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Thread: Troubleshooting a "Refreshed" Fender SF Pro Reverb

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    Troubleshooting a "Refreshed" Fender SF Pro Reverb

    Repairing a 1975 Fender SF Master Volume Pro Reverb that has been sitting, unused for probably 30 years. My brother, the original owner, asked me to clean it up and refresh the capacitors, etc. When I removed the amp, I noted the chassis had been bent near the Master Volume control.
    I straightened this as best I could, reference my posting here:

    https://music-electronics-forum.com/...ighlight=sleet

    I finished my initial repairs and decided to test the amp and ran into some issues:

    The amp is quite save for some crackling noise when I exercise the MV control. When I plug in with a guitar on the normal channel, initially it was loud and sounded pretty good. Moving over the the vibrato channel was not as good. I found that the volume level was much lower and the tremelo did not seem to be working. Working the volume, treble, bass and MV had little to no affect. The volume could be quiet at "1" but would only increase a small amount while at "10". I noted that the MV with it's push-pull switch position (in or out) made no difference to the sound on the normal channel, not distortion was introduced. The MV was apparently damaged as part of the chassis trauma. I noticed this and attempted to source a replacement, but found that the part received would not fit due to the slant of the front of the chassis. I had hoped to keep the amp original and just replace the MV. The other issue now is that both channels suffer from crackling when I aggressively attack the strings on the guitar, less so with gentle strumming. The foot switch seemed to make little difference on the sound on the vibrato channel.

    For diagnostic purposes, I know I need to bypass the MV and see if the crackling and low output problems are solved. There are probably other issues as well. I think my brother would be okay with bypassing the MV, and leaving the control in place for cosmetic reasons. I've also considered repurposing the MV control for other things like a reverb dwell control or the like.

    Summary of work performed:

    Replaced other electrolytics (coupling caps, bias supply, etc)

    Cleaned the pots.

    Other information:

    Original tube complement which I was unable to test, I'm going to check the bias on the 6L6's and have some NOS tubes ready to go if need be.

    Thanks.

    fender_cbs_45w_mstrvol_pullsw_super-pro-bmstr_rev_sch.pdf_1.pdf

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    Quote Originally Posted by Sleet View Post
    For diagnostic purposes, I know I need to bypass the MV and see if the crackling and low output problems are solved.
    The Master Volume on this amp should effect the volume of both channels. If it has failed both channels would be quiet not just the vibrato channel. Crackling usually only happens when a control is turned and not when just passing a signal.

    Be sure to clean and tighten the tube socket connections and the front and rear panel jacks. You can also try swapping preamp tubes from the normal to the vibrato channel. If you turn up the reverb control a little and shake the reverb tank is the spring sound loud and clear through the speakers?

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    Another thing you might try is disconnecting the resistive side of the tremolo bug.

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    Quote Originally Posted by g1 View Post
    Another thing you might try is disconnecting the resistive side of the tremolo bug.
    Good call. I had a Twin with a shorted photocell last week. Not much problem when the vibrato depth was all the way down, but signal disappeared as depth was dialed up, whether the lamp end of the bug was blinking or not.

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    Thanks for all of the replies. I see your point about the master volume if malfunctioning, would affect both channels. I will tighten and clean the tube sockets in an effort to eliminate the crackling.

    Lifting the resistive side of the trem bug would be lifting the leg(s) of the 10meg and 100K resistors? This would remove the bug from the circuit to see if it's the problem?


    Trem Bug Schematic Closeup.pdf

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    No, those are the terminals for the neon side, the resistive element connects to 0V and the intensity control wiper.

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    Okay, so I'm back having experimented with the suggestions made upstream. I removed the resistive side of the "bug" and is seemed to have no effect on the output of the vibrato channel, it's still quite low with the volume and master volume dimed. Looking at the circuit, I'm hoping for some more ideas about what to check (voltages, signals). Just to be on the safe side, I'm going to order a new "bug" and install it. Here is the schematic for the amp itself:

    Click image for larger version. 

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    Bent Member Chuck H's Avatar
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    What affect do the other controls have on the sound in the vibrato channel? Do the TMB controls all work as expected? Does the reverb work? Is the reverb at full volume even though the main signal is not? etc. Any other symptoms to report please.

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    Quote Originally Posted by Chuck H View Post
    What affect do the other controls have on the sound in the vibrato channel? Do the TMB controls all work as expected? Does the reverb work? Is the reverb at full volume even though the main signal is not? etc. Any other symptoms to report please.
    Chuck,

    Thanks for the reply. So I've gone back and checked. The vibrato channel responds to treble and bass controls (i can hear it as I strum). I removed the bug from the circuit and used jumpers to connect to the resistive part of the circuit and the neon bulb is blinking. I can hear it "ticking" as well when I use the pedal to switch in and out for tremelo. The normal channel sounds pretty good and the amp is very quiet in general. The reverb channel is not working at all. I've removed the tank and plan on testing it and the connecting cables. When the tank was installed and connected I couldn't hear the reverb.

