Boss FA-1 Rehash Circuit Dilemna (Dilemma) Need Help

[bluelagoon]

Hoping I can receive some well needed assistance here to nut out the present conundrum in this Boss FA-1 rehash circuit. Circuit includes optional selectable Mid Range tone control, plus optional selectable voltage range on all op amps after IC1.
Since you guys here on this forum are best in the biz for troubleshooting, I think I've come to the right place.

Here's the lowdown, The effect sounds great, works flawlessly in the +9 Volt power select mode.
Trouble comes when I up the voltage selector to run at the higher voltage rate of +27V, with +18V -9V and VR +4.5V on all last 4 op amp stages, it still works well and sounds good with the extra headroom emphasis from the higher power supply, but trouble comes from what this higher voltage supply brings when switched in.
It adds +7.1V DC to the output signal, and thus some extra loud clicking and thumping when switching in and out.
I cant understand why it is happening, since when in 9 volt mode there is zero volts DC on the in and out signal lines,

It uses 2 Hitachi HA12017 SIP7 op amps, while these weren't used in the original Boss FA-1 as much as the more popular primarily used HA1457W they were still used in some of the original Boss FA-1's non the less, and give much the same results as the HA1457W with just a few components differing.

Any light to shine on this dilemna is much appreciated. Have included the main circuit detail in the schematic.
Thanks for looking and hope you can help me out. Cheers.

[antonis]

It adds +7.1V DC to the output signal,

Meaning you measure 11.6V DC on IC3C output..??

or

you measure 7.1V on C10/R12 junction..??

[bluelagoon]

Hi Antoni, Yes it is showing 7.1V at C10, R12 output junction, every time that 27V power selection is made. I don't know why, and another thing, even though I have omitted in the schematic, is that I have an optional selectable Electra Fuzz circuit that is switched in or out after C10 and R12, and when the Electra Fuzz gets switched in the DC voltage at the output then goes back to zero. the electra circuit is set at fixed voltage +9V

See attached Schem for where the Electra Fuzz is placed in the circuit.

[antonis]

when the Electra Fuzz gets switched in the DC voltage at the output then goes back to zero.

Zero or about 200 - 300 mV..??
(one Ge diode voltage drop..)

In case of zero, there is a build up voltage on 10μF negative plate (without obvious reason) where in case of a Ge forward voltage drop there is a permanent supply there..

[bluelagoon]

The DC on the output when the Electra Fuzz is switched in with the voltage at 27V selected, is
25mV with no diodes attached, with germanium pair is around 45mV with silicon pair around 90mV
As I have the diodes switchable in and out.
But even where the whole Electra Circuit is disconnected switched out and bypassed, the voltage at the output still exists as the main problem.
Another strange phenom, is that when in the 27V and effect switched on and midrange selected, the voltage at the output varies from 5 volt with midrange pot full off anti clock, up to 7 volt full on clockwise.
Thanks for investigating this with me, appreciated, hopefully we will get to the solution. Cheers.

[duck_arse]

what physical form is this circuit in thusfar, breadbord? pcb? cause I look and see that switch, with ground and -9V on one side, and Sw- on common, and I think to myself - if that wasn't wired as shown ............

[bluelagoon]

Hello Duck A,  The physical form of this is a fully fabricated PCB assembly. And As for the power select switch it all tests out correctly, presenting appropriate voltages to the op amps as per selected, +9 and Ground, as one side of switch and +18 and -9 on other side of switch going to the last 4 op amps in circuit, all keeping the same voltage reference bias +4.5V.
Its a mystery thus far, hoping someone can help me solve, Thanks.

[duck_arse]

ahh, well then, you would be showing us photos of the thing, no? and layout drawings?

[bluelagoon]

Okay, here are some images of the item -

[Rob Strand]

Hi Antoni, Yes it is showing 7.1V at C10, R12 output junction, every time that 27V power selection is made. I don't know why, and another thing, even though I have omitted in the schematic, is that I have an optional selectable Electra Fuzz circuit that is switched in or out after C10 and R12, and when the Electra Fuzz gets switched in the DC voltage at the output then goes back to zero. the electra circuit is set at fixed voltage +9V

See attached Schem for where the Electra Fuzz is placed in the circuit.

Check C10 is in the right way around.  If C10 is working you can't get a constant DC at the output unless something is wrong.

You might expect some sort of transient thud on the output when you switch supply rails but not a constant DC voltage.

FYI, you can minimize some of the switch-over thud by placing say 100 ohm + 47uF cap across the sw+/sw- supplies.  What that does is hold-up the opamp supplies during switch over and make a slow transistion between the two voltages.  High R and low C are safer because when you switch from 18V to 9V you don't want the charge from the added caps to feed too much current back into the +9V rail and fry the converter.  You could add another level of protection by adding diodes in series with the 100R resistors.

When I look at your schematic it seems there's cap missing from D3's cathode to ground, maybe 100uF.  The missing cap could cause havoc.

Something else the 100k (R12) should be on the C10 side of the switch.  The another resistor (R12B) on the Electra output, 1MEG would do there unless you like the sound of 100k

[bluelagoon]

Hi Rob, Good pickup, I had redrawn that schematic and forgot to put that cap back in there at D3, fixed now and reloaded the correct schematic. Had a look at C10, seems to be oriented the correct way around, I might pull it from the board and check and replace, to see if any difference.

Also had in mind to do a second power board and omit the charge pump and 10V zener, to then test it at 18V and 24V on the +9V power select switch setting, to see if it still puts the extra voltage at the output. My main issue is the heavy clicking and thunking with the footswitch when changing states, on what is otherwise a reasonably silent relay bypass system using the VFE circuit and MCU chip.

