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Post subject: Re: Fender 75 Reverb Problem??
Posted: Thu Dec 24, 2015 9:15 am
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Harris Norton wrote:
This is what I figured. Thanks for that good explanation. I bet I just never noticed it before cause I never turned it up that much. I'm fine with leaving it the way it is. Thanks again for everyone's help!


There are few venues where you could dime such an amp with any expectation that you'd be invited back to play again. Thus the issue becomes a non sequitur for all intents and purposes. Enjoy your amp -- it's a great one.

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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 04, 2016 5:22 pm
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Don't mean to beat a dead horse here, but I just discovered something. I opened the reverb tank, it's a 3 spring tank, and saw that one of the springs is missing. Would this have any affect on my problem or is it not relevant? The reverb seems to work fine but I'm not happy with an incomplete tank. I'm considering replacing it.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 04, 2016 5:59 pm
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Is that 3-spring tank original to the amp?

Something tells me it's not......

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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 04, 2016 6:12 pm
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yeah I believe it is. It's a USA made Accutronics so it seems to be from the right time. Doesn't look like it was changed out.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 04, 2016 8:12 pm
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Regardless, I doubt that the missing spring is the root of your reverb problem.

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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 25, 2016 8:37 am
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Ok one more stab at this. The caps were bad so I replaced the filter caps, bypass, caps, etc. amp sounds even better now but I can still here that reverb even with the volume down at normal levels, it's really bugging me now. If I unplug the tank, it goes away so somewhere in there some signal is going through the tank where it's not supposed to. I also discovered that when I turn the lead channel on, the reverb goes away, which is supposed to happen considering they both use the same tube so they can't be on at the same time, that's my understanding anyway. However, I can turn the reverb on with the lead channel on and it goes away completely when I turn the reverb knob all the way down. Does this sound like faulty wiring possibly? or maybe something to do with the optocouplers, aren't those the things that control the channel switching? just thought I throw this out there if it makes sense to anyone. Sounds like something is going on with this signal path and isn't a "normal" reverb leak problem.

Also, there is also some added buzz/hum when the reverb is clicked on and turned up. not sure if that is related but is also annoying.

Thanks again for the help.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 25, 2016 9:22 am
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Harris, the way the Fender 75 has the reverb section of the amp wired, you will have a very difficult time getting the reverb to go to absolute quiet. There is only one input section in your amp (purple dot). That means the reverb section (within the yellow square) will always see signal. The reverb section of the amp is ALWAYS on. The 12AT7 driver and 12AX7 recovery tubes are always energized.

The on/off foot switch connects where the green dot is. By the mere nature of a long foot switch cord and the contact of the jack and switch --- you can never get 0-ohms (complete ground out --- at blue point) --- in other words complete dumping of reverb signal to ground can only happen if the resistance of green point (through the on/off foot switch) to the blue point is 0-ohms. Thus, sending 100% of the reverb signal to ground.


Image

If there is any potential in this line from the jack point of the foot switch-to the chassis ground ---> you will get bleed-through of the reverb. The closer you can get these points to 0-ohms the better the ground out of the reverb signal. Try this... get a 1/4-inch plug (or whatever plug your foot switch uses to connect to the amp). Solder the center (hot) point to the ground strap on the plug (make a grounding plug). Now, plug this into the jack, where the foot switch would go. My bet is the reverb tone will be more quiet. You'll need to crank the amp up more, to hear the residual reverb signal.

Most BF and SF Fender amps avoid bleeding of the reverb (and tremolo, if the amp has both) --- by having TWO inputs. The NORMAL input does not feed the reverb and/or tremolo sections of the amp. Therefore, much less chance of bleeding of either EFX into the output stage. Ultimately, into the speakers.

For the Fender 75, Fender saved money by not having the separate NORMAL and VIBRATO (or REVERB) inputs. They probably thought that most ppl, during the era when the amp first came out, used reverb --- so there is no need to do elaborate circuitry to insure that the reverb signal is 0%.


Super Reverb schemo. But most Fender amps of this era have this type of circuitry (wrt the EFX sections).

Image


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 25, 2016 9:48 am
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ok great, thanks for the detailed explanation. Here's what I've done so far. I've tried the grounding plug, that didn't make a difference, but I think I did the ring to ground, not the tip. Was that the wrong part to ground? I've also measured resistance from the jack ring and tip and several other places in the amp to ground and I get anywhere from 0.02 ohms to 1.2, not sure why it changes but that's what I've seen and that seems pretty low to me. so it looks like it's grounding out most of the signal, unless 1.2 ohms is enough to let it through.

Any idea about the lead channel part and why I can turn on reverb and why it goes away completely with lead on? The reverb also passes through those optocouplers, right? If this is how it is then I'm fine accepting that, but this new finding has me questioning whether or not there is more too it. but if I'm wrong then that's fine.

I appreciate your help with this. I'm a dumb newbie at this but I'm trying to learn and see if I can figure this out, if possible.

And if V2 is the reverb recovery tube, what does V4 do? I've been wondering this and couldn't figure it out since V2 is actually before V3, the 12AT7 instead of after.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 25, 2016 11:49 am
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Harris, 1.2-ohms difference between the contact and ground prolly not enough potential to bleed signal into the output stage. I agree, the layout of the gain stages is much more complicated than first appears. Thanks to the crazy way Fender drew the schematic. Need a few questions answered to maybe help find out how this amp works.

The EFFECT IN/OUT switch operates the reverb? What other EFX does this foot switch control?

What does pedal jack RED and pedal jack PLAIN control?


My guess: V2A = reverb recovery. V2B = gain stage after the reverb. V4A = pulls signal off V2B. Prolly another gain and/or mixer stage.

