Passac EC-100 Acoustic Preamp

mediy · Post by **mediy** » 22 Oct 2022, 04:17

Not really a stompbox, but related. This is an external piezo preamp that I have since the early 90's - back when the controls on an acoustic with a pickup were passive volume and tone. I believe that the company was bought out by one of the big players and that these designs were incorporated into the early onboard preamps. Anyway, the schematic doesn't seem to be out there that I could find so here are a few photos of the boards.

I am short on time now but will come back and put a few values of components that are hard to see.

mediy · Post by **mediy** » 22 Oct 2022, 09:45

Q3 BC559
Q4 BC549
C8 (red WIMA cap near the ribbon cable) 0.1uF
C6 (red cap in the middle) 0.15
C2 (red can next to the pots) is hard to read because the value is printed on the side right next to the pot but it looks like 0.1uF

The bass/treble and presence pots both have centre detents. I am pretty sure the tone pot is some form of tilt EQ

The LED is designed to illuminate for a few seconds when the input jack is inserted and then goes out.

According to the manual the removable jumper changes the input impedance from 1 MOhm (installed) to 3 MOhm (removed)

The resonance and balance controls are about tuning the preamp to remove dead spots. The process is to set balance to zero and play up and down on the bass strings. When you hear one note that sounds sounds dead or lacks timbre you then set the resonance control to the frequency for that note - 9 o clock is 110Hz, 12 o clock is 131 Hz, 3 o clock is 160Hz. Increse the balance level until the dead note takes on the same timbre as the others. A/B the acoustic sound with the amplified sound and if there is a noticeable boost on either adjacent fret then re-tune the resonance.

I will leave this out of the box for a bit in case there are any other Q's about it.

mediy · Post by **mediy** » 22 Oct 2022, 11:25

Oh yeah, the 2 electro caps are 100uF. The blue wima cap is marked 1500, so I assume 1500pF

cspar · Post by **cspar** » 24 Oct 2022, 01:19

I just finished checking the pics of the main board against the clone I made with EagleCad.
Here's the schematic.

Passac-EC-100.sch: (112.56 KiB) Downloaded 36 times

It's interesting how the signal seems to ride the rail of the op amp. I personally haven’t seen that before.

I'll probably trace the smaller board in the morning and tidy it up some but I'm done tracing for the moment, on to other things.

cspar · Post by **cspar** » 24 Oct 2022, 16:51

I've started on the smaller board but I have quite a few questions hopefully you haven't reboxed it yet mediy.

For the resistors I have from the transistors downwards;
220k
100k
270k
And the one by the diodes 33k.

The rest of the board's values/part # are a mystery.

Can you possibility get the transistor #'s, look for diode #'s and the electrolytic value.

Also on the top of the first picture, above the 7's on the sticker there are three solder spots.

It is unclear to me if they are all connected together it seems so but it also seems that there might be a break as well as a thin trace that goes under the sticker that I can't really see.

It probably is nothing but I might as well ask.

Thanks for the gut shots.

mediy · Post by **mediy** » 25 Oct 2022, 09:43

The transistors are above: Q3 is BC559 and Q4 is BC549.

The electro cap is another 100uF

I can't see any markings on the glass diode in the middle, but forward voltage drop measures 0.6V in circuit

The blue diode puzzles me. I can read B2X on it, which I think makes it a zener. with the unit powered up I read 9V on one end (anode) and 8.55 on the cathode. with power off forward voltage reads as 0.7V in circuit.

Those 3 solder spots are all connected and there is no thin trace from there under the sticker. It looks like the 9V line, and all 3 pads are 9V connected

mediy · Post by **mediy** » 26 Oct 2022, 09:28

The LED on this hasn't worked for a bit, and I have now tested it and it does seem to be just a broken LED. I have a busy weekend coming up, but I am going to try and find a few minutes to get into the shed and replace the LED. If that goes well with no risk of lifting pads etc, then I will try lifting one end of each of the diodes so I can read whatever marking are hidden on the bottom.

mediy · Post by **mediy** » 28 Oct 2022, 08:26

Except that when I got my son to help me test a replacent LED using the "here, plug this in while I carefully hole the component on these pads" trick, the original LED worked fine, and has ever since, so I won't be replacing it.

Manfred · Post by **Manfred** » 28 Oct 2022, 11:30

I made approximated PCB Clones for tracing, the main board is in prorgress.
I found the following circuit structure of the PCB with the jacks.

cspar · Post by **cspar** » 28 Oct 2022, 15:57

Cheers Manfred.

