Exar - MT-1 Metal  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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massaada
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Post by massaada »

My first reversed stompbox
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Schematic Exar Metal MT-1
Schematic Exar Metal MT-1

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mirosol
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Post by mirosol »

Due to newfound interest on Exar designs, i decided to try this circuit out. The feedback idea of the gyrator stage is something i don't think i've seen before.

No. It won't work as drawn above. I think there needs to be a coupling cap between the output of the first opamp stage and the 10K resistor before the clipping diodes shunt to vref. Without this cap, most of the signal will be dump to virtual ground - and the output level will be extremely low. Assuming that there is an output. The output buffer has no reference voltage on base and thus, it won't pass signal.

Tried adding 150n cap in series with the 10K going to shunt clippers. And placed a 470K resistor from vref to T2 base.

With those additions, it works. But i had somewhat underwhelming results. It's loud enough, but the gyrator controls are quite mild and otherwise this sounded a bit like a watered down Ibanez TM5. So no, i'm fairly certain this won't be your low parts count replacent build for HM-2.

Had fun playing around with this circuit though.
http://tagboardeffects.blogspot.com/
http://mirosol.kapsi.fi/
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VitaliiBobrov
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Post by VitaliiBobrov »

mirosol wrote: 14 Nov 2022, 18:12 Due to newfound interest on Exar designs, i decided to try this circuit out. The feedback idea of the gyrator stage is something i don't think i've seen before.

No. It won't work as drawn above. I think there needs to be a coupling cap between the output of the first opamp stage and the 10K resistor before the clipping diodes shunt to vref. Without this cap, most of the signal will be dump to virtual ground - and the output level will be extremely low. Assuming that there is an output. The output buffer has no reference voltage on base and thus, it won't pass signal.

Tried adding 150n cap in series with the 10K going to shunt clippers. And placed a 470K resistor from vref to T2 base.

With those additions, it works. But i had somewhat underwhelming results. It's loud enough, but the gyrator controls are quite mild and otherwise this sounded a bit like a watered down Ibanez TM5. So no, i'm fairly certain this won't be your low parts count replacent build for HM-2.

Had fun playing around with this circuit though.
I actually have an original Exar MT-1, I'll try to verify the schematic on weekend.

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Post by VitaliiBobrov »

So I took some time during the weekend to check the schematic and I found mistakes in it, the one from the first post won't work at all. Here is my unit
Exar MT-1 top
Exar MT-1 top
Exar MT-1 bottom
Exar MT-1 bottom
PCB in the series 1 Exars is single-sided, but unfortunately not "transparent" in the light. My unit was made in 1993 or later due to dates on potentiometers (93-04).
Exar MT-1 PCB bottom
Exar MT-1 PCB bottom
The first thing that was brought to my attention IC and transistors are not the same as listed in the first schematic. I have TL072, BC550B, and BC547B - these might be the changes during the production, as Series 1 was produced starting from the late 80s.
Exar MT-1 PCB TL072
Exar MT-1 PCB TL072
Exar MT-1 PCB top
Exar MT-1 PCB top
There were a few differences in capacitors/resistors values as well, but the most important mistake was the connection after the EQ state back to the gain stage. I attached a fixed schematic (omitted bypass switching parts) with components numeration as in the first schematic (PCB doesn't have any silkscreen, so components are not enumerated on it).
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exar-mt-1-schematic.pdf
Exar MT-1 Metal schematic
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Last edited by VitaliiBobrov on 26 Nov 2022, 20:57, edited 1 time in total.

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mirosol
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Post by mirosol »

Very nice work again! Thank you!
You wouldn't happen to have any of the Exar Fuzzes? :)

Here's a stripboard layout for the circuit. Didn't build it myself yet, but seems to be okay.
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Exar-MT1-Metal.png
http://tagboardeffects.blogspot.com/
http://mirosol.kapsi.fi/
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Post by VitaliiBobrov »

mirosol wrote: 24 Nov 2022, 15:26 Very nice work again! Thank you!
You wouldn't happen to have any of the Exar Fuzzes? :)

Here's a stripboard layout for the circuit. Didn't build it myself yet, but seems to be okay.
No, they made something called Super Fuzz, but I don't have it yet.

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Post by Lithium_Grim »

I think the schematic has some minor errors. The pins should be reversed on the Level knob (so pin 1 to Vr instead of pin 3) and the polarity of the 1uf on the output should be reversed.
I only give negativ feedback and y'all love it!
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Post by VitaliiBobrov »

Lithium_Grim wrote: 25 Nov 2022, 10:15 I think the schematic has some minor errors. The pins should be reversed on the Level knob (so pin 1 to Vr instead of pin 3) and the polarity of the 1uf on the output should be reversed.
You're right regarding the Level pot, just forgot to mirror the symbol. Thanks, will upload updated version soon. However the polarity of the capacitor is as it placed in the pedal 😉.

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