The two 013556 appear to be associated with the RED and the PLAIN sections. Does this amp have lights that go on with regard to those foot switch positions?

Thanks! We'll figure this one out. :fingerscrossed:


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 25, 2016 12:50 pm
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I hope so! I appreciate your trying anyway. Here we go...

EFFECTS IN/OUT is a separate jack for an effects loop, not related to reverb, lead or anything else, just a send/return for external effects using a TRS y-cable splitter. This is not my amp but you can see the effects jack, and the footswitch plugs and reverb plugs

Image

This amp has 2 "channels", Normal and Lead. The RED jack is the input for the RED plug on the footswitch which controls the LEAD Channel on/off. The PLAIN jack is for the other plug on the footswitch which is reverb on/off. here is the schematic of the footswitch. both effects are intertwined in the wiring but are separated by the jacks. So the effects are off when the footswitch sends either signal to ground, as you said earlier. But that ground looks good when I measured. I don't have the exact footswitch below, I made my own but it works fine switching both effects on and off.

Image

Image

Image

Yes the footswitch has 2 led's that go on/off when LEAD or REVERB are switched on and off. Those part numbers are those optocouplers I think, you can see them in the picture below of my amp

Image

I hope this helps!!


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 25, 2016 3:07 pm
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Thanks! Big help. Very nice job on that footswitch.


Since the PLAIN jack is the where the reverb foot switch attaches, this is the problem area. The jack has two internal attachments to the amp's circuit. One longer lead (+) on the schematic, to V2B signal grid. Attached at point on the reverb pot control. This is where V2A injects post reverb tank signal into the reverb control pot. And the shorter (+) lead to the 013556 LED lamp (opto-coupler).

Seems that both leads carry reverb signal. My guess is that if you have checked all points from the footswitch (FS) to chassis ground --- that is: 1.) measured static DCR from metal body of FS to amp's chassis, 2.) Measured DCR from (+) of switch in FS to chassis ground, with reverb switch in FS in off position, 3.) measured the (-) side of the FS jack to chassis ground --- and all measurements are 2-ohms or less... problem may lie in that 013556 LED or circuitry around it.

There are some cap-resistor networks around that 013556. Could have a leaky 0.0022 mfd cap. This cap maybe there to block reverb signal going to ground (back flow from V2B grid). Or the 013556, itself, could be leaking signal to ground. If reverb signal (even weak signal) rides on the ground... later gain circuitry may pick it up & amplifying it.

You could try measuring VAC (in mV) between the ground point just left of that 013556 (on the schematic) and say where the green earth ground from the power cord attaches to the chassis. Try the measurement with the tank connected and not connected. Plug the guitar in and pluck a few notes to see if the VAC readings jump with tank connected and not connected. A difference of 10-50 mVAC maybe enough to inject reverb into the final stages.

A scope and signal generator would really be nice. But, I think the DVM may give you the answer. In any event, if the 0.0022 mfd cap is good --- you would need to rule-out the 013556. I'd try lifting one connection. Maybe, the one to ground and see if that resolves the reverb bleeding issue. I have no idea where you can buy a new 013556.

As for V4A, it looks like another input/impedance matching stage. For effects put into the amp via this jack. Not quite sure what EFX you plug into this jack, but it seems like the signal is 180° out-of-phase with the main signal (goes to cathode of V4A). Maybe to compensate where the EFX is injected, V4B. Which could be out-of-phase with the original signal.


Good luck! Keep us posted. :mrgreen:


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Mon Jan 25, 2016 6:03 pm
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Awesome, thanks! This gives me a new place to start looking. How do I know if one of the caps is bad? I don't think I have a capacitor setting on my DVM. I will also check all those spots that you mentioned to make sure the signal is being grounded. I'll let you know what happens. Thanks again.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Tue Jan 26, 2016 8:27 am
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I'd just replace the 0.0022mfd cap. Need at least 400VDC rated. The cap and the 220k-ohm resistor are in some kind of RC or crossover network, to limit the frequency of what is being sent to the LED coupler and to ground. I have a bunch of caps laying around, so it is easy to do that mod.

In general, caps can fail with time and heat. More so, than resistors. That LED is suspect, though. As these and other solid-state devices can also be heat and time sensitive.


Good luck! Keep us posted.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sat Jan 30, 2016 2:55 pm
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Ok update. I tested all those points to ground again and everything was within 0.5-1.5 ohms so still good there. I wasn't able to test the signal because my DVM doesn't do AC mV, I can try with another one but I didn't see anything when I tried that. I haven't changed the capacitor or resistor yet because I wanted to test something else first and I'm getting warmer. I borrowed a much newer reverb tank and hooked it up. It's not the exact same model but it works and...the reverb signal is much less noticeable with that one. I can barely hear it when playing normally and my new test is to just slap the strings so no notes come out but I can hear the reverb splashing. It's much more pronounced with my tank rather than the other one. so maybe my tank is going bad? maybe that missing spring in mine, there's only 2 out of 3 in there, is causing some signal to get through and not be grounded all the way/not completely the path? either way it might be time for a new tank, but I don't want to spend the money if that's not really going to fix but it definitely made a difference. I can hear a tiny faint splash with the new tank but nowhere nears as much as my current one. Thoughts? I'll definitely look into changing those caps and resistors if necessary.


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Post subject: Re: Fender 75 Reverb Problem??
Posted: Sat Jan 30, 2016 4:09 pm
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I think you need to determine whether or not your tank type is original to the amp and if it's not, buy the correct model for it. Not all tanks are created equal. Aside from the amount of delay, there are input and output impedances and the type of grounding scheme to consider. Each tank is designed for a specific application and violating the protocols I've cited may cause the performance problems you've encountered.

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