead:
    .250k Ohms on the primary
    0 Ohms on the secondary
    What reverb tank is in the amp? According to the posted schematic, the tank needs to have both input and output jacks grounded. Or the drive cable must be grounded at the circuit end.

    And have you read the resistance of the two tank transducer coils?

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    I'd bet my bottom dollar it is the original stock tank, looks like a Gibbs or a Hammond, with 6647 stamped on top.
    I can crash the tank, so I'm thinking it is in fairly decent order. Also, when I get a signal pop testing caps etc., I can hear the reverb on it.
    I also get a pop when I plug in the output line when the amp is live.

    The Line to the output of the tank (P2, on the schematic) is grounded at the circuit board end; however the schematic does not show the input lead (P3) as grounded just a bare end, no connection, which is how it is wired physically.

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    What kind of voltage should I have at the reverb intensity knob? I'm showing almost nothing...between 1-4 mV

    I tested the .001uF cap, there is 108VDC coming from Pin 1 of V5 to the cap and then basically nothing after that. I actually get a negative reading on my multi-meter, which I don't know how to interpret.
    The start of the lead that runs to the pot shows the same at the terminal on the back of the pot.

    When I test the cap live, I get echo (reverb) on the pop.

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