Troubleshooting a "Refreshed" Fender SF Pro Reverb

[Sleet]

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

[52 Bill]

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?

[g1]

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

[Leo_Gnardo]

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.

[Sleet]

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

[pdf64]

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

[Sleet]

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:

[Chuck H]

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.

[Sleet]

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.

[Chuck H]

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.

[g1]

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.

[Chuck H]

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.

[pdf64]

...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?

...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.

[Sleet]

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.

[Chuck H]

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.