Cornerstone - Royal Antique

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
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insidecircles
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Post by insidecircles »

I kinda wish I'd tried something easier for my first trace. Does anything look very wrong here?

This looks like a TS (with some mods) with a switch that adds in extra gain on the op amp plus a JFET boost on the back. There's also a separate JFET boost stage on an external footswitch - I've added that as a separate trace below, but it's a duplicate of the output boost on the main pedal. I believe it's also the same circuit in the Flame Boost by Cornerstone.
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royal antique.jpg
Flame boost.png
PXL_20230524_070530282.jpg
PXL_20230524_075926810.jpg
PXL_20230524_075934674.jpg
PXL_20230524_075939805.jpg
PXL_20230524_075944590.jpg

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mauman
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Post by mauman »

insidecircles wrote: 07 Jun 2023, 19:11 I kinda wish I'd tried something easier for my first trace. Does anything look very wrong here?
The biasing arrangement for U2.2 looks a little odd (blue oval in the center), there's DC continuity from U2.1 which will serve to bias U2.2. Could there be a series DC blocking capacitor missing after U2.1, maybe before or after the 1K?

The diode/EL cap arrangement in the power section (blue oval, upper right)will protect the cap from reverse polarity, but won't help with DC noise reduction. Putting the cap after the series 8.2R resistor would do more.

<edit> my apologies, I forgot to upload the marked schematic with the mysterious blue ovals! It's attached.
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royal antique.jpg
Last edited by mauman on 08 Jun 2023, 19:03, edited 1 time in total.

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insidecircles
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Post by insidecircles »

mauman wrote: 07 Jun 2023, 23:42 The biasing arrangement for U2.2 looks a little odd (blue oval in the center), there's DC continuity from U2.1 which will serve to bias U2.2. Could there be a series DC blocking capacitor missing after U2.1, maybe before or after the 1K?
Good spot, I've taken another look, but pin 1 connects directly to the 1k and the other end connects directly to pin 5. So, no DC blocking between those stages.

I have re-counted the caps and realise I've missed a couple. The Wima looks like it's in parallel with the 82n off the tone pot (I can't get the value without removing the part). Fine-tuning that tone response, I guess - maybe 82n is just a bit too small and 100n is just a bit too big. And there's also a 100n from the power rail to ground. I've updated the schematic with those below.
mauman wrote: 07 Jun 2023, 23:42 The diode/EL cap arrangement in the power section (blue oval, upper right)will protect the cap from reverse polarity, but won't help with DC noise reduction. Putting the cap after the series 8.2R resistor would do more.
Yes, I thought the power section looked weird. As I say, I've added a missing 100n, but I'd really expect that 100u in that spot. I've deliberately left off circuitry around the relay, and there's a few power components for that in this area. There's a 78L05 connected to the node between the diode and the 100u for that. Maybe I've simplified weirdly - maybe I should leave out the diode/100u from this schematic entirely. But I'm still left wondering why there's no (large) filter cap on 9V.
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Royal antique trace_2.png

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Post by bajaman »

The 8r2 resistor needs to be connected to the junction of the 1N4001 and the 100uf capacitor surely ?
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Post by insidecircles »

bajaman wrote: 08 Jun 2023, 11:37 The 8r2 resistor needs to be connected to the junction of the 1N4001 and the 100uf capacitor surely ?
You'd think. Image below shows trace/node at 9V/8R2/diode.

I did however mess up the position of the other diode, which is after the series resistor. Updated schematic below.

ETA: Argh, it's 82R, not 8R2. Yesterday, I was convinced I'd ironed out all my errors in this.
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PS.png

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Post by Manfred »

The series connection of the 100uF electrolytic capacitor and the 1N4001 diode makes no sense.
When the voltage is applied to the circuit, the capacitor is charged up to this voltage and then it has no more purpose,
because no current can be taken from it by the diode in reverse direction and the stabilization does not work any more.
I think the circuit is rather in this way:
PSU-Cornerstone - Royal Antique.jpg
PSU-Cornerstone - Royal Antique.jpg (14.78 KiB) Viewed 1370 times
Last edited by Manfred on 09 Jun 2023, 00:16, edited 1 time in total.

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Post by bmxguitarsbmx »

MPSA18 bias resistor (510k) should go to some positive voltage. Likely connected to Vr.

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Post by insidecircles »

bmxguitarsbmx wrote: 08 Jun 2023, 23:51 MPSA18 bias resistor (510k) should go to some positive voltage. Likely connected to Vr.
Good call - you're right. Updated below.
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Flame boost_2.png
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Royal antique trace_4.png

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Post by insidecircles »

Manfred wrote: 08 Jun 2023, 19:10 The series connection of the 100uF electrolytic capacitor and the 1N4001 diode makes no sense.
When the voltage is applied to the circuit, the capacitor is charged up to this voltage and then it has no more purpose,
because no current can be taken from it by the diode in reverse direction and the stabilization does not work any more.
I think the circuit is rather in this way:
I completely agree. It's just... that's not what is on the board. :?

If I were to build this myself, I'd definitely just use a 'normal' power section, like this.

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Post by MikeMikeMike »

bmxguitarsbmx wrote: 08 Jun 2023, 23:51 MPSA18 bias resistor (510k) should go to some positive voltage. Likely connected to Vr.
Good catch! Same treatment should be applied to n°1 and n°2 "2N5088".
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Post by Dirk_Hendrik »

After the MPSA to Vr has been corrected som posts back, the same comment applies to the 2 other NPN transistors. The biasing resistors should go to Vr, not GND.
Sorry. Plain out of planes.

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