    I know that this shouldn't be that hard to solve this problem, I'm just at standstill.

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    Bent Member Chuck H's Avatar
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    No need to remove the tank yet. It could be that whatever is causing the low volume is also causing a lack of drive to the reverb. And anyway, the tank has nothing to do with the channels dry signal volume. Don't undo things that may not need undoing. It may complicate matters right now. Let's fix the drive level problem first.

    Ok... So the tone stack is working but the reverb is not and there's a lack of volume thereafter. Also, the normal channel is working. And the treble and bass controls are driven by V2A and that seems to be fine. The reverb drive comes from V2B. So maybe there's a problem with this stages circuit. You'll want to start by checking DC voltages for pins 6, 7 and 8 on V2.

    V4 is also exclusive to the reverb channel. Since we're working on the dry signal volume just now you should also check DC volts for pins 6, 7 and 8 of V4.

    Please check voltages for these circuits and report back.

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Sleet View Post
    I removed the bug from the circuit and used jumpers to connect to the resistive part of the circuit and the neon bulb is blinking. I can hear it "ticking" as well when I use the pedal to switch in and out for tremelo. The normal channel sounds pretty good and the amp is very quiet in general. The reverb channel is not working at all.
    I'm not sure what you mean about using jumpers to connect the resistive side of where the bug was, that would kill the signal.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by g1 View Post
    I'm not sure what you mean about using jumpers to connect the resistive side of where the bug was, that would kill the signal.
    This confused me too. My thinking was that it was only temporarily connected to see if the trem bug was flashing.?. But we can't be sure. I don't get the impression that he disabled the trem and then clipped the circuit back in before performing the requested tests. It seems to obvious that it would be a mistake.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Sleet View Post
    ...Summary of work performed:

    Replaced other electrolytics (coupling caps, bias supply, etc)...
    Just noticed the above, 'coupling caps' seems odd, was that just a terminology error, or have ecaps been used for coupling purposes?

    Quote Originally Posted by Sleet View Post
    ...I removed the bug from the circuit and used jumpers to connect to the resistive part of the circuit and the neon bulb is blinking...
    Confused by this, the neon couldn't flash if it's been removed from circuit?

    My take on the thread is that
    1/ the amp seems to have been modded as the master doesn't seem to affect the normal channel (true?); so need to verify the circuit to its intended schematic.
    2/ from the bent front panel, the master volume may have received a massive bash, may be broken, and given the symptoms must be replaced, or just taken out of circuit. Hardwire the pull boost switch in the non boost mode.
    3/ a voltage survey and idle plate / cathode current measurements (eg resistance and volt drop from OT CT to each plate) would be very useful.

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    Quote Originally Posted by Chuck H View Post
    No need to remove the tank yet. It could be that whatever is causing the low volume is also causing a lack of drive to the reverb. And anyway, the tank has nothing to do with the channels dry signal volume. Don't undo things that may not need undoing. It may complicate matters right now. Let's fix the drive level problem first.

    Ok... So the tone stack is working but the reverb is not and there's a lack of volume thereafter. Also, the normal channel is working. And the treble and bass controls are driven by V2A and that seems to be fine. The reverb drive comes from V2B. So maybe there's a problem with this stages circuit. You'll want to start by checking DC voltages for pins 6, 7 and 8 on V2.

    V4 is also exclusive to the reverb channel. Since we're working on the dry signal volume just now you should also check DC volts for pins 6, 7 and 8 of V4.

    Please check voltages for these circuits and report back.
    Chuck,

    Here are the voltages I recorded:

    V2: Pin 6 = 241V
    Pin 7 = 0V
    Pin 8 = 1.96V

    V4: Pin 6 = 359V . (Note: When I probed this pin there was a noticeable static, scratchy sound through the speakers)
    Pin 7 = 0V
    Pin 8 = 0V . (Note this pin has no wire connected to it)

    I did temporarily remove the bug from the circuit to get a closer look at it out of the chassis. I then reconnected it back to its back to the appropriate locations on the tag board with jumpers. I reinstalled it to the board prior to taking the above measurements. The Master Volume does seem (best I can tell) with the regular channel and does seem to impact the volume on the vibrato channel a little (it needs to be turned up all the way to hear the reduced output that I do get)

    I have considered removing or disabling the MV, but wasn't sure on how to proceed with that.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Sleet View Post
    V4: Pin 8 = 0V . (Note this pin has no wire connected to it)
    Ok. The question now is *Why not?*

    Because it should. According to the schematic it should be tied to pin 3 of that same socket and then to a 25uf cap parallel with an 820r resistor on the board that are to ground.

    With that pin not connected the tube will not conduct. This could explain the low volume and lack of reverb.

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    Last edited by Chuck H; 11-17-2019 at 06:44 PM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    Ok. The question now is *Why not?*

    Because it should. According to the schematic it should be tied to pin 3 of that same socket and then to a 25uf cap parallel with an 820r resistor on the board that are to ground.