Your trace as always is definitely more tidy than mine in form as I was just going for the connections instead of a scale accurate clone.

I had set my schematics to the side to come back and combine the boards into a circuit block independent of layout.

I'll have some time today to do that.

Both of our approaches have their own value.

I noticed a few things on your small board schematic that could use corrections other than the values of the diodes.

You forgot the green 3mm LED.

The 270R does not connect to ground, it connects to the LED.

The electrolytic is 100uf.

Also although implied it is a little unclear on the schematic how the power off/on switching is done via the ring to sleeve connection of a mono jack in the output socket, not the input socket.

Perhaps a bit of notation about the power switching on the schematic would be helpful for the historical record that you are creating.

Once again, cheers.

mauman · Post by **mauman** » 28 Oct 2022, 17:34

According to the stencil on the front, the battery switch is part of the input jack. The battery negative is wired to a third lug on the Input jack (not Tip, not Ground, may or may not be Ring.)

: EC-100 front view

cspar · Post by **cspar** » 28 Oct 2022, 20:22

I stand corrected about which jack is which and the exact way the switching is.

The above comment was made after glancing at Manfred's schematic without looking at my trace at all.

My schematic needs tidying as I didn't mark my sockets as in or out nor label the ribbon pins on either board. I left off at tracing those with the help of an additional gut shot from reverb.com.

Without the full trace I assumed that the volume pot was on the output not the input.

My folly... as was my earlier typo of 270K instead of 270E.

I'm sure of my trace as I made clones of the boards in eagle without the tracks being right angled and just using pins instead of the ribbon and sockets.

It's quite apparent now that I am bridging the gap of the ribbon in more sense than that the middle goes straight to a socket that I'm going to have to tidy the 1st schematic I posted so that the complete circuit block shows propper flow of of the signal.

Manfred · Post by **Manfred** » 28 Oct 2022, 23:56

You forgot the green 3mm LED.
The 270R does not connect to ground, it connects to the LED.

Thanks for the hint Cspar.

mediy · Post by **mediy** » 28 Oct 2022, 23:59

Can confirm that the input jack does the switching, and the timing thing with popping the LED on for a second or so and then off again. After seeing the mock up of the input/output/power board above I also got out the multimeter and can confirm that the thin trace in the centre that runs between the electro cap pads does not in fact connect to either of them, so that is correct

mediy · Post by **mediy** » 29 Oct 2022, 00:03

Some time later today I will measure the voltages across that suspected zener again. It looks as if it has something to do with turning the LED on and then off, so the voltages may change over the first few seconds after power up. Watching a full cycle of that may give a bit better idea about what it is?

mediy · Post by **mediy** » 29 Oct 2022, 05:16

So, measuring the cathode end of the diode marked B2X as the jack is inserted to turn on the device - the voltage goes up to 5.8 or 5.9V and then drops back to a few hundred millivolts as the LED turns off again.

mediy · Post by **mediy** » 29 Oct 2022, 10:09

I cranked out my TC1 component tester. The diode in the middle measures 0.707 Vf. To me it looks a lot like 1N4148. I'd be surprised if that didn't work in that spot. The other one measures as a pair of diodes back to back. One way measures 0.73 Vf. The other way measures 4.3 Vf. I assume that is further evidence of a zener with zener voltage of around 4.2V?

mediy · Post by **mediy** » 29 Oct 2022, 10:28

Looking at data sheets B2X is probably BZX and it looks like BZX55 or BZX85 4V3 zener. The packaging looks like it was made by Thomson.

Manfred · Post by **Manfred** » 29 Oct 2022, 23:49

The value of the resistor parallel to the diode is 33 Kiloohms not 3.3 Kiloohms, I read it wrong.

I found a similar battery indicator circuit.

flashing-led-battery-status-indicator.pdf: (102.79 KiB) Downloaded 22 times

I am now ready with the approximate clone of the main PCB.
Please, could you please check the values and fill in the missing values of the red framed parts.

mediy · Post by **mediy** » 30 Oct 2022, 01:36

The 2 trimpots are marked 103B - so 10K linear.
The main control pots from left to right are 105k 103b and 103b - I assume !Meg linear, 10k linear and 10K linear.
The cap values are in posts above - C8 (red WIMA cap near the ribbon cable) 0.1uF
C6 (red cap in the middle) 0.15
C2 (red can next to the pots) is hard to read because the value is printed on the side right next to the pot but it looks like 0.1uF
the 2 electro caps are 100uF. The blue wima cap is marked 1500, so I assume 1500pF

Passac EC-100 Acoustic Preamp [schematic]