Aside from that it still gets the sound coming out alright as expected, just would imagine it cant be healthy on some other level with that excess DC voltage at the output, had a feeling it could be detrimental to effects coming after, although I guess that is what the coupling caps are there to block. Thanks for looking at this with me. Cheers, and ทำเต็มที่ "do your best"

[bluelagoon]

Just tried something, Placed another 10uf on the signal path negative leg directly onto the output jack signal out, with a 100k to ground , plugged it all together and tested on the work bench.
Seems to have fixed the issue, it is reading zero for any DC on the out signal now when effect is engaged.
Will test through the amp tomorrow to confirm if its worked or not.
Sure cannot figure the reason for it, but then I am no electronics engineer.

Scrap that last thought on putting that 10uF on the out jack, Duh, that was a dumb move. Forgot that the true bypass needs to go to the same point, which makes it a bad move, have since pulled that cap and resistor, but it did show promise in that it reduced the DC voltage to zero at the output when effect was engaged, so right idea just wrong placement,

Will try the same concept again with the correct placement next attempt.

[antonis]

Sure cannot figure the reason for it, but then I am no electronics engineer.

But you can easily put the blame on C10 / DPDT switch order..

[duck_arse]

one thing about your circuit diagram - you have a point marked "+9V" on the left side of R25, and another point right of R25 marked "+9V". you need to change the name of one of those, or the layouter will connect them together and short the resistor, negating the filter it is part of.

if you measure the voltage between a jack sleeve and the 'ground' end of R12, what reading do you get? when you work that switch, what reading? if you do the same measure between your "-9V" point and the 'ground' end of R12, what results?

also, there is a relay on your board marked RLY1. where on the circuit diagram is it? one of the relay pins has a track that runs down by sw5, looks like it might have a connection to the left side common. is this so, or my eyes?

[bluelagoon]

Hello Stephen, You don't mean C25 that you were referencing to?
Have put an image to show a bit more detail at that point, other than that not too sure what you were alluding to.



As for the Relay 1, Yeah that is not included in the schematic I put up, as It was just going to get too convoluted putting every detail on that schematic, I really only just wanted to reference the main detail where there was an issue that needed addressing, as for that relay section, plus the Electra Fuzz add on at the end, plus some extra switching detail pertaining to the fuzz and the relay, they are all very much not a part of the issue that's presented, as far as I can see, just mainly peripherals that I already ruled out of the equation,  so didn't concern to include in the schematic, only the basic context of the circuit, and the problem that is inherent.

But since you've asked, the relay is part of a switching circuit that includes a Coda Bypass MCU and momentary switch that totally bypasses the Baxandall Treble and Bass, and with the other switch that bypasses the Mid range allows for the complete Baxandall and mid shelving filter to be switched out, leaving just an uncolored boosted signal through,

It is very much a take on the JHS Clover pedal only with some extras.

Will do a check on those voltages you mentioned tomorrow, as for now I need some sleep.

Cheers

[Rob Strand]

Hi Rob, Good pickup, I had redrawn that schematic and forgot to put that cap back in there at D3, fixed now and reloaded the correct schematic. Had a look at C10, seems to be oriented the correct way around, I might pull it from the board and check and replace, to see if any difference.

Also had in mind to do a second power board and omit the charge pump and 10V zener, to then test it at 18V and 24V on the +9V power select switch setting, to see if it still puts the extra voltage at the output. My main issue is the heavy clicking and thunking with the footswitch when changing states, on what is otherwise a reasonably silent relay bypass system using the VFE circuit and MCU chip.

Aside from that it still gets the sound coming out alright as expected, just would imagine it cant be healthy on some other level with that excess DC voltage at the output, had a feeling it could be detrimental to effects coming after, although I guess that is what the coupling caps are there to block. Thanks for looking at this with me. Cheers, and ทำเต็มที่ "do your best"

If C10 is OK then maybe check where the ground side of R12 (the 100k at the output) is going.  You could measure the DC voltage on both sides of that resistor.    I both sides are at a DC voltage then it's likely to come from the ground side.

[bluelagoon]

Checked the R12 resistor, the ground side has continuity with the rest of Ground, and has the same 7V across it with the Effect engaged.
Will check What Duck A mentioned next. Cheers.

Checked the Voltage between -9 and ground end R12, with black probe on ground end R12 red at -9, Gives a reading of same voltage as at output -7.1V

And No traces from Relay 1 connected or bridged at Power Select switch top Right on PCB, see pic -




Seems to be something indicative, arising from the Power Pump, with its extra power rail -9.1V, as it gives that +7.1V at the signal output, which is getting close to what the supply reading drops down to under power load with the effect on. Something to ponder and contemplate.

[bluelagoon]

Just wondering if the DC on the output has to do with the 100nF Decoupling Caps coming off IC2-(C16) and IC3-(C22) at their + power pins to Ground.

When they receive the +18V and the -9V at their power pins, with the +4.5 VR, just thinking , wondering, is this an imbalance having those decoupling caps at ground?

[duck_arse]

one thing about your circuit diagram - you have a point marked "+9V" on the left side of R25, and another point right of R25 marked "+9V". you need to change the name of one of those, or the layouter will connect them together and short the resistor, negating the filter it is part of.

[bluelagoon]

Okay thanks Duck, I get ya now, the one on the left is just the DC Jack power in, Thought it would suffice, but then agasin, clarity is always nice.