Fender Deluxe 85 reverb issue

[Apexelectric]

Have a late 80s/early 90s Fender Deluxe 85 on the bench with the reverb not working properly. I suspect there is an issue in the multiplexer switching system as the reverb effect will work briefly when powering down the amp. It won’t work with the footswitch plugged in or without it plugged in. The indicator led on the footswitch won’t light up either but the other two functions and indicators work.

I’m new to multiplexors and really can’t figure out how the mono 1/4” jack is able to trigger three functions separately. Could use some help in understanding the system and why I’m getting the symptoms I am. If I had to guess, the reverb SPDT switch in the multiplexor has failed and is stuck in the normally open position but not sure if that explains the led on the footswitch not working as well.

Attached is a schematic of the Studio 85 which has an almost identical switching system. The schematic for the Deluxe 85 is attached but the only copy I can find is rather illegible.

Any help is greatly appreciated

[PRR]

The pedal is fed AC. The pedal contains diodes which short-out part of the wave depending. U5A/B sort out the logic to SELA and SELB.

Reverb returns through U6A, CMOS 4053 with level-shift. Monitor pin 11. Does it go up/down as the pedal commands? I'd expect 'low' to be under 1V and 'high' to be over 6V, but just report what you get.

[Apexelectric]

Thanks for the reply,

On the Deluxe 85 the SEL pin for the reverb is on pin 9 and the voltage at the pin stays at around -8V regardless of the footswitch position. The other two channels toggle between positive and negative 8V

[PRR]

Follow 'SELB' back to its opamp (U5A?). What are the voltages on all three pins +In -In Out?

[Apexelectric]

Thanks again for the reply,

Looks like 0.6V in pos -4.4v in Meg and -16.1v out. On this amp it’s a LM339N 4 ch comparator rather than a dual opamp on the studio schematic.

[PRR]

> 0.6V in pos -4.4v in Meg and -16.1v out.

Is that +0.6V or -0.6V?

If "+": Very strange. The 'pos' input is biased by R51-R53. -16V at one end, -16V at the other end, so how is it at a negative voltage?

If "-": that's about the right answer. Does it change as the footswitch works?

The inverting input should change from -4V to about zero V as the foot switch works.

I assume the footswitch is verified somehow?

[Apexelectric]

Will double check the voltage on the positive again but I thought it was a positive 0.6 volts.

The reverb wasn’t working with or without the footswitch so I would assume it’s not part of the issue. I think, based on the schematic, that the effect should be active if the footswitch is not in use. I’ll verify the change a little later today, if any, when the footswitch is in use.

[Apexelectric]

My bad. Was counting pins wrong. Voltage on pin 8 is -4.4 with no footswitch present and 0V with it plugged in. Pin 9 is -.5 without or -.7V with footswitch plugged in. Pin 14 is 8V without and -16 with. No changes when pressing the switch

[PRR]

> No changes when pressing the switch

I wonder how that can be?

This is a special switch, isn't it?

[Apexelectric]

Just a SPDT footswitch that’s functioning as a SPST in this circuit. There is an associated diode for each of the three. The switch works according to my meter.

It seems to me that the reverb should work without the presence of the footswitch and it is there during the powering down of the amp so when full voltage is present the switching of the multiplexor to short out the reverb signal takes place when it should be allowing signal to pass.

Not sure why that is happening. I have a feeling there isn’t an issue with the footswitch.

[shooter]

withOUT the FS, does the led light when you flip the switch?

[Apexelectric]

Hey Shooter!

The only LED associated with the reverb is on the foot switch and it does not light up. The other two LEDs on the foot switch do light up.

[shooter]

so all this stuff is inside the FS?

[PRR]

Just a SPDT footswitch that’s functioning as a SPST

But it does two things? Looks like Channel/Distortion and Reverb? So two buttons plus some diodes to differentiate them?

[Apexelectric]

I posted two schematics, one is the deluxe 85 which is what I have on the bench but the schematic is hard to read, the other is the studio 85 which is similar and the schematic is legible.

[shooter]

do you have a signal source, IF so, inject it into the amp, set the volume so it's on the edge of annoying


with a meter set to measure LOW voltage AC, or a scope, what do you have at U4A pin 1?  The note says about 1/2voltAC

[Apexelectric]

Shooter, Plenty of signal at that point. Can get it up to about a volt without blowing out my eardrums.