    With that pin not connected the tube will not conduct. This could explain the low volume and lack of reverb.
    I see exactly what you're saying when looking at the schematic about pin 8. I've taken some pictures of the chassis today so you can get a better look at the wiring for V4.Click image for larger version. 

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    Ok!!! Well, somebody's been in there eh?

    The orientation of the reverb transformer looks to be 90 different from what I'm use to in earlier amps. That threw me off for a second. I think those cathode circuits going to the RCA jack frames for ground isn't stock, but I've not been inside one of these amps before. Still, the component values for the V4 cathode circuit look right so try connecting pin 8 to pin 3.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    Ok!!! Well, somebody's been in there eh?

    The orientation of the reverb transformer looks to be 90 different from what I'm use to in earlier amps. That threw me off for a second. I think those cathode circuits going to the RCA jack frames for ground isn't stock, but I've not been inside one of these amps before. Still, the component values for the V4 cathode circuit look right so try connecting pin 8 to pin 3.
    Thanks for your input Chuck. So (just to review) I need to connect pin 8 to pin 3 using a jumper wire, correct?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Sleet View Post
    Thanks for your input Chuck. So (just to review) I need to connect pin 8 to pin 3 using a jumper wire, correct?
    Correct.

    No guarantees how it will work out though. I can't know what sort of changes may have been made to the amp. No risk of damage to anything.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Am I seeing things or is pin1 connected to a heater wire? (pin8 is without connection)


    Click image for larger version. 

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    Last edited by g1; 11-18-2019 at 02:21 AM. Reason: add pic
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by g1 View Post
    Am I seeing things or is pin1 connected to a heater wire?
    It looks like that in one of the shots. Furthermore, it's hard to tell otherwise in all but one of the other shots. But I don't think so.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    see edit, post #20

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    Quote Originally Posted by g1 View Post
    Am I seeing things or is pin1 connected to a heater wire? (pin8 is without connection)


    Click image for larger version. 

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    Yes, that's how it is currently connected, or not, in the case of pin 8. Pin 8 appears to have some traces of solder on it. The amp, to my novice eyes didn't seem to been modified in the past. Having said that, it certainly doesn't seem to be wired correctly..... I attempted to get as much detail in the photos as I could.

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    " I think those cathode circuits going to the RCA jack frames for ground isn't stock, but I've not been inside one of these amps before."

    Actually, this is indeed stock. I have one in my shop right now that is this way.

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by g1 View Post
    Am I seeing things or is pin1 connected to a heater wire? (pin8 is without connection)


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    Quote Originally Posted by Chuck H View Post
    It looks like that in one of the shots. Furthermore, it's hard to tell otherwise in all but one of the other shots. But I don't think so.
    I see what you mean now Chuck. That heater wire isn't going to pin1, but tucking around under and going to pin 4/5.

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    Quote Originally Posted by pdf64 View Post
    Just noticed the above, 'coupling caps' seems odd, was that just a terminology error, or have ecaps been used for coupling purposes?


    Confused by this, the neon couldn't flash if it's been removed from circuit?

    My take on the thread is that
    1/ the amp seems to have been modded as the master doesn't seem to affect the normal channel (true?); so need to verify the circuit to its intended schematic.
    2/ from the bent front panel, the master volume may have received a massive bash, may be broken, and given the symptoms must be replaced, or just taken out of circuit. Hardwire the pull boost switch in the non boost mode.
    3/ a voltage survey and idle plate / cathode current measurements (eg resistance and volt drop from OT CT to each plate) would be very useful.
    I just reread all of the comments and postings, thanks to one and all. I would like to narrow the field of possibilities, a process of elimination to get to a solution. I seem to have a partially functioning amp with the power section working and part of the preamp working. The reverb and vibrato channel are malfunctioning. Based on the schematic I think I need to troubleshoot the signal path and confirm the function of the master volume.

    I do have a signal generator and scope to help out. Voltage and current survey as pdf64 suggested might also be enlightening. Any pointers or starting points would be greatly appreciated.

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    Did you tie pin 3 & 8 of V4 and now are still having the problem?
    (sorry about the heater wiring distraction)

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by g1 View Post
    Did you tie pin 3 & 8 of V4 and now are still having the problem?
    THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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    Quote Originally Posted by Chuck H View Post
    THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    I went and tied pin 8 to pin 3 and sure enough I'm now getting full output from the vibrato channel. I believe that in process of changing out the capacitor tied to pin 3 I seem to have forgotten to connect the capacitor lead across to pin 8. All of the tone controls and the MV are all functioning with both channels. I haven't had time to reconnect the reverb tank (getting late), but the I'm pretty sure it's going to work.

    Thanks so much to all the contributors in this thread. I may be back with a few more questions related to this amp

    4 Not allowed! Not allowed!

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