Somehow the reverb started to work in this test process. I can now get the effect without the footswitch and it stays on even with the footswitch plugged in which still seems to do nothing. No indicator light, but the led, switch and zener diode all test fine.

I’m pretty confident I can chase down this issue if I just had a better understanding of how the switching system worked. Just not able to comprehend how the footswitch triggers three functions, through the opamps into the multiplexor, separately, on the same wire. How do the opamps differentiate between which of the three functions is being activated or deactivated if they’re all being fed down the same line.
PRR mentioned a clip of the AC wave by the diode which is a bit of a hint but doesn’t paint the complete picture.

Maybe if you teach me to fish, I can feed myself😎

[shooter]

get me the actual schematic of the FS, (the lake), i'll kick in the fishing poles, you dig the worms, we'll be eating panfry by nightfall 

[Apexelectric]

Thanks Shooter! I’ll see what I can “dig” up on the foot switch but it’s compromised of three SPDT and three diodes, one for each switch. Let me see if I can come up with the actual schematic though.

[Apexelectric]

This should help. S2 is for the reverb. Looks like CR1 and CR2 are just switching diodes and CR3 is a 6.2V zener

[shooter]

No indicator light, but the led, switch and zener diode all test fine.

can you "call out" the parts, helps allot, like LED2, CR33, since there appears to be switches in the fs, out of the fs, diodes everywhere.


the diodes from FS jack to SEL_ are used for "steering" and "clamping", an ancient form of logic that dates back to the '50s.  I entered the fray as DTL was phased out for the modern ('70s) TTL logic, but the oem's had so many diodes left over they couldn't just waste 'em.  when they broke-down under load, but tested good with testers, everything became illogical   

[Apexelectric]

Will do. I was just saying that CR2 and S2 and LD2 in the footswitch were testing ok according to the meter

[Apexelectric]

Took it a step further and put voltage to LD2 and swapped out CR2. LD2 woks and there is no change in the operation of the foot switch after swapping CR2. Looks like the switching issue is something on the main board.

[shooter]

there's still confusion on my part;
guessing those parts you replaced were inside the FS
because the schematic snippet in reply #12 also has a LD2
on that same schematic it shows S2A, does that exist in the amp?  it's pretty straight fwd to get LD2 to lite, vac2 on one side LD2, switch set for ground on the other side.

[Apexelectric]

My previous reply was regarding the foot switch assembly. Looks like there are repeated part numbers between the amp and the foot switch assembly.

If it’s a switch with an A or B designation, it wouldn’t be in the amp as they are all SPDT and not DPDT.

Bear in mind, the parts on the Studio 85 schematic are not the same as the ones in the Deluxe 85 which is what I’m dealing with. Only included it for the sake of analyzing a more legible schematic with a switching system that was similar to the Deluxe 85.

[shooter]

this should be a spst slider, not sure why it's S2A since i don't see an S2b,c,d...
the other one is S1A used for mid-boost, same thing, appears to be a spst slider


if you have the Deluxe 85 schematic, even if it's crappy, post a pic of the switching circuit RVB-jack to SELA SELB

[Apexelectric]

The first post of the string has the schematic along with the rest of the service manual but here are a couple of cropped pics

The resolution might be better on the pdf at the top.

[Apexelectric]

Shooter,

Been out of touch lately due to the wife having spine surgery. Have a little time here and there to check things out.

Let me know if this pics helped at all

Thanks!

[shooter]

the image made it to my computer but it's July here for the next couple days so I been playing hooky 

[Apexelectric]

Nice!

No huge rush, as always, thanks for the assistance.

[shooter]

ya, til Monday when it's back to snow, 85 to 35 
anyway;
I created a TP1, measure both VAC and VDC at that junction WITHOUT the FS plugged in. leave meter hooked to that TP.
with a NON metallic object, or a OPEN dummy plug, break the "short" on the FS jack.  (simulating a FS, but without the electronics getting in the way)


re-check both VAC and DVC at TP1

[Apexelectric]

Sounds like the Colorado weather here. Will check that